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  • Coral geochemical signals and growth responses to coseismic uplift during the great Sumatran megathrust earthquakes of 2004 and 2005
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-24
    Saori Ito; Atsuko Yamazaki; Yuichi Nishimura; Eko Yulianto; Tsuyoshi Watanabe

    The annual banded skeletons of reef corals potentially record past earthquakes events. We examined cores of five living Porites coral heads in Simeulue Island, Indonesia, near the epicenter of the 2004 Sumatra-Andaman and the 2005 Nias-Simeulue earthquakes. These sites showed 0.4-1.4 m of uplift. We measured skeletal strontium, magnesium, and calcium; carbon and oxygen isotopic ratios; and skeletal density, extension, and calcification rates, from 1994 to 2010. Coral geochemistry fluctuates more than are expected from strictly environmental causes; however, stress bands, reduced growth rates, and changed skeletal δ13C appear to reflect the tsunami and seismic uplift (the step-change in skeletal δ13C results equated to 0.31 ± 0.10 ‰/m in response to the 2004 uplift and 0.23 ± 0.03 ‰/m to the 2005 uplift).

    更新日期:2020-01-24
  • Geochemically heterogeneous Martian mantle inferred from Pb isotope systematics of depleted shergottites
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-24
    Ryota Moriwaki; Tomohiro Usui; Minato Tobita; Tetsuya Yokoyama

    Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (238U/204Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding 147Sm/144Nd, 176Lu/177Hf, ɛ182W, and ε142Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

    更新日期:2020-01-24
  • Timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 Vesta
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-24
    F. Jourdan; T. Kennedy; G.K. Benedix; E. Eroglu; C. Mayer

    Eucrites are extraterrestrial basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of the HED (Howardite-Eucrite-Diogenite) parent body, thought to be the asteroid 4 Vesta. Unbrecciated eucrites offer the best way to understand the igneous, metamorphic and cooling processes occurring in the crust of Vesta following accretion since they were not substantially affected/altered by secondary impact processes. The 40Ar/39Ar system of unbrecciated eucrites should be in a relatively pristine state, and thus can inform us on the early volcanic and thermal history of the HED parent body, and, in particular, the cooling history of various crustal parts below the ∼300 °C isotherm, which represents the average closure temperature of the Ar diffusion in plagioclase. We analysed plagioclase and pyroxene (± groundmass) separates of two cumulate (Moore County and Moama), and five (Caldera, BTN 00300, EET 90020, GRA 98098, QUE 97053) equilibrated basaltic eucrites with the 40Ar/39Ar technique using a Thermo© ARGUS VI multi-collection mass spectrometer. The two cumulate unequilibrated gabbros also gave cooling ages of 4531 ± 11 Ma and 4533 ± 12 Ma and combined with a fast cooling rate estimated from lamella thicknesses, suggest that magmatic activity persisted up to 4533 ± 11 and 4535 ± 12 Ma and that the plutons were intruded in a relatively shallow part of the crust, above the metamorphosed regions. Four equilibrated eucrites yielded a well-defined cluster of ages between 4523 ± 8 Ma to 4514 ± 6 Ma. Those ages indicate when the part of the upper crust, where those eucrites probably resided (∼10-15 km deep), cooled below ∼300°C at a rate of 17.3 ± 3.6°C/Ma (2σ). Such a slow cooling rate combined with available global thermal models, supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows. Finally, an age of 4531 ± 5 Ma was obtained for metamorphosed eucrite EET 90020 and, combined with petrographic observations, indicates the age of a major crustal excavation event by impact. 40Ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by 40Ar/39Ar.

    更新日期:2020-01-24
  • Textural and Geochemical Investigation of Pyrite in Jacobina Basin, São Francisco Craton, Brazil: Implications for Paleoenvironmental Conditions and Formation of pre-GOE Metaconglomerate-Hosted Au-(U) Deposits
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-24
    Guilherme S. Teles; Farid Chemale; Janaína N. Ávila; Trevor R. Ireland; Airton N.C. Dias; Daniele C.F. Cruz; Carlos J.L. Constantino

    The Jacobina Basin has a well-preserved sedimentary record, including continental and marine deposits, and hosts Au-(U)-Py mineralization in metaconglomerate beds similar to the Witwatersrand gold province. Based on petrographic observations, in situ trace-elements, and multiple sulfur isotope analyses (32S, 33S, 34S, and 36S) on pyrite from alluvial and marine facies, several types of pyrite were recognized. The pyrite grains identified in the alluvial metaconglomerates resemble those found in several pre-GOE gold-bearing metaconglomerates, including detrital and epigenetic varieties. Detrital inclusion-bearing pyrite is enriched in several redox-sensitive trace-metals as well as Au, which indicate an association with carbonaceous shales. On the other hand, the sources of detrital massive pyrite are more variable and include igneous, metamorphic, and hydrothermal sources from the Paleoarchean hinterland of Jacobina Basin. Epigenetic pyrite in metaconglomerates formed during metamorphism by the recrystallization of detrital pyrite with negligible contributions from external hydrothermal fluids to the basin. Diagenetic and epigenetic pyrite are found in marine lithologies. In a metapelite sample, the pyrite grains formed near the sediment-water column interface, with S sourced from the photolytic sulfate (SO42-, Δ33S < 0) dissolved in the water column. A quartzite sample, in turn, has detrital pyrite grains that were likely reworked from continental deposits, as well as pyrite formed by the assimilation of elemental sulfur (S8, Δ33S > 0) that accumulated in sediment pore water during diagenesis. Significantly, the pyrite associated with terrestrial metasediments shows a wide range in δ34S values but quite restricted ranges in Δ33S and Δ36S values, whereas pyrite associated with the marine metasedimentary rocks exhibits limited δ34S values but has a wide range in Δ33S values and correlated Δ36S values close to the Archean array. These data suggest distinct preservation routes for MIF-S from atmospheric SO42- and S8 in terrestrial and marine environments. Conditions on the terrestrial surface resulted in re-equilibration of distinct S sources, diminishing the amplitude of the Archean atmospheric signal. In contrast, SO42- dissolved in shallow marine settings while S8 settled to the floor, favoring the preservation of MIF-S isotopic signatures. Such processes may also explain the apparent differences in interpretations of atmospheric conditions derived from uncharacterized pyrites from Archean sources. Our data suggest that the Earth’s atmosphere remained anoxic, and terrestrial conditions were such as to allow the syngenetic accumulation of gold, as recently proposed for the Witwatersrand gold deposits.

    更新日期:2020-01-24
  • Controls on the shuttling of manganese over the northwestern Black Sea shelf and its fate in the euxinic deep basin
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-23
    W.K. Lenstra; M.J.M. Séguret; T. Behrends; R.K. Groeneveld; M. Hermans; R. Witbaard; C.P. Slomp

    Manganese (Mn) is an essential micronutrient for phytoplankton and its cycling interacts with that of iron (Fe). Continental shelf sediments are a key but poorly quantified source of Mn to marine waters. In this study, we investigate Mn release from shelf sediments, its lateral transport (”shuttling”) in the oxic water column over the northwestern Black Sea shelf and its fate in the adjacent euxinic deep basin. We find a high release of Mn from organic-rich, bioirrigated coastal sediments, but negligible mobilization and release of Mn from sediments in offshore shelf regions, because of a low input of organic matter. Most Mn in the water column is present in dissolved form. We suggest that this dissolved Mn is released from coastal sediments and subsequently transported offshore through physical processes. Surface sediments at open shelf and shelf edge stations are highly enriched in Mn when compared to coastal and deep basin stations. Only part of the surface enrichment can be explained by oxidation of porewater Mn. The remainder of this enrichment is likely the result of oxidative removal of dissolved Mn from the water column and deposition as Mn oxides. Using X-ray spectroscopy we show that Mn in surface sediments in this area predominately consists of Mn(IV) oxides (phyllo- and/or tectomanganates). A key difference between Mn versus Fe shuttling is the form in which the metal is transported: while dissolved Mn dominates in the water column over the shelf, most Fe is present in particulate form. Sediment trap data indicate that the vertical transport of both Mn and Fe through the euxinic water column is correlated and is associated with the sinking flux of biogenic particulate matter following the spring and fall phytoplankton blooms. In the sediments of the euxinic basin, Mn is enriched when compared to a detrital Mn background and its burial correlates with that of Fe. This suggests that Mn could be incorporated in pyrite in the euxinic water column. Our results highlight the critical role of organic matter input as a driver of Mn and Fe shuttling over the Black Sea continental shelf and particulate matter as the carrier of Mn and Fe into the euxinic basin.

    更新日期:2020-01-23
  • Sulfur isotope composition of individual compounds in immature organic-rich rocks and possible geochemical implications
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Lubna Shawar; Ward Said-Ahmad; Geoffrey S. Ellis; Alon Amrani

    We applied compound-specific sulfur isotope analysis (CSSIA) to organic matter (OM) extracted from ancient and immature organic-rich rocks from the Cretaceous Ghareb (Shefela Basin locality, Israel) and Miocene Monterey (Naples Beach locality, California, USA) Formations. Large variations in the δ34S values of different organosulfur compounds (OSCs), that reach up to 28‰ and 36‰, were observed in the Ghareb and Monterey samples, respectively. Additionally, some common OSCs in both locations showed consistent 34S trends relative to each other. The consistent enrichment in 34S of C35 hopane thiophene relative to iC20 thiophene in the studied sections probably resulted from differences in the timing of OM sulfurization. Reactive organic precursors quickly consume the most 34S-depleted reduced S, while less reactive species incorporate the heavier residual S at a later time. Despite the differences in the depositional environments, ages, and the initial δ34S values of the reduced S (represented by the δ34S of pyrite) between the Ghareb and the Monterey Formations, the sulfurization order of common organic compounds seems to be similar. All of the δ34S values of OSCs are 34S enriched relative to that of the coexisting pyrite with the exception of the C25 highly branched isoprenoid (HBI) thiophene in several samples from the Monterey Formation. The existence of 34S-depleted sulfurized HBI may point to OM sulfurization that occurred at or near the sediment-water interface during the deposition of the Monterey. Moreover, the δ34S of steroid sulfides shows an inverse trend with the pristane/phytane ratio, which may indicate that the sulfurization mechanism of these OSCs are affected by redox conditions. Further investigation of CSSI values in immature rocks from other basins may help constrain the OM sulfurization process, timescale, and depositional conditions and their possible use as paleoenvironmental proxies.

    更新日期:2020-01-23
  • Cycling Phosphorus on the Archean Earth: Part I. Continental weathering and riverine transport of phosphorus
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Jihua Hao; Andrew H. Knoll; Fang Huang; Robert M. Hazen; Isabelle Daniel

    Phosphorus (P) is the key nutrient thought to limit primary productivity on geological timescales. Phosphate levels in Archean marine sediments are low, but quantification of the P cycle and how it changed through a billion years of recorded Archean history remain a challenge, hindering our understanding of the role played by P in biosphere/geosphere co-evolution on the early Earth. Here, we design kinetic and thermodynamic models to quantitatively assess one key component of the early P cycle – continental weathering – by considering the emergence and elevation of continents, as well as the evolution of climate, the atmosphere, and the absence of macroscopic vegetation during the Archean Eon. Our results suggest that the weathering rate of apatite, the major P-hosting mineral in the rocks, was at least five times higher in the Archean Eon than today, attributable to high levels of pCO2,g. Despite this, the weathering flux of P to the oceans was negligible in the early Archean Eon, increasing to a level comparable to or greater than the modern by the end of eon, a consequence of accelerating continental emergence. Furthermore, our thermodynamic calculations indicate high solubilities of primary and secondary P-hosting minerals in the acidic weathering fluids on land, linking to high Archean pCO2,g. Thus, weathering of P was both kinetically and thermodynamically favorable on the Archean Earth, and river water could transport high levels of dissolved P to the oceans, as also supported by the observed P-depletion in our new compilation of Archean paleosols. Lastly, we evaluated the relative rates of physical erosion and chemical weathering of silicates during the Archean Eon. The results suggest that continental weathering on the early and middle Archean Earth might have been transport-limited due to low erosion rates associated with limited subaerial emergence and low plateau elevations; by the late Archean, however, continental weathering would have transited to kinetically-limited state because of continental emergence, increased plateau elevation, and weakening weathering rates. Overall, our weathering calculations together with paleosol evidence indicate an increasing flux of bioavailable P to the oceans through time, associated with late Archean continental emergence, reaching levels comparable to or higher than modern values by the end of the eon. Increased P fluxes could have fueled increasing rates of primary production, including oxygenic photosynthesis, through time, facilitating the irreversible oxidation of the Earth’s atmosphere early in the Proterozoic Eon. (388 words)

    更新日期:2020-01-23
  • Effect of altitude on the stable carbon and oxygen isotopic compositions of land snails at the margin of the East Asian monsoon
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Rui Bao; Xuefen Sheng; Chenglong Li; Hua Shen; Liping Tan; Ling Sun; Chunqian Li; Hongke Peng; Ling Luo; Min Wu; Huayu Lu; Junfeng Ji; Jun Chen

    The carbon and oxygen isotopic composition of land snail shells (δ13Cshell and δ18Oshell) has been used to reconstruct paleoenvironmental changes worldwide. Adding to the limited calibration information available for the monsoon marginal regions, we present δ13C and δ18O results for modern snails, plants and soils, collected from the Helan Mountains, in North China, spanning an altitudinal gradient of 1733-2851 m a.s.l.. Linear regression analysis show that δ13Cshell is positively correlated with altitude on both slopes, with similar rates of altitudinal variation; in contrast,δ18Oshell is negatively correlated with altitude on the western slopes, but this effect is absent on the eastern slopes. The variations in the integrated δ13C signals of living vegetation and the flux of absorbed atmospheric CO2 attributed to changes in metabolic rates are likely responsible for the altitudinal effect on δ13Cshell above 2000 m a.s.l., while changes in vegetation type and/or carbonate ingestion by snails may be important in inducing the positive shift in δ13Cshell below 2000 m a.s.l. δ18Oshell is determined by the rainwater δ18O and by the fractionation processes between shell carbonate and snail body water, and both are controlled by the temperature of the alpine environment. In addition, evaporative enrichment of the body fluid or of the environmental water prior to ingestion by snails exerts an important influence on δ18Oshell. Notably, both δ13Cshell and δ18Oshell in the study region respond more sensitively to climate than in the monsoon core area. The rate of change of δ13Cshell in response to temperature and precipitation above the altitude of 2000 m a.s.l. are -0.87‰/℃ and +2.76‰/100mm, while the corresponding rates for δ18Oshell on the western slopes are +0.41‰/℃ and -1.29‰/100mm. Our results expand the range of application of δ13Cshell and δ18Oshell as climatic indicators, and emphasize the differences in the climatic sensitivity of these proxies between the monsoon margin and core areas. In addition, the differences in shell isotope composition in response to environment changes between adjacent locations (i.e. on different slopes) in mountainous terrain need to be considered in future studies.

