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  • Origins of high δ18O in 3.7–3.6 Ga crust: A zircon and garnet record in Isua clastic metasedimentary rocks
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-17
    Laure Gauthiez-Putallaz; Allen Nutman; Vickie Bennett; Daniela Rubatto

    Elevated δ18O is used as a marker for the presence of continents and surficial alteration in the Eoarchean and Hadean. This study establishes a timeline for δ18O enrichment in Eoarchean metasedimentary rocks of the Isua supracrustal belt in Greenland. The source-rocks for the protolith of these metasedimentary rocks are mafic to intermediate magmatic rocks of ≥3709 ± 4 Ma, based on the age of zircons found in volcanogenic layers. The δ18O of +5.4 ± 0.4‰ of the zircon crystals indicate that the sources had a primary mantle-derived signature. However, garnet in two metasediments yields higher δ18O values of +8.7 to +9.7‰, in equilibrium with a whole-rock of +11 to +12‰ at 500–600 °C. This requires that the mafic protolith was weathered at surficial conditions, in agreement with previous conclusions based on major element geochemistry. The garnet grains with high δ18O record four growth zones, assigned to I) arc-building thermal metamorphism, II–III) terrane assembly at medium to high-pressure conditions, estimated to occur at 3660–3690 Ma and IV) late-Archaean overprint likely at ca. 2690 Ma. This shows that material with originally mantle-like δ18O was altered at low temperature (near-surface) to generate elevated oxygen isotope signatures and then recycled to middle-crustal conditions within 10–50 million years of crystallization in the Eoarchean. We propose that melting of such rocks could produce the zircon crystals with high δ18O that are found in the detrital and magmatic record in the Archean.

    更新日期:2020-01-17
  • Re-Os geochronology and isotope systematics, and organic and sulfur geochemistry of the middle–late Paleocene Waipawa Formation, New Zealand: Insights into early Paleogene seawater Os isotope composition
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-16
    Enock K. Rotich; Monica R. Handler; Sebastian Naeher; David Selby; Christopher J. Hollis; Richard Sykes

    In the middle–late Paleocene, a marine, organic-rich sedimentary unit (Waipawa Formation [Fm]) in which the organic matter was derived mainly from terrestrial plants was deposited in many of New Zealand's sedimentary basins. The unique organofacies of this formation has not been identified in any other time interval within the geological history of the Southwest Pacific, indicating that unusual climatic and oceanographic conditions likely prevailed during this time. It has, therefore, attracted wide scientific interest due to its significance for regional and global reconstruction of the early Paleogene transitional climate as well as potential for oil and gas production. Scarcity of age-diagnostic fossils, presence of unconformities and lack of volcanic interbeds have, however, hindered precise dating and correlations of all the known occurrences of the formation. Here, rhenium‑osmium (Re-Os) geochronology has yielded the first radiometric age for the formation (57.5 ± 3.5 Ma), which is consistent with available biostratigraphic age determinations (59.4–58.7 Ma). Further, a comparison of Re-Os, bulk pyrolysis, sulfur and palynofacies data for the Waipawa Fm with those of more typical marine sediments such as the underlying Whangai Fm supports the interpretation that the chelating precursors or fundamental binding sites responsible for uptake of Re and Os are present in all types of organic matter, and that these elements have a greater affinity for organic chelating sites than for sulfides. The results also indicate that sedimentation rate may not play a dominant role in enhanced uptake of Re and Os by organic-rich sedimentary rocks. The initial 187Os/188Os values for the Waipawa (~0.28) and Whangai (~0.36) formations are broadly similar to those reported for coeval pelagic sediments from the central Pacific Ocean, further constraining the low-resolution marine 187Os/188Os record of the Paleocene. We present a compilation of 187Os/188Os values from organic-rich sedimentary rocks spanning the period between 70 and 50 Ma which shows that seawater Os gradually became less radiogenic from the latest Cretaceous, reaching a minimum value in the earliest late Paleocene (~59 Ma) during the deposition of Waipawa Fm, and then increased through the later Paleocene and into the early Eocene. The composite Os isotope record broadly correlates with global temperature (δ18O and TEX86) and carbon isotope (δ13C) records from the middle Paleocene to early Eocene, which is inferred to reflect climate-modulated changes in continental weathering patterns.

    更新日期:2020-01-17
  • New frontiers in calcium stable isotope geochemistry: Perspectives in present and past vertebrate biology
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-15
    Théo Tacail; Sandrine Le Houedec; Joseph L. Skulan

    Beyond their established uses in Earth and Planetary sciences, calcium (Ca) isotopes have a promising future in the study of the biology of present and past vertebrates, including humans. Early work paved the way to the ongoing research on the potential of Ca isotopes as relevant tools to disciplines other than geology, including palaeobiology, bioarchaeology and biomedical research. In this article, we first review the rationale behind the cycling of Ca isotopes in vertebrate organisms. We then summarize and discuss the use of Ca isotopes as dietary tracers from trophic reconstructions in past vertebrate ecosystems to the tracking of early life dietary transitions. Next, we review and examine the research outcomes on the potential of Ca isotopes as biomarkers of bone loss in physiological and pathological conditions such as bone cancers and osteoporosis. While emphasizing the needs of future research in each of these applications, we suggest new potential uses of Ca isotopes in vertebrate biology. Finally, we identify challenges and barriers faced when developing such interdisciplinary projects and suggest how these can be overcome.

    更新日期:2020-01-15
  • Genesis of glauberite sedimentation in Lop Nur Salt Lake - Constraints from thermodynamic simulation of the shallow groundwater in the Tarim River Basin, China
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-15
    Ruiqin Li; Chenglin Liu; Haiming Xu; Pengcheng Jiao; Yufei Hu; Li Fan; Xiaohong Sun

    This study was carried out in Lop Nur Salt Lake, which is one of the largest closed playa lakes in the world and is located in the lowest part of the Tarim Basin at the east end. The objective of this study was to further reveal the genesis of glauberite sedimentation in Lop Nur Salt Lake by thermodynamic simulation of the shallow groundwater in the Tarim River Basin with the Equilibrium and Evaporation (EQL/EVP) program, as the shallow groundwater of the Tarim River Basin may represent the recharging water body of Lop Nur. The shallow groundwater of the Tarim River Basin is of the sodium sulfate subtype and the magnesium sulfate subtype and is dominated by the sodium sulfate subtype. The shallow groundwater is poor in Ca2+ but is rich in SO42−. The EQL/EVP program, an equilibrium model that simulates the evaporation of dilute waters as well as of concentrated brines, simulated the evaporation of the shallow groundwater, and the results showed good agreement between predicted and observed mineral sequences and assemblages. The simulation results with the EQL/EVP showed that a small amount of glauberite can be formed under an open system, a large amount of glauberite can be precipitated under a closed system, and that the amount of glauberite precipitated under the closed system is approximately 100 times higher than that under open system, indicating that a closed system is more conducive to the formation of glauberite. However, even in the closed system, the amount of glauberite precipitated by simulated evaporation is much less than the observed amount that is deposited in Lop Nur. Therefore, it is inferred that Lop Nur must also be recharged by deep calcium-rich waters. With the Tarim River Basin and deep calcium-rich waters as the recharge sources and under arid climate conditions, the water of Lop Nur was concentrated by evaporation, minerals began to accumulate over a long period of time, and a large amount of glauberite was formed, which provides storage space for potassium-rich brine in Lop Nur.

    更新日期:2020-01-15
  • On the utility of quantitative modeling to the interpretation of Ca isotopes
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-15
    Jennifer L. Druhan; Laura Lammers; Matthew S. Fantle

    Though the utility of calcium isotopes is broad, examples of application are still relatively sparse. Some of this, as with many metal isotope systems, may derive from a perception that the analytical tool is difficult to access. Alternatively, potential users may be deterred by a sense that calcium isotopes are ambiguous, difficult to interpret, and/or complicated by too many interrelated processes. Here we provide an overview of the quantitative modeling approaches available to facilitate interpretation of the distribution and signatures of calcium isotopes in Earth systems, in particular their ability to push interpretations beyond the qualitative. While no method is perfect, the variety of approaches described here offer an accessible toolset that is incredibly valuable. We suggest that the calcium isotope community should be expanding the use of such techniques, noting their limitations and striving to improve upon current methods over time.

    更新日期:2020-01-15
  • Calcium and strontium isotope dynamics in three polluted forest ecosystems of the Czech Republic, Central Europe
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-15
    Martin Novak; Chris Holmden; Juraj Farkaš; Pavel Kram; Jakub Hruska; Jan Curik; Frantisek Veselovsky; Marketa Stepanova; Yulia V. Kochergina; Vojtech Erban; Daniela Fottova; Martin Simecek; Leona Bohdalkova; Eva Prechova; Petra Voldrichova; Vladimir Cernohous

    Calcium and strontium isotope ratios were used to identify Ca sources for bulk soil, soil solutions and runoff in the stressed forest ecosystems of Central Europe. All three study sites are underlain by Ca-poor crystalline bedrock (orthogneiss, leucogranite and serpentinite, respectively), but differ in historical rates of acid deposition. UDL, the spruce die-back affected site at the Czech–Polish border underlain by orthogneiss, received six times more acidifying compounds from the atmosphere than LYS and PLB, two paired catchments near the Czech–German border. The paired catchments near the Czech–German border, whose forests were only mildly damaged by industrial pollution, differ in their acid buffering capacity (extremely low for LYS leucogranite, extremely high for PLB serpentinite). At the spruce die-back affected catchment UDL, 19 years of monthly hydrochemical monitoring revealed six times higher atmospheric deposition and runoff fluxes of Ca, compared to the paired catchments LYS and PLB. Across the three sites, the mean δ44Ca values increased in the order: spruce bark (−0.84‰) < spruce xylem (−0.31‰) < spruce fine roots (0.0‰) < bedrock (0.08‰) < soil (0.11‰) < spruce needles (0.31‰) < open-area precipitation (0.68‰) < spruce throughfall (0.71‰) < runoff (0.74‰) < soil water (1.11‰). At all three sites, Ca from atmospheric deposition was isotopically similar to Ca in runoff, indicating export of recent atmospheric Ca. All three catchments had isotopically lighter Ca in silicate bedrock and exported isotopically heavier Ca via runoff. Trees mostly represented the isotopically lightest Ca pool, whereas soil water collected by lysimeters contained isotopically heavy Ca. The 87Sr/86Sr isotope ratios were nearly uniform at the serpentinite site PLB (0.710). The 87Sr/86Sr ratios across the two felsic sites (UDL and LYS) increased in the order: open-area precipitation (0.710) < spruce canopy throughfall (0.715) < spruce xylem (0.723) < spruce needles (0.725) < soil water (0.734) < runoff (0.740) < soil (0.862) < bedrock (0.881). Collectively, the δ44Ca and 87Sr/86Sr isotope systematics indicated that organic Ca cycling, along with atmospheric input of isotopically heavy Ca, largely contributed to the high-δ44Ca values of runoff. The studied silicate bedrock had significantly lower δ44Ca values than the reported world-wide average. While bulk soil Ca was likely affected by bedrock Ca even after the period of acid rain, the δ44Ca difference between soil and soil water indicated a major role of recycled organic Ca in supplying nutrition to the trees. Future Ca-Sr isotope research in headwater catchments should include isotope analysis of sequentially leached Ca fractions from weathered minerals in soil to better assess the geogenic Ca inputs to runoff. In catchments currently recovering from pollution, Ca-Sr isotope fingerprinting of dust emitted from coal-fired power plants will be needed. Isotope analysis of local coal can indicate to what extent Ca-Sr isotope composition of past deposition contributes to the isotope signatures of present-day runoff.

    更新日期:2020-01-15
  • Porewater flow patterns in surficial cold seep sediments inferred from conservative tracer profiles and early diagenetic modeling
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-15
    Jurjen Rooze; Leigha Peterson; Richard N. Peterson; Christof Meile

    Porewater flow at cold seeps controls methane oxidation and carbonate precipitation rates, and also plays an important role in gas hydrate dynamics. We constrain the porewater flow rates in the upper sediment of Gulf of Mexico cold seeps by fitting measured 226Ra and Cl− profiles. The approach exploits their natural increase with depth through shallow sediments at non-brine (226Ra) and brine (Cl−) seeps. Vertical flow velocities at non-brine and brine seeps are found to be similar in magnitude, typically on the order of 0.1 m y−1. The accuracy of the flow estimates is assessed and shown to depend substantially on uncertainties in both concentrations and sediment tortuosity. Model simulations further reveal that the natural heterogeneity of sediment permeability can cause considerable lateral variability in vertical flow at the scale of a sediment core, which implies that the coring location and core size can have a substantial effect on the estimated porewater velocities. The formation of gas hydrates and calcium carbonates reduce sediment permeability, and decrease the spatial variability in flow over time, as the formation rates are higher along preferential flow paths. This analysis of the effect of spatiotemporal variability on chemical profiles improves the assessment of uncertainties in the estimated magnitude of flow, and is important for the evaluation of benthic-pelagic coupling at seeps.

