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  • Upper mantle slab under Alaska: Contribution to anomalous core-phase observations on south-Sandwich to Alaska paths
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2020-01-15
    Daniel A. Frost; Barbara Romanowicz; Steve Roecker

    Observations of travel time anomalies of inner core-sensitive PKPdf seismic body waves, as a function of path orientation with respect to the earth's rotation axis, have been interpreted as evidence of anisotropy in the inner core. Paths from earthquakes in the South Sandwich Islands to stations in Alaska show strongly anomalous travel times, with a large spread that is not compatible with simple models of anisotropy. Here we assess the impact of strong velocity heterogeneity under Alaska on the travel times, directions of arrival and amplitudes of PKPdf. We use 3D ray-tracing and 2.5D waveform modelling through a new, high-resolution tomography model of the upper mantle beneath Alaska. We find that the structure beneath Alaska, notably the subducting slab, is reflected in the patterns of these PKPdf observations, and this can be replicated by our model. We also find similar patterns in observed teleseismic P waves that can likewise be explained by our slab model. We conclude that at least 2 s of the travel time anomaly often attributed to inner core anisotropy is due to slab effects in the upper mantle beneath Alaska.

    更新日期:2020-01-15
  • Source parameters of the 1926 and 1927 Jersey earthquakes from historical, instrumental, and macroseismic data
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2020-01-15
    D. Amorèse; J. Benjumea; M. Cara

    English Channel Islands are located off Normandy coast of France within an intraplate area not associated with high seismicity rate and active tectonics. However, in July 1926 a damaging and well-documented earthquake occurred there, followed by a strongly felt event in February 1927. In this paper, we reprocess macroseismic observations, analog seismograms and bulletin data in order to re-appraise the location and magnitude of these two earthquakes. We find that the macroseismic epicentre of the 1926 Jersey earthquake is shifted to the East when compared with the location offshore South of Jersey given in both the French database SisFrance and the recent French catalog FCAT-17. Arrival-times from published data, together with our own onset-time readings, are processed in order to obtain probabilistic hypocentral locations. The maximum-likelihood instrumental epicentre of the 1926 event is located about 15 km East of Jersey Island (49.20°N, 1.82°W). This epicentre is well constrained within a 10 km radius area. The location of the 1927 epicentre is slightly south of Jersey, at a location similar to that of SisFrance, but this latter epicentre is less constrained as fewer observations are available (the Probability Density Function for the 1927 epicentre is relatively “diluted”). Focal depth remains very poorly determined for both the 1926 and 1927 events. Analysis of historical seismograms is also performed in order to determine both surface-wave and moment magnitudes, MS and MW. We find MS = 5.6 ± 0.2 for the 1926 event and Ms. = 5.0 for the 1927 event. Waveform fitting of some of the highest quality seismograms of the 1926 event gives us MW in the range [5.0, 5.5], depending on the chosen focal depth and focal mechanism. The 1927 Jersey earthquake is expected with an intensity magnitude MI about 0.6 smaller.

    更新日期:2020-01-15
  • One-dimensional seismic velocity model of the sub-basin of Chalco, Mexico
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2020-01-08
    Filiberto Vergara-Huerta; Jorge Aguirre-González

    In this work, a one-dimensional (1-D) seismic velocity model is proposed for the central portion of the sub-basin of Chalco, located in the southeast (SE) sector of the basin of Mexico. The seismic model was integrated from spectral analysis of the seismic ambient vibrations (microtremors or seismic noise) records, through the methodologies of Modified Spatial Autocorrelation (MSPAC) and H/V spectral ratios. The one-dimensional (1-D) seismic model is the result of the simultaneous inversion of the H/V ratios and the phase-velocity dispersion curves, corresponding with Rayleigh waves and contained in the vertical component of the microtremor records. The scheme used in the inversion process was Monte Carlo. Which allowed us to establish a best fit model configured by four representative lithological units, congruent with the amplitude and dominant periods identified in this sector of the basin of Mexico.