    更新日期:2020-01-22
  • Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Simone Tumiati; Carla Tiraboschi; Francesca Miozzi; Alberto Vitale-Brovarone; Craig E. Manning; Dimitri Sverjensky; Sula Milani; Stefano Poli

    Organic matter, showing variable degrees of crystallinity and thus of graphitization, is an important source of carbon in subducted sediments, as demonstrated by the isotopic signatures of deep and ultra-deep diamonds and volcanic emissions in arc settings. In this experimental study, we investigated the dissolution of sp2 hybridized carbon in aqueous fluids at 1 and 3 GPa, and 800°C, taking as end-members i) crystalline synthetic graphite and ii) X-ray amorphous glass-like carbon. We chose glass-like carbon as an analogue of natural “disordered” graphitic carbon derived from organic matter, because unlike other forms of poorly ordered carbon it does not undergo any structural modification at the investigated experimental conditions, allowing approach to thermodynamic equilibrium. Textural observations, Raman spectroscopy, synchrotron X-ray diffraction and dissolution susceptibility of char produced by thermal decomposition of glucose (representative of non-transformed organic matter) at the same experimental conditions support this assumption. The redox state of the experiments was buffered at ΔFMQ ≈ –0.5 using double capsules and either fayalite-magnetite-quartz (FMQ) or nickel-nickel oxide (NNO) buffers. At the investigated P–T–fO2 conditions, the dominant aqueous dissolution product is carbon dioxide, formed by oxidation of solid carbon. At 1 GPa and 800°C, oxidative dissolution of glass-like carbon produces 16–19 mol% more carbon dioxide than crystalline graphite. In contrast, fluids interacting with glass-like carbon at the higher pressure of 3 GPa show only a limited increase in CO2 (fH2NNO) or even a lower CO2 content (fH2FMQ) with respect to fluids interacting with crystalline graphite. The measured fluid compositions allowed retrieving the difference in Gibbs free energy (ΔG) between glass-like carbon and graphite, which is +1.7(1) kJ/mol at 1 GPa–800°C and +0.51(1) kJ/mol (fH2NNO) at 3 GPa–800°C. Thermodynamic modeling suggests that the decline in dissolution susceptibility at high pressure is related to the higher compressibility of glass-like carbon with respect to crystalline graphite, resulting in G–P curves crossing at about 3.4 GPa at 800°C, close to the graphite–diamond transition. The new experimental data suggest that, in the presence of aqueous fluids that flush subducted sediments, the removal of poorly crystalline “disordered” graphitic carbon is more efficient than that of crystalline graphite especially at shallow levels of subduction zones, where the difference in free energy is higher and the availability of poorly organized metastable carbonaceous matter and of aqueous fluids produced by devolatilization of the downgoing slab is maximized. At depths greater than 110 km, the small differences in ΔG imply that there is minimal energetic drive for transforming “disordered” graphitic carbon to ordered graphite; “disordered” graphitic carbon could even be energetically slightly favored in a narrow P interval.

    更新日期:2020-01-22
  • Dissolved silicon isotope dynamics in large river estuaries
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Zhouling Zhang; Zhimian Cao; Patricia Grasse; Minhan Dai; Lei Gao; Henning Kuhnert; Martha Gledhill; Cristiano M. Chiessi; Kristin Doering; Martin Frank

    Estuarine systems are of key importance for the riverine input of silicon (Si) to the ocean, which is a limiting factor of diatom productivity in coastal areas. This study presents a field dataset of surface dissolved Si isotopic compositions (δ30SiSi(OH)4) obtained in the estuaries of three of the world’s largest rivers, the Amazon (ARE), Yangtze (YRE), and Pearl (PRE), which cover different climate zones. While δ30SiSi(OH)4 behaved conservatively in the YRE and PRE supporting a dominant control by water mass mixing, significantly increased δ30SiSi(OH)4 signatures due to diatom utilization of Si(OH)4 were observed in the ARE and reflected a Si isotopic enrichment factor 30ε of −1.0±0.4‰ (Rayleigh model) or −1.6±0.4‰ (steady state model). In addition, seasonal variability of Si isotope behavior in the YRE was observed by comparison to previous work and most likely resulted from changes in water residence time, temperature, and light level. Based on the 30ε value obtained for the ARE, we estimate that the global average δ30SiSi(OH)4 entering the ocean is 0.2-0.3‰ higher than that of the rivers due to Si retention in estuaries. This systematic modification of riverine Si isotopic compositions during estuarine mixing, as well as the seasonality of Si isotope dynamics in single estuaries, needs to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle.

    更新日期:2020-01-22
  • Secular variation in the elemental composition of marine shales since 840 Ma: Tectonic and seawater influences
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Wei Wei; Thomas J. Algeo

    Marine shales, the most common lithology in the stratigraphic record, represent an archive of long-term changes in sediment production, weathering intensity, and seawater chemistry through Earth history whose potential has been underexploited to date. In this study, we compiled compositional data for 14,531 samples from 268 shale and mud units ranging in age from the early Cryogenian (840 Ma) to the Recent. We examined secular variation in the Al-normalized concentrations of ten elements (Mg, Na, K, Ba, Sr, Y, Th, Rb, Nb, and Zr) selected for (1) their presence in typical X-ray fluorescence (XRF) datasets (thus providing sufficient data), and (2) their lack of redox sensitivity (thus excluding elements whose primary control is aqueous redox conditions). Relationships among the study elements were quantified using principal component analysis (PCA) and cluster analysis (CA), revealing several significant groupings: (1) a subset of “detrital elements” (K, Rb, Th, Nb, Y, and Zr) whose concentrations are primarily controlled by subaerial weathering processes; (2) a subset of “hydrogenous elements” (Mg, Na, and Sr) whose concentrations are significantly influenced by seawater chemistry, and (3) Ba, which is subject to multiple controls. The dominant influence on the detrital elements appears to have been not chemical weathering intensity per se but the ratio of Al-poor silt to Al-rich clay in the particulate weathering flux. Although possibly modulated by factors such as climate and land plant evolution, silt:clay ratios appear most closely linked to tectonism, with higher values associated with major orogenic events. The concentrations of the hydrogenous elements in marine shales were likely related to their seawater inventories, which were modulated at long time scales by rates of submarine hydrothermal alteration. Ba in marine shales may have been significantly influenced by atmospheric pO2 and seawater sulfate levels via their control of barite saturation in seawater. In summary, our findings document significant relationships of marine shale composition to first-order geological controls, suggesting that long-term secular changes in sediment and seawater chemistry can be reconstructed from fine-grained siliciclastic sediments.

    更新日期:2020-01-22
  • Two distinct episodes of marine anoxia during the Permian-Triassic crisis evidenced by uranium isotopes in marine dolostones
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Feifei Zhang; Shu-zhong Shen; Ying Cui; Timothy M. Lenton; Tais W. Dahl; Hua Zhang; Quan-feng Zheng; Wenqian Wang; Karl Krainer; Ariel D. Anbar

    The end-Permian mass extinction (EPME; ca. 251.94 Ma) is the most severe mass extinction in the geological record. Detailed paleobiological investigations show a very rapid EPME event, and recently published δ238U data show a large negative excursion and thus a massive shift to globally expanded anoxia at the main extinction phase in the latest Permian. The negative shift in δ238U is in correlation with a globally characterized negative δ13C excursion near the Permian-Triassic boundary (PTB). In some highly expanded PTB carbonate sections, however, there are two distinct negative δ13C excursions whereas uranium isotopes (δ238U) from such sections have not yet been examined, leaving a gap in the understanding of the global perturbations of marine redox conditions immediately following the EPME. Here, we present a new δ238U study of syn-depositional dolostones from a well-characterized and highly expanded drill core, which recorded two pronounced negative δ13C excursions across the PTB, from the Carnic Alps, Austria. This drill core extends 330-meters across the PTB and provides a unique opportunity to explore the detailed timing, duration, and extent of marine redox chemistry changes before, during, and immediately after the EPME. Our new δ238U record shows two negative shifts, which are correlated with the two negative δ13C excursions. The first negative δ238U excursion preceding the EPME confirms the recently published δ238U records from across the EPME and support that syndepositional marine dolostones can record δ238U trends of seawater similar to that of limestones. Modeling of uranium isotope cycling in the latest Permian and earliest Triassic oceans suggests two distinct stages of expanded marine anoxia separated by a brief interval (∼100 kyr) of reoxygenation across the PTB. The first anoxic episode lasted for ∼60 kyr while anoxic seafloor area expanded to cover >18% of the entire seafloor, coeval with the main EPME horizon, agreeing with marine anoxia as a proximate kill mechanism for the EPME. The second anoxic event was less intense compared to the first anoxic pulse but sustained for a longer duration. A global modeling of coupled C, P, and U cycles show that two pulses of volcanic carbon injection that drives global warming and increased phosphorus weathering rate can reasonably reproduce our data to match two phases of anoxia. The model also demonstrates that the loss of terrestrial vegetation in the EPME is crucial to generating an intervening interval of oxygenated ocean. Our new study adds to a growing body of evidence that the global marine redox conditions underwent rapid oscillations during the EPME event and continued afterward, which may have played a central role in delaying the marine ecosystem recovery in the Early Triassic.

    更新日期:2020-01-22
  • Field biotite weathering rate determination using U-series disequilibria
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-22
    Rihs Sophie; Gontier Adrien; Voinot Alexandre; Chabaux François; Pelt Eric; Lemarchand Damien; Turpault Marie-Pierre

    The chemical weathering rate of silicates is a key parameter of several geochemical processes; however, the long-term field measurement of these rates remains challenging. Currently, there are several methods for quantifying field weathering rates, but each presents it's own unique set of difficulties. In this study uranium-series nuclides were used to investigate long-term biotite weathering in a soil profile. For this purpose, pseudomorph grains of biotites (including weathering products) were hand-picked from four horizons of a soil profile and from underlying granitic saprolite from the experimental Breuil-Chenue site (France). Unexpected behaviors of U and Th nuclides were recorded from these samples. An unambiguous leaching of 232Th occurs during the weathering of biotite, while reduced U release relative to Th was observed, implying an efficient redistribution of U between the primary mineral and weathering products within the pseudomorphs. The measured U-series activity ratios reflect a regular and self-consistent pattern evolution. However, the “U-series-derived-weathered-stage” is not always coherent with the weathering features of major elements and/or with the location of the samples within the soil profile. For instance, biotite from the saprolite displays U-series activity ratios typical of significantly weathered samples, while major elements present rather limited signs of weathering. These results suggest an incongruent leaching of U and Th isotopes and suggest that U- and Th-series nuclides probe some water-mineral interactions that occur before macroscopic mineral tansformation. Using an open-system leaching model, coupled (234U/238U), (234U/230Th), and (226Ra/230Th) disequilibria measured from the samples allow us to calculate a weathering duration range of 6 to 52 ka for most weathered samples of this soil profile. The biotite weathering rate deduced from these data ranges from 7.8×10-17 mol.m-2.s-1 to 6.5×10-16 mol.m-2.s-1, which is consistent with the range of field rates previously reported. This rate was compared to the weathering rate of biotites induced by the substitution of vegetation occuring 35 years ago in a nearby soil profile, which was about 2 - 3 orders of magnitude higher than the long-term rate reported above. This feature is perfectly consistent with the decrease in the weathering rate with time that was previously widely documented but that still remains debated. Our results allude to a valuable use of U-series isotopes for mineral-weathering field rate determination and report a decrease in silicate weathering rates over long- (10 000s of years) and short-term (10s of years) weathering events recorded from the same site. The results show that short term laboratory experiments can be representative of field processes, and suggest that some forms of mineral weathering reactivation, similar to that observed during laboratory experiments, may naturally occur in soils in response to high frequency perturbation processes such as land cover change.

    更新日期:2020-01-22
  • Clumped isotope geochemistry of carbonatites in the north-western Deccan igneous province: Aspects of evolution, post-depositional alteration and mineralisation
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Benjamin R. Fosu; Prosenjit Ghosh; Shrinivas G. Viladkar

    Carbonatites crystallise along a wide range of solidus temperatures and are commonly affected by post-magmatic textural re-equilibration and diagenesis. Further insights into the formation and modification of carbonatites are provided using carbon, oxygen and clumped isotope (Δ47) data of rocks from spatially associated Amba Dongar and Siriwasan alkaline complexes in the north-western Deccan igneous province, India. We derive apparent equilibrium blocking temperatures to help constrain the thermal evolution of the different rock types found within the alkaline complexes in a petrographic context. The apparent temperatures for the carbonatites are significantly low but are consistent with reports on other global carbonatites and model predictions. Rapidly cooled Oldoinyo Lengai natrocarbonatite yielded similar low temperatures, even in the absence of bulk isotopic alteration. The isotopic proxies and petrographic observations favour both isotopic exchange reactions and diagenesis in altering Δ47 values in calciocarbonatites. Diagenetic reactions are however strongly favoured, as secondary calcites in nephelinites and ferrocarbonatites record much lower temperatures than in the calciocarbonatites, highlighting the effect of fluids and diagenetic reactions in 13C-18O bond ordering in carbonatites. Variations in the C-O isotope data reveal the coupling of fractional crystallisation and post-magmatic fluid-rock interactions on bulk rock composition. After emplacement, the resetting of clumped isotope signatures in carbonatites is facilitated by post-magmatic processes in both open and closed systems.

    更新日期:2020-01-22
  • Constraining multiple controls on planktic foraminifera Mg/Ca
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Kate Holland; Oscar Branson; Laura L. Haynes; Bärbel Hönisch; Katherine A. Allen; Ann D. Russell; Jennifer S. Fehrenbacher; Howard J. Spero; Stephen M. Eggins

    The Mg/Ca of planktic foraminifera is widely used to determine past surface ocean temperatures but temperature is not the only factor that controls test Mg/Ca. Here we quantify the combined influence of seawater temperature, carbon chemistry, and cation chemistry on Orbulina universa Mg/Ca, based on experimental cultures where these factors were varied independently and simultaneously. We fit a new empirical multi-parameter model that quantifies the effects of each of these variables on the Mg/Ca of O. universa: Mg/CaO. universa=Mg/CaswA∙DICswB∙expC∙Casw+D∙T+E We extend our approach to published Mg/Ca from cultured and sediment trap collected Globigerinoides ruber, and show that a similar equation can be used to describe test Mg/Ca, using [CO32-]sw or pH in place of DIC. The cause of this difference is unknown, but the sensitivity of these two ecophysiologically similar species to different carbon chemistry parameters highlights the uncertainty inherent in applying modern Mg/Ca calibrations to uncalibrated and/or extinct species. In both O. universa and G. ruber, Mg/Casw, [Ca]sw and carbon chemistry (DIC, [CO32-]sw or pH) each modulate the sensitivity of Mg/Ca to temperature. These variables are not constant in the ocean over time-scales relevant to palaeotemperature reconstructions, and must be accounted for when deriving temperature from foraminiferal Mg/Ca. Over time-scales longer than the residence times of Mg and Ca, secular changes in Mg/Casw and [Ca]sw will exert a primary influence on foraminiferal Mg/Ca and its sensitivity to temperature. On shorter timescales, co-variance between temperature and seawater carbon chemistry will influence Mg/Ca-derived temperature estimates. This is particularly relevant to transient hyperthermal events, where the Mg/Ca palaeothermometer is used to evaluate the relationship between atmospheric CO2 and ocean temperature. We explore the potential impact of these effects in an illustrative re-interpretation of Mg/Ca records across the Paleocene-Eocene Thermal Maximum.