    更新日期:2020-01-15
  • Environmental conditions of deposition of the Lower Cretaceous lacustrine carbonates of the Barra Velha Formation, Santos Basin (Brazil), based on stable carbon and oxygen isotopes: A continental record of pCO2 during the onset of the Oceanic Anoxic Event 1a (OAE 1a) interval?
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-14
    Raphael Pietzsch; Leonardo R. Tedeschi; Daniel M. Oliveira; Camila Wense Dias dos Anjos; Joselito C. Vazquez; Milene F. Figueiredo

    This study presents new stable carbon and oxygen isotope data from Lower Cretaceous lacustrine carbonate rock samples recovered from a well drilled in the Santos Basin, offshore southeast Brazil. These samples represent a record of a continental environment just prior to the opening of the South Atlantic Ocean and the ultimate break-up of Gondwanaland. The geochemical data, along with carbonate mineralogy, indicate repeated cycles of lake level variation that could be attributed to climatic oscillations. Despite the absence of correlations between δ13C and δ18O values, facies analysis and the isotopic and mineralogical data suggest that lake hydrology was essentially closed for most of the depositional interval studied here. The existence of persisting trends of nearly constant δ13C values with a spread in δ18O values though, suggests long water residence times in the palaeolake, equilibrium between atmosphere and lake water CO2, as well as significant evaporation of water. The overall geological model that emerges unveils a more comprehensive picture of the depositional conditions that favoured the continuity of a significant carbonate factory in the middle of the Gondwanan continent, corroborating previous studies that suggested the lasting existence of a large and somewhat shallow endorheic lake in the area during the Early Cretaceous. As a result of this recorded trend strongly suggesting equilibrium between lake waters DIC (dissolved inorganic carbon) reservoir and atmospheric CO2, the data are most consistent with lacustrine deposition rather than precipitation of travertine, contrasting with some suggestions for the genesis of the carbonates of the Barra Velha Formation. Finally, this apparent equilibrium with the atmosphere likely left a preserved record in the continental carbonates of the final stages that preceded a major global environmental disturbance associated with an increase in atmospheric CO2, known for this time as the Oceanic Anoxic Event (OAE) 1a. If this is correct, it also helps to put further time constraints on this studied interval, which should not be younger than Barremian age, and to provide a regional continental perspective on a global event.

    更新日期:2020-01-14
  • Introduction to calcium isotope geochemistry: Past lessons and future directions
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-13
    Elizabeth M. Griffith; Matthew S. Fantle

    Calcium (Ca) isotopes have been utilized as geochemical tools since the 1970s, increasing in popularity and scope since the mid- to late 1990s. Research conducted over the past thirty years has revealed a range of applications for Ca isotopes that are highlighted in this introduction and Special Issue, as well as a series of interesting and fundamental problems that have yet to be resolved. For such a chemically-simple system, in comparison to redox-active elements for instance, the level of complexity is both surprising and intellectually attractive. While the advent of multiple collector inductively coupled plasma mass spectrometers made metals such as Fe, Mg, and Zn considerably more accessible analytically, the precision and ease of Ca isotopic analysis did not radically improve in a comparable manner. For a variety of reasons, one of which may be perceived analytical difficulty, the application of Ca isotopes to geological questions remains in the shadows, to some extent, in the Geosciences. In this introduction, we identify (i) areas that we think are most fertile for the application of Ca isotopes as tools and (ii) outstanding questions and future directions that can move the field towards increased utility and broader acceptance by the Geosciences community.

    更新日期:2020-01-14
  • Exploration of driving mechanisms of equilibrium boron isotope fractionation in tourmaline group minerals and fluid: A density functional theory study
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-13
    Yin-Chuan Li; Hong-Wei Chen; Hai-Zhen Wei; Shao-Yong Jiang; Martin R. Palmer; T.G.M. van de Ven; Simon Hohl; Jian-Jun Lu; Jing Ma
    更新日期:2020-01-14
  • Titanite petrochronology linked to phase equilibrium modelling constrains tectono-thermal events in the Akia Terrane, West Greenland
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-10
    C.L. Kirkland; C. Yakymchuk; N.J. Gardiner; K. Szilas; J. Hollis; H. Olierook; A. Steenfelt
    更新日期:2020-01-10
  • Evaluation of zircon U-Pb geochronology as a tool to determine soil provenance in a limestone terrane, Middle TN, USA
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-09
    Xiaomei Wang; Nathan Katsiaficas; Jeong Hyun Nam; Camille Lasley; Xiaoming Liu; John C. Ayers

    Soil can form by bedrock weathering or by deposition of alluvial or aeolian material. To test the utility of detrital zircon UPb geochronology for determining soil provenance, and to determine the provenance of soils in Middle Tennessee, samples of soil and underlying bedrock were collected from two sites, along with alluvium from a nearby river and loess from West Tennessee. Multi-dimensional scaling plots were used to compare complex detrital zircon LA-ICP-MS UPb sample age distributions. Concentration ratios of immobile elements and Nd and Hf isotope compositions were also used as provenance indicators for comparison. At one site zircon UPb ages and bulk sample trace element and isotopic compositions of soil were similar to the underlying limestone bedrock, indicating soil formation by bedrock weathering. In contrast, at a second site provenance tests indicate that soil from a late Pleistocene river terrace was sourced from alluvium rather than the underlying bedrock. Provenance inferences based on zircon UPb ages were generally consistent with evidence from Zr/Hf ratios and Hf isotope compositions, which are all dominated by zircon, and Nd isotope compositions, which are not, although the best matches for different provenance indicators did not always agree. Zircon geochronology should be used with other soil provenance indicators to reduce effects of bias introduced by sedimentary sorting and sample processing.

    更新日期:2020-01-10
  • Element and isotopic signature of re-fertilized mantle peridotite as determined by nanopowder and olivine LA-ICPMS analyses
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-09
    Christopher J.M. Lawley; D. Graham Pearson; Pedro Waterton; Alex Zagorevski; Jean H. Bédard; Simon E. Jackson; Duane C. Petts; Bruce A. Kjarsgaard; Shuangquan Zhang; Donald Wright

    The lithospheric mantle should be depleted in base- and precious-metals as these elements are transferred to the crust during partial melting. However, some melt-depleted mantle peridotites are enriched in these ore-forming elements. This may reflect re-fertilization of the mantle lithosphere and/or sequestering of these elements by residual mantle phase(s). Both processes remain poorly understood because of the low abundances of incompatible elements in peridotite and the nugget-like distribution of digestion-resistant mantle phases that pose analytical challenges for conventional geochemical methods. Herein we report new major and trace element concentrations for a suite of mantle peridotite and pyroxenite samples from the Late Permian to Middle Triassic Nahlin ophiolite (Cache Creek terrane, British Columbia, Canada) using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analysis of nanoparticulate powders and olivine. Compatible to moderately incompatible element concentrations suggest that Nahlin ophiolite peridotites represent residues after ≥20% melt extraction. Pyroxenite dykes and replacive dunite bands are folded and closely intercalated with residual harzburgite. These field relationships, coupled with the presence of intergranular base metal sulphide, clinopyroxene and Cr-spinel at the microscale, point to percolating melts that variably re-fertilized melt-depleted mantle peridotite. Radiogenic Pb (206Pb/204Pb = 15.402–19.050; 207Pb/204Pb = 15.127–15.633; 208Pb/204Pb = 34.980–38.434; n = 45) and Os (187Os/188Os 0.1143–0.5745; n = 58) isotope compositions for a subset of melt-depleted peridotite samples further support metasomatic re-fertilization of these elements. Other ore-forming elements are also implicated in these metasomatic reactions because some melt-depleted peridotite samples are enriched relative to the primitive mantle, opposite to their expected behaviour during partial melting. New LA-ICPMS analysis of fresh olivine further demonstrates that a significant proportion of the highly incompatible element budget for the most melt-depleted rocks is either hosted by, and/or occurs as trapped inclusions within, the olivine-rich residues. Trapped phases from past melting and/or re-fertilization events are the preferred explanation for unradiogenic Pb isotope compositions and Paleozoic to Paleoproterozoic Re-depletion model ages, which predate the Nahlin ophiolite by over one billion years.

    更新日期:2020-01-09
  • Sulfur isotope ratios in co-occurring barite and carbonate from Eocene sediments: A comparison study
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-08
    Kotaro Toyama; Adina Paytan; Ken Sawada; Takashi Hasegawa
    更新日期:2020-01-08
  • Modeling singular mineralization processes due to fluid pressure fluctuations
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-08
    Yihui Xiong; Renguang Zuo; Keith C. Clarke; Stephen A. Miller; Jian Wang

    Mineralization in the Earth's crust can be regarded as a singular process resulting in large amounts of mass accumulation and element enrichment over short time or space scales. The elemental concentrations modeled by fractals and multifractals show self-similarity and scale-invariant properties. We take the view that fluid-pressure variations in response to earthquakes or fault rupture are primarily responsible for changes in solubility and trigger transient physical and chemical variations in ore-forming fluids that enhance the mineralization process. Based on this general concept, we investigated mineral precipitation processes driven by rapid fluid pressure reductions by coupling mineralization to a cellular automaton model to reveal the nonlinear mechanism of the orogenic gold mineralization process using simulation. In the model, fluid pressure can increase to the rock failure condition, which was set as lithostatic pressure at a depth of 10 km (270 MPa), due to either porosity reduction or dehydration reactions. Rapid drops in pressure resulting from fault rupture or local hydrofracture may induce repeated gold precipitation. The geochemical patterns generated by the model evolve from depletion to enrichment patterns, and from spatially random to spatially clustered structures quantified by multifractal models and geostatistics. Results show how metal elements self-organize to form high metal concentration patterns displaying self-similarity and scale-invariance. These transitions are attributed to the growth and coalescence of sub-networks with different fluid pressures up to the percolation threshold, resulting in a wide range of fluid pressure reductions and gold precipitation in the form of clusters. The results suggest that cyclic evolution of fluid pressure and its effects on gold precipitation systems can effectively mimic the repeated mineralization superposition process, and generate complex geochemical patterns characterized by a multifractal model. The nonlinear behavior exhibits scale-invariance and self-organized critical threshold, where mineral phase separations result from fluid pressure reductions associated with fault failure.

    更新日期:2020-01-08
  • 更新日期:2020-01-07
  • Time-dependent shapes of a dissolving mineral grain: comparisons of simulations with microfluidic experiments
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-07
    Filip Dutka; Vitaliy Starchenko; Florian Osselin; Silvana Magni; Piotr Szymczak; Anthony J.C. Ladd
    更新日期:2020-01-07
  • Ab initio molecular dynamics simulation of Nd3+ incorporation in calcite
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-07
    Sebastien N. Kerisit; Micah P. Prange

    Trivalent rare-earth elements (REEs) readily incorporate in Ca-bearing minerals such as calcite (CaCO3), a phenomenon with important implications for the use of REEs as biogeochemical tracers as well as for environmental remediation. Despite substantial efforts, determining the incorporation modes of trivalent REEs in calcite has remained challenging due to their presence in multiple coordination environments. This is particularly problematic for the interpretation of extended X-ray absorption fine structure (EXAFS) spectroscopy. In this work, ab initio molecular dynamics (AIMD) simulations of Nd3+ incorporated in calcite were performed to resolve its incorporation modes. Nd3+ and Ca2+ have very similar ionic radii but the additional positive charge requires a charge compensation scheme (CCS). Five CCSs were considered in AIMD simulations, and the resulting atomic trajectories were then used to compute EXAFS spectra for direct fitting to a published experimental spectrum. The results of the fits indicated that Nd3+ incorporates at the Ca2+ site in calcite in a mixture of six- and seven-fold coordination environments and through three main incorporation modes: associated with a protonated nearest-neighbor Ca2+ vacancy, incorporated as NdOH2+, and charge compensated by a remote species (on the spatial scale probed by EXAFS). The incorporation of Nd3+ in calcite via multiple modes was consistent with previous work, but the AIMD-EXAFS approach allowed for resolving the nature, proportion, and structure of the different coordination environments.