    更新日期:2020-01-08
  • Application of wavelets for specifying tsunami from tide: A comprehensive study on the 1945 Makran tide gauge records
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2020-01-07
    Ahmad Ala Amjadi; Mohammad Mokhtari

    In tide gauge records, the separation of tidal from non-tidal contents is in general done by specialists after applying high pass or bandpass filtering techniques. However, there is always a tradeoff between losing some valuable contents of the signal while trying to remove the tide. In this manuscript, we deal with two significant tide gauge records of the 1945 Makran tsunami. The importance of these data is well known for researchers since these are the only valid tide gauge data for any evaluation of tsunami in the western Makran region. Wavelet decomposition technique was applied as a tool by us to minimize the influence of tidal contents on these data. Several parameters were set and applied to several types of wavelets in various temporal series and frequency ranges. The goal was to find the most appropriate wavelet parameter and type, to distinguish tsunami data from tidal contents. Our results show that the application of wavelets in comparison to conventional methods yields to lesser RMS errors before the arrival of the tsunami wave and therefore produces minor data loss after applying. Moreover, after applying suitable wavelets to data, we have noticed a sudden decrease in the sea level before the arrival of tsunami waves, which is consistent with eyewitness reports. The result of our work is beneficial for further evaluation of tsunami modeling in western Makran. The procedure introduced can be applied at any tide gauge data for tidal analysis with wavelet decomposition techniques.

    更新日期:2020-01-07
  • P-wave upper-mantle tomography of the Tanlu fault zone in eastern China
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-11-18
    Jianshe Lei; Dapeng Zhao; Xiwei Xu; Mofei Du; Qi Mi; Mingwen Lu

    The Tanlu fault zone is the most significant active fault in eastern China, which generated the great 1668 Tancheng earthquake (M 8.5). It is still unclear whether or not there is a link between the great earthquake generation and the upper-mantle structure. To address this issue, we study P-wave upper-mantle tomography beneath eastern China using 44,047 teleseismic P-wave arrival times. Our results show that at depths <150 km, high-velocity (high-V) anomalies appear west of the Tanlu fault zone, whereas low-velocity (low-V) anomalies are visible east of the fault zone. Strong lateral heterogeneities are revealed along the fault zone. At depths of 230–470 km, northwest of the Tanlu fault zone, there are obvious low-V anomalies that may reflect hot and wet mantle upwelling, whereas to the east high-V anomalies are visible, which may reflect the detached Eurasian lithosphere (downwelling). In the mantle transition zone (MTZ), both high-V and low-V anomalies are revealed, and the widespread high-V anomalies may reflect the stagnant Pacific slab. Beneath the hypocenter of the 1668 Tancheng earthquake, intermittent low-V anomalies are revealed in the upper mantle down to the MTZ depth, which may reflect hot and wet mantle upwelling flow. Integrating the present results with previous findings, we deem that the Tancheng earthquake was affected by fluids from the hot and wet mantle upwelling associated with the lithospheric delamination. Complicated mantle convection, including both upwelling and downwelling flows, may occur under the Tanlu fault zone in the big mantle wedge above the stagnant Pacific slab in the MTZ.

    更新日期:2020-01-04
  • Calcium carbonate at high pressures and high temperatures: A first-principles investigation
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-11-19
    Samuel S.M. Santos; Michel L. Marcondes; João F. Justo; Lucy V.C. Assali

    We present a theoretical investigation, within the density functional theory and the quasi-harmonic approximation framework, on the stability properties of calcium carbonate (CaCO3) at the thermodynamic conditions of the Earth's lower mantle. We initially explored the structural properties of a low-pressure phase (aragonite) using three different approximations for the exchange-correlation potential, and find that all properties are better described, when compared to recent experimental data, by using the one that takes into account the van der Waals interactions. We used this approximation to compute the free energy of this mineral in several crystalline phases, and explored the respective thermodynamic properties at high temperatures and high pressures. The results on phase stability, thermal expansivity, specific heat, bulk modulus, and mass density were compared to available experimental data. Then, we built the phase diagram for this mineral and discussed its impact on the Earth's mantle properties.