    更新日期:2020-01-22
  • Quadruple sulfur isotopic fractionation during pyrite desulfidation to pyrrhotite
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Li Liu; John Mavrogenes; Peter Holden; Trevor Ireland

    The chemical and mineralogical conversion of pyrite to pyrrhotite has been studied in many aspects, but the associated quadruple sulfur isotopic fractionation between these phases has not been investigated previously. In this study, Ruttan pyrite and pyrite from a Neoarchean carbonaceous phyllite drill core were heated to 675 °C, and the released sulfur was reacted with iron metal, forming compact sulfide rims. The products of both desulfidized pyrite and sulfidized iron metal are pyrrhotite, following the equations n(1-x)FeS2 = nFe1-xS + (1-2x)Sn and Sn + n(1-x)Fe = nFe1-xS. The mean δ34S of the Ruttan experiment products has shifted slightly with the residue becoming slightly heavier and the sulfidized iron becoming slightly lighter than the starting Ruttan pyrite. However, the shift is small (0.5 ‰ and 1.2 ‰, respectively), yet the range of δ34S increases in the pyrrhotite products, suggesting the breakdown and evaporation of sulfur is not occurring uniformly. A shift in δ34S between the initial Archean pyrite and resultant pyrrhotite products is likely a real difference in composition of the analyzed pyrite and the pyrite extracted for the experiment. The compositions of the pyrrhotite run products are quite close with Δ33S and Δ36S being within ca. 0.1 ‰. They differ slightly from the initial Archean pyrite but this is likely due to heterogeneity in the original pyrite. The experiments indicate that the fractionation in Δ33S and Δ36S during pyrite desulfidation, sulfur evaporation transfer and condensation and iron metal sulfidation is negligible. Therefore the preservation of Δ33S and Δ36S between initial pyrite and released sulfur during pyrite desulfidation is demonstrated, establishing Δ33S and Δ36S as effective tools of tracing sulfur sources of Archean gold deposits and opening up pyrrhotite as an appropriate phase for studying Archean S-MIF.

    更新日期:2020-01-22
  • Effect of deformation on helium storage and diffusion in polycrystalline forsterite
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Rémi Delon; Sylvie Demouchy; Yves Marrocchi; Mohamed Ali Bouhifd; Julien Gasc; Patrick Cordier; Sanae Koizumi; Pete G. Burnard

    Although recent studies have investigated He behavior in undeformed mantle minerals, the effect of defects generated by plastic deformation on He storage and transport remains unconstrained. For this purpose, synthetic dense aggregates of fine-grained iron-free forsterite were deformed under 300 MPa confining pressure at 950, 1050, and 1200 °C using a Paterson press. Three deformed samples and one undeformed sample were then doped with He under static high-pressure (1.00 ± 0.02 GPa) and high-temperature (1120 ± 20 °C) conditions for 24 h in a piston cylinder. Uraninite was used as a source of noble gases. The samples were subsequently analyzed using a cycled step heating protocol coupled with noble gas mass spectrometry to investigate He storage and diffusion in the deformed polycrystalline forsterite aggregates. Results show complex diffusive behaviors that cannot be fitted by a single linear regression. Nevertheless, individual step heating cycles can be fitted by several linear regressions determined by a F-test, suggesting that diffusivities follow Arrhenius law within the given temperature ranges. Our results highlight the complex diffusive behavior of He in deformed forsterite aggregates, which is due to the competition between several diffusion mechanisms related to different He storage sites (Mg vacancies, interstitial sites, dislocations, and grain boundaries). Diffusion parameters (activation energy Ea and pre-exponential factor D0) for He diffusion in grain boundaries were refined from literature data (Ea = 36 ± 9 kJ·mol–1 and D0 = 10–10.57 ± 0.58 m2·s–1), and those of He diffusion in interstitials (Ea = 89 ± 7 kJ·mol–1 and D0 = 10–8.95 ± 1.16 m2·s–1) and Mg vacancies (Ea = 173 ± 14 kJ·mol–1 and D0 = 10–5.07 ± 1.25 m2·s–1) were defined from our results and literature data. Furthermore, we determined Ea = 56 ± 1 kJ·mol–1 and D0 = 10–9.97 ± 0.37 m2·s–1 for He diffusion along dislocations. These results suggest that a maximum He fraction of only 1.2% can be stored along dislocations in mantle minerals, which is negligible compared to 22% in grain boundaries as reported by previous studies. This implies that bulk lattice diffusivities are barely affected by the presence of dislocations, whereas the proportion of He stored in grain boundaries can significantly enhance the bulk diffusivities of mantle rocks. Thus, deformation processes can significantly increase He storage capacity by decreasing grain size (i.e., via dynamic recrystallization), but will not sufficiently increase the dislocation density to induce a change in He storage and mobility within the crystallographic lattice. Furthermore, rapid redistribution of He between the mineral lattice and grain boundaries could enhance the bulk He concentrations of deformed peridotites upon equilibration with nearby undeformed (or less-deformed) peridotites.

    更新日期:2020-01-22
  • Spatial U–Pb age distribution in shock-recrystallized zircon – a case study from the Rochechouart impact structure, France
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Cornelia Rasmussen; Daniel F. Stockli; Timmons M. Erickson; Martin Schmieder

    Age determination of impact structures via the zircon U–Pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on U-Pb geochronology. It is, therefore, crucial to link the observed zircon microtextures, including their temperature and pressure conditions associated with their formation, directly to the U–Pb ages preserved. Here, we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium-sized Rochechouart impact structure in the northwestern Massif Central of France. For the Rochechouart impact structure the impact age (206.92 ± 0.32 Ma [40Ar/39Ar]) as well as the tectono-themal history is well established making this study site ideal to test concepts about U-Pb systematics in shocked zircon and to differentiate between shock-driven age resetting and pre-impact crystallization and metamorphic overprinting. Zircon microstructures were studied using scanning electron imaging, cathodoluminescence imaging, and electron backscatter diffraction (EBSD) mapping. Further, we conducted U–Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) depth-profiling analysis, allowing us to interpret the resultant age data in discrete steps with increasing ablation time/depth. The U–Pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre-impact (pre-Variscan and Variscan) regional tectonic ages. Our results also reveal that the granular crystals encountered contain microstructural evidence for “former reidite in granular neoblastic” (FRIGN) zircon, reflecting both high-pressure (⩾30GPa) and high-temperature (⩾1200℃) conditions. This signifies that FRIGN zircon is now known from an additional, medium sized, impact structure further supporting the hypothesis that this impact induced microstructure is commonly preserved. In addition, FRIGN zircon has a high potential to preserve completely impact-reset crystal domains (∼204 to 207 Ma) that can be identified by our combined analytical approach of U–Pb depth profiling and EBSD mapping, and thus are suitable for determining reliable impact ages.

    更新日期:2020-01-22
  • Inverse correlation between the molybdenum and uranium isotope compositions of Upper Devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Brian Kendall; Jieying Wang; Wang Zheng; Stephen J. Romaniello; D. Jeffrey Over; Yvonne Bennett; Liyan Xing; Alexandra Kunert; Cameron Boyes; Jian Liu

    Coupled Mo-U isotope data from the Upper Devonian Kettle Point Formation (Ontario, Canada) provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales. In the Gore of Chatham core, the Kettle Point black shales have high Mo concentrations (48–473 μg/g) and consistently high Mo/U ratios (≥3 times the Mo/U ratio of modern seawater), suggesting a euxinic depositional environment. These shales yield an inverse correlation (r = 0.89, p < 0.001) between authigenic U isotope (δ238U = –0.3‰ to +0.6‰ relative to CRM145 = 0‰) and Mo isotope (δ98Mo = +0.5‰ to +2.0‰ relative to NIST SRM 3134 = 0.25‰) compositions and a stratigraphic trend upsection towards lower δ98Mo and higher δ238U. These stratigraphic trends were probably not caused by global ocean redox variation, which should shift both isotope systems in the same direction. Instead, the inverse correlation between the δ98Mo and δ238U of the black shales indicates that changes to local depositional conditions in the Chatham Sag affected both isotope systems, consistent with the lithological and elemental evidence for variable paleohydrographic conditions (paleosalinity and sea-level). Black shales with the highest δ98Mo and lowest δ238U likely capture the most quantitative removal of Mo and U from more intensely sulfidic bottom waters, and thus most closely approximate global seawater isotope compositions. Lower δ98Mo and higher δ238U in the black shales likely record less quantitative Mo and U removal from weakly euxinic bottom waters and thus capture greater sediment-seawater isotope fractionation (lighter Mo isotopes and heavier 238U were preferentially removed from seawater to the sediments). Some samples with low δ98Mo (0.5-1.0‰) have high V and Mo enrichments, suggesting delivery of V and isotopically light Mo to the sediments by Fe-Mn particulates, but this process does not significantly affect U isotopes (because of weak adsorption of U onto Fe-Mn particulates) and is thus not the major driver of the inverse correlation. The highest δ98Mo (2.0‰) and lowest δ238U (–0.3‰) from the lower Kettle Point Formation may thus represent minimum and maximum estimates of the Mo and U isotope compositions, respectively, of early Famennian global seawater. The slope of the inverse Mo-U isotope correlation for the Kettle Point black shales broadly parallels data from modern euxinic basins but is shifted to slightly lower isotopic compositions. Hence, Famennian seawater likely had δ98Mo (about 2.0‰ to 2.2‰) and δ238U (about –0.7‰ to –0.4‰) values that were slightly lower than modern global seawater (δ98Mo = 2.3‰; δ238U = –0.4‰), suggesting a mildly greater extent of euxinia (up to 5% of the seafloor) in the Famennian oceans. The opposite stratigraphic trends in δ98Mo and δ238U through the Kettle Point Formation may have caused misinterpretation of global ocean redox conditions if either isotope system was used individually. Hence, paired Mo and U isotope analyses are recommended to determine if stratigraphic trends through euxinic black shales are caused by local depositional changes or global ocean redox variations.

    更新日期:2020-01-22
  • A Chemo-mechanical Snapshot of In-situ Conversion of Kerogen to Petroleum
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Arash Abarghani; Mehdi Ostadhassan; Paul C. Hackley; Andrew E. Pomerantz; Siamak Nejati

    Organic matter (OM) from various biogenic origins converts to solid bitumen in-situ when it undergoes thermal maturation. It is well documented that during this process, the ratios of both hydrogen and oxygen to carbon will decrease, resulting in an increase in OM aromaticity and molecular chemo-mechanical homogeneity. Although there have been extensive efforts to reveal molecular alteration occurring to OM during conversion, in-situ and continuous observation of such alterations on naturally occurring samples is missing. Therefore, evaluation of previous results cannot be made independent from natural sample variability. In this study, we identified OM particles (Tasmanites) that are evolving in-situ into solid bitumen in the Bakken Formation. This in-situ bituminization allows examination of a continuous transformation in OM molecular structure at micron-scale using AFM based IR spectroscopy applied at the transition/interface zone. Moreover, contact mode in the AFM was employed to reveal and relate changes in mechanical properties at a similar scale of measurement. Understanding these chemical and mechanical alterations is important to understand shale reservoir properties and better explain hydrocarbon generation, expulsion, and migration processes at the microscale.

    更新日期:2020-01-22
  • Large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO2 in a mountain stream
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Hao Yan; Zaihua Liu; Hailong Sun

    Understanding the process of CO2 degassing during precipitation of calcite from a Ca2+-HCO3– solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO2(g) and HCO3– due to breaking of C-O bond, with an equilibrium fractionation of ca. -9‰ at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (δ13C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on δ13C of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO2 in a supersaturated solution where rapid calcite precipitation drives CO2 degassing out of isotopic equilibrium. Here we show the data of water chemistry and carbon isotope compositions of DIC and carbonate precipitates in a mountain stream at Baishuitai, China. Results from numerical models show there exist large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO2 via HCO3– dehydration and dehydroxylation. Average carbon isotope fractionation between CO2(g) and HCO3– (εCO2(g) - HCO3-) calculated from our dataset is about -20‰ which is much lower than the equilibrium value. Moreover, an inverse correlation between εCO2(g) - HCO3- and precipitation rate was observed, indicating higher precipitation rates cause εCO2(g) - HCO3- farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO2(g) and HCO3– may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of εCO2(g) - HCO3- has special implications for speleothem archives from ventilated caves. As partial pressure of CO2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO2 and DIC and thus perturb the time-series of speleothem’s δ13C records for paleo-environmental reconstruction.

    更新日期:2020-01-22
  • Characterisation of shallow groundwater dissolved organic matter in aeolian, alluvial and fractured rock aquifers
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Liza K. McDonough; Helen Rutlidge; Denis M. O'Carroll; Martin S. Andersen; Karina Meredith; Megan I. Behnke; Robert G.M. Spencer; Amy M. McKenna; Christopher E. Marjo; Phetdala Oudone; Andy Baker

    Groundwater organic matter is processed within aquifers through transformations such as the adsorption of dissolved organic matter (DOM) to minerals and biodegradation. The molecular character of DOM varies according to its source and this can impact its bioavailability and reactivity. Whilst the character of DOM in riverine and oceanic environments is increasingly well understood, the sources, character and ultimately the fate of groundwater DOM remains unclear. Here we examine groundwater DOM from contrasting hydrogeological settings in New South Wales, Australia. For the first time, we identify the distinct molecular composition of three groundwater DOM end-members including a modern terrestrial input, an aged sedimentary peat source, and an aged stable by-product pool. We also identify and characterise the processing pathway of DOM in semi-arid, low sedimentary organic carbon (OC) environments. Based on size exclusion chromatography, ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), isotopic analyses (13C, 14C and 3H) and principle component analysis (PCA), we show that in higher rainfall temperate coastal peatland environments, large amounts of aged sedimentary organic carbon can leach into groundwater resulting in higher molecular weight (500 g mol−1 < molecular weight > 1000 g mol−1) and highly aromatic groundwater DOM with high O / C ratios and low H / C ratios. We show that in semi-arid environments with low rainfall rates and high groundwater residence times, groundwater dissolved organic carbon (DOC) is processed into increasingly low molecular weight (< 350 g mol−1), low aromaticity DOM with low O / C ratios and high H / C ratios by subsurface processing mechanisms such as biodegradation and adsorption. We provide the first comprehensive study of groundwater DOM characterisation based on multiple analytical techniques, and highlight the impact of source inputs and processing on groundwater DOM composition at a molecular level.

    更新日期:2020-01-22
  • 40Ar/39Ar systematics of melt lithologies and target rocks from the Gow Lake impact structure, Canada
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    A.E. Pickersgill; D.F. Mark; M.R. Lee; G.R. Osinski

    The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ 40Ar/39Ar analyses we have sought to understand the 40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40Ar/39Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of ‘extraneous’ 40Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ 40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40Ar by using isotope correlation plots to define the initial trapped 40Ar/36Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster.