    更新日期:2020-01-07
  • 更新日期:2020-01-07
  • Magmatic fluids play a role in the development of active gas chimneys and massive gas hydrates in the Japan Sea
    Chem. Geol. (IF 3.618) Pub Date : 2020-01-07
    Glen T. Snyder; Yuji Sano; Naoto Takahata; Ryo Matsumoto; Yoshihiro Kakizaki; Hitoshi Tomaru
    更新日期:2020-01-07
  • Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-30
    Marta Sośnicka; Volker Lüders

    Upper Permian Zechstein carbonate Ca2 gas reservoirs in the southern part of the Pompeckj Block in the North German Basin locally contain up to 36 vol% hydrogen sulfide (H2S) produced by thermochemical sulfate reduction (TSR). TSR was triggered by migration of dry to extremely dry coal gas from Upper Carboniferous into the Zechstein carbonate reservoirs. Methane reacted with dissolved sulfate at temperatures of <150 °C, as inferred from fluid inclusions hosted in fracture-filling minerals and cements in the carbonate reservoir rocks. Such low temperatures for methane-dominated TSR are unique and were not observed so far, as it was widely believed that alteration of super dry methane requires much higher temperatures. Here we present detailed compositional and carbon isotope data of reservoir gases as well as those of gases trapped in fluid inclusions hosted in cements and fracture-filling minerals in Zechstein Ca2 carbonate reservoir rocks. We constrained the P-T conditions of gas entrapment, hydrocarbon reactivity and the lower temperature limit for TSR. The results of this study decipher three major stages of gas migration in the Pompeckj Block. Stage I commenced in the Late Triassic during burial when Zechstein Ca2 reservoirs were charged with dry CH4-CO2 ± N2 gas sourced from mature Upper Carboniferous coals. Burial continued through the Jurassic and caused alteration of Ca2 reservoir gas by sulfate reduction reactions due to increasing temperatures. Entrapment of CH4-H2S-CO2-N2 gases in fluid inclusions, hosted in cements and fracture-filling minerals, occurred at temperatures between 100 and 152 °C and was related to Stage II uplift in the Early Lower Cretaceous. In the Late Cretaceous (Stage III) deep burial of the Pompeckj Block led to charge of the Zechstein Ca2 carbonate reservoirs with Upper Carboniferous-derived CH4-CO2 ± N2 ± C2+ coal gas and/or dilution of existing reservoir gas at temperatures of 144–167 °C. Highly variable δ13CCH4 values from −18.7 to −8.7‰ and very negative δ13CCO2 values (−22.4 to −18.9‰) of H2S-rich fluid inclusion gases as well as negative δ13C values (−10.4 to −4.6‰) of host calcites reveal compelling evidence for participation of methane in TSR. Fluid inclusions imply that CH4-dominated TSR proceeded at Tmin of 135 °C in the presence of catalyzers such as H2S and dissolved Mg2+. This study demonstrates that fluid inclusions serve as an excellent and accurate tool for tracing H2S concentrations in hydrocarbon gases through time and space, which is not possible using the present-day compositions of natural reservoir gases. It also contributes to the understanding of carbonate reservoir-hosted hydrocarbon-bearing fluid systems and processes that significantly control the quality of reservoir gases.

    更新日期:2019-12-30
  • Controls on granitic weathering fronts in contrasting climates
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-30
    Nick R. Hayes; Heather L. Buss; Oliver W. Moore; Pavel Krám; Richard D. Pancost

    Granitic weathering profiles display highly diverse morphologies, reflecting the complex relationships between climate and weathering rates. Some profiles exhibit abrupt transitions from fresh bedrock to highly weathered material over short (<1 m) distances, while others exhibit only limited weathering extending 10s of meters into the bedrock. Although granitic weathering processes have been well studied, the controls on profile morphology and weathering rates within granitic, and many other lithologies remain poorly understood; these are likely influenced by a range of both intrinsic and extrinsic factors, which in turn will have crucial implications for understanding, for example, climate-weathering feedbacks. In this study we present multi-scale elemental and mineralogical analyses of a >30 m granitic weathering profile from the cool, temperate, Lysina catchment in the NW Czech Republic, from which we calculated mass transfer, weathering indices, and mineral specific weathering rates. The Lysina profile exhibits limited weathering extending >30 m into fractured bedrock, dominated by albite weathering at a rate of 9.3 × 10−17 mol m2 s−1. To identify environmental and geological controls on weathering front morphology and chemical weathering rates, Lysina was compared to previously published granitic weathering profiles from around the world. Weathering front morphology and weathering rates were calculated for the additional sites from published data and were correlated to mean annual precipitation (MAP), mean annual temperature (MAT), and erosion rates, with MAP having the strongest relationship. Higher MAP likely promotes lower saturation indices in pore waters, allowing weathering reactions to occur further from equilibrium. Comparison of erosion rates amongst the granitic catchments revealed an inconsistent effect on chemical weathering rates, but high erosion rates may promote weathering by reducing the thickness of the regolith and exposing the bedrock to reactive fluids. Mean annual temperatures appear to only have significant impacts on weathering fronts in environments with high precipitation and high erosion rates. Fractured bedrock profiles (Lysina and Río Icacos) have higher weathering intensities, than the other sites studied here. High connected porosity in fractured rocks enhances water movement allowing more efficient removal of weathering products, thus reducing thermodynamic saturation, increasing weathering rates, and producing sharper weathering gradients. These findings indicate that CO2 drawdown on geological timescales is also likely to be governed by precipitation rates, as well as temperature, and that much of the climate-significant weathering may occur within very narrow zones of the Earth's surface.

    更新日期:2019-12-30
  • Sources and radiocarbon ages of organic carbon in different grain size fractions of Yellow River-transported particles and coastal sediments
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-30
    Tiantian Ge; Yuejun Xue; Xueyan Jiang; Li Zou; Xuchen Wang

    The elemental (TOC, TN and C/N) and carbon isotope (Δ14C and δ13C) compositions of organic matter were measured in different grain size fractions of particles transported by the Yellow River and surface sediments in the Bohai Sea and the Yellow Sea. In the riverine particle and sediment samples, high OC contents were associated with small grain size fractions consisting mainly of clay minerals. The δ13C and Δ14C values of the bulk riverine particulate organic carbon (POC) collected from the Lijin and Xiaolangdi sites were relatively constant but varied significantly (−21.9‰ to −26.0‰ and −325‰ to −620‰, respectively) among the different size fractions. In comparison, large spatial variations in δ13C (−20.6‰ to −24.5‰) and Δ14C (−188‰ to −646‰) values, which increased seaward due to the difference in source carbon, were found for bulk TOC Δ14C preserved in the surface sediments, but no significant differences were observed among the values in the different size fractions in most sediments. The different carbon isotopic values of the riverine POC and sedimentary TOC reflect differences in the sources, degradation and cycling time scales of the OC. The Yellow River exports very old (5220 ± 295 yrs) POC that is much older than the TOC (2457 ± 676 yrs) preserved in the surface sediments in the Bohai and Yellow seas. Calculations using a dual-isotope three end-member model indicate that pre-aged soil OC and ancient fossil OC represent major proportions (57 ± 16% and 30 ± 8%, respectively) of the riverine POC and that terrestrial biomass OC represents a minor proportion (13 ± 11%). The drainage environment of the river plays important roles in controlling the sources and ages of the riverine POC. In contrast, the TOC in the grain size fractions of the surface sediments in the Bohai and Yellow seas is dominated by marine-derived modern OC (47 ± 13%), followed by pre-aged soil OC (29 ± 9%) and ancient fossil OC (25 ± 14%). The ages of the TOC are determined mainly by the source input, rapid sedimentation, sediment minerology and decomposition of OC during early diagenesis in these large river-influenced marginal seas.

    更新日期:2019-12-30
  • Near-field hydrothermal plume dynamics at Brothers Volcano (Kermadec Arc): A short-lived radium isotope study
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-18
    René Neuholz; Bernhard Schnetger; Charlotte Kleint; Andrea Koschinsky; Karsten Lettmann; Sylvia Sander; Andreas Türke; Maren Walter; Rebecca Zitoun; Hans-Jürgen Brumsack

    The naturally occurring radium (Ra) isotope quartet was measured for the first time at an intraoceanic arc hydrothermal setting along the Kermadec Island Arc (SW Pacific). 224Ra (3.7 d half life) and 223Ra (11.4 d half life) are assumed to be tracers solely originating from hydrothermal discharge and have been used to resolve near-field hydrothermal plume dynamics by applying both advection and eddy diffusion-based dispersion models. Assuming dominant advective processes, we identified different horizontal plume age structures at Brothers volcano, with a slow moving plume inside the caldera (0.060 cm/s) and a faster moving plume outside the caldera (0.323 cm/s), indicating a longer plume residence time within the caldera structure. The decreasing trend in Ra activity also allows the calculation of horizontal eddy diffusion coefficients inside and outside of the caldera, with 0.164 m2/s and 3.545 m2/s for 224Ra, respectively. These trends are in accordance with the radial horizontal eddy diffusion model. Our results reveal that net advection might not necessarily play a major role at Brothers, as radial horizontal eddy diffusion reproduces the observed Ra activity distribution to the same degree. Additional model simulations based on a combination of advection and diffusion, favor slow net advection to explain the Ra activity distribution. Within the rising non-buoyant plume, Ra isotopes suggested constant dilution along the first 170 m of the vertical water column and a vertical transport velocity on the order of 0.156 cm/s for altitudes up to 150 m above the vent and around 0.004 cm/s for altitudes above 150 m. When a common source of diffuse and focused venting is assumed and only small amounts of Ra are added to the ascending plume, due to recoil originating from high water to rock ratios, fluids from diffuse vents show a slower migration rate through the subsurface than fluids from focused vents. Our data imply that the fluids from diffuse vents needed approximately five days longer to percolate through the underlying subsurface to reach the water column plume, compared to the fluids from direct vents which emanate through chimney structures. This study demonstrates the potential of short-lived Ra isotopes as tracers of near-field plume dynamics, which will improve our understanding of hydrothermal vents as sources for various trace elements to the global ocean.

    更新日期:2019-12-29
  • Sulfur cycling in the Yellow River and the sulfate flux to the ocean
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-27
    Dong Zhang; Zhi-Qi Zhao; Yongbo Peng; Bailing Fan; Lili Zhang; Jianlin Li; Aichun Chen
    更新日期:2019-12-27
  • Aerobic release and biodegradation of dissolved organic matter from frozen peat: Effects of temperature and heterotrophic bacteria
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-27
    Dahédrey Payandi-Rolland; Liudmila S. Shirokova; Paty Nakhle; Marawit Tesfa; Ahmed Abdou; Carole Causserand; Bruno Lartiges; Jean-Luc Rols; Frédéric Guérin; Pascale Bénézeth; Oleg S. Pokrovsky

    Understanding the conditions of dissolved organic matter (DOM) release from thawing peat in the Arctic regions and identifying the pathways of processing DOM by soil and aquatic heterotrophic bacteria are critical in the context of rapid climate change. Until now, experimental approaches did not allow quantitative predictions of temperature and biota effects on carbon release from peat in permafrost-affected aquatic environments. In this study, we incubated frozen peat and its aqueous leachate at various temperatures (4, 25 or 45 °C), with and without culturable heterotrophic bacteria Iodobacter sp., extracted from thermokarst lakes, to quantify the release and the removal rate of organic carbon (OC) with time. The metabolic diversity of the native microbial community associated with the substrates involved in OC processing was also characterized. Transmission electron microscopy revealed that, after degradation, the associated bacteria are mostly located in the inner parts of plant cells, and that the degradation of organic matter around bacteria is more pronounced at 4 and 25 °C compared to 45 °C. The metabolic diversity of heterotrophic bacteria was equally high at 4 and 25 °C, but lower at 45 °C. Regardless of the microbial consortium (native community alone or with added culturable heterotrophs), both the OC release from peat and the OC removal from peat leachate by bacteria were similar at 4 and 25 °C. Very low apparent activation energies of DOM biodegradation between 4 and 25 °C (−4.23 ± 12.3 kJ mol−1) suggest that the short-period of surface water warming in summer would have an insignificant effect on DOM microbial processing. Such duration (1–3 weeks) is comparable with the water residence time in peat depressions and permafrost subsidences, where peat degradation and DOM microbial processing occur. This questions the current paradigm of a drastic effect of temperature rise on organic carbon release from frozen peatlands, and should be considered for modelling short-term climate impacts in these regions.

    更新日期:2019-12-27
  • Platinum enrichment and phase associations in marine ferromanganese crusts and nodules based on a multi-method approach
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-24
    Andrea Koschinsky; James R. Hein; Dennis Kraemer; Andrea L. Foster; Thomas Kuhn; Peter Halbach

    Marine ferromanganese crusts and nodules host high concentrations of many economically interesting metals such as platinum (Pt), with Pt concentrations in the range of 44 to 3207 ppb in 182 analyzed samples from various locations in the global ocean. Different mechanisms have been proposed to explain this strong enrichment in the MnFe oxide phases compared to seawater, including reduction of seawater Pt(II) to Pt(0) or oxidation to Pt(IV) and surface adsorption on the Fe-oxyhydroxide or Mn-oxide phases. To shed more light on the process of Pt enrichment in nodules and crusts, we applied a multi-method approach including bulk analyses, statistics (correlations and Q-mode factor analyses), sequential leaching, sorption experiments, and XANES and EXAFS measurements. Our analyses lend new support to the heterogeneous oxidation/sorption mechanism advanced by Maeno et al. (2016) to explain the initial Pt(II) enrichment on FeMn crusts, with Pt oxidation only taking place on the Mn-oxide phase but not on the Fe-oxyhydroxide phase of crusts. Nodules show lower concentrations and a lesser association of Pt with the Mn phase as the latter is partly of diagenetic origin while the origin of Pt is hydrogenetic. Our results also indicate that sorption of Pt2+(aq) to FeO(OH) is possible if Pt2+(aq) exists under conditions typical of nodule and crust growth in the oceans. Platinum concentrations are consistently highest in phosphatized FeMn crusts where they appear to be related to the occurrence of a 10 Å manganate, so they are likely to be the best exploration target for Pt globally in marine FeMn crusts. However, further work is needed to understand the geochemical reactions that dominate mass transfer of Pt under conditions under which crusts are phosphatized and to identify the exact mineral type which hosts the Pt enrichment. The complex Pt geochemistry in marine FeMn crusts and nodules due to the different phase associations and enrichment processes on the different carrier phases make a straightforward interpretation of Pt data in specific samples difficult. Our findings suggest that only spatially resolved, species-specific methods available via synchrotron-based X-ray and modern electron-probe micro-analyzers (in development) could provide the combination of spatial resolution and species-specificity to determine which of several proposed mechanisms are actually responsible for Pt enrichment in FeMn crusts.