    更新日期:2020-01-04
  • High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-11-21
    Jannes Binck; Stella Chariton; Michal Stekiel; Lkhamsuren Bayarjargal; Wolfgang Morgenroth; Victor Milman; Leonid Dubrovinsky; Björn Winkler

    The stability of Fe-poor dolomite, CaMg0.98Fe0.02(CO3)2, was studied by Raman spectroscopy and single-crystal X-ray diffraction at high pressures (P < 60 GPa) and high temperatures (T < 2300 K). Density functional theory calculations were employed to complement the experimental study. Between 40 and 60 GPa and 1800–2300 K, we observed the formation of a high P,T phase, “dolomite-V”, which, after quenching to ambient temperature, remained stable down to 12 GPa. The dolomite-V phase crystallizes in the space group C2/c with Z = 4 formula units. The structure of the high pressure polymorph “dolomite-IIIc” was solved, which crystallizes in the space group P1¯ with Z = 8 formula units. The combined experimental and theoretical findings show that at high pressures and low to moderate temperatures Dol-IIIc is formed, while at high pressures and high temperatures Dol-V becomes stable. Assuming that thermodynamic equilibrium is obtained at high-pressure, high-temperature conditions, the current study extends our understanding of the phase stability of Fe-poor dolomite polymorphs at upper and lower mantle conditions.

    更新日期:2020-01-04
  • Upper mantle tomography of the Western Junggar: Implications for its geodynamic evolution
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-11-26
    Yuanyuan Hua; Yixian Xu; Dapeng Zhao; Rong Huang; Xiaohuan Jiang

    A high-resolution 3-D P-wave velocity model of the upper mantle beneath the Western Junggar is determined by inverting a large number of relative travel-time residuals of teleseismic events. Our results reveal a prominent high-velocity (high-V) anomaly beneath the western Junggar basin and a low-velocity (low-V) anomaly beneath the Darbut belt. The NE-SW striking high-V anomaly at depths of 50–200 km beneath the western Junggar basin dips toward the northwest, which represents a fossil oceanic slab. The low-V anomaly may reflect an intraoceanic arc-related terrain that has experienced strong metasomatism by hot upwelling asthenosphere in the late Paleozoic, resulting in adakitic intrusions and concentration of porphyry CuAu deposits in the Darbut belt. These results shed new light on the geodynamic evolution of the Western Junggar and formation mechanisms of the adakites and porphyry CuAu deposits.

    更新日期:2020-01-04
  • Regional variation of coda wave attenuation in Northeast India: An understanding of the physical state of the medium
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-11-25
    Rabin Das; Sagarika Mukhopadhyay

    In this study an attempt has been made to estimate spatial variation in attenuation characterisitics in Northeast India using coda Q. The entire study region is divided into three sub-regions for this purpose. Estimated average frequency dependency of coda wave attenuation for 30 s window length are Qc(f) = 135 ± 7f0.99±0.03, Qc(f) = 109 ± 7f1.10±0.03 and Qc(f) = 90 ± 2f1.04±0.02 for Shillong Plateau, Mikir hills and surrounding River valley, and Indo-Burma Ranges respectively. It is observed that Q0 is greater for the Shillong Plateau than the other sub-regions. This indicates lower attenuation due to more rigid high-density material present in this area than the other sub-regions. The depth variations of the Qc, Q0 and n values were also examined. It is observed that the rate of increase of Q0 with depth is not uniform for all the sub-regions. Indo-Burma Ranges has the smallest Q0 and the largest n values at all depth levels among the three sub-regions. These results indicate that central part of Indo-Burma Ranges is the most attenuative, seismically active and heterogeneous in nature. However, this region has smaller Qc values than the other two sub-regions for all window lengths up to the 6 Hz. This means at lower frequencies the subsurface beneath this area is more attenuative compared to the other two sub-regions. Similar trends are observed at 8, 10 and 12 Hz, up to 45 s window lengths. For window lengths ≥55 s, central part of Indo-Burma Ranges has higher Qc values at 10 and 12 Hz compared to Shillong plateau. Qc values are lower for Shillong Plateau compared to the other two regions for window length ≥ 55 s at 10 and 12 Hz, which corresponds to depth levels ≥90 km. Such a complicated variation in Qc values is a manifestation of complex nature of tectonic regime in Northeast India.