    更新日期:2020-01-22
  • Uranium reduction and isotopic fractionation in reducing sediments: Insights from reactive transport modeling
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Kimberly V. Lau; Timothy W. Lyons; Kate Maher

    Uranium concentrations and isotopic ratios (238U/235U, denoted as δ238U) have been used to provide quantitative information about the degree of oxygenation and de-oxygenation of past oceans. The potential to constrain changes in global redox conditions, in contrast to many other proxies that reflect local conditions, is a particular strength of the uranium isotope approach. Because uranium reduction primarily occurs in sediments underlying anoxic water columns rather than in the anoxic water column itself, the removal of uranium in organic-rich shales is the largest lever on seawater δ238U. Accordingly, accumulation and isotopic fractionation are modulated by local variations in productivity, basin connectivity, sedimentation rate, and bottom-water redox conditions. To isolate the processes at the sediment-water interface that control δ238U and uranium accumulation in reducing sediments, we constructed a reactive transport model that couples biogeochemical reactions to diffusive transport and the burial of solutes and minerals. Using the model framework, we test the sensitivity of authigenic uranium isotopic fractionation and accumulation to oxygenation, permeability, sedimentation rate, organic carbon delivery, and basin restriction. Our results demonstrate that these external forcings produce diagnostic patterns in isotopic fractionation. Specifically, the model predicts that authigenic δ238U is sensitive to productivity because of the associated organic carbon burial rate. Moreover, our results suggest that the isotopic offset does not vary significantly due to changing bottom-water O2 concentrations, but the amount of accumulation does—a result that differs from previous estimates. Water column uranium reduction adds additional complexity to the ultimate δ238U value. The predictive patterns derived from model results can offer insight into local depositional conditions, such as sedimentation patterns. Collectively, these effects—including bottom-water redox conditions and related reducing sediments—alter the isotopic signature of the overlying water column according to the authigenic δ238U value and the diffusive fluxes arising from porewater concentration gradients. More broadly, this work provides important new constraints on the major controls on the δ238U of sediments while also supporting its use as a proxy for global marine redox conditions.

    更新日期:2020-01-22
  • The composition of Mars
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-21
    Takashi Yoshizaki; William F. McDonough

    Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.

    更新日期:2020-01-22
  • Far from equilibrium basaltic glass alteration: The influence of Fe redox state and thermal history on element mobilization
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-20
    Marius Stranghoener; Stefan Dultz; Harald Behrens; Axel Schippers

    Elemental release from basaltic glasses at far from equilibrium conditions was investigated as a function of the Fe redox state (Fe(II)/Fetot = 0.35 and 0.80) and thermal history (quenched ↔ annealed). A flow-through column setup was used to ensure disequilibrium of basaltic glass and solution during the entire runtime. Percolation experiments were performed at 25 °C for up to 500 h with intermediate sample collection. Two different pH values were adjusted, and the effect of organic matter was tested by adding oxalic acid. Element concentrations in the percolate were measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). After an initial high release of elements from fresh glass surface, steady state was achieved after 240-430 h for Si at pH 5-7 and after 170 -240 h for Al at pH 2. At near neutral conditions (pH 5-7) mobilization of Si is relatively high, while Fe and possibly Al are retained in precipitates. Under more acidic conditions (pH 2), the Si concentration of the solutions was very low compared to the other main constituents of the glass. Large amounts of Si-rich residues are formed after glass dissolution at pH 2. On the other hand, Fe (and Mn) is very mobile under acidic conditions, favored by complex formation with oxalic acid and chlorine. In the initial phase of the pH 2 experiments, the element release from reduced glasses is higher than from oxidized glasses. However, this trend is reversed when approaching steady state. Higher dissolution rates for oxidized glasses are predicted due to the progressive replacement of strong Si-O-Si with weaker Fe(III)-O-Si. At pH 5-7 the concentrations of elements in the percolate are too low to establish a systematic difference between oxidized and reduced glasses. Looking at the total amount of mobilized elements, the thermal history of the glasses has no significant effect in the case of oxalate-free solutions, but a noticeable increase of element release in the case of rapidly quenched glasses was observed when using 1mM oxalic acid solution. The strong effect of oxalate on dissolution of quenched glasses is probably related to the more open glass network structure in quenched glasses.

    更新日期:2020-01-21
  • Glacial influence on the iron and sulfur cycles in Arctic fjord sediments (Svalbard)
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-16
    Alexander B. Michaud; Katja Laufer; Alyssa Findlay; André Pellerin; Gilad Antler; Alexandra V. Turchyn; Hans Røy; Laura M. Wehrmann; Bo Barker Jørgensen

    Arctic fjord sediments of Svalbard receive terrestrial material from glacial runoff and organic matter from marine primary productivity. Organic carbon mineralization proceeds primarily through sulfate and iron reduction in the fjord sediment. The ongoing retreat of glaciers in the high Arctic is altering the input of glacial material to the fjords, with unknown consequences for the iron and sulfur cycles in the fjord sediments. We measured sulfate reduction rates in sediment cores and analyzed porewater geochemistry, then compared these results to long-term sediment incubations to determine the rates of iron reduction and sulfide oxidation in three glacially influenced fjords on the west coast of Spitsbergen, Svalbard. Despite an abundance of glacially-sourced Fe(III)-oxide minerals, active sulfate reduction took place throughout the sediment. Analyses of the sulfur and oxygen isotopic composition of porewater sulfate and sulfate concentrations suggest that sulfide produced from biological sulfate reduction is reoxidized to sulfate. Long-term sediment incubations indicated sulfide oxidation at all three stations. The rate of sulfide oxidation was controlled by both the rate of sulfate reduction and the quantity and reactivity of Fe(III)-oxides. In our experimental incubations, we detected a decrease in Fe(III) content of the 0.5 M HCl and ascorbate-extractable fractions, but not in the 6 M HCl fraction, indicating that the highly reactive Fe(III) fraction is utilized by microorganisms and serves as the oxidant for sulfide reoxidation. Our results show that sulfide oxidation in glacially-influenced fjord sediments is a wide-spread geochemical process. Further warming will drive glacial retreat onto land, where sediment-laden glacial meltwater will be altered during flow through proglacial streams and lakes before entering the marine environment. Fjord sediments will likely become more sulfidic, as glaciers deliver less particulate, highly-reactive metal oxides to the marine environment.

    更新日期:2020-01-17
  • The role of Te, As, Bi, Sn and Sb during the formation of platinum-group-element reef deposits: examples from the Bushveld and Stillwater Complexes
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-16
    Eduardo T. Mansur; Sarah-Jane Barnes

    The distribution of platinum-group element (PGE) and Te, As, Bi, Sb and Sn (TABS) in whole-rock samples, and in disseminated base metal sulfides (BMS) pentlandite, pyrrhotite and chalcopyrite from the Bushveld and Stillwater Complexes are reported. The samples are from: the Merensky Reef (Bushveld), the J-M Reef (Stillwater), Picket Pin deposit (Stillwater), and also barren sulfide-bearing samples, from outside the reef intervals from both intrusions. The objective of the study was to document the distribution of PGE and TABS in PGE-reef deposits, and to investigate whether TABS play a significant role during the formation of PGE-reef deposits. The whole-rock concentrations of PGE and TABS (except for As) correlate with S and PGE, and thus their distribution appear to be controlled by BMS. The distribution of As, and to a lesser extent Sb, correlate with incompatible elements and with changes in K-phologopite compositions, suggesting that these elements are controlled both by the amount of trapped liquid in cumulate rocks, and the amount of sulfides. The possible role of TABS in forming pre-nucleation clusters (nanonuggets) to enrich the reefs in PGE is considered and discarded, because the ratio of TABS/PGE <0.3 is too low for TABS to form pre-nucleation clusters with PGE. A model where both the PGE and TABS are initially collected by a base metal sulfide liquid is favoured. During equilibrium crystallization of the sulfide liquid, Pd, Pt and the TABS were incompatible with the crystallizing BMS, and concentrated in the fractionated liquid eventually crystallizing as TABS-rich PGM. However, a portion of Pd, Pt and TABS also partitioned into the crystallizing BMS. During cooling and S-loss they exsolved from the BMS as PGM included in the BMS. In the reefs, the ratio of PGE/TABS is very high, and thus during exsolution all of the TABS could be accommodated in PGM, and consequently the BMS of the reefs have very low TABS concentrations. In contrast, outside the reefs the PGE/TABS ratios are low, and thus some TABS remained in the BMS, resulting in the BMS outside the reefs having higher TABS concentrations than those from within the reefs.

    更新日期:2020-01-17
  • Experimental determination of Ni isotope fractionation during Ni adsorption from an aqueous fluid onto calcite surfaces
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-16
    Cristina Castillo Alvarez; Ghylaine Quitté; Jacques Schott; Eric H. Oelkers

    The fractionation of Ni isotopes during Ni adsorption from aqueous fluids onto calcite surfaces was measured at 25 oC and as a function of pH from 7.7 to 8.9. Experiments showed that the percent Ni adsorbed and the degree of Ni isotope fractionation attained constant values in less than 30 hours after the calcite was exposed to the Ni bearing, calcite saturated aqueous solution. The percentage of Ni adsorbed from the fluid onto the calcite surfaces increased from 9 to 67% as the pH increased over this range. Calcite preferentially adsorbs light Ni isotopes during adsorption, resulting in a fractionation between adsorbed and aqueous, Δ60Nicalcite-fluid, of -0.52±0.16‰. This value is pH independent, within uncertainty, over the experimental pH range. The preferential adsorption of light Ni isotopes into calcite likely results from the change in coordination environment between adsorbed and aqueous nickel; the Ni-O length in the Ni-CO3 bond formed at the calcite surface is greater than that in the Ni2+ aquo ion.

    更新日期:2020-01-17
  • Influence of particulate versus diffusive molybdenum supply mechanisms on the molybdenum isotope composition of continental margin sediments
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-16
    Sümeyya Eroglu; Florian Scholz; Martin Frank; Christopher Siebert

    The sedimentary concentration and stable isotope composition of molybdenum (Mo) is widely used as a proxy for paleo redox conditions in the marine environment. However, the behavior of Mo during early diagenesis is still not fully understood, which complicates the application of the Mo proxy in ancient continental margin environments. Here, we present Mo concentrations and isotope compositions of sediment and pore water samples from the Guaymas Basin in the Gulf of California. Our sample set covers a broad range of depositional environments, including sediments from within the eastern equatorial Pacific oxygen minimum zone (OMZ), from a semi-restricted oxic graben, and from near a hydrothermal vent-field. By investigating Mo cycling in these different settings, we provide new insights into different modes of Mo fixation and the associated isotope fractionation. Sediments from the OMZ have authigenic Mo concentrations (Moauth) between 3.3 and 17.2 µg/g and δ98Mo between +1.64 and +2.13 ‰. A linear decrease in pore water Mo concentrations to the depth were hydrogen sulfide accumulates along with sedimentary authigenic δ98Mo values (δ98Moauth) close to seawater indicate diffusion of Mo from the bottom water into the sediment with little isotope fractionation during quantitative Mo removal. Sediments from the site with oxic bottom water within the basin reveal Moauth concentrations ranging from 1.2 to 14.7 µg/g and δ98Moauth signatures between –1.39 to +2.07 ‰. Pore water Mo concentrations are generally higher than ambient bottom water concentrations and the light δ98Moauth signatures of the pore waters between +0.50 and +0.80 ‰ and of the sediments indicate continuous Mo exchange between the pore water Mo pool and Mn and Fe oxides during early diagenesis. Sediment samples from the vent field mainly consist of black smoker debris and are characterized by Moauth concentrations ranging from 8.6 to 33.2 µg/g and δ98Moauth values as high as +2.20 ‰. The relatively high Mo concentrations and seawater-like δ98Mo can be explained by near-quantitative Mo scavenging from hydrothermal solutions with little isotope fractionation at high temperatures. Comparison of our new data for the OMZ sediments in the Gulf of California with previously published data for sediments from the Peruvian OMZ highlights that Mo isotope compositions in this kind of setting strongly depend on how Mo is delivered to the sediment. If Mo delivery into the sediment contributes to Mo accumulation in the solid phase, as is the case in the Guaymas Basin, sedimentary Moauth concentrations are relatively low but the isotope values are close to the δ98Mo signal of seawater. If Mo is exclusively delivered by particles, like on the Peruvian margin, much higher sedimentary Moauth concentrations can be attained. In the latter case, Moauth isotope values will be lighter because the sediments preserve the isotopic offset that was generated during adsorption or uptake of Mo by particles. Our findings de-emphasize the role of dissolved Mo speciation in pore waters but highlight the importance of the mode of Mo delivery for the Mo concentration and isotope composition preserved in the paleo-record.

    更新日期:2020-01-17
  • The spatial footprint of hydrothermal scavenging on 230ThXS-derived mass accumulation rates
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-16
    Jennifer L. Middleton; Sujoy Mukhopadhyay; Kassandra M. Costa; Frank J. Pavia; Gisela Winckler; Jerry F. McManus; Marcelle D'Almeida; Charles H. Langmuir; Peter J. Huybers

    Mid-ocean ridges are valuable archives of sedimentary flux records used to investigate atmospheric, oceanographic, and solid Earth responses to climate variability. Constant flux proxies, such as extraterrestrial helium-3 (3HeET) and excess thorium-230 (230ThXS), constrain vertical mass accumulation rates independent of the biases associated with lateral sediment transport and age model resolution. However, thorium scavenging by hydrothermal particles can perturb local 230ThXS deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary 230ThXS using coupled 3HeET-230ThXS analyses in cores from the Mid-Atlantic Ridge and the Juan de Fuca Ridge. Samples deposited >10 km from the Juan de Fuca Ridge indicate reliable off-axis behavior of both constant flux proxies. In contrast, samples deposited <10 km from the Juan de Fuca Ridge axis and within the axial valley of the Mid-Atlantic ridge suggest 50-80% deficits in sedimentary 230ThXS relative to its production rate. These deficits contrast with sedimentary 230ThXS surpluses recently observed on the East Pacific Rise. The spatial footprint of hydrothermal scavenging varies globally and temporally, likely as a function of the intensity of local hydrothermal activity. The combined ridge data suggest that near-vent sediments (within ∼5 km, but variable by ridge) receive relatively high 230ThXS deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low 230ThXS deposition rates due to diffusive loss of overlying seawater 230ThXS towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical 230ThXS deposition rates at distances greater than ∼10 km of the ridge axis. However, 230ThXS systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined 3HeET-230ThXS studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary 230ThXS anomalies to record large-scale variability in past hydrothermal activity.

    更新日期:2020-01-17
  • Experimental study on mafic rock dissolution rates within CO2-seawater-rock systems
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-11
    Chiara Marieni; Juerg M. Matter; Damon A.H. Teagle

    Far-from-equilibrium batch experiments have been performed to study the low temperature dissolution potential of crystalline submarine basalts (from Juan de Fuca Plate and Mid-Atlantic Ridges) and of a highly altered gabbro from the Troodos ophiolite (Cyprus) in presence of seawater and carbon dioxide (CO2). The experiments have been carried out at 40 °C for up to 20 days with initial pH of ∼4.8 and under ∼1 bar pCO2 to identify the progressive water-rock interactions. Elemental steady-state release rates from the rock samples have been determined for silicon and calcium, the solution concentrations of which were found to be the most effective monitors of rock dissolution. Mass balance calculations based on dissolved Si and Ca concentrations suggest the operation of reaction mechanisms focussed on the grain surfaces that are characteristic of incongruent dissolution. Also, basic kinetic modelling highlights the role of mass-transport limitations during the experiments. Ca release rates at pH ∼5 indicate significant contributions of plagioclase dissolution in all the rocks, with an additional contribution of amphibole dissolution in the altered gabbro. Si release rates of all solids are found to be similar to previously studied reactions between seawater and basaltic glass and crystalline basalt from Iceland, but are higher than rates measured for groundwater-crystalline basalt interaction systems. This comparison with previous experimental results resumes the debate on the role of experimental variables, such initial rock mass and crystallinity, pCO2, and fluid chemistry on dissolution processes. Our new data suggest that CO2-rich saline solutions react with mafic rocks at higher rates than fresh water with low pCO2, at the same pH. Most significantly, both ophiolitic gabbro and Juan de Fuca basalts show Si and Ca release rates similar or higher than unaltered crystalline basalt from Iceland, highlighting the potential substantial role that ophiolitic rocks and offshore mafic reservoirs could play for the geological storage of CO2.