    更新日期:2019-12-25
  • Development of a protocol to obtain the composition of terrigenous detritus in marine sediments -a pilot study from International Ocean Discovery Program Expedition 361
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-20
    Margit H. Simon; Daniel P. Babin; Steven L. Goldstein; Merry Yue Cai; Tanzhuo Liu; Xibin Han; Anne A. Haws; Matthew Johns; Caroline Lear; Sidney R. Hemming

    The geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments. International Ocean Discovery Program Expedition 361 drilled sites along the eastern margin of southern Africa that potentially provide archives of rainfall on the continent as well as dispersal in the Agulhas Current. We used standard methods to remove carbonate and ferromanganese oxides and Stokes settling to isolate the clay fractions. In comparison to most previous studies that aimed to extract the detrital signal from marine sediments, we additionally applied a cation exchange wash using CsCl as a final step in the sample preparation. The motivation behind the extra step, not frequently applied, is to remove ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure. Either would alter the composition of the detrital fraction if no cation exchange was applied. Moreover, using CsCl will provide an additional measure of the cation exchange capacity (CEC) of the samples. However, no study so far has evaluated the potential and the limitations of such a targeted protocol for marine sediments. Here, we explore the effects of removing and replacing adsorbed cations on the clay surfaces with Cs+, conducting measurements of the chemical compositions, and radiogenic isotopes on a set of eight clay sample pairs. Both sets of samples underwent the same full leaching procedure except that one batch was treated with a final CsCl wash step. In this study, organic matter was not leached because sediments at IODP Site U1478 have relatively low organic content. However, in general, we recommend including that step in the leaching procedure. As expected, significant portions of elements with high concentrations in seawater were replaced by Cs+ (2SD 2.8%.) from the wash, including 75% of the sodium and approximately 25% of the calcium, 10% of the magnesium, and 8% of the potassium. Trace metals such as Sr and Nd, whose isotopes are used for provenance studies, are also found to be in lower concentrations in the samples after the exchange wash. The exchange wash affected the radiogenic isotope compositions of the samples. Neodymium isotope ratios are slightly less radiogenic in all the washed samples. Strontium and Pb isotopes showed significant deviations to either more or less radiogenic values in different samples. The radiogenic isotopes from the CsCl-treated fractions gave more consistent correlations with each other, and we suggest this treatment offers a superior measure of provenance. Although we observed changes in the isotope ratios, the general trend in the data and hence the overall provenance interpretations remained the same. However, the chemical compositions are significantly different. We conclude that a leaching protocol including a cation exchange wash (e.g. CsCl) is useful for revealing the terrestrial fingerprint. CEC could, with further calibration efforts, be useful as a terrestrial chemical weathering proxy.

    更新日期:2019-12-20
  • In situ Raman spectroscopic quantification of aqueous sulfate: Experimental calibration and application to natural fluid inclusions
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-19
    Ye Qiu; Yuanxian Yang; Xiaolin Wang; Ye Wan; Wenxuan Hu; Jianjun Lu; Guoliang Tao; Zhen Li; Fanwei Meng

    Sulfate is an important component of geological fluids, which is often encapsulated in fluid inclusions (FIs). Analyses of the composition of such FIs can provide important information on the origin and properties of the fluids inside. However, conventional microthermometric analyses are often hampered due to the complex composition of such fluids. Alternatively, in situ Raman spectroscopy is a powerful method to determine the aqueous sulfate concentration of FIs. Unfortunately, the effect of a common component, chloride, on the analytical results has not been well addressed yet. In addition, disagreements also exist in the current spectrum-processing method, which also prevents the application of this method in various geological cases. In this study, a series of fused silica capillary capsules (FSCCs) containing aqueous Na2SO4 (0.01–1.5 mol/kg) and Na2SO4-NaCl (mNa2SO4 = 0.05 and 0.5 mol/kg, mNaCl = 0.1–4.0 mol/kg) solutions were prepared as standards. In situ Raman spectra for the stretching vibration of SO42− (v1-SO42−), the bending vibration (v2-H2O) and the stretching vibration (vs-H2O) of water were collected at room temperature. Results show that, for sulfate solutions, both v2-H2O and vs-H2O bands can be used as internal standards for the determination of aqueous sulfate concentration (mSO42−). The peak area (A) ratios between v1-SO42− and v2-H2O (or vs-H2O) bands increased nearly linearly with mSO42− in Na2SO4 solutions. Consequently, the relationships between mSO42− and the spectral parameters (i.e., A(v1-SO42−)/A(v2-H2O) and A(v1-SO42−)/A(vs-H2O)) were established. However, the calculated mSO42− will be underestimated when applying the above calibration curve to NaCl-Na2SO4 solutions, using A(v2-H2O) as an internal reference. In this case, the influence of chloride on the calculated mSO42− will be largely eliminated when using A(vs-H2O) as an internal reference, or fitting the vs-H2O band with two Gaussian components at ~3230 cm−1 and ~3450 cm−1, and then using the total peak area as internal reference. It should be noted that our calibration results allow comparisons across different laboratories, because the spectral intensity was corrected to reduce the response function of the spectrometer, and the temperature and wavenumber dependence. Then, the A(v1-SO42−)/A(vs-H2O)-mSO42− relationship was applied to natural FIs in fluorite from the Dalucao hydrothermal REE deposit (southwestern China), and in halite from the Paleogene evaporite in the Jianghan basin (South China). The mSO42− in pre-REE stage fluorite-hosted FIs can reach 0.37 mol/kg, indicating that sulfate are responsible for the hydrothermal transport of REE. The mSO42− in the halite-hosted FIs were found to be in close association with the mineral assemblage, serving as an indicator for the degree of evaporation or sedimentary facies.

    更新日期:2019-12-20
  • Elucidating modern geochemical cycles at local, regional, and global scales using calcium isotopes
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-19
    Elizabeth M. Griffith; Anne-Désirée Schmitt; M. Grace Andrews; Matthew S. Fantle

    In Earth's surface environment, calcium (Ca) is an important mobile metal that is actively and passively transported in solution and within organic and mineral phases, being cycled and recycled during various biogeochemical processes. With the development of modern mass spectrometric techniques small variations in the stable and radiogenic isotopic compositions of Ca can be measured, revealing insight in these complex biogeochemical cycles and tracing and quantifying components across a range of spatial and temporal scales similar to other more routine isotope systems. More than three decades of work reveal systematic variations in the partitioning of Ca isotopes due to both abiotic and biological processes. An overview of processes that fractionate Ca isotopes at local, regional, and global scales is outlined here. We present detailed examples of instances in which Ca isotopes have provided unique insight into the functioning of Earth surface processes and the cycling of Ca at multiple scales. Future studies should target questions for which Ca isotopic analysis provide unique insight and, when combined with other isotope and trace element multi-proxy studies, better constrain the system of interest. At the same time, we challenge the scientific community to explore new frontiers including polar regions and other extreme environments.

    更新日期:2019-12-19
  • Efficient sequestration of terrigenous organic carbon in the New Britain Trench
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-19
    Wenjie Xiao; Yunping Xu; Negar Haghipour; Daniel B. Montluçon; Binbin Pan; Zehua Jia; Huangmin Ge; Peng Yao; Timothy I. Eglinton

    The fate of terrigenous organic carbon (OCterr) in the ocean remains an enigma for four decades. Hadal trenches, the deepest ocean realm (6–11 km deep), were recently proposed to be OC depocenters, but whether and how much OCterr was sequestrated there remain elusive. Here we conducted comprehensive analyses for four sediment cores from the New Britain Trench (NBT) close to Papua New Guinea to assess source, translocation and burial of OC. The bulk and molecular radiocarbon data suggest that the NBT landward slope and axis sediments mainly receive young and biogenic rather than petrogenic OC. The three-endmember mixing model based on Δ14C, δ13C and OC contents reveals that sediments of the NBT axis (8225 m) comprise relatively high OC contents (0.66 ± 0.08%), of which biogenic OCterr accounts for 62 ± 10%. The high proportion of biogenic OCterr was attributed to the selective translocation of OCterr-enriched coarse particles and rapid delivery of sediments supported by unique V-shape feature of the trench. In contrast, the sediment OC at the oceanward slope is primarily of a marine origin, suggesting that OCterr was efficiently trapped in the trench bottom. It is estimated that the burial rate is 2.75 ± 0.32 g C m−2 yr−1 for OC and 1.69 ± 0.41 g C m−2 yr−1 for OCterr in the NBT. Given a fact that many trenches are close to the landmasses, we propose that the hadal trenches may contribute significantly to the burial of OCterr in the ocean.

    更新日期:2019-12-19
  • Halogens (Cl, Br, and I) geochemistry in Middle Triassic carbonates: Implications for salinity and diagenetic alteration of I/(Ca + Mg) ratios
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-17
    Lichao Wang; Wenxuan Hu; Xiaolin Wang; Jian Cao; Suping Yao

    As an important family of elements, halogens (Cl, Br, and I) have been widely used to study geological processes, particularly in high-temperature environments, showing great prospects either in fundamental or in applied studies. However, little work has been conducted for sedimentary and low-temperature diagenetic environments re-construction although these elements have the potential in theory. Here we perform a pilot study on this issue using a case study of Middle Triassic marine dolomitization in South China. Our results show that the average Cl concentrations in limestone, dolomitic limestone, calcitic dolomite and dolomite are 88.3 ppm, 114.7 ppm, 121.3 ppm and 142.9 ppm respectively, and the corresponding Br concentrations are 1.15 ppm, 1.42 ppm, 1.59 ppm and 2.13 ppm, respectively. Both the concentrations of Cl and Br increased with the increasing dolomitization degree. The increasing tendency of Cl and Br is similar to the seawater evaporation trajectory before halite precipitation, indicating that dolomitization occurs in fluids with elevated salinity. Meanwhile, the difference of Cl/Br ratios between our carbonate samples and seawater may be ascribed to the input of terrestrial water, which has variable Cl/Br ratios. As for the concentration of I, the mean values sharply decreased from 0.36 ppm in limestone to 0.13 ppm in dolomitic limestone samples, and it further lowered to 0.09 ppm and 0.08 ppm in calcitic dolomite and dolomite samples, respectively. The calculated I/(Ca + Mg) ratios in limestones vary between 0.05 and 0.58 μmol/mol with an average value of 0.29 μmol/mol, indicating that limestone formed in open ocean oxygen-minimum zone (OMZs) or anoxic basin environment. The average I/(Ca + Mg) ratio decreases from 0.29 μmol/mol in limestone to 0.06 μmol/mol in dolomite, serving as an important case that dolomitization decreases I/(Ca + Mg) ratios of carbonates rocks. Halogens in carbonates therefore have promising potential for investigating the sedimentary and diagenetic environments of carbonate rocks.

    更新日期:2019-12-18
  • Helium concentrations and isotope compositions in 10 km deep groundwaters
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-12
    M.A. Gannibal; V.V. Kolobov; P.H. Barry; R.L. Tyne; S.V. Tarakanov; I.N. Tolstikhin

    Helium isotopes are useful for tracing terrestrial water movement and calculating residence times, which have important implications for radioactive waste disposal and carbon capture and storage projects. Helium concentrations are generally measured directly in water samples; however, this is not always possible. In this scenario, the He concentration equilibrium between pore waters and He accessible volumes (e.g., fluid inclusions, hereafter HAV) in minerals (e.g., quartz) can be utilized to determine He concentrations. In this study, the He partial pressure was measured in HAVs of quartz grains collected from cores of the Kola Super Deep Borehole (KSDB) using isothermal He extraction/saturation experiments. A large fraction of He was released from these samples during the first interval of isothermal heating, indicating a destruction of the unstable HAVs, likely due to decrepitation of fluid inclusions. We present a new approach to calculate He partial pressures using only the He fraction released from the stable HAVs. This approach gives a He partial pressure pHe of ≅ 1.5 atm at depths of ~ 10 km. When combined with solubility data, this value suggests a high He concentration in the deep pore waters nearby the KSDB, ≅ 1.3 × 10−6 mol (cc H2O)−1, indicating a long residence time of He atoms in a deep water – mineral system, close to the age of the regional metamorphism (≅1700 Ma). The 3He/4He in the deep KSDB quartz samples (and the pore waters) are ~ 3 × 10−8, typical of radiogenic He generated in 10 km deep Archean rocks of the KSDB. Importantly, external fluxes are not needed to explain He isotope abundances in rocks, minerals and pore waters at these depths.