    更新日期:2020-01-04
  • JULOC: A local 3-D high-resolution crustal model in South China for forecasting geoneutrino measurements at JUNO
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-12-06
    Ruohan Gao; Zhiwei Li; Ran Han; Andong Wang; Yufeng Li; Yufei Xi; Jingao Liu; Xin Mao; Yao Sun; Ya Xu

    Geothermal energy is one of the keys for understanding the mechanisms driving the plate tectonics and mantle dynamics. The surface heat flux, as measured in boreholes, provides limited insights into the relative contributions of primordial versus radiogenic sources of the interior heat budget. Geoneutrino, electron antineutrino that is produced from the radioactive decay of the heat producing elements, is a unique probe to obtain direct information about the amount and distribution of heat producing elements in the crust and mantle. Cosmochemical, geochemical, and geodynamic compositional models of the Bulk Silicate Earth (BSE) individually predict different mantle neutrino fluxes, and therefore may be distinguished by the direct measurement of geoneutrinos. Due to low counting statistics, the results from geoneutrino measurements at several sites are inadequate to resolve the geoneutrino flux. However, the JUNO detector, currently under construction in South China, is expected to provide an exciting opportunity to obtain a highly reliable statistical measurement, which will produce sufficient data to address several vital questions of geological importance. However, the detector cannot separate the mantle contribution from the crust contribution. To test different compositional models of the mantle, an accurate a-priori estimation of the crust geoneutrino flux based on a three-dimensional (3-D) crustal model is important. This paper presents a 3-D crustal model over a surface area of 10° × 10° grid surrounding the JUNO detector and a depth down to the Moho discontinuity, based on the geological, geophysical and geochemical properties. This model provides a distinction of the thickness of the different crustal layers together with the corresponding Th and U abundances. We also present our predicted local contribution to the total geoneutrino flux and the corresponding radiogenic heat. Compared to previous studies where the surface layer is subdivided into a few geologic units and each of them is considered to have the same geochemical property, our method has provided an effective approach to reduce the uncertainty of geoneutrino flux prediction by constructing the composition of the surface layer through cell by cell which are independent to each other.

    更新日期:2020-01-04
  • Comparisons between measurements and predictions of Rayleigh wave amplification across the contiguous United States
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-12-02
    Celia L. Eddy; Göran Ekström

    We present local surface-wave-amplification maps spanning the contiguous United States for Rayleigh waves between 35 s and 125 s using data recorded on the USArray between 2006 and 2015. We isolate the effect of local structure from those of the earthquake and propagation using a previously developed method based on ratios of amplitudes measured at adjacent stations. To assess the ability of our technique to resolve surface-wave amplification, we perform a parallel synthetic-tomography experiment. We determine amplification from a large dataset of SPECFEM synthetic seismograms calculated for USArray stations and compare the measurements with direct predictions of local amplification. Correlations between synthetic and predicted amplification are high, with correlation coefficients ranging from 0.77 at 40 s to 0.95 at 100 s, indicating that we are able to resolve well local variations in amplification, particularly at long periods. The remaining differences between synthetic and directly predicted amplification maps suggest the influence of finite-frequency effects on surface-wave amplitudes. Observed Rayleigh wave amplification factors reflect local elastic structure in the region surrounding each station and could be used as a complementary constraint to image the structure of the Earth's crust and upper mantle.

    更新日期:2020-01-04
  • Examination of the repeatability of two Ms6.4 Menyuan earthquakes in Qilian-Haiyuan fault zone (NE Tibetan Plateau) based on source parameters
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-12-02
    Xiaohui He; Yipeng Zhang; Xuzhang Shen; Wenjun Zheng; Peizhen Zhang; Dongli Zhang

    Repeating earthquakes are key evidence for understanding earthquake recurrence and help improve long-term seismic risk assessment. In 1986 and 2016, two Ms6.4 earthquakes occurred in Menyuan region, near the middle segment of Qilian-Haiyuan fault zone, NE Tibetan Plateau. Long period waveforms of the two events at teleseismic stations show high degree of similarity, however, previous solutions on earthquake location and focal mechanism show remarkable discrepancies, especially for the 1986 event, as well the ruptured faults also remain ambiguous. In this study, we determined the source parameters of these two earthquakes, including the relative location, focal depth, focal mechanism, source duration and rupture directivity. The results show that two earthquakes are thrust events in shallow crust and are located within 15 km, and the 2016 earthquake ruptured down-dip for ~6 km along the southwest dipping plane. Combing the source parameters, geological data and deep seismic-reflection profiles, we infer that these two earthquakes occurred on the same secondary fault of Lenglong Ling fault (the middle segment of Qilian-Haiyuan fault), but they ruptured on different sections of the fault. Although the two events are probably not repeating earthquakes of Qilian-Haiyuan fault zone, the seismic risk of ~M6 thrust earthquake on these secondary faults and even larger strike-slip earthquake on Lenglong Ling fault are not negligible based on the source parameters and ten-year GPS observations.