    更新日期:2020-01-13
  • Examination of crystal dissolution in 3D: a way to reconcile dissolution rates in the laboratory?
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-11
    Catherine Noiriel; Matthias Oursin; Damien Daval

    Surface reactivity is a major parameter controlling mineral reactivity, and microscopic techniques investigating surface retreat with time have pointed at the heterogeneous and/or anisotropic reactivity of minerals, in relation with the diversity and stochastic distribution of energetic sites. However, in view of the discrepancies between rates determined in the laboratory, a thorough 3D approach of a whole crystal reactivity might be particularly attractive to evaluate the respective contributions of single faces and crystal edges to the dissolution flux, and to fill the gap between the rates derived from face-specific, topography observations at micro-scale (i.e., with no contribution of the edges to dissolution) and those determined on crystal powders in continuously stirred reactors (with an overcontribution of the edges and surface defects to dissolution). Here, we provide a detailed 3D characterization of the geometry evolution and dissolution rate of a single crystal of calcite at pH 4.5 and 4.0 using X-ray micro-tomography (XMT) with a pixel size of 0.325 µm. Evaluation of the retreat and mapping of the reaction rates at the 3D crystal surface reveals a large range of dissolution rates reflecting the specific contributions of the different regions of the crystal. During dissolution and against all expectation, etch pits forming at the crystal surface progressively annihilate, primarily by intersecting with trains of steps coming from the near edge regions. The global rate determined at the crystal scale integrates the contribution of the local rates of all the crystal features, with r¯corner′ > r¯edge′ > r¯cleavage′ > r¯macrostep′ ∼ r¯pit′ > r¯macrostepbase′. Crystal rounding reveals that contribution from the crystal edges progressively dominates the dissolution process over pit formation at the {101¯4} surfaces. The contribution of the edges to dissolution increases the crystal dissolution rate by at least 1.6 to what would be a face-specific dissolution, and will be size- and time-dependent, as suggested by a simple geometric model based on uniform or non-uniform dissolution of the faces of a model crystal. Finally, comparison of the method to vertical scanning interferometry measurements and scanning electron microscopy observations on surface portions shows that XMT imaging is robust, suggesting that its application to the dissolution/precipitation of other minerals would be highly beneficial to determine reliable rates that can be further used to model mineral reactivity.

    更新日期:2020-01-13
  • Kinetics of As(V) and carbon sequestration during Fe(II)-induced transformation of ferrihydrite-As(V)-fulvic acid coprecipitates
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-10
    Shiwen Hu; Yuzhen Liang; Tongxu Liu; Fangbai Li; Yang Lu; Zhenqing Shi

    The dynamic interactions among iron (Fe) oxides, organic matter (OM) and heavy metals/metalloids play crucial roles in controlling the geochemical cycling of carbon (C) and heavy metals/metalloids in natural environments. Although the inhibitory effects of arsenate (As(V)) or OM on the ferrihydrite transformation process have been studied previously, there is still a lack of mechanistic and quantitative understanding on the kinetics of As(V) and C sequestration during the Fe(II)-induced ferrihydrite transformation. In this study, we employed a suite of techniques to elucidate the underlying mechanisms accounting for the temporal changes of As(V) and C distributions and speciation on Fe oxides during the Fe(II)-induced ferrihydrite transformation process. Characterizations with X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and the spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) at different times indicated that the presence of As and/or FA resulted in the formation of more lepidocrocite than goethite. Besides surface adsorption, a portion of As(V) may be incorporated into the lattice structures of newly formed crystalline Fe oxides, and the amount of As(V) sequestration within Fe oxides were correlated with the formation of crystalline Fe oxides. In comparison, FA molecules were either adsorbed on the surfaces of goethite or diffused into the defects or nano pore spaces of lepidocrocite. Our results shed the light on different nanoscale mechanisms accounting for the sequestration of C and As(V) on Fe oxides. The defects or nano pore spaces formed in the structures of lepidocrocite may provide an effective way to sequester C through physical isolation, while the crystal structures of Fe oxides may sequester As(V) through isomorphic substitution. The knowledge on the dynamic coupling between Fe oxide transformation and C/As(V) sequestration provided the basis for accurately predicting the geochemical cycling of trace elements and OM.

    更新日期:2020-01-10
  • An experimental assessment of the chalcophile behavior of F, Cl, Br and I: implications for the fate of halogens during planetary accretion and the formation of magmatic ore deposits
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-10
    E.S. Steenstra; F. van Haaster; R. van Mulligen; S. Flemetakis; J. Berndt; S. Klemme; W. van Westrenen

    The elemental and isotopic abundances of halogens (F, Cl, Br, I) are used to constrain planetary volatile loss and volatile delivery processes, but their behavior during magmatic differentiation in general, and sulfide liquid segregation in particular, is currently not well constrained. To test whether sulfide liquid segregation could affect halogen behavior during magmatic processes, we performed high-pressure experiments to systematically quantify the sulfide liquid - silicate melt partition coefficients (Dsulliq-silmelt values, defined as the ratio between the wt.% concentration of the halogen in the sulfide liquid and silicate melt, respectively) of F, Cl, Br and I at a pressure of 1 GPa and temperatures of 1683–1883 K. Results show that dry-polishing target surfaces is crucial for obtaining representative halogen concentrations of sulfide liquids. The results also show that no appreciable amounts of F partition into sulfide liquids, whereas Cl, Br and I behave increasingly chalcophile with increasing atomic radius (i.e.,DFsulliq-silmelt < DClsulliq-silmelt < DBrsulliq-silmelt < DIsulliq-silmelt), presumably as a result of an increasingly covalent nature of Fe-halogen bonds with increasing radius. This results in I behaving chalcophile (DIsulliq-silmelt >1) in several experiments. In contrast to previous observations, DCl/Brsulliq-silmelt was found to be <1. The DCl,Br,Isulliq-silmelt predominantly vary with sulfide liquid melt composition, showing an increase with increasing O in the sulfide liquid, which itself is correlated with more oxidizing conditions (i.e., higher fO2) or silicate melt FeO contents. The DCl,Br,Isulliq-silmelt values remain constant and/or potentially decrease again at the highest O concentrations of the sulfide liquids in this study (∼2.5 wt.% O). Results indicate that the magnitude of halogen depletions in the terrestrial, martian and lunar mantle are not strongly affected by segregation of sulfide liquids during their accretion, given the expected low modal abundance of sulfide liquids and/or relatively low DCl,Br,Isulliq-silmelt values. Core formation remains the most important process in establishing iodine depletion in the terrestrial mantle, whereas volatility-related loss seems most likely for F, Cl, Br and I, in case of the martian mantle. However, segregation of sulfide liquids during accretion could have resulted in a relative increase of the offset between the mantle depletions of the lighter and heavier halogens. The experimental results confirm the previously proposed feasibility of sulfide liquids as reservoirs for halogens in magmatic sulfide ore environments. As proposed by Mungall and Brenan (2003), fractional crystallization of these sulfide liquids in the absence of a silicate melt can lead to the formation of halide melts or fluids, consistent with the association between halide minerals and magmatic sulfide ores in some localities.

    更新日期:2020-01-10
  • Metastable equilibrium of substitution reactions among oxygen- and nitrogen-bearing organic compounds at hydrothermal conditions
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-07
    Kirtland J. Robinson; Kristopher M. Fecteau; Ian R. Gould; Hilairy E. Hartnett; Lynda B. Williams; Everett L. Shock

    Measured abundances of organic compounds can reveal information about the environments in which they formed. Since organic compounds can be mobilized and released from geologic and planetary settings, they have the potential to provide insights into environments that are difficult to observe directly. To advance our understanding of these environments, this study identifies organic reactions that approach metastable equilibrium in experiments, so that future studies can predict geochemical conditions in remote settings (e.g., deep Earth, extraterrestrial bodies) by monitoring reaction ratios of compounds involved in similar organic reactions. At high temperatures organic redox reactions can equilibrate, which allows comparisons with thermodynamic properties to yield estimates of reaction conditions. However, redox reactions may equilibrate too slowly to be applicable to lower temperature systems. To explore metastable equilibria at lower temperatures, we studied substitution reactions that tend to be faster than redox reactions. In this study, we demonstrate that oxygen- and nitrogen-bearing organic compounds at hydrothermal conditions undergo a series of simultaneous substitution reactions that rapidly approach steady-state ratios indicative of metastable equilibrium. Four sets of aqueous experiments were performed at 250°C and 40 bar, each beginning with a single organic reactant: benzyl alcohol, benzylamine, dibenzylamine, or tribenzylamine. All reactant solutions were prepared with identical bulk elemental compositions by adjusting the concentrations of the starting organic compounds and adding ammonia as needed. After 72 hours, reaction ratios between the model compounds converged in all four sets of experiments, consistent with approaches toward metastable equilibrium. Reaction ratios for ether and aldehyde formation reactions were also observed to group within a relatively small range, but without a clear convergence pattern, suggesting other non-redox reactions may approach metastable equilibrium. The approach to metastable equilibrium among the initial organic reactants could be observed and quantified even in the presence of competing redox reactions whose mechanisms are less understood, including dibenzylimine and toluene formation, which did not appear to reach steady-states. These findings identify classes of organic compounds and reactions that can reflect the conditions at which they last equilibrated and should be targeted for analysis in natural systems.

    更新日期:2020-01-07
  • Selenium isotope fractionation during adsorption by Fe, Mn and Al oxides
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-07
    Wenpo Xu; Jian-Ming Zhu; Thomas M. Johnson; Xiangli Wang; Zhi-Qing Lin; Decan Tan; Haibo Qin

    Adsorption plays an important role in the biogeochemical cycling of selenium (Se) in natural environments and Se isotope fractionation during adsorption is a significant but poorly studied part of Se isotope system. This paper examined Se(IV) and Se(VI) adsorption onto four naturally occurring metal oxides (hematite, manganese dioxide (β-MnO2), and α and γ-alumina oxides) and revealed Se isotope fractionation as a function of the absorbents used, Se species adsorbed, exposure time, and pH. The results show that Se(IV)/(VI) adsorption was initially rapid and was accompanied with kinetic isotope fractionations as large as 3‰, but slowed down as adsorption equilibrium was approached, eventually approaching isotopic equilibrium. The Se(IV) adsorption onto the studied metal oxides took at least 12 hours to reach adsorption equilibrium, longer than Se(VI) adsorption (< 60 minutes). At adsorption equilibrium, Se(IV) adsorption onto Fe and Mn oxides induces significant isotope fractionation, with lighter Se isotopes preferentially adsorbed, whereas Se(IV) adsorption onto Al oxides causes only small fractionation: Δ82/76Sedissolved-adsorbed = δ82/76Sedissolved -δ82/76Seadsorbed is 0.87 ± 0.12 ‰ for hematite, 1.24 ± 0.05 ‰ for β-MnO2, 0.08 ± 0.10 ‰ for α-alumina, and 0.05 ± 0.09 ‰ for γ-alumina at pH 5. In contrast to Se(IV) adsorption, Se(VI) adsorption does not induce detectable Se isotope fractionation. The contrasting Se isotope fractionation between Se(IV) and Se(VI) adsorption is likely related to the mechanism of adsorption onto metal oxides, which causes a structural difference between dissolved and adsorbed Se(IV)/Se(VI). In addition, pH had a strong influence on Se isotope fractionation during Se(IV) adsorption onto β-MnO2: Δ82/76Sedissolved-adsorbed varied from 1.24‰ to -0.08‰ as pH increased from 5 to 8. However, there was little pH effect on Se isotope fractionation during adsorption onto Fe (consistently at 0.7-0.9‰) and Al oxide (consistently at ∼0‰). Our findings show that, beside abiotic and biotic reduction, Se(IV) adsorption onto Fe-Mn oxides is potentially another important process that can induce Se isotope fractionation in Earth’s surface environments. This moves an important step toward correctly reconstructing the Se isotopic composition of seawater using Fe-Mn nodules or crusts.

    更新日期:2020-01-07
  • Gold solubility in alkaline and ammonia-rich hydrothermal fluids: insights from ab initio molecular dynamics simulations
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-07
    Yuan Mei; Weihua Liu; Joël Brugger; Qiushi Guan

    Knowledge of speciation and thermodynamic properties for aqueous gold complexes are important for both the understanding and numerical modelling of the formation of hydrothermal gold deposits. While gold speciation in chlorine- and sulfur-rich hydrothermal fluids has been investigated by considerable theoretical and experimental studies, the complexation of gold with other ligands that may be important for transporting gold in hydrothermal fluids, such as hydroxyl (OH–) and ammonia (NH3) complexes, have received limited attention so far. In this study, we conduct ab initio molecular dynamics (MD) simulations to calculate the coordination structures of Au(I)-OH and Au(I)-NH3 complexes. Our simulations show linear structures of Au(I) complexes with two bonded ligands (OH–/NH3/H2O), consistent with previous experimental and theoretical studies of Au(I) complexation (e.g., Liu et al., 2014). The new speciation models show that Au(I)-OH–/NH3/H2O complexes are stable in hydrothermal fluids and vapors. We also use thermodynamic integration to determine the formation constants of these species at temperatures up to 350 °C and at water-saturated pressures; furthermore, we extrapolate to higher temperatures and pressures based on the Modified Ryzhenko–Bryzgalin equation of state parameters. In particular, this study, for the first time, reports the speciation and formation constants of mix-ligand [Au(NH3)(OH)]0(aq) complexes. The derived formation constants show that the stability of Au(I)-NH3 complexes decreases progressively with increasing temperature. The quantitative modelling of gold solubility based on the new thermodynamic data shows that gold can be transport as Au(I)-OH–/NH3 complexes in sulfur-poor and ammonia-rich hydrothermal fluids. In sulfur-bearing ore fluids, ammonia is only likely to transport small amounts of gold as [Au(NH3)(OH)]0(aq) in hydrothermal fluids in the near-neutral to alkaline fluids under extremely reduced conditions.