    更新日期:2019-12-13
  • Standardizing the reporting of Δʹ17O data from high precision oxygen triple-isotope ratio measurements of silicate rocks and minerals
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-10
    Martin F. Miller, Andreas Pack, Ilya N. Bindeman, Richard C. Greenwood

    Accurate and precise Δʹ17O data are important for quantifying and understanding processes responsible for variations of oxygen triple-isotope ratios in nature. Without universally agreed numerical values of λRL and γRL in the definition Δʹ17O = ln(1 + δ17O) – λRLln(1 + δ18O) – γRL however, as is currently the situation for measurements on rocks and minerals, reported Δʹ17O data may not be directly comparable. Furthermore, the accuracy of Δʹ17O measurements is linked to calibration (at the ppm level) of individual laboratory’s ‘working standard’ O2 relative to the VSMOW water reference or to the VSMOW-SLAP scale, whereas few laboratories have the facility to make high precision δ17O and δ18O measurements on silicates and waters. Both points may be addressed by defining a reference line from measurements of a pair of silicates which differ substantially in oxygen isotopic composition (comparable to the difference between VSMOW and SLAP), rather than by assigning λRL and γRL values arbitrarily. The resulting reference line has no physical significance. However, because Δʹ17O values reported relative to it are independent of the calibration of the working standard O2, this greatly facilitates Δʹ17O reproducibility and comparability. We propose silicate standards for this purpose and report comparative measurements – from two institutions – of the respective δ17O and δ18O values relative to VSMOW, together with Δʹ17O data relative to various assigned reference lines. We also report Δʹ17O measurements of UWG-2 garnet, San Carlos olivine and NBS 28 quartz relative to our reference line. The proposed standards are: (1) KRS (Khitostrov Rock Standard, δ18O = –25.20 ‰), which consists of sintered, fine-grained constituents (primarily garnet) of a very low-δ18O amphibolite-grade Palaeoproterozoic rock from Khitostrov, Karelia, Russia; (2) SKFS (Stevns Klint Flint Standard, δ18O = 33.93 ‰), consisting of thermally dehydrated grains of a high-δ18O nodule of flint sampled from just below the exposed Cretaceous–Palaeogene boundary at Stevns Klint, southeast Zealand Island, Denmark. Both silicates fluorinate readily, using standard laser-assisted procedures, and the associated low blanks allow for overnight pre-treatment. Small quantities of these materials may be supplied, on request, for standardizing Δʹ17O measurements of silicate rocks and minerals according to the proposed procedure.

    更新日期:2019-12-11
  • Investigating the molybdenum and uranium redox proxies in a modern shallow anoxic carbonate rich marine sediment setting of the Malo Jezero (Mljet Lakes, Adriatic Sea)
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-09
    Elvira Bura-Nakić, Ivan Sondi, Nevenka Mikac, Morten B. Andersen

    The molybdenum (Mo) and uranium (U) isotope compositions recorded in carbonate rich sediments are emerging as promising paleo-redox proxies. However, the effects of early diagenetic effects within the sediments on these isotope systems are not well constrained. We examined the Mo and U isotopic systematics in anoxic carbonate rich sediments in a semi enclosed karstic marine lake (Malo Jezero) of the Island of Mljet, Adriatic Sea. Measurements of water column redox behavior in the lake since the 1950s, have shown a transition from anoxic-sulfidic conditions in the deeper water column to more oxic conditions and anoxia refined to the sediment and pore-waters. A 50 cm long sediment core from the deepest part of the lake, show a transition from moderate to high authigenic Mo and U accumulation with depth, consistent with the changing lake redox environment in the past. In the deep euxinic part of the core, the authigenic Mo and U are isotopically lighter and heavier, respectively, than seawater, following similar systematics as observed in other modern euxinic basins, with high, but non-quantitative, Mo and U uptake into the sediments. Based on Bahamas bank carbonate sediments, it has been suggested that the 238U/235U ratio is ~+0.25‰ higher compared to seawater from the effects of early carbonate sediment diagenesis and this carbonate vs. seawater off-set is applicable to carbonate rich sediments across the geological past. The shallower part of lake sediment core was deposited under similar redox conditions as the Bahamas sediments, and these sediments show an average 238U/235U ratio + 0.31 ± 0.01‰ (2SE) higher than seawater. Although the average 238U/235U ratios for these two carbonate rich settings are similar, caution is necessary when inferring seawater 238U/235U compositions from such sediments, as they contain U from different sources (e.g. diagenetic uptake and carbonate-bound). The Mo isotope compositions within the same Malo Jezero sediments are variable but approaches the seawater composition at low pore-water H2S concentrations. This show the potential of using the Mo isotope composition from carbonate rich sediments to infer the seawater composition, however, further work is required to establish the link between the Mo isotope composition and the chemistry of the pore water environment.

    更新日期:2019-12-09
  • Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-07
    Le Xiong, Xinfu Zhao, Junhao Wei, Xiaoye Jin, Lebing Fu, Zuwei Lin

    The source(s) of lode gold deposits formed in Precambrian cratons related to accretion/collision and cratonic reactivation, formerly attributed to either supercrustal volcano-sedimentary sequences or deep crustal/sub-crustal origin, remains controversial largely because gold deposits are spatially related to metamorphosed rocks, but geochemical data somewhat indicate a poorly understood deep sources. Reconciling such conflicts is important to better understand the main factor controlling the formation of ore deposits and their genetic link with specific tectonic settings. Giant Late Mesozoic lode gold provinces in North China Craton (NCC) were formed ca. 1.7 Ga later than cratonization metamorphism, and contemporaneous with intensively felsic to mafic magmatism related to cratonic reactivation. In this study, we conduct a comprehensive Pb isotope study on major gold deposits from the eastern Yanshan belt, northern margin of the NCC. In order to constrain the source(s) of gold, we attempt to map regional Pb isotope variations of lower continental crust (LCC), and develop a two stage quantitative model (punctuated by three prominent geological events at 2.80 Ga, 1.85 Ga and 0.16 Ga) to reproduce time-integrated Pb isotopic signatures of deep-seated lithospheric reservoirs to make a comparison with Pb isotopic signatures of the gold mineralization. Gold-bearing pyrites from different types of host rocks have relatively uniform Pb isotopic ratios, which are significantly different to high-grade metamorphosed host rocks, but similar to those of spatially associated Late Mesozoic granitic rocks. Our data show that Pb isotopic signatures of gold deposits vary consistently with presumed regional Pb isotopes of the LCC during the Late Mesozoic. Pb isotopic heterogeneity of the LCC was likely caused by underplating of mafic magmas derived from mantle sources. During underplating highly chalcophile elements (e.g. Au, Ag and Cu) were concentrated at the base of the LCC due to sulfide saturation from mafic magmas. Integrating petrological, geochemical, geochronological, and considering chalcophile element solubility, we propose a new genetic model to describe the formation of Late Mesozoic gold deposits in the NCC: (1) early formation of sulfide-bearing cumulates with high Au/Cu ratios during magma differentiation at the base of the LCC; (2) subsequent fluid-fluxed remelting of these cumulates at the onset of lithospheric extension and release and ascent of ore-forming fluids to the site of precipitation. Considering the metallogenic characteristics of other Au ore-forming systems, we suggest that metal fertilization in deep-seated reservoirs and subsequent tectonic decompression are important factors controlling the development of Au-rich ore deposits worldwide. This study demonstrates how Pb isotope can be employed to trace source(s) of gold deposits.

    更新日期:2019-12-07
  • Protocols for in situ measurement of oxygen isotopes in goethite by ion microprobe
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-06
    H.S. Monteiro, P.M. Vasconcelos, K.A. Farley, J.N. Ávila, H.B.D. Miller, P. Holden, T.R. Ireland

    We present protocols for high-spatial resolution measurement of oxygen isotope ratios of goethite (α-FeOOH) with the Sensitive High Mass Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) and propose a natural sample as a potential goethite reference material (RM) for ion microprobe analysis. We assess the effects of goethite chemical composition, crystallographic orientation, and texture on the accuracy and repeatability of SHRIMP-SI δ18O (δ18OSIMS) results. Synthetic goethites evaluated as potential δ18OSIMS RM are powdery, porous, and finely crystalline; they do not yield repeatable results. A dense colloform goethite from the Capão topaz mine, Minas Gerais, Brazil, fulfills major prerequisites: it is stoichiometrically relatively pure, yields repeatable oxygen isotope results, and occurs in abundance to produce a RM for long-term use. We use an average laser fluorination δ18OLF-VSMOW value of −17.3 ± 0.3‰ (1SD) obtained for five aliquots of this RM to normalize all δ18OSIMS measurements. Multiple δ18OSIMS analyses of a large fragment of the Capão L4 (CL4) RM analyzed in three different runs yield an overall repeatability of −17.3 ± 0.5‰ (2SD, n = 294) for all three runs combined. Natural variability and crystal orientation effects are the main reasons for the excess spread of the δ18OSIMS results compared to the spot internal precision (ca. 0.2‰). All δ18OSIMS analyses (n = 1027) in various aliquots of CL4, randomly oriented and analyzed in 26 sessions during eight distinct runs, yield an overall repeatability of ±0.7‰ (2SD), confirming that CL4 is a suitable SIMS RM. After ascertaining its suitability as a RM, we used CL4 to standardize analyses of other natural goethite samples with the SHRIMP-SI and compared δ18OSIMS and laser fluorination results to test the relationship between natural properties (e.g., porosity, minor elements substituting for Fe), preparation procedures (e.g., polish and relief), instrument conditions, and the overall precision and accuracy of the SIMS analyses. Samples containing minor elements substituting for Fe (e.g., Al, Mn, Cu, Zn, etc.) or as contaminants (e.g., Si, P) require significant matrix corrections. Because we could not find homogenous natural goethite samples showing a large range in metal concentrations, we extrapolate our calibration curves beyond the composition of our calibration goethite samples. δ18OSIMS results corrected for instrument mass fractionation (using CL4) and compositionally dependent matrix effects (using several calibration goethites of known elemental composition) are less precise but statistically indistinguishable from their laser fluorination results. However, porous samples are unsuitable for SHRIMP-SI δ18O analysis. Dense colloform samples yield repeatable results for individual growth bands, showing that the high spatial resolution, moderate precision, and speed of analysis of the SHRIMP-SI can resolve variations in oxygen isotope composition acquired during sample growth. (UTh)/He geochronology of equivalent aliquots from the same goethite samples reveal that the combination of the two methods permits the extraction of temporal variation in the isotopic compositions of meteoric solutions in the geological past.

    更新日期:2019-12-06
  • Effect of microbial growth rate on temperature and metabolic water recorded in 18O/16O ratios of PO4 in DNA
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-05
    Fei Wang, Chan Yu, Hui Li, Sae Jung Chang, Ruth E. Blake
    更新日期:2019-12-05
  • Oxygenation of acid sulfate soils stimulates CO2 emission: Roles of acidic dissolution and hydroxyl radical oxidation
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-05
    Peng Zhang, Songhu Yuan, Rong Chen, Xiaochuang Bu, Man Tong, Qiaoyi Huang

    Emission of CO2 from acid sulfate soil regions has received great attention in recent years. Production of CO2 due to microbial respiration has been documented by previous studies, but the abiotic mechanisms are poorly understood. In this study, CO2 production was measured from oxygenation of 60 acid sulfate soil samples that were collected from a typical acid sulfate soil region. The soil samples were characterized by low pH (3.3–5.8) and high sulfate content (0.51–5.03 mg/g). The cumulative concentrations of CO2 were 0.76–54.54 μmol per g dry soil upon oxygenation for 24 h. When 0.1% HgCl2 was added into soil suspension to kill microbes, the yield of CO2 decreased by 8.7%–41.1%, which suggests that both microbial respiration and abiotic pathways contributed to CO2 production and abiotic pathway played a major role. Both acidic dissolution and hydroxyl radical (OH) oxidation were identified to be the abiotic mechanisms for CO2 production. For acidic dissolution, CO2 was produced from the decomposition of carbonate by H+ generated. Production of CO2 from acidic dissolution was dependent on the content of carbonate and the generation of H+. A combination of Fe speciation sequential extraction and kinetic model shows that H+ was mainly generated from the oxidation of both Fe2+ and pyrite by O2. For OH oxidation, CO2 was produced from the oxidation of organic carbon by OH produced and its relative importance for CO2 production was estimated to be 14.6%–27.6%. OH was mainly produced from exchangeable Fe(II) oxidation in sulfuric horizon but from pyrite oxidation in potential acid sulfate soils. This study presents the overlooked abiotic mechanisms, acidic dissolution and OH oxidation, for CO2 emission in acid sulfate soils when perturbed by O2. Our findings implicate that contribution of microbial respiration for CO2 production is not as large as previously thought to be in acid sulfate soils, and instead abiotic mechanisms contribute largely, particularly for oxygenation of potential acid sulfate soils.