    更新日期:2020-01-04
  • Constraints on S-wave velocity structures of the lithosphere in mainland China from broadband ambient noise tomography
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-12-14
    Jie Peng; Jinli Huang; Zhikun Liu; Kang Xing

    In this study, a 3-D S-wave velocity model of the lithosphere in mainland China down to 150 km depth was determined from ambient noise data. First, we collected 2-year continuous waveforms recorded by 1031 broadband stations of the China Regional Seismic Network and NECESSArray. Then, by applying the procedures of noise cross-correlation and time-frequency domain phase-weighted stacking, we obtained the interstation empirical Green's functions of Rayleigh waves. We measured the group and phase velocity dispersions at periods of 5–125 s from the EGFs and inverted both group and phase velocities for the S-wave velocity model. Our model revealed that the lithospheric thickness in the northern Songliao basin is ~70–110 km. A weak high-velocity anomaly that is discontinuous in the vertical direction was detected beneath the southern Songliao basin, which may indicate that the lithosphere is delaminated in this area. The model also revealed that deep (>150 km) high-velocity lithospheric roots exist beneath the Ordos and Sichuan basins. The lithospheric thicknesses in eastern parts of the Northeast China Block, the North China Craton, and the South China Block are only 60–100 km, which may be the result of a series of deep processes caused by the subduction of the Pacific plate to the Eurasian plate that produced strong lithospheric thinning in eastern China. The lithospheric thickness within the Tanlu fault belt is generally thinner than that in the adjacent areas of the fault belt, and the S-wave velocity of the mantle lithosphere beneath the fault belt is also lower than that beneath the adjacent areas, indicating that the Tanlu fault belt is a deep fault belt cutting through the lithosphere.

    更新日期:2020-01-04
  • Deformation of two welded half-spaces caused by a centre of rotation source
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-12-18
    Nishu Verma; Kuldip Singh

    Closed-form analytic expressions for Papkovich-Neuber displacement potentials, displacements, and stresses due to the centre of rotation source in an elastic half-space overlying an elastic half-space are obtained. The elastic results are valid for different values of Poisson's ratio and a change in the rigidity of the two half-spaces. Further, the correspondence principle of linear viscoelasticity is used to obtain the corresponding solutions in an elastic half-space overlying a viscoelastic half-space. The viscoelastic results are valid for different values of relaxation time and a change in the rigidity of the two half-spaces. The variation of the displacements and stresses with the epicentral distance as well as with the relaxation time are studied. The magnitude of displacement and normal stress components assume a maximum value for the Maxwell model and minimum value for the Kelvin model whereas the shear stresses assume a minimum value for the Maxwell model and maximum value for the Kelvin model.

    更新日期:2020-01-04
  • Possible H2O storage in the crystal structure of CaSiO3 perovskite
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-12-23
    H. Chen; K. Leinenweber; V. Prakapenka; C. Prescher; Y. Meng; H. Bechtel; M. Kunz; S.-H. Shim

    The lower mantle is believed to contain much less hydrogen (or H2O) because of the low storage capacity of the dominant mineral phases, such as bridgmanite and ferropericlase. However, possible hydrogen storage in the third most abundant mineral in the region, CaSiO3 perovskite (Ca-Pv), is not well unknown. We have synthesized Ca-Pv from different starting materials with varying H2O contents at 19–120 GPa and 1400–2200 K in laser-heated diamond-anvil cell. While cubic perovskite structure is stable at the mantle-related pressures-temperatures (P−T) in anhydrous systems, we found non-cubic diffraction peak splitting in Ca-Pv even at high temperatures when it is synthesized from hydrous starting materials. In-situ high-pressure infrared spectroscopy showed OH vibration possibly from Ca-Pv. The unit-cell volume of hydrothermally synthesized Ca-Pv is systematically smaller than that of anhydrous Ca-Pv at high pressures. These observations suggest possible H2O storage in Ca-Pv at mantle-related P−T conditions. We also found the formation of separate δ–AlOOH and Ca-Pv phases from Al-bearing CaSiO3 glass starting materials in an H2O medium at 60 GPa and 1400 K. Ca-Pv still showed non-cubic peak splitting at high temperatures in this experiment. Therefore, it is possible that hydrous phases may coexist together with hydrous Ca-Pv in the lower mantle.