    更新日期:2020-01-07
  • Zircon-hosted melt inclusion record of silicic magmatism in the Mesoproterozoic St. Francois Mountains terrane, Missouri: Origin of the Pea Ridge iron oxide-apatite-rare earth element deposit and implications for regional crustal pathways of mineralization
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-07
    Kathryn E. Watts; Celestine N. Mercer

    Voluminous silicic magmatism was coeval with iron ore mineralization in the St. Francois Mountains terrane in southeast Missouri, part of the broader Mesoproterozoic Granite-Rhyolite province along the eastern margin of Laurentia. Some of the iron deposits contain extraordinary endowments of critical elements, such as the Pea Ridge iron oxide-apatite (IOA) deposit, which has an average grade of ∼12 wt% rare earth oxides in breccia pipes that flank the ore body. To assess the role of silicic magmatism in the genesis of the Pea Ridge deposit, we present a high-spatial resolution study of zircon-hosted melt inclusions from rhyolitic ash-flow tuffs. Melt inclusion data are combined with textural, geochemical, and geochronological analyses of zircon hosts to elucidate the magmatic-hydrothermal evolution of the Pea Ridge system. Two contemporaneous silicic igneous centers in the St. Francois Mountains terrane, Bourbon and Eminence, were studied for comparison. Pea Ridge melt inclusions are trachydacitic to rhyolitic (∼63-79 wt% SiO2, ∼5.6-11.7 wt% Na2O+K2O) with very high Cl in the least-evolved and most alkaline melt inclusions (∼2,000-5,000 ppm Cl). Rare earth elements (REE) in melt inclusions have identical chondrite-normalized patterns to the mineralized breccia pipes, but with systematically lower absolute concentrations. Haplogranite ternary pressures range from ∼0.5-10 kbar, with an average of ∼2-3 kbar (7-12 km depth), and liquidus temperatures are ∼850-950 °C, with an average of ∼920 °C. Silicate and phosphate mineral inclusions have compositions that overlap minerals from the iron ore body and breccia pipes, recording a transition from igneous to hydrothermal zircon growth. Igneous iron oxide inclusions have compositions that indicate Pea Ridge magmas were reduced to moderately oxidized (log fO2 of -0.8 to -1.84 ΔNNO). Zircons from two Pea Ridge samples have 207Pb/206Pb concordia ages of 1456 ± 9 Ma and 1467 ± 13 Ma that overlap published ages for the breccia pipes and iron ore zones of the Pea Ridge deposit. A population of texturally and chemically disrupted zircons have discordant domains that correspond to high Fe, U, and REE concentrations, consistent with the unique geochemical attributes of the IOA-REE ore body. Inherited cores in Pea Ridge and Bourbon zircons have concordant 207Pb/206Pb dates of 1550-1618 Ma, providing direct evidence of cratonic basement beneath these centers. Oxygen isotope data for inherited and autocrystic igneous zircons span from mantle to crustal values (δ18Ozircon=5.5-7.9‰). Our data are consistent with a model in which metasomatized mantle components were mixed with cratonic and accreted crustal material in a back-arc or rifted segment of a volcanic arc, with ore fluids derived from Cl-rich melts to transport Fe and REE in a long-lived (tens of Myr), pulsed, magmatic-hydrothermal system. Bourbon, which also possesses IOA mineralization, shares key petrologic similarities with the Pea Ridge system, whereas Eminence, which is not mineralized, has disparate geochemical and isotopic signatures that indicate it formed in a different crustal setting. The location of Pea Ridge and Bourbon along a cratonic margin may have been important in focusing silicic melts and mineralization in the upper crust, serving as a guide for future exploration efforts.

    更新日期:2020-01-07
  • Constraints on mantle evolution from Ce-Nd-Hf isotope systematics
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-03
    Michael Willig; Andreas Stracke; Christoph Beier; Vincent J.M. Salters

    Mantle evolution is governed by continuous depletion by partial melting and replenishment by recycling oceanic and continental crust. Several important unknowns remain, however, such as the extent of compositional variability of the residual depleted mantle, the timescale, mass flux and composition of recycled oceanic and continental crust. Here, we investigate the Ce-Nd-Hf isotope systematics in a globally representative spectrum of mid ocean ridge and ocean island basalts. Using a Monte Carlo approach for reproducing the observed Ce-Nd-Hf isotope variation shows that the type and age of depleted mantle and recycled crust have the dominant influence on the slope, scatter, and extent of the modeled Ce-Nd-Hf isotope array. The model results suggest a relatively young (<1.5 Ga) average depletion age of the depleted mantle, consistent with Nd and Os isotope model ages of abyssal peridotites, and an apparent moderate extent of incompatible element depletion. The latter, however, is deceiving, because it reflects a natural sampling bias, resulting from melting an inherently heterogeneous depleted mantle. In principal, recycling of oceanic crust can explain most of the isotopic range of the isotopically enriched end of the Ce-Nd-Hf mantle array, but only if the entire compositional variability of the recycled crust is preserved during recycling, residence in the mantle, and re-melting. The latter is unlikely, however, because many sources of internal chemical variance average out on the scale of the bulk oceanic crust, during residence in the mantle, and subsequent sampling by partial melting. Moreover, both the slope and limited scatter of the observed Ce-Nd-Hf mantle array show that recycling of bulk oceanic crust, that is, both the extrusive basalts and intrusive gabbros of the lower oceanic crust must be considered, and are key to better understand crust-mantle cycling in general. The Monte-Carlo simulation also indicates that the return flux from the continental crust into the mantle mainly derives from the lower continental crust, consistent with current models of continental crust evolution, which all require that a substantial amount of the mafic lower continental crust must be recycled into the mantle to maintain the average andesitic composition of the continental crust.

    更新日期:2020-01-04
  • Labile Fe(III) from sorbed Fe(II) oxidation is the key intermediate in Fe(II)-catalyzed ferrihydrite transformation
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2020-01-02
    Anxu Sheng; Juan Liu; Xiaoxu Li; Odeta Qafoku; Richard N. Collins; Adele M. Jones; Carolyn I. Pearce; Chongmin Wang; Jinren Ni; Anhuai Lu; Kevin M. Rosso

    Ferrihydrite (Fh) is a major Fe(III)-(oxyhydr)oxide nanomineral distinguished by its poor crystallinity and thermodynamic metastability. While it is well known that in suboxic conditions aqueous Fe(II) rapidly catalyzes Fh transformation to more stable crystalline Fe(III) phases such as lepidocrocite (Lp) and goethite (Gt), because of the low solubility of Fe(III) the mass transfer pathways enabling these rapid transformations have remained unclear for decades. Here, using a selective extractant, we isolated and quantified a critical labile Fe(III) species, one that is more reactive than Fe(III) in Fh, formed by the oxidation of aqueous Fe(II) on the Fh surface. Experiments that compared time-dependent concentrations of solid-associated Fe(II) and this labile Fe(III) against the kinetics of phase transformation showed that its accumulation is directly related to Lp/Gt formation in a manner consistent with the classical nucleation theory. 57Fe isotope tracer experiments confirm the oxidized Fe(II) origin of labile Fe(III). The transformation pathway as well as the accelerating effect of Fe(II) can now all be explained on a unified basis of the kinetics of Fe(III) olation and oxolation reactions necessary to nucleate and sustain growth of Lp/Gt products, rates of which are greatly accelerated by labile Fe(III).

    更新日期:2020-01-02
  • Magnesium isotope fractionation during hydrothermal seawater–basalt interaction
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-31
    Martin Voigt; Christopher R. Pearce; David M. Fries; Andre Baldermann; Eric H. Oelkers

    Fluid-rock interactions in hydrothermal systems at or near mid-oceanic ridges (MOR) play a major role in determining the composition of the oceanic crust and seawater. To quantify the processes that govern cation exchange in these environments we have experimentally studied the isotopic evolution of δ26/24Mg in the fluid phase during seawater-basalt interaction at 250 and 290 °C. Mass balance constraints indicate that isotopically heavy Mg was preferentially incorporated into non-exchangeable (octahedral) sites in secondary clay minerals such as saponite (Mg-rich smectite), leaving residual fluids enriched in light Mg isotopes. The magnitude of fractionation observed during smectite precipitation in our experiments (εSmectite-Liquid26/24) ranged from 0.35‰ to 0.42‰. This observation, which contrasts with the preferential uptake of light Mg isotopes into biogenic and inorganic marine carbonates, highlights the potential utility of Mg isotopes as tracers of the precipitation dynamics of authigenic Mg-silicate and Mg-carbonate phases. Furthermore, although Mg isotopic fractionation is often masked by the almost complete removal of Mg in high temperature marine hydrothermal systems, our experiments demonstrate that it does become significant at lower temperatures where Mg removal by clay formation is incomplete. Under such conditions, this fractionation will create isotopically light fluids due to smectite precipitation, thus potentially represents an important component of the marine Mg isotope inventory.

    更新日期:2019-12-31
  • Transformation of amorphous precursor to crystalline carbonate: insights from Mg isotopes in the dolomite-analogue mineral norsethite [BaMg(CO3)2]
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-31
    Chuan Liu; Weiqiang Li

    Crystallization from an amorphous precursor is an important pathway of carbonate precipitation in nature. However, the mechanistic details of the transformation from an amorphous phase to a crystalline phase of carbonates remain a topic of intense debate. Two competing mechanisms, including solid-state transition and coupled dissolution-reprecipitation, have been proposed to explain this transformation process. Magnesium is a common element in carbonate crystal lattices and its isotopes may provide unique insights into this problem. In this study, we investigated the transformation of the amorphous carbonate (AC) precursor for norsethite [BaMg(CO3)2], a dolomite analogue mineral, by in situ XRD analysis and isotope exchange experiments using a 25Mg enriched tracer coupled with high precision isotope analyses of δ26Mg and δ25Mg values for aqueous and solid phases. In situ XRD experiments revealed that the AC can transformed to crystalline norsethite at various temperatures (25℃, 50℃ and 70℃) and no intermediate mineral formed during the AC transformation process. 25Mg tracers indicated that near-complete Mg isotope exchange occurred in all exchange experiments during AC transformation. More importantly, after the AC transformation, the system showed surprising apparent non-mass dependent fractionation relationship, that the δ25Mg value of solid phase became greater than that of aqueous solution from a lower value, producing positive Δ25Mgsolid-aq fractionation, whereas the Δ26Mgsolid-aq fractionation remained negative. We numerically modeled the behavior of Mg isotopes (in both δ26Mg and δ25Mg) for the experimental system according to the two competing mechanisms of AC transformation. The results suggest that the apparent non-mass dependent isotope behavior can only be explained by the coupled dissolution-reprecipitation process. Therefore, this study does not support the solid-state transition mechanism for AC transformation. Further, this study rigorously proves that norsethite can form by precipitation from aqueous solution without replacement, and implies that Mg2+ in aqueous solutions can be efficiently dehydrated and incorporated into a well ordered dolomite-group mineral (norsethite) under abiotic, low temperature conditions, thus providing new insights for understanding dolomite precipitation in nature.

    更新日期:2019-12-31
  • Tracing subduction zone fluids with distinct Mg isotope compositions: Insights from high-pressure metasomatic rocks (leucophyllites) from the Eastern Alps
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-30
    Yi-Xiang Chen; Attila Demény; Hans-Peter Schertl; Yong-Fei Zheng; Fang Huang; Kun Zhou; Qi-Zhen Jin; Xiao-Ping Xia

    Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of -0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from -0.64‰ to -1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from -0.29‰ to -0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about -0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg (<-1.3‰) fluid produced by dissolution of mainly Mg-calcite at HP conditions; (2) a high δ26Mg (>0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite mélange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as -1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries.

    更新日期:2019-12-30
  • Decoupling feldspar dissolution and precipitation rates at near-equilibrium with Si isotope tracers: Implications for modeling silicate weathering
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-27
    Chen Zhu; J. Donald Rimstidt; Yilun Zhang; Jinting Kang; Jacques Schott; Honglin Yuan

    Here we show that the isotope tracer experimental method for kinetic studies, aided by the recent advance and accessibility of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analysis for non-traditional stable isotopes, can provide unidirectional dissolution rates at near-equilibrium conditions. For a long time, the only rates available at near-equilibrium were net reaction rates—dissolution rates minus precipitation rates. This is because the conventional experimental method of kinetic studies is based on element concentrations and can only provide net rates. The availability of unidirectional rates allows us to re-examine some fundamental concepts and practices of modeling weathering in geochemistry. In this study, we used the 29Si isotope tracer to conduct albite and K-feldspar dissolution experiments at near-equilibrium conditions in near-neutral pH solutions at 50 °C. Results show that the saturation indices (SI) of solutions approached zero with respect to albite and K-feldspar after ∼240-360 hours (h), but 29Si/28Si ratios of the experimental solutions indicated continual dissolution for another 720 h to 1440 h. The rates of total Si precipitation were much smaller than the rates of Si dissolution. The experimental solutions were supersaturated with respect to amorphous Al(OH)3, gibbsite, quartz, allophane, imogolite, and kaolinite. The SI of the solutions remained constant with respect to these phases while Al concentrations slightly decreased and Si concentrations slightly increased, indicating the coupled feldspar dissolution and precipitation of secondary phases, such as albite →amorphous Al(OH)3 + quartz or albite → solution + Al-Si phase(s), instead of significant albite and K-feldspar precipitation (the reverse reaction) at 50 °C. Reaction path modeling of the temporal evolution of Si, Al, Na, and pH revealed that albite dissolution (without significant backward reaction) coupled with the precipitation of a secondary phase with a Si:Al ratio of ∼2:1 can successfully match the experimental data. Given the negligible feldspar precipitation reactions in low-temperature systems (e.g., T<100 oC), we recommend modeling feldspar weathering using unidirectional forward rates together with secondary phase precipitation rates in near-equilibrium, feldspar-undersaturated systems. This can be accomplished with minor modifications in geochemical modeling software or input files. The coupled feldspar dissolution with secondary phase precipitation arrests the system in a near-equilibrium steady state. Using affinity-based rate equations such the classical linear Transition State Theory rate law or the Burch empirical relation together with far-from-equilibrium rate data will predict significant feldspar precipitation in solutions undersaturated but close to equilibrium with respect to feldspars, which is unlikely at near ambient temperatures.

    更新日期:2019-12-27
  • Origin of Al-rich chondrules in CV chondrites: Incorporation of diverse refractory components into the ferromagnesian chondrule-forming region
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-26
    Mingming Zhang; Yangting Lin; Guoqiang Tang; Yu Liu; Ingo Leya

    Aluminum-rich (Al2O3 > 10 wt%) chondrules (ARCs) are important chondritic components that petrologically link two other major chondritic components, ferromagnesian chondrules (FMCs) and calcium-aluminum-rich inclusions (CAIs), which formed in different regions of the protoplanetary disk. They are closely related to FMCs as indicated by their similar igneous textures, mineral assemblages, and Al-Mg isotope systematics; meanwhile, they have genetic relationship with CAIs as indicated by their distinctly Al2O3-rich compositions and occasional occurrences of relict CAI minerals. In order to further understand their formation mechanism and genetic relationships to FMCs and CAIs, nine ARCs and three ARC-related objects from Allende (CV3 oxidized), Leoville (CV3 reduced), and the ungroup Ningqiang carbonaceous chondrites were studied for petrology, mineralogy, bulk compositions, rare earth element (REE) abundances, and in situ oxygen isotopic compositions. Our results suggest that (i) ARCs crystallized from incompletely molten droplets with crystallization sequences mainly determined based on their bulk compositions. Projection of their bulk compositions onto the forsterite-saturated tridymite-diopside-spinel diagram allows us to classify them into Al-rich [Sp], Al-rich [En], and Al-rich [Plag]; (ii) ARC precursors are mixtures of refractory materials and the precursors of FMCs, in which the refractory materials have diverse sources rather than a single type of CAI/AOA (amoeboid olivine aggregate); this is inferred from the bulk compositions, relict minerals (both coarse- and fine-grained spinel, olivine, and Al-Ti-diopside), and various CAI-like REE patterns (unfractionated Group I/ III and highly fractionated Group II/ II-like) of ARCs. The source include AOAs and igneous Type B/C CAIs; (iii) ARCs were melted in the FMC-forming region, possibly by the same heating mechanism or during the same transient heating event, which is consistent with the similar oxygen isotopic compositions of their phenocrysts (Δ17O = -5.2 ± 1.7‰, 2SD). Thus, we consider that ARCs formed by melting of mixtures of diverse refractory components with the FMC precursors in the FMC-forming region.