    更新日期:2019-12-05
  • On the controls of mineral assemblages and textures in alkaline springs, Samail Ophiolite, Oman
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-05
    Manolis Giampouras, Carlos J. Garrido, Wolfgang Bach, Catharina Los, Dario Fussmann, Patrick Monien, Juan Manuel Garcia-Ruiz
    更新日期:2019-12-05
  • Multi-site surface complexation modelling of Se(IV) sorption on biotite
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-04
    Xiaodong Li, Eini Puhakka, Longcheng Liu, Wenzhong Zhang, Jussi Ikonen, Antero Lindberg, Marja Siitari-Kauppi

    A surface complexation model of Se(IV) sorption on biotite with one type of strong sorption sites and two types of weak sorption sites were developed based on experimental data obtained from titration, sorption edge and sorption isotherm experiments. Titration data was collected using a batch-wise manner together with backtitration to calibrate the effect of mineral dissolution in 0.01 M KClO4 background electrolytes from pH 3 to 11 in an inert atmosphere glovebox. Further calibrations of the titration curve include proton exchange and cation exchange in which the calculations of cation occupancies on biotite surfaces were taken into account. The sorption edge measurements were determined by measuring the sorption of 10−9 M total Se with a radioactive Se-75 tracer on converted biotite in 0.01 M KClO4 solution from pH 3 to 11. Se sorption was observed to be strongly dependent on pH. Surface complexation modelling was performed by deriving a set of optimized parameters that can fit titration, sorption edge and sorption isotherm (at pH ~7.7) experimental data. A CASTEP code implemented into Materials Studio was used to calculate the site densities and site types on the biotite surfaces. Weak sorption sites with site densities of 3.2 sites/nm2 and 1.4 sites/nm2 were derived from the codes and used in the sorption model. A computer code that coupled PHREEQC with Python was developed for the fitting and optimizing processes. The model was validated by sorption data at pH ~9.5. The results show that the model can provide quantitative predicts of Se(IV) sorption in groundwater conditions of a deep geological repository and help improve the performance assessments by giving more convincing estimates of the release of radionuclides towards aquifers and biosphere.

    更新日期:2019-12-04
  • Proxy evidence from the Gartnerkofel-1 core (Carnic Alps, Austria) for hypoxic conditions in the western Tethys during the end-Permian mass-extinction event
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-03
    Michael R. Rampino, Eva Baransky, Sedelia Rodriguez

    The most severe mass extinction in the Phanerozoic in the latest Permian Period (251.9 Ma), has been attributed to environmental effects related to the massive coeval Siberian flood-basalt eruptions. A potential causal link between the eruptions and the marine extinctions arises from lethal global warming from greenhouse-gas emissions and the resulting development of widespread hypoxic conditions in the warm oceans. The 331-m Gartnerkofel-1 core (Carnic Alps, Austria) penetrated the Permian-Triassic boundary interval in a western Tethys shallow carbonate-ramp setting. We calculated U/Th ratios and element-enrichment factors (for U, Cr, V, Co, and Ni) using the previously published raw elemental data from the core. These redox-sensitive proxy calculations provide evidence for an ~60-ky long episode of anoxic to euxinic ocean conditions in the shallow western Tethys at the time of the abrupt end-Permian mass extinction (EPME). Subsequent fluctuating dysoxic conditions were coincident with a negative excursion of δ13Ccarb that is estimated to have lasted ~450 ky in the earliest Triassic at the GK-1 site. These findings in a high-deposition-rate section support previous reports of widespread anoxia in the western Tethys, and in the oceans in general, coincident with the EPME and its aftermath.

    更新日期:2019-12-03
  • Life on the edge: Microbial biomineralization in an arsenic- and lead-rich deep-sea hydrothermal vent
    Chem. Geol. (IF 3.618) Pub Date : 2019-12-02
    Si-Yu Hu, Stephen J. Barnes, Anais Pagès, Joanna Parr, Ray Binns, Michael Verrall, Zakaria Quadir, William D.A. Rickard, Weihua Liu, Denis Fougerouse, Kliti Grice, Louise Schoneveld, Chris Ryan, David Paterson

    Unravelling complex microbial activity in modern hydrothermal vents can provide crucial insights into the evolution of ancient life on Earth. It is well established that microorganisms in hydrothermal vents have a significant impact on the cycling of metals and mineral formation. However, the detailed roles played by microorganisms in driving sulfide deposition and cycling of toxic metals, like arsenic (As) and lead (Pb), in high-temperature deep-sea hydrothermal vents remain unknown. The understanding of these mechanisms in extreme environments is of particular importance as As has been postulated as a driver of microbial activities on the early Earth. Here, we present the first geologic evidence of PbAs rich microbial filamentous clusters observed in a modern high-temperature black smoker from the Manus back-arc basin, Papua New Guinea. The clusters occur as net-like structures on the surface of barite and sulfides and are composed of multiple filaments and fine-grained PbAs sulfosalt. Each of the filaments includes an As-Pb-rich sulfosalt core and organic-rich shell structure with elevated carbon, nitrogen and phosphorus. Further synchrotron X-ray absorption near edge structure analysis shows that the clusters contain a mixture of As (II) and As (III). Additionally, those filaments show a close association with realgar (As4S4), by penetrating and dissolving this As-sulfide mineral. We interpret the filaments to be a result of As-related microbial activity in As- and Pb-enriched hydrothermal environments. The findings show possible processes through which extremophiles live in Pb and As-rich environments during chimney growth. In addition, as hydrothermal vents are regarded as modern analogs of ancient volcanogenic massive sulfide deposits, the observed biominerals present the potential to be used as proxies to trace the signatures of early life in ancient geological systems.

    更新日期:2019-12-02
  • An internally consistent triple oxygen isotope calibration of standards for silicates, carbonates and air relative to VSMOW2 and SLAP2
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-30
    Jordan A.G. Wostbrock, Erick Cano, Zachary D. Sharp

    High precision triple oxygen isotope measurements are becoming a more common analysis in laboratories. There is a lack of calibrated standards to use for triple oxygen isotope measurements and this has led to data being presented on different scales rather than to the traditional VSMOW2-SLAP2 scale. Here we present triple oxygen isotope values of standard carbonates, CO2 liberated from carbonates, silicates and air calibrated to the VSMOW2-SLAP2 scale. We analyzed VSMOW2 and SLAP2 to calibrate our reference gas. Our measured δ18O value of SLAP2 is −55.55‰, indistinguishable from the accepted value of −55.5‰. Our Δ′17O value of SLAP2 (λ = 0.528) is not zero, but rather −0.015‰, corresponding to a δ17O value of −29.741‰. The Δ′17O values of carbonate standards NBS19, IAEA603 and NBS18 are −0.102, −0.100 and − 0.048‰, respectively (±0.010). For CO2 of calcite liberated by phosphoric acid digestion at 25 °C, the θACID value at 25 °C is 0.5230 ± 0.0003. These results can be used to correct triple oxygen isotope measurements of CO2 released by phosphoric acid digestion in other laboratories. We present triple oxygen isotope values for UW Garnet-2, NBS-28, San Carlos Olivine (NM-SCO), and our in-house quartz standard (NM-Q). Aliquots of NM-Q and NM-SCO are available from the Center for Stable Isotopes (CSI), New Mexico for interlaboratory comparison. Our δ17O and δ18O values for air are 12.178‰ (±0.066) and 24.046‰ (±0.117), respectively, with a corresponding Δ′17O value of −0.441 ± 0.012‰. With the availability of common standards, all laboratories making δ17O-δ18O measurements can calibrate their reference gas relative to the VSMOW2-SLAP2 scale. Laboratories making triple oxygen isotope measurements on CO2 released from carbonates using phosphoric acid digestion can correct to the bulk carbonate value.

    更新日期:2019-11-30
  • The formation of graphite-rich eclogite vein in S.W. Tianshan (China) and its implication for deep carbon cycling in subduction zone
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-27
    Jianjiang Zhu, Lifei Zhang, Renbiao Tao, Yingwei Fei

    The balance of carbon flux in subduction zones is critical to the deep carbon cycle. Carbonate-bearing lithologies are the major carbon carriers transported from Earth's surface into its interior at subduction zones. Recently, a number of studies have showed that carbon can be released from the subducting slab through metamorphic decarbonation and dissolution into C-H-O fluids. However, the evolution of the released C-H-O fluids during subduction-zone metamorphism is ambiguous and poorly explored. In this study, we found graphite-rich eclogite veins (VE) in the carbonated eclogites from the Southwestern (S.W.) Tianshan subduction zone. The observed graphite with high crystallinity and graphite-bearing fluid inclusions indicate the fluid-deposited origin. Phase equilibrium modelling for the host carbonated-eclogite (HE) in a closed system indicates that it has experienced a retrograde P-T path involving decompression with heating from 26.5 kbar at 487 °C to 20.6 kbar at 565 °C. The calculation showed that about 0.92–2.03 wt% of CO2 (0.25–0.55 g C per 100 g rock) could be released from the carbonated eclogite during its exhumation process, which is enough to provide the carbon source for graphite precipitation in the VE. Combined with petrological and isotopic results, we suggest that the graphites in the VE were precipitated from carbon-bearing fluids derived from the carbonated eclogites during exhumation metamorphism. The overall redox reaction is: FeO (in silicate, Grt, Omp or Gln) + FeS + (H2O + CO2) (released from Law and Dol) → Fe2O3 (in Ep, Andradite or Hematite) + C (graphite) + SO42− + HCO3– + CO32−. Mass balance calculation indicates that carbonates could also be re-precipitated in the VE during fluid-rock interaction, in addition to the graphite precipitation. The finding of fluid-deposited graphite in the carbonated eclogites provides new insights into the fate of carbonic fluids formed in the subducted oceanic crust. We suggest that carbonic fluids formed in the carbonated eclogites by decarbonation or carbonate dissolution may also precipitate abiotic graphite or carbonates under favorable conditions during their migration in addition to the commonly recognized transportation to the mantle wedge.

    更新日期:2019-11-27
  • SIMS oxygen isotopes indicate Phanerozoic fluids permeated a Precambrian gold deposit
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-26
    Erik L. Haroldson, Philip E. Brown, Akizumi Ishida, John W. Valley

    The Reef Deposit is an anomalous AuCu occurrence in the Paleoproterozoic terranes of northern Wisconsin, better known as host to significant CuZn volcanogenic massive sulfide (VMS) deposits. Previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a VMS deposit during the Penokean orogeny (~1.9–1.8 Ga), with the most recent mineralization/remobilization activity associated with late Paleozoic Mississippi Valley-type (MVT) fluid flow (Haroldson et al., 2018a, 2018b). Here we use the oxygen isotope history of the Reef Deposit to verify and further examine the deposit's protracted development. Laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in δ18O from 6.8 to 10.0‰ (VSMOW), and a trend is observed of increasing δ18O values of parallel vein zones along a 400-m traverse from northwest to southeast, likely from a temperature gradient during initial formation. Temperature estimates for a VMS deposit setting (230 to 400 °C) are consistent with a hydrothermally shifted formation fluid, using seawater δ18O estimates during the Penokean Orogeny and evolving to higher δ18O by incorporation of magmatic fluids or interaction with local crust. In situ oxygen isotope measurement by Secondary Ion Mass Spectrometer (SIMS) of cross-cutting quartz and carbonate range in δ18O from 19.4 to 28.4‰ for quartz, 25.3 to 28.1‰ for dolomite, and 9.6 to 29.3‰ (VSMOW) for calcite. High δ18O values (>19‰) are measured in a crustiform-textured quartz stockwork microveinlet, in dolomite observed in late carbonate microveinlets, and calcite in settings associated with the late quartz and dolomite and a separate calcite-only setting directly linked with gold mineralization/remobilization.

    更新日期:2019-11-27
  • U-loss associated with laser-heating of hematite and goethite in vacuum during (U-Th)/He dating and prevention using high O2 partial pressure
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-09
    Florian Hofmann, Jonathan Treffkorn, Kenneth A. Farley

    Single-aliquot (U-Th)/He dating of hematite has been used to study iron-oxide precipitation in various environments, but we show there is an important challenge to the method: highly retentive hematite samples require temperatures of > 1000° C to be completely degassed, whereas the temperature for major U-loss is ∼980° C. This leads to erroneously high (U-Th)/He ages. Through the analysis of U, Th, and Sm of hematite and goethite samples, we show the degree of U-loss at this temperature and demonstrate that prolonged heating at temperatures of 950° C can lead to U-loss. We show that loss of U in goethite and hematite samples is associated with phase change from hematite to magnetite as Fe is reduced. The onset temperature of vacuum reduction of hematite can be increased from about 800-900° C in vacuum to approximately 1250° C in an oxygen partial pressure of 100 mbar. We show that samples can be outgassed to extract helium at 1150° C without U-loss in an O2-rich atmosphere during heating, which does not increase the analytical blanks. We describe our implementation and automation of the procedure. An average age calculated on a reference hematite sample from replicate aliquots (n=12), which were analyzed using this procedure, has a relative uncertainty of 2% (1σ), and is within uncertainty of the previously measured two-aliquot age. We suggest this O2 degassing procedure as a way to precisely and reproducibly determine single-aliquot hematite and goethite (U-Th)/He ages.