    更新日期:2020-01-04
  • Identification of paleomagnetic remanence carriers in ca. 3.47 Ga dacite from the Duffer Formation, the Pilbara Craton
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2020-01-03
    Yoichi Usui; Masafumi Saitoh; Kenichiro Tani; Manabu Nishizawa; Takazo Shibuya; Chie Kato; Tomoyo Okumura; Teruhiko Kashiwabara

    The ca. 3.47 Ga Duffer Formation has been considered to carry one of the oldest paleomagnetic records. Yet, the lack of rock magnetic data limits the interpretation of the nature of the remanence. We conducted a rock magnetic and paleomagnetic investigation on columnar dacite of the Duffer Formation. The main magnetic minerals are phenocrysts of titanomagnetite and magnetite, and secondary hematite in groundmass. Detailed thermal demagnetization revealed more complex natural remanence than previously estimated, consisting of four components with typical unblocking temperature of 200 − 350, 200 − 500, 590, and 690 °C. Combined with alternating field demagnetization and rock magnetic data, they are attributed to titanomagnetite, coarse-grained magnetite, fine-grained magnetite, and hematite, respectively. The comparison of unblocking temperature and coercivity suggest that the previously proposed secondary component is carried by fine-grained magnetite as well as hematite, while the putative primary component is carried by coarse-grained magnetite and titanomagnetite. Microscopic observations showed that coarse-grained magnetite and titanomagnetite are primary crystals, although this does not necessarily indicate they preserve primary remanence. The remanence directions of all components revealed higher scatter than the previous studies, suggesting the need for caution in interpretation. The low unblocking temperature of tittanomagnetite suggests that if their remanence is truly primary, the rocks must have kept below ∼ 250 °C for ∼3.47 billion years.

    更新日期:2020-01-04
  • Extremely young melt infiltration of the sub-continental lithospheric mantle
    Phys. Earth Planet. Inter. (IF 2.615) Pub Date : 2019-10-23
    Simon Turner; Michael Turner; Bernard Bourdon; Kari Cooper; Don Porcelli

    It has long been inferred that mantle metasomatism and the incompatible element enrichment of the continents both require movement of melts formed by very low degree melting of the mantle. Yet establishing the presence of these melts and whether this process is on-going and continuous, or spatially and temporally restricted, has proved difficult. Here we report large U-Th-Ra disequilibria in metasomatised, mantle xenoliths erupted in very young lavas from the Newer Volcanics Province in southeastern Australia. The 226Ra-230Th disequilibria appear to require reappraisal of previous estimates for the age of eruption that now seems unlikely to be more than a few kyr at most. We propose that infiltration of carbonatitic melts/fluids, combined with crystallization of pargasite, can account for the first order U-series disequilibria observations. Irrespective of the exact details of the complex processes responsible, the half-lives of the nuclides require that some of the chemical and isotopic disturbance was extremely young (« 8 kyr) and potentially on-going at the time of incorporation into the alkali basalts that transported the xenoliths to the surface. This provides evidence for the presence and possibly continuing migration of small melt fractions (~0.02%) in the upper convecting mantle that may contribute to the seismic low velocity zone. By implication, it appears that the asthenosphere must lie close to its solidus, at least in this region. Pressure-temperature estimates indicate that the small degree melts identified could infiltrate as far as 25 km upwards into the sub-continental lithospheric mantle leading to strong incompatible element enrichment and the recent timing of this event this urges a reappraisal of the meaning of 300–500 Ma Nd model ages in mantle xenoliths from this region. In principle, the resultant metasomatised mantle could provide a component for some ocean island basalts, should the sub-continental lithospheric mantle be returned to the asthenosphere by convective removal at some later time.

    更新日期:2020-01-04
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