    更新日期:2019-12-27
  • Transformation of Ni-containing birnessite to tectomanganate: Influence and fate of weakly bound Ni(II) species
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-26
    Zhongkuan Wu; Bruno Lanson; Xionghan Feng; Hui Yin; Zhangjie Qin; Xiaoming Wang; Wenfeng Tan; Zhongjun Chen; Wen Wen; Fan Liu

    The geochemical behavior of nickel, an essential trace metal element, strongly depends on its interactions with Mn oxides. Interactions between the phyllomanganate birnessite and sorbed or structurally incorporated Ni have been extensively documented together with the fate of Ni along the transformation of these layered species to tunnel Mn oxides (tectomanganates). By contrast, interactions of phyllomanganates with weakly bound Ni species [hydrated Ni, Ni (hydr)oxides], that possibly prevail in natural Ni-rich (>10% NiO) manganates, have received little attention and the influence of these Ni species on the phyllomanganate-to-tectomanganate transformation remains essentially unknown. A set of phyllomanganate precursors with contrasting contents of Ni was thus prepared and subjected to a reflux treatment mimicking the natural phyllomanganate-to-tectomanganate conversion. Layered precursors and reflux products were characterized with a combination of diffractometric, spectroscopic, thermal, and chemical methods. Ni is essentially present as hydrated Ni(II) and Ni(II) (hydr)oxides in Ni-rich layered precursors whereas Ni(II) sorbed at particle edges prevail at low Ni content. No Ni sorbed at layer vacancy sites or structurally incorporated was detected in the initial vacancy-free layered precursors. Consistent with the high content (≈1/3) of Jahn-Teller distorted Mn(III) octahedra in layered precursors, which is favorable to their conversion to tectomanganates, Ni-free samples fully convert to todorokite, a common tectomanganate with a uniform 3×3 tunnel structure. Contrastingly and despite similar high Mn(III) contents in Ni-rich precursors, hydrolysis of interlayer Ni2+ and polymerization of Ni(OH)2 in phyllomanganate interlayers is kinetically favored during reflux process. Asbolane, a phyllomanganate with an incomplete – island-like – octahedral layer of metal (hydr)oxides, is thus formed rather than todorokite. A nitric acid treatment, aiming at the dissolution of the island-like interlayer Ni(OH)2 layer, allows an easy and unambiguous differentiation between asbolane and todorokite, the latter being unaffected by the treatment. Both compounds exhibit indeed similar interplanar periodicities and can be confused when using X-ray diffraction, despite contrasting intensity ratios. Migration rate of Mn(III) out of the MnO2 layer relative to the metal hydrolysis and polymerization rate determines the formation of todorokite or asbolane. Here, Ni(OH)2 polymerization hampers the formation of tectomanganates and likely contributes to the prevalence of phyllomanganates over tectomanganates in natural Ni-rich environments. Most Ni is retained during the reflux process, part of Ni (≈20%) being likely structurally incorporated in the reaction products, thus enhancing the sequestration of Ni in Mn oxides.

    更新日期:2019-12-27
  • A combined first principles and classical molecular dynamics study of clay-soil organic matters (SOMs) interactions
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-24
    Yingchun Zhang; Xiandong Liu; Chi Zhang; Xiancai Lu

    In this study, clay-soil organic matters (SOMs) interactions were investigated by combining first principles molecular dynamics (FPMD) and classical molecular dynamics (CMD) techniques. FPMD was employed to quantify the binding mechanisms of reactive SOMs groups on clay surfaces and to derive parameters for the bonding of the reactive groups on edge surfaces. By integrating the derived parameters with CVFF–CLAYFF force fields, CMD simulations were carried out to investigate the structures of large models of clay-SOMs associations. All possible reactive SOMs groups, including carboxylate, phosphate, quinone species and ammonium groups were taken into account. FPMD results showed that all these groups are important to the complexation of SOMs on clay surfaces under dry conditions but only chemical bonding of carboxylate and phosphate and cation bridging are of significance under wet conditions. pH dependence of carboxylate and phosphate and Eh-pH dependence of quinone species are presented. CMD simulations showed that SOMs bound on clay surfaces via direct bonding of carboxylate and Ca2+ bridging and revealed the effect of water on structures of the clay-SOMs association. Based on the computational results, a procedure was proposed for constructing realistic molecular models for soils. Possible applications and further improvements of these models are discussed.

    更新日期:2019-12-25
  • Paleo-redox context of the Mid-Devonian Appalachian Basin and its relevance to biocrises
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-24
    Ruliang He; Wanyi Lu; Christopher K. Junium; Charles A. Ver Straeten; Zunli Lu

    The Devonian Period witnessed the expansion of vascular land plants and an atmospheric oxygenation event associated with enhanced organic mass burial. The deposition of organic-rich shales (e.g. black shales of the Marcellus subgroup) and several biotic crises in the marine realm have been linked to Devonian ocean anoxia. However, it is not clear how redox conditions evolved in different parts of the water column in such a context of dynamic changes in the atmosphere-ocean system. To address this problem, we use the bulk carbonate I/Ca proxy on core samples from Yates County, NY, in order to reconstruct the water column redox history through the Onondaga Limestone into the lower Marcellus shale. On the secular scale, the range of I/Ca values support the notion of a Devonian rise in atmospheric oxygen, relative to time intervals earlier in the Paleozoic. In terms of Eifelian Stage stratigraphic trends, I/Ca ratios are generally stable and high in the Onondaga Formation but show large fluctuations in lower Marcellus strata. Low I/Ca ratios are found near the onset of organic-rich shale deposition indicating relatively reducing subsurface waters. The pattern of redox changes resembles that of contemporaneous sea-level changes. Finally, the reconstructed oxygenation changes are correlated to three biotic transitions in Devonian marine systems.

    更新日期:2019-12-25
  • Synthetic fluid inclusions XXII: Properties of H2O-NaCl±KCl fluid inclusions trapped under vapor- and salt-saturated conditions with emphasis on the effect of KCl on phase equilibria
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-24
    Pilar Lecumberri-Sanchez; Maocheng Luo; Matthew Steele-MacInnis; Simone E. Runyon; D. Matthew Sublett; Yury Klyukin; Robert J. Bodnar

    Hydrodynamic and thermodynamic modeling of fluid evolution in shallow magmatic hydrothermal systems based on phase equilibria for the system H2O-NaCl predicts that the fluid becomes halite saturated at some point in its evolution. A review of the published fluid inclusion data also supports halite saturation in these systems. In this study, we synthesized fluid inclusions at known pressure-temperature-composition (PTX) conditions in the H2O-NaCl and H2O-NaCl-KCl systems such that vapor+halite or liquid+vapor+halite existed at the time of trapping. Our results show that fluid inclusions trapped in the liquid+vapor+halite field in experiments along an evolving temperature path best match the phase ratios and thermometric behavior of fluid inclusions commonly reported in magmatic-hydrothermal systems (particularly in porphyry-style deposits). These results are consistent with the hypothesis that magmatic-hydrothermal fluids in porphyry settings are commonly trapped under conditions of liquid+vapor+halite equilibrium, which in turn has consequences for fluid flow and mineralization. Our results also show that the presence of KCl in the system significantly improves the agreement between microthermometric behaviors reported in natural systems and experimental results. The major effect of adding KCl is to increase the thermodynamic variance (degrees of freedom) of coexistence of liquid+vapor+halite from univariant in the system H2O-NaCl to divariant (occupying an area, rather than a line, in pressure-temperature space). Simulations based on the system H2O-NaCl predict that halite saturation occurs predominantly in the vapor+halite field with localized halite precipitation during the transition from the liquid+vapor to the vapor+halite field. By rendering the liquid+vapor+halite coexistence space as divariant, the addition of KCl also prevents an abrupt termination of liquid stability upon intersecting the liquid+vapor+halite boundary. Simulations using the system H2O-NaCl commonly predict pressure-temperature pathways constrained to the univariant liquid+vapor+halite curve, owing to volumetric and latent heat constraints. In contrast, the presence of KCl relaxes these constraints and expands the ranges of pressure and temperature over which liquid, vapor and halite can coexist. This phenomenon has major implications for the chemical and hydrological evolution in magmatic-hydrothermal systems as related to the ability of fluids to transport metals (ore deposition) and the potential loss of porosity/permeability as a result of salt and quartz precipitation.

    更新日期:2019-12-25
  • The Pb isotope evolution of Bulk Silicate Earth: constraints from its accretion and early differentiation history
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-24
    Alessandro Maltese; Klaus Mezger

    Constraining the evolution of Pb isotopes in the bulk silicate Earth (BSE) is hampered due to the lack of a direct determination of Earth’s U/Pb and initial Pb isotope composition. All estimates of these parameters are strongly model dependent and most Pb evolution models start with a meteoritic source, i.e., the primordial Pb composition determined in troilite from the Canyon Diablo iron meteorite. During the condensation of the elements in the solar nebula, accretion of the Earth, and its subsequent chemical evolution, the U/Pb was modified. Different models make different assumptions about the timing and extent of this U-Pb fractionation during Earth’s chemical evolution that cannot always be related to known global geological processes at the time of this modification. This study explores geochemical constraints that can be related to known geological processes to derive an internally consistent model for the evolution of the U-Th-Pb systematics of the silicate Earth. Lead is chalcophile, moderately volatile, and as a result strongly depleted in the BSE compared to primitive meteorites. Any process affecting the abundance and isotope composition of Pb in Earth throughout its early history has to be consistent with the abundance of elements with similar chemical and physical properties in the same reservoir. The abundances of refractory to moderately and highly volatile elements in the BSE imply that the proto Earth was highly depleted in volatile elements and therefore evolved with a very high U/Pb (238U/204Pb = µ ≥100) prior to collision with the Moon-forming giant impactor. This impactor had close to chondritic abundances of moderately to highly volatile elements and delivered most of Earth’s volatile elements, including the Pb budget. Addition of this volatile rich component caused oxidation of Earth’s mantle and allowed effective transfer of Pb into the core via sulfide melt segregation. Sequestration of Pb into the core therefore accounts for the high µBSE, which has affected ca. 53 % of Earth’s Pb budget. In order to account for the present-day Pb isotope composition of BSE, the giant impact must have occurred at 69 ±10 Myr after the beginning of the solar system. Using this point in time, a model-derived µ-value, and the corresponding initial Pb isotope composition of BSE, a single stage Pb isotope evolution curve can be derived. The result is a model evolution curve for BSE in 208Pb-207Pb-206Pb-204Pb-isotope space that is fully consistent with geochemical constraints on Earth’s accretionary sequence and differentiation history. This Pb-evolution model may act as a reference frame to trace the silicate Earth’s differentiation into crust and mantle reservoirs, similar to the CHUR reference line used for other radio-isotope systems. It also highlights the long-standing Th/U paradox of the ancient Earth.

    更新日期:2019-12-25
  • Anomalous 182W in high 3He/4He Ocean Island Basalts: Fingerprints of Earth’s core?
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-24
    A. Mundl-Petermeier; R.J. Walker; R.A. Fischer; V. Lekic; M.G. Jackson; M.D. Kurz

    The short-lived 182Hf-182W isotope system (t1/2 = 9 Ma) left evidence in both ancient and modern terrestrial rock record of processes that took place during the earliest stages of Earth’s accretionary and differentiation history. We report µ182W values (the deviation of 182W/184W of a sample from that of laboratory standards, in parts per million) and corresponding 3He/4He ratios for rocks from 15 different hotspots. These rocks are characterized by µ182W values that range from ∼0 to as low as -23 ± 4.5. For each volcanic system that includes rocks with negative µ182W values, the values tend to be negatively correlated with 3He/4He. The W-He isotopic characteristics of all samples can be successfully modeled via mixing involving at least three mantle source reservoirs with distinct µ182W-3He/4He characteristics. One reservoir has 3He/4He ≈ 8 R/RA and μ182W ≈ 0, which is indistinguishable from the convecting upper mantle. Based on high 3He/4He, the other two reservoirs are presumed to be relatively un-degassed and likely primordial. One reservoir is characterized by µ182W ≈ 0, while the other is characterized by µ182W ≤-23. The former reservoir likely formed from a silicate differentiation process more than 60 Myr after the origin of the solar system, but has remained partially or wholly isolated from the rest of the mantle for most of Earth history. The latter reservoir most likely includes a component that formed while 182Hf was extant. Mass balance constraints on the isotopic composition of the core suggest it has a strongly negative µ182W value of ∼ -220. Thus, it is a candidate for the origin of the negative µ182W in the plume sources. Mixing models show that the direct addition of outer core metal into a plume rising from the core-mantle boundary would result in collateral geochemical effects, particularly in the abundances of highly siderophile elements, which are not observed in OIB. Instead, the reservoir characterized by negative µ182W most likely formed in the lowermost mantle as a result of core-mantle isotopic equilibration. The envisioned equilibration process would raise the W concentration and lower the µ182W of the resulting silicate reservoir, relative to the rest of the mantle. The small proportion (<0.3 %) of this putative core-mantle equilibrated reservoir required to account for the µ182W signatures observed in OIB is insufficient to result in observable effects on most other elemental and/or isotopic compositions. The presumed primordial reservoirs may be linked to seismically distinct regions in the lower mantle. Seismically imaged mantle plumes appear to preferentially ascend from the vicinity of large low-shear velocity provinces (LLSVPs), which have been interpreted as thermochemical piles. We associate the LLSVPs with the primordial reservoir characterized by high 3He/4He and µ182W = 0. Smaller, ultra-low velocity zones (ULVZs) present at the core-mantle boundary have been interpreted to consist of (partially) molten lower mantle material. The negative µ182W signatures observed in some plume-derived lavas may result from small contributions of ULVZ material that has inherited its negative µ182W signature through core-mantle equilibration.