    更新日期:2019-11-27
  • 更新日期:2019-11-26
  • Understanding the microscale spatial distribution and mineralogical residency of Re in pyrite: Examples from carbonate-hosted Zn-Pb ores and implications for pyrite Re-Os geochronology
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-26
    Danny Hnatyshin, Robert A. Creaser, Sebastien Meffre, Richard A. Stern, Jamie J. Wilkinson, Elizabeth C. Turner

    Accurate and precise geochronology using the Re-Os isotopic system in pyrite is an invaluable tool for developing and confirming genetic models of ore systems. However, as a bulk method, the results produced by pyrite Re-Os geochronology are commonly complex, and many imprecise isochrons exist in the literature. Using LA-ICPMS methods it is now possible to map and quantify Re distribution at the ppb level, allowing an unprecedented look into the Re-Os systematics of pyrite-bearing ore. Two samples from the Lisheen Zn-Pb ore deposit in Ireland showing disparate Re-Os isotopic behavior were investigated. In-situ sulfur isotope measurements using SIMS, an analytical technique not previously attempted on the Irish deposits, was used to supplement the Re-Os dataset. A massive pyrite sample from the Main Zone produced a precise, low-scatter isochron (346.6 ± 3.0 Ma, MSWD = 1.6). The Re distribution in this sample is relatively homogeneous, with the Re budget dominated by pyrite containing 1–5 ppb Re, but the δ34S varies significantly from −45.2‰ to 8.2‰. A second, more paragenetically complex, sample from the Derryville Zone produced a younger age with high scatter (322 ± 11 Ma, MSWD = 206) and this also displays a large variation in δ34S (−53‰ to +4‰). The cores of grains of main-stage iron sulfide are depleted in trace elements and show low Re abundances (<10 ppb) but have been altered in an irregular fashion leading to Re-enriched domains that exceed 100 ppb. Additionally, micron-scale molybdenite crystals, found in close association with altered sulfides, contain Re at levels that locally exceed 10 ppm. The highly scattered (MSWD = 206) and younger age (322 Ma), produced by the Derryville Zone sample are interpreted to result from mixing of different generations of sulfide, potentially involving fluids associated with Variscan deformation (<310 Ma). Therefore, the Re-Os data produced from the Derryville Zone sample does not reflect the timing of iron sulfide mineralization, even though a relatively precise age was obtained. A second Re-Os dataset from Zn-Pb mineralization at Hawker Creek, Nunavut, Canada was produced from massive pyrite that displays low Re concentrations (<1 ppb). However, on grain boundaries and in fractures, silicate-rich material contains Re at levels that can locally exceed 500 ppb. Analyses of fracture-free pyrite produced by bulk separation using magnetic separation yielded the oldest model age (1083 Ma), whereas mineral separates containing the highest fracture density produced the youngest age (413 Ma). In general, therefore, the complexities of pyrite Re-Os geochronology can result from impurities in mineral separates. Attempts to eliminate impurities through different mineral separation techniques (e.g. crushing, heavy liquid separation, magnetic separation, acid leaching) are frequently only partially successful and therefore full characterization of any resulting mineral separates is extremely important. We conclude that LA-ICPMS mapping of Re and Mo distributions is essential for the identification of such impurities. Although other trace element LA-ICPMS maps, in-situ sulfur isotope measurements, and petrographic evidence were of limited use in assessing the Re budget of a sample, they are invaluable in linking the documented Re distribution obtained through LA-ICPMS to Re-Os geochronological results.

    更新日期:2019-11-26
  • Three wood isotopic reference materials for δ2H and δ13C measurements of plant methoxy groups
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-25
    Markus Greule, Heiko Moossen, Max K. Lloyd, Heike Geilmann, Willi A. Brand, John M. Eiler, Haiping Qi, Frank Keppler

    Methoxy groups (OCH3) of plants show specific stable carbon and hydrogen isotope patterns that are used for applications in biogeochemical, atmospheric, paleoclimatic and food research. The method of choice for determining stable hydrogen and carbon isotope values of methoxy groups (δ2HOCH3 and δ13COCH3 values) is the conversion to gaseous iodomethane (CH3I) and subsequent measurement by stable isotope ratio mass spectrometry. However, comparative measurements particularly for stable hydrogen isotopes of plant methoxy groups are limited due to the lack of suitable reference materials. We have prepared three batches of powdered wood samples (birch HUBG3, beech HUBG4, and tineo HUBG5) collected from different geographical locations to serve as long-term reference materials for normalization of δ2HOCH3 but also δ13COCH3 values. Methoxy contents of the three wood samples range between 4.7 and 5.4%. Methoxy groups from subsamples of the three homogenized wood samples were quantitatively converted to CH3I and δ2HOCH3 and δ13COCH3 values of this CH3I were calibrated against international reference substances by high-temperature conversion- and elemental analyzer isotope ratio mass spectrometry. The δ2HOCH and δ13COCH3 values of HUBG3 and HUBG4 at 1σ uncertainty calibrated to the VSMOW and VPDB isotopic δ-scale, respectively are −272.9 ± 1.5 mUr and −29.40 ± 0.13 mUr; and −239.1 ± 1.4 mUr and −30.17 ± 0.13 mUr, respectively. In addition, the calibrated δ2HOCH value of HUBG5 is −191.7 ± 0.8 mUr. Whilst the δ2HOCH3 values of the three wood samples span a relatively wide range (~−280 to −190 mUr) suitable for normalization of δ2HOCH3 values of most plant samples that have been reported so far, δ13COCH3 values are restricted to a composition (~−30 mUr) typical of wood methoxy groups. We suggest that the three investigated wood materials HUBG3–5 are ideally suited for long-term usage, inter-laboratory comparison, and together with the two recently reported methyl sulfate salts (HUBG1 and HUBG2) complete a new set of solid methoxy reference materials that cover almost the full range of plant methoxy groups reported so far. Therefore, we recommend replacing liquid CH3I and instead use the solid reference materials set HUBG1–5 for normalizing δ2HOCH3 and δ13COCH3 values to the respective δ-scales.

    更新日期:2019-11-26
  • Evaluating estuarine sediment provenance from geochemical patterns in upper Chesapeake Bay
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-23
    Emily Russ, Cindy Palinkas, Jeff Cornwell

    Geochemical patterns in upper Chesapeake Bay sediments can reflect a variety of important processes. Spatial and down-core geochemical patterns are useful for understanding the underlying conservative (sediment provenance) and non-conservative (salinity, redox, anthropogenic influences) processes driving these patterns. Distinguishing conservative and non-conservative processes to quantify sediment sources will help develop management strategies to limit excess sedimentation in estuaries. In this study, elemental concentrations were measured via inductively coupled plasma mass spectroscopy (ICP-MS) and analyzed using principal component analysis. The sediment geochemistry data were then analyzed to evaluate contributions of Susquehanna River and shoreline erosion to bottom sediments in the upper Bay using a sediment-provenance analysis. Elements associated with aluminosilicate minerals, which were sensitive to grain-size changes, and rare earth elements, which were sensitive to salinity changes, explained the most variability in the dataset. Variability in heavy metal concentrations demonstrated decadal changes in anthropogenic inputs. The results from the sediment-provenance analysis indicated that the Susquehanna River is the dominant source of fine sediment in the upper Bay.

    更新日期:2019-11-26
  • Clumped and stable isotopes of land snail shells on the Chinese Loess Plateau and their climatic implications
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-22
    Jibao Dong, John Eiler, Zhisheng An, Naiqin Wu, Weiguo Liu, Xiangzhong Li, Nami Kitchen, Fengyan Lu

    We report stable (δ13Cshell, δ18Oshell) and clumped isotope (Δ47) compositions of modern and last glacial fossil snail shell carbonates from the Luochuan and Weinan sections on the central and southern Chinese Loess Plateau (CLP). Our study reveals that the average Δ47 temperature (T47) of modern snails is consistent with monitored temperatures during the snail growing season at the studied locations and is ~10 °C higher than that of fossil snails from glacial time. Moreover, the average δ13Cshell of modern snails is more depleted than that of fossils. We argue that the δ13Cshell cannot record changes in plant communities (i.e., the C3/C4 ratio) on the CLP and may mainly indicate arid conditions with depleted values reflecting reduced aridity. Additionally, the reconstructed snail body water δ18O (δ18Owater) of modern snails is more enriched than δ18O in modern growing season precipitation and δ18Owater of fossils. This contrast may be related to the high degree of evaporative enrichment of environmental water 18O in the body/ingested by modern snails under warm conditions. Therefore, we suggest that using δ18Oshell to directly reconstruct the oxygen isotopes of precipitation is difficult and that higher δ18Oshell and δ18Owater values probably indicate higher environmental temperature/stronger evaporative enrichment on glacial-interglacial timescales on the CLP.

    更新日期:2019-11-22
  • AWESOME OCIM: A simple, flexible, and powerful tool for modeling elemental cycling in the oceans
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-20
    Seth G. John, Hengdi Liang, Tom Weber, Tim DeVries, Francois Primeau, Keith Moore, Mark Holzer, Natalie Mahowald, Wilford Gardner, Alexey Mishonov, Mary Jo Richardson, Yannice Faugere, Guillaume Taburet

    Here we present a new flexible modeling tool for simulating the distribution of tracers in the modern ocean. A Working Environment for Simulating Ocean Movement and Elemental cycling within an Ocean Circulation Inverse Model, the AWESOME OCIM, is a transport matrix model (TMM) which is specifically designed to be easy, accessible, and intuitive, even for scientists without prior modeling experience. The AWESOME OCIM comes with a variety of selectable biogeochemical functions, including sources (atmospheric dust, hydrothermal vents, and seafloor nepheloid layers), internal cycling processes (biological uptake, remineralization, and scavenging), and sinks (radioactive decay and burial of particles in the sediments). A wide variety of elements can be simulated through different combinations of this suite of processes. We anticipate that the AWESOME OCIM will be a valuable tool for interpreting transect data from ocean surveys, particularly the trace-elements and isotopes distributions mapped by the ongoing GEOTRACES program. This manuscript provides an introduction to the philosophical, mathematical, and functional basis of the AWESOME OCIM.

    更新日期:2019-11-21
  • Precipitation of protodolomite facilitated by sulfate-reducing bacteria: The role of capsule extracellular polymeric substances
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-20
    Deng Liu, Qigao Fan, Dominic Papineau, Na Yu, Yueying Chu, Hongmei Wang, Xuan Qiu, Xingjie Wang

    The origin of dolomite has long puzzled geologists. It has recently been documented that sulfate-reducing bacteria (SRB) are capable of catalyzing the formation of protodolomite, a previously proposed precursor of ordered sedimentary dolomite. However, the catalytic mechanism of SRB remains incompletely understood. This experimental study is aimed at probing the effect of capsule extracellular polymeric substances (capsule EPS) from SRB on the crystallization of protodolomite in vivo. The capsule EPS tested herein was isolated from a protodolomite-mediating SRB, Desulfotomaculum ruminis, and added into a solution wherein the degree of oversaturation was close to the growth medium of D. ruminis at stationary phase. The solid products were characterized with X-ray diffraction (XRD), Raman spectroscopy and, scanning and transmission electron microscopy (SEM and TEM). Our results indicated that aragonite emerged in the reactors without capsule EPS, while Ca-Mg carbonates (Mg-calcite and protodolomite) were produced in the systems amended with capsule EPS. The incorporation amount of Mg2+ in Ca-Mg carbonates was enhanced with the increasing concentration of capsule EPS. The predominant occurrence of protodolomite was found in the reactor with 140 mg/L capsule EPS. These resulting protodolomites were spherical in shape, and composed of numerous nano-particles. The catalytic influence of capsule EPS on the precipitation of protodolomite might be attributed to their strong Mg2+ binding capacity, potentially diminishing Mg-hydration, which is a potent inhibitor of protodolomite crystallization. The results of Fourier transformation infrared (FT-IR) spectra showed that Mg2+ was bonded with carboxyl and hydroxyl groups on capsule EPS. This inferred adsorption capacity of capsule EPS was also supported by new calculations of complexation chemistry between Mg-H2O complex and organic compounds present in capsule EPS.