    更新日期:2019-12-25
  • Hydrogen isotopic composition of CI- and CM-like clasts from meteorite breccias – sampling unknown sources of carbonaceous chondrite materials
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-23
    Markus Patzek; Peter Hoppe; Addi Bischoff; Robbin Visser; Timm John

    Volatile-rich, CI- and CM-like clasts occur in different brecciated achondrite and chondrite groups. The CI-like clasts in HEDs, polymict ureilites, as well as ordinary, CR, and CB chondrites have a similar mineralogy, indicating a similar alteration history. However, when viewed in detail, their mineral chemistry shows some minor differences between the clasts from different meteorite groups. For CM-like clasts found in HED meteorites, the clasts are, based on their mineralogy, clearly fragments of CM chondrites. To be able to decipher whether CI- (or CM-)like clasts from different meteorite groups are related to certain meteorite classes known to contain volatiles, we obtained D/H ratios of several clasts from the meteorite groups mentioned above and compared them with those of CI and CM chondrites as well as to unique carbonaceous chondrites such as Bells, Essebi, and Tagish Lake. Considering the δD-values, CM-like clasts in HEDs span a similar range compared to bulk values of CM chondrites, further indicating that CM-like clasts are fragments of CM chondrites. For CI-like clasts a clear distinction can be made: While CI-like clasts in HEDs and ordinary chondrites show a very similar range in their δD-signatures compared to “common” CI chondrites, meaning that these clasts are likely related to CI chondrites, the CI-like clasts in polymict ureilites are enriched in D up to 3000 ‰; a similarly high enrichment is found for the CI-like clasts in CR chondrites. Thus, although the CI-like clasts in ureilites and CR chondrites likely experienced similar alteration histories as the CI-like clasts found in the other meteorite types, these clasts probably formed in a different region than the CI chondrites and, thus, are more accurately referred to as C1 clasts. Overall, the existence and isotopic signatures of the C1 clasts in several meteorite groups proves the existence of additional primitive, volatile-rich material in the (early) Solar System besides the matter we study as the CI, CM, and CR chondrites. This material was distributed throughout the Solar System very early and might have played an important role for the volatile inventory of the terrestrial planets.

    更新日期:2019-12-23
  • Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in Paleozoic black shales
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-23
    Michael A. Kipp; Thomas J. Algeo; Eva E. Stüeken; Roger Buick

    Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.

    更新日期:2019-12-23
  • Reactive alteration of a Mt. Simon Sandstone due to CO2-rich brine displacement
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-20
    Gabriela Dávila; Laura Dalton; Dustin M. Crandall; Charlotte Garing; Charles J. Werth; Jennifer L. Druhan

    We report a series of acidified brine flow-through experiments designed to quantify the coupled alteration of geochemical, structural and fluid transport properties of a Mt. Simon sandstone core recovered at a depth of 2110.5 m as part of the Illinois Basin Decatur Project (IBDP). Flow-through experiments were completed at representative in-situ conditions to isolate the stages of initial CO2 injection: first, a single-phase (CO2–saturated brine, Stage 1) followed by a second multi-phase (CO2–saturated brine and supercritical CO2, Stage 2) experiment. During both stages, effluent major and trace cation concentrations were tracked through time. Two imaging methods were employed to analyze the structural alterations of the rock core induced by the percolation of CO2-saturated brine and supercritical CO2: (1) scanning electron microscopy-petrography before Stage 1 and after Stage 2 and (2) computed tomography (CT) scans before and after Stage 2. The time series of Stage 1 effluent solutes were used to constrain a reactive transport simulation of the system. Modeling results suggest the evolution of the solute composition is a result of coupled dissolution of K-feldspar, calcite, illite and pyrite, and precipitation of montmorillonite, mesolite, alunite, diaspore, goethite and muscovite. The model predicted a net opening of pore space and associated increased permeability at the inlet. However, across the whole core, an overall decrease in permeability of approximately 23% ± 0.01 after Stage 1 was determined experimentally. CT analysis confirmed a corresponding decrease in porosity. A comparable permeability decrease was directly measured during Stage 2, concurrent with a decrease in the volume of macro-pores based on multiple CT-resolution methods. In total, this coupled approach demonstrates that geochemical alterations exert a first order control on the evolution of fluid transport properties through time at the earliest stages of in-situ CO2 injection and suggest that chemical dynamics ultimately influence both the magnitude and timing of alterations to the physical integrity of Mt. Simon reservoir over these timescales.

    更新日期:2019-12-21
  • Primordial organic matter in the xenolithic clast in the Zag H chondrite: Possible relation to D/P asteroids
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-19
    Yoko Kebukawa; Michael E. Zolensky; Motoo Ito; Nanako O. Ogawa; Yoshinori Takano; Naohiko Ohkouchi; Aiko Nakato; Hiroki Suga; Yasuo Takeichi; Yoshio Takahashi; Kensei Kobayashi

    Some xenolithic clasts in meteorites may have originated from unique primitive Solar System bodies. These clasts would provide novel insights into the early evolution of the Solar System. We conducted multiple analyses of organic matter (OM) in a CI-like xenolithic clast in the Zag (H5) meteorite including bulk elemental and isotopic analysis, FTIR, STXM/XANES, and NanoSIMS. The bulk C and N abundances in the Zag clast were +5.1 ± 0.4 wt.% and +0.26 ± 0.01 wt.%, respectively, which were the highest observed among various chondrite groups. The bulk δ13C value of the Zag clast was +23.0 ± 4.1 ‰ which was close to the value of the Tagish Lake meteorite; the δ15N value was +300 ± 3 ‰ which was close to the values of CR chondrites and Bells (a unique CM). The δD values of C-rich regions obtained by NanoSIMS were approximately +600 to +2000‰ which were close to the values of IOM from CI, CM and Tagish Lake. Some isotopic “hot spots” were observed with δD values up to ≈ +4000‰ and δ15N values up to ≈ +5500‰. The infrared transmission spectrum of the Zag clast was consistent with the abundant phyllosilicates and carbonates observed in the clast. The STXM showed abundant OM in various forms. C-XANES spectra from the OM were generally similar to CI/CM/CR chondrites. However, some variations existed in the molecular structures. OM in the Zag clast was partially associated with carbonates. The functional group, elemental and isotopic signatures of the OM in the Zag clast support the idea that the Zag clast is unique among known carbonaceous chondrite groups and originated from the outer Solar System such as aqueously-altered D/P type asteroids.

    更新日期:2019-12-19
  • Calibrating equilibrium Fe isotope fractionation factors between magnetite, garnet, amphibole, and biotite
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-19
    Hui Ye; Changzhi Wu; Matthew J. Brzozowski; Tao Yang; Xiangping Zha; Shugao Zhao; Bingfei Gao; Weiqiang Li

    Equilibrium isotope fractionation factors are the basis for application of stable isotopes in geological studies. Experimental calibration and theoretical calculations have been employed to determine Fe isotope fractionation factors for a variety of minerals, however, these methods have their limitations. An alternative approach to calibrating inter-mineral Fe isotope fractionation factors is to use well-characterized geological samples; this approach has unique advantages over the other methods including attainment of equilibrium at relatively low temperatures. In this study, we investigated the Fe isotope composition of magnetite (Mt), garnet (Grt), amphibole (Amp) and biotite (Bt) from the metamorphosed Yingshan iron formation in South China. Two independent geothermometers, quartz–magnetite oxygen isotope and amphibole–garnet–biotite Fe–Mg exchange geothermometers, give a consistent metamorphic temperature of 538 ± 39℃. The Fe isotope composition of the different Fe-bearing minerals is highly variable in different ironstone samples, with δ56Fe values (relative to IRMM-014) ranging from -0.23 to +0.37‰ in magnetite, -0.44 to +0.09‰ in amphibole, -0.78 to +0.02‰ in garnet, and -0.61 to +0.04‰ in biotite. Despite that, the offsets of δ56Fe values for mineral pairs are consistent, implying attainment of equilibrium isotope fraction between these minerals. Inter-mineral Fe isotopic fractionations (±2 standard deviation) measured from the multiple mineral pairs are Δ56FeMt–Grt = +0.55 ± 0.08‰, Δ56FeMt–Amp = +0.25 ± 0.06‰, Δ56FeMt–Bt = +0.42 ± 0.11‰, Δ56FeAmp–Grt = +0.34 ± 0.15‰, Δ56FeAmp–Bt = +0.26 ± 0.04‰, and Δ56FeBt–Grt = +0.13 ± 0.08‰. Based on the well-defined metamorphic temperature (538 ± 39 ℃) and internally consistent inter-mineral fractionation factors, the temperature-dependent functions for equilibrium Fe isotope fractionation between the following mineral pairs are derived: 103lnαMt–Grt = 0.36(±0.05)×106/T2, 103lnαMt–Bt = 0.28(±0.07)×106/T2, 103lnαMt–Amp = 0.16(±0.04)×106/T2, 103lnαAmp–Grt = 0.22(±0.10)×106/T2, 103lnαAmp–Bt = 0.17(±0.03)×106/T2, and 103lnαBt–Grt = 0.09(±0.05)×106/T2,where Fe3+/ΣFe ratio is 0.05 ± 0.02 in garnet, 0.29 ± 0.04 in biotite, and 0.24 ± 0.06 in amphibole for the above functions. The equilibrium Fe isotope fractionation factors derived in this study enable estimation of the metamorphic temperature of rocks that contain these mineral pairs, and identification of secondary processes that may have induced disequilibrium Fe isotope distribution in rocks, such as retrograde metamorphism and hydrothermal alteration.

    更新日期:2019-12-19
  • Molybdenum and boron isotopic evidence for carbon-recycling via carbonate dissolution in subduction zones
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-19
    Yunying Zhang; Chao Yuan; Min Sun; Jie Li; Xiaoping Long; Yingde Jiang; Zongying Huang

    Subduction zones are critical sites for carbon cycling between Earth’s surface and interior. However, how subducted carbon is released and transferred to the surface is not well understood, especially regarding the role of slab-derived fluids in the deep carbon cycle. Here we report Mo and B isotopic data for the Silurian normal arc andesites and adakitic andesites from the Chinese North Tianshan, which represent partial melts of fluid-modified mantle wedge and dehydrated oceanic crust, respectively. The normal arc andesites yielded δ98Mo values (0.33–1.08‰) significantly higher than that (about –0.20‰) of the depleted mantle. Because their limited range of SiO2 (53.8–55.3 wt.%) precludes differentiation as a cause for their variable δ98Mo values and Mo isotopic fractionation solely by fluid mobilization is limited (≤ 0.3‰), the elevated δ98Mo values could be ascribed to the incorporation of crustal material with heavy Mo isotopes in the mantle source. Since marine carbonate is featured by both heavy Mo and B isotopes and our normal arc andesites also give heavy δ11B (–1.63 to +4.00‰) values, we consider that marine carbonate was possibly involved as a component of the subducted slab, which modified Mo–B isotopic compositions of the mantle source. The positive correlations between δ98Mo and δ11B and between δ98Mo and Ba/Rb suggest transport of subducted carbonates to the overlying mantle wedge via slab fluids, thus providing robust evidence for transfer of subducted carbon to the overriding plate by carbonate dissolution. In contrast, the younger adakitic andesites have light δ98Mo (–0.48 to –0.27‰) and δ11B (–9.43 to –2.05‰) values, implying an isotopically Mo- and B-light source. Given the preferential transport of heavy 98Mo and 11B to the fluid phase during slab dehydration, their remarkably light δ98Mo and δ11B values support a dehydrated oceanic crust as their magma source. The contrasting Mo–B isotopes for such two kinds of andesites highlight that most carbonates can be removed from the subducted slab to the overriding plate during oceanic subduction.

    更新日期:2019-12-19
  • The combined Zr and Hf isotope inventory of bulk rock and sequentially leached chondrite samples
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-17
    Bo-Magnus Elfers; Sprung Peter; Messling Nils; Münker Carsten

    The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present for the first time combined isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites for the geo- and cosmochemically similar elements Zr and Hf. Our analyses reveal significant Zr and Hf isotope anomalies that (i) are complementary between acid leachates and residues and (ii) well-correlated with each other. The observed Zr and Hf anomalies strongly suggest variable contributions of common s-process carrier phases to the different leachates and residues. Ratios of r- (and p-process) Zr and Hf isotopes appear to be uniform in leachates and residues. In contrast to the well-correlated anomalies found in our leaching experiments, nucleosynthetic Zr and Hf isotope signatures seem to be decoupled on the bulk rock scale. This contrast may result from the heterogeneous distribution of neutron-rich Zr material devoid Hf, or alternatively be caused by the presence of anomalous CAI material which overprinted s-process deficits that were initially correlated. In contrast to a previous study, we find no direct evidence for the presence of a third isotopically distinct nucleosynthetic Zr component.

    更新日期:2019-12-18
  • Carbon and oxygen isotope systematics in cave environments: Lessons from an artificial cave “McMaster Cave”
    Geochim. Cosmochim. Acta (IF 4.258) Pub Date : 2019-12-17
    Mohammed I. EL-Shenawy; Sang-Tae Kim; Henry P. Schwarcz

    Understanding carbon and oxygen isotope systematics in cave environments is a prerequisite for the interpretation of stable isotopes in speleothem-based paleoclimate records. Here we present a series of experimental data collected under laboratory conditions with controlled temperature, relative humidity, drip water chemistry, flow rate and cave pCO2, simulating the growth of speleothems in natural cave settings. Drip water with high pCO2 and low calcium concentration (Ca2+ = 2 mmol/L) flowed along a three-step glass path, similar to a stalactite-stalagmite-pool route in natural caves, forming a thin water film that allowed CO2 degassing and CaCO3 precipitation as a result of the pCO2 gradient between the drip water and ambient cave atmosphere. The experiments were conducted at 15, 25 and 32 °C and flow rates of 700, 270 and 125 ml/d. The growth rate of calcite on the stalagmite-like settings increases linearly with increasing flow rate and/or temperature. The δ13CCc and δ18OCc of calcite formed on the stalagmite-like settings increases with decreasing flow rate (corresponding to increasing residence time of water) at a given temperature, indicating non-equilibrium isotopic effects between calcite, water and dissolved inorganic carbon (DIC). Nevertheless, these non-equilibrium isotope effects still display regular temperature dependence under a constant flow rate. This suggests that non-equilibrium isotope effects in natural stalagmites might be used to provide useful qualitative paleoclimate information (such as differentiating wet/dry and warm/cold climate conditions). The non-equilibrium carbon and oxygen isotope effects in the stalagmite-like settings were most likely caused by rapid CO2 degassing and CaCO3 precipitation that rapidly consume the available DIC pool in the thin water film. Furthermore, CO2 exchange between DIC and cave atmosphere quickly amplified the observed non-equilibrium carbon isotope effects in the precipitated calcite. In the pool-like settings, calcite was buffered by oxygen isotope exchange between DIC species and water, and slowly precipitated at or near to oxygen isotopic equilibrium with the temperature dependence of 1000ln18αCc-H2O = 18.33 (103/T) – 33.31 regardless of flow rate. This fractionation relation agrees with that determined by Kim and O’Neil (1997) when a newly recommended value for the acid fractionation factor for calcite is used (i.e., Kim et al., 2015). Carbon isotope fractionation between calcite and bicarbonate was temperature-independent between 15 and 32 °C and the average magnitude was 1000ln13αCc-HCO3 = 1.7 ± 0.7 ‰. Observed variability of 1000ln18αCc-H2O in modern calcite speleothems from natural cave settings lies in the range predicted by this study, between the predicted maximum non-equilibrium deviation at the stalagmite-like settings and the equilibrium 1000ln18αCc-H2O achieved in the pool-like settings.

    更新日期:2019-12-18
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