    更新日期:2019-11-20
  • What do Ce anomalies in marine carbonates really mean? A perspective from leaching experiments
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-18
    Cheng Cao, Xiao-Ming Liu, Clement P. Bataille, Chao Liu

    Yttrium and rare earth elements (REY) in aqueous environments have been intensively studied because they record important geochemical information. For example, the cerium anomaly, measured in marine sedimentary rocks, has been widely applied as a paleoredox indicator. Marine carbonates are the main substrate used to reconstruct REY signal in paleo-oceans with the underlying assumption that REY incorporated into carbonate minerals preserve the authigenic seawater signal. However, extracting authigenic REY signals from carbonate rocks are challenging. It requires dissolving carbonate phases using acid leaching procedures without contamination from other non‑carbonate phases such as Mn/Fe oxides (hydroxides), clay minerals, siliciclastic and organic phases. Multiple dissolution protocols, especially partial leaching, were proposed to eliminate potential contaminations. Yet, the efficiency of these procedures in reducing contamination remains ambiguous. Additionally, no systematic study on how each non‑carbonate phase can modify the authigenic REY signal from carbonates. First, we systematically investigated the efficiency of using ammonium acetate buffer pre-treatment to remove exchangeable ions prior to dissolution. We tested a range of concentration, pH, and reaction time of the buffer and we found that ammonium acetate of 1 M with a pH slightly <7, and 30 mins for reaction time works best for efficiency. Second, we tested a range of acid species and molarity to dissolve common non‑carbonate contamination phases including USGS standards NOD-A-1 (manganese nodule) and SBC-1 (Brush Creek shale), as well as montmorillonite, kaolinite, illite from the Clay Minerals Society. We used three batches of acetic and nitric acids with different molarity and found that both the acid species and molarity are key parameters to avoid leaching of non‑carbonate phases. We observed a negative Ce anomaly during leaching of Mn nodule using acetic acid versus a positive anomaly using nitric acid suggesting that nitric acid may introduce false positive Ce anomalies into samples. We also demonstrated that it is essential to filter the leachate to prevent the later dissolution of floating particles. For instance, Mn oxides, which carry positive Ce anomalies, can overprint the carbonate REY signal because the nitric acid (i.e. 2% v/v) used during later ICP-MS analysis could dissolve tetravalent Ce. Third, we conducted a series of leaching experiments on mixed samples consisting of 80% pure modern limestone and 20% mixed pure contaminants to identify the best protocol to avoid contamination. We demonstrate that using acetic acid with a molarity lower than 0.5 M is sufficient to avoid contaminations from Mn oxides, clay minerals, and siliciclastic phases. Finally, we present a new sequential leaching protocol for calcite (>75% CaCO3) using two steps of ammonium acetate pre-leaching followed by three steps of 0.3 M acetic acid leaching.

    更新日期:2019-11-18
  • Influence of provenance and transport process on the geochemistry and radiogenic (Hf, Nd, and Sr) isotopic composition of Pleistocene glacial sediments, Minnesota USA
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-18
    Chad Wittkop, Julie K. Bartley, Russell Krueger, Audrey Bouvier, R. Bastian Georg, Alan R. Knaeble, Katherine St. Clair, Christian Piper, Andy Breckenridge

    We develop a broad geochemical dataset from 50 samples of Pleistocene glacial till matrix (including three lacustrine samples) representing four sediment provenances collected from across Minnesota, USA. Such a dataset is useful both in the context of regional studies of glaciation, and in efforts to understand how provenance and glacial processes impact the geochemistry of sediment archives. The principal sediment sources of the four provenances include materials from the Archean-Proterozoic Canadian Shield, the Proterozoic Midcontinent Rift, Paleozoic carbonates, and Cenozoic and Mesozoic clastic sediments. We measured major element geochemistry in all till samples in both <2 mm and <63 μm size fractions, the trace element geochemistry in most samples, radiogenic isotopic compositions (Sr, Nd) in 13 representative samples, and Hf-isotope composition in 6 samples. Differences in source rock composition explain the primary variations in the geochemistry of our samples. In untreated (carbonate-bearing) samples, Na2O + K2O versus Fe2O3 or CaO are distinct in tills sourced from crystalline rocks versus sedimentary basins, and mechanical mixing from different source areas is evident on 100-km glacial transport length scales. Glacial materials originating from sedimentary rocks have higher chemical index of alteration (CIA) relative to materials sourced from the predominately igneous and metamorphic Canadian Shield. Increased values of Rb + Sr, Zr/Sc, and Cr + Ni are also associated with tills derived from crystalline rocks relative to tills derived from sedimentary rocks. Both the Hf- and Nd-isotopic composition of glacial sediments distinguish crystalline rock sources (less radiogenic) versus sedimentary rock sources (more radiogenic). The Sr-, and, to a lesser degree, the Hf-isotopic composition of lacustrine samples is influenced by subtle changes in sample mineral composition, reflecting both source rock variability and the sorting of clay and heavy mineral components during sediment transport. A carbonate-free composite of our till samples was found to be broadly representative of a Canadian Shield source. The ability to discern provenance and sediment transport process controls on glacial sediment geochemistry presents an opportunity to extend our understanding of past ice-sheet dynamics, and validates approaches that use tills as a proxy for continental crustal composition, provided that the influences of sediment recycling are carefully considered.

    更新日期:2019-11-18
  • Examining pelagic carbonate-rich sediments as an archive for authigenic uranium and molybdenum isotopes using reductive cleaning and leaching experiments
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-18
    Matthew O. Clarkson, Kim Müsing, Morten B. Andersen, Derek Vance

    Novel metal isotope systematics are increasingly used to understand environmental change in geological history. On a global scale, the isotopic budgets of these metals respond to a range of environmental processes, allowing them to trace complex changes in the global climate system and carbon cycle. In particular, uranium (U) and molybdenum (Mo) isotopes are useful tools for quantifying the global extent of oceanic anoxia and euxinia respectively. The oceanic signature of these metals is recorded in contemporaneous marine sediments. Whilst, traditionally, organic-rich anoxic ‘black shales’ have provided a useful archive of these metals, carbonate sediments are increasingly being used as a passive recorder of ocean chemistry. The majority of published U and Mo isotope studies come from shallow water platform environments. By contrast, pelagic carbonate sediments are an under-explored archive for these metals, yet are widely available for important periods of Earth history. Despite their advantages, carbonates are a complex archive, containing multiple ‘contaminant’ components such as Mn-oxides, organic matter and detrital minerals. Each of these phases can have different metal concentrations and isotopic signatures, giving the potential to distort or bias the true oceanic signature recorded by the carbonate. Reductive cleaning procedures and selective leaching protocols can be used to avoid these contaminant phases, and are tested here on modern and ancient samples to judge their efficacy in isolating a ‘carbonate-bound fraction’. To this end, leaching experiments were performed using different concentration acetic acid, HCl and HNO3, on reductively cleaned and uncleaned sample pairs. The data demonstrate that Mn-oxide coatings and exchangeable phases have a large impact on the Mo isotopic signature (δ98Mo) of carbonates, even when weak leaching techniques are used to preferentially dissolve them. Furthermore, detrital sources of Mo are also easy to liberate with different leaching protocols, and exert a significant control on leachate isotopic composition. The leaching studies identify that the pelagic carbonate end-member has a relatively high δ98Mo, but the precise relationship to seawater compositions remains unclear. For U, significant contributions from non‑carbonate phases can clearly be identified in higher concentration leaching acids using U/Ca ratios. However, U isotopes (δ238U) show no resolvable difference with different leaching procedures and are not affected by reductive cleaning. This result probably reflects (a) the low potential for leaching refractory residual detrital U phases (e.g., zircon) that contain the majority of U in the sample and (b) the low U inventories of Mn oxides versus those of Mo. Instead, leaching likely extracts U that is mineralogically bound in carbonates and authigenic clays, which share a common isotopic signature. These new data suggest that U incorporation into pelagic carbonates may be dominated by adsorption, and be offset from seawater by ~−0.15‰, in a similar manner to that seen for clays.

    更新日期:2019-11-18
  • Fluorine surface speciation in South Andean volcanic ashes
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-16
    G. Bia, L. Borgnino, G. Zampieri, M.G. Garcia

    Fresh volcanic ash samples collected during the eruptions of the Hudson (1991), Chaitén (2008), Puyehue (2011), Calbuco (2015) and Copahue (2016) volcanoes were analyzed in order to identify the fluorine (F) bearing phases. XPS and HR-TEM was used to determine the solid speciation of F. In order to decrease the surface differential charging of samples, XPS data have been collected using an electron flood gun during spectral acquisition. Ash leaching at different pH conditions was also performed with the aim of understanding the factors that control the fluoride release to aqueous reservoirs. The results indicates that in the studied Patagonian ashes, F is concentrated onto the ash surface, likely in the form of FNa and FCa salts but also within the glass lattice forming FSi and FAl bonds. Although the main F-source in the studied volcanic ashes is the aluminosilicate glass, this phase is stable at the neutral pH that dominates the aqueous reservoirs, thus the contribution to the pool of dissolved fluoride is minor. Higher contributions are associated with soluble fluoride salts. In addition, the presence of coatings of FCa compounds inhibits the release of fluoride within the volcanic glass lattice, at least during the initial stages of the ash-water interaction.

    更新日期:2019-11-18
  • Lawsonite-rich layers as records of fluid and element mobility in subducted crust (Sivrihisar Massif, Turkey)
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-16
    Katherine F. Fornash, Donna L. Whitney

    Lawsonite is an abundant hydrous mineral in subducted oceanic crust and sediments and is an important carrier of water (∼11.5 wt%) and trace elements (REE, U, Th, Sr, Pb) into the mantle. Despite its widespread stability at the high-pressure/low-temperature conditions of subduction, it is rarely preserved in the geologic record, particularly in eclogite. Therefore, scarce fresh lawsonite eclogites in which the composition of lawsonite can be directly determined are important geochemical archives of fluid-mediated processes during subduction. Of particular interest are lawsonite-rich veins and layers that may represent former fluid pathways or metasomatic zones and that therefore may record element mobility related to fluid-rock interaction during subduction and/or incipient exhumation. In the Sivrihisar (Turkey) subduction complex, lawsonite-rich veins and layers comprised of ∼30-50% lawsonite occur at pod margins where lawsonite eclogite has been partially transformed to lawsonite blueschist. To understand the petrogenesis of these lawsonite-rich assemblages, we conducted a petrographic and geochemical study of a representative lawsonite-rich layer at the margin of a meter-scale lawsonite eclogite pod. Bulk rock and mineral major and trace element analyses were conducted along a transect consisting of the lawsonite-rich layer (Lws + Grt + Ph), its glaucophane-rich margin (Gln + Grt + Lws + Ph + Rt), and the lawsonite eclogite host at varying distances from the layer (∼1 – 2 cm away and >10 cm away). The bulk-rock composition of the lawsonite eclogite indicates a basaltic protolith that experienced interactions with (meta-)sedimentary rocks before or during the crystallization of high-pressure phases such as garnet and rutile. Integrated major and trace element composition and zoning in high-pressure phases indicate that the lawsonite-rich layer and its associated glaucophane-rich margin likely formed at or near peak eclogite-facies conditions (2.2 – 2.4 GPa, 520 °C) as a result of fluid-mediated processes that scavenged Al2O3, MnO, Y, Th, HFSE, and REE from the eclogite immediately adjacent to the vein (∼1 – 2 cm away). Mass balance calculations also suggest the addition of LILE and transition metals (Ni, Cr, Zn) to the lithologic layers at the pod margin; these elements were likely supplied from an external source, such as serpentinites and/or sediments, both of which occur interspersed with mafic rocks in the Sivrihisar complex. Fluid-rock interaction may have also driven changes in fO2, as omphacite, glaucophane, lawsonite, and phengite from the pod margin record core-to-rim increases in Fe3+. These changes in bulk composition preferentially stabilized glaucophane-rich (blueschist) and LREE- and LILE-enriched lawsonite-rich assemblages at the pod margin.

    更新日期:2019-11-18
  • An expanded shale δ98Mo record permits recurrent shallow marine oxygenation during the Neoarchean
    Chem. Geol. (IF 3.618) Pub Date : 2019-11-14
    Chadlin M. Ostrander, Brian Kendall, Stephanie L. Olson, Timothy W. Lyons, Gwyneth W. Gordon, Stephen J. Romaniello, Wang Zheng, Christopher T. Reinhard, Moutusi Roy, Ariel D. Anbar

    Multiple attempts have been made using the ancient shale record to track the molybdenum isotope composition (δ98Mo) of seawater during the final two-hundred million years of the Archean Eon (2.7 to 2.5 billion-years-ago, or Ga). This seawater δ98Mo value is important because it should have scaled with levels of ocean oxygenation during the runup to the Great Oxidation Event. Unfortunately, however, it is difficult to tell if the majority of the existing late-Archean shale δ98Mo record tracks an ancient seawater value. Here, we further attempt to track pre-GOE seawater δ98Mo using an expanded and well-characterized shale sample set from Western Australia (Jeerinah, Wittenoom, and Mt. Sylvia Formations) and South Africa (Nauga and Klein Naute Formations). Most importantly, and in contrast to most previous Mo isotope studies of similarly aged shales, local redox conditions for our shales have been independently constrained using the iron (Fe) speciation proxy. Local redox conditions are an important parameter because transfer of the seawater δ98Mo to sediments today is shown to be dependent on these conditions. According to our updated sedimentary δ98Mo record, seawater δ98Mo commonly exceeded 1.0‰ between ~2.69 Ga and 2.50 Ga. In order to drive such a heavy seawater δ98Mo, there must have been a marine sink with a strong preference for lighter-mass Mo isotopes frequently present over this timeframe. The operation of some anaerobic processes in late-Archean marine settings could theoretically explain the heavier seawater δ98Mo. Such processes are known to promote the preferential retention of lighter-mass Mo isotopes in marine sediments today (e.g., interactions between Mo and organic matter or the formation of thio-complexes). Alternatively, or in addition, adsorption of lighter-mass Mo isotopes to Fe and Mn oxide minerals formed in oxygenated marine environments can explain the heavier seawater δ98Mo. A compilation of previous work suggests that oxygenated shallow marine environments were fairly common during the late-Archean, and thus Mo adsorption to FeMn oxides formed in these settings probably played an important role in driving heavy seawater δ98Mo over this timeframe.

    更新日期:2019-11-14
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