Composition Of Crust Research Articles

Nitrogen deposition stimulated winter nitrous oxide emissions from bare sand more than biological soil crusts in cold desert ecosystem

Dryland ecosystems are often nitrogen-limited, and small nitrogen inputs may produce large responses to dryland ecological processes, such as gaseous nitrogen emission. The effect of increased anthropogenic nitrogen deposition on N2O and NO emissions in desert ecosystems is unclear, especially in non-growing seasons when the surface is covered with snow. In this study, nitrogen applications were performed on biological soil crusts (lichen crust and moss crust, bare sand for control) in the Gurbantunggut Desert, Northwest China. We measured the fluxes of N2O and NO and related nitrogen cycle functional gene abundances in winter for three-years period. Nitrogen addition significantly affected N2O emissions and increased the abundances of key functional gene for nitrogen cycle, while it only slightly influenced NO emissions. These effects of nitrogen addition depended on composition of biological soil crusts. For bare sand and lichen crust, nitrogen addition significantly increased N2O emissions, whereas for moss crust, only a negligible effect was observed. Meanwhile, significant differences in nitrogen cycle functional gene abundances were found among different composition of biological soil crusts. Abundance of amoA, narG, and nosZ genes were highly related to N2O and NO emissions. Thus, our results indicate that gaseous nitrogen emissions were generally increased by nitrogen addition through their effects on related functional microbial groups. The effects were regulated by composition of biological soil crusts which can buffer the effects of increasing nitrogen addition during winter.

The magnesium isotopic compositions of the crust and mantle: A study on the Oman ophiolite

Magnesium isotope compositions (δ26Mg) can provide valuable insights into Mg cycling through surface fluid-rock interactions. Isotopic variations of Mg within the oceanic lithosphere imply incorporation of isotopically different Mg, but the underlying mechanisms remain uncertain. In this study, we measure the Mg isotopic composition of the oceanic crust (n = 32) and mantle peridotite (n = 13) from the Oman ophiolite, which has experienced magmatism at the spreading center and subduction zone. The crustal section records δ26Mg values that range from −0.58 to −0.04 ‰. In the upper crustal section, higher Mg-bearing mineral abundance tends to have lighter δ26Mg values. However, the lower crustal section has no correlation with the δ26Mg values, indicating that the hydrothermal Mg sink formed by seawater was significant within the upper 2 km of the oceanic crust. In the mantle section, the δ26Mg values range from −0.39 ‰ to −0.07 ‰. Seven peridotite samples, with a loss of MgO by up to 5.3%, have slightly higher δ26Mg values than the global mantle, which may be attributed to low-temperature weathering. In contrast, three peridotite samples with higher melting degrees (spinel Cr# = Cr/(Cr + Al) atomic ratio) than the adjacent rocks are considered to have been affected by subduction-related interactions, forming lighter δ26Mg values. The isotopically light Mg source was subducted sediments, but its low concentration of Mg requires a water/rock ratio >> 1. Therefore, we conclude that the contribution of slab-derived Mg and modification of mantle δ26Mg values were localized.

Constant iron isotope composition of the upper continental crust over the past 3 Gyr

Constant iron isotope composition of the upper continental crust over the past 3 Gyr

Partially accreted crusts of neutron stars

Context. Neutron stars in low-mass binary systems are subject to accretion when material originating from the companion star accumulates on the surface. In most cases, the justified and common assumption in studying the properties of the neutron star crust is the fully accreted crust approximation. However, observations of some X-ray transient sources indicate that the original crust has not been completely replaced by accreted material, but is partly composed of the compressed original crust. Aims. The crust of an accreting neutron star beyond the fully accreted crust approximation was studied; a two-part (or hybrid) crust made of the original crust that is compressed and of the accreted material crashing onto it was reconstructed as a function of the accretion stage. The differences in the composition and energy sources for the fully accreted and hybrid crusts influence the cooling and transport properties. Methods. A simple semi-empirical formula of a compressible liquid drop was used to compute the equation of state and composition of the hybrid crust. Calculations were based on the single-nucleus model, with a more accurate treatment of the neutron drip point. We compared the nuclear reactions triggered by compression in the original crust and in the accreted matter part of the hybrid crust. We discuss another crust compression astrophysical phenomenon related to spinning neutron stars. Results. The compression of the originally catalyzed outer crust triggers exothermic reactions (electron captures and pycnonuclear fusions) that deposit heat in the crust. The heat sources are cataloged as a function of the compression until the fully accreted crust approximation is reached. The pressure at which neutron drip occurs is a nonmonotonic function of the depth, leading to a temporary neutron drip anomaly. The additional potential source of energy for partially accreted crusts is the occurrence of a density inversion phenomenon between some compressed layers. Conclusions. The original crust of a neutron star cannot be neglected for the initial period of accretion, when the original crust is not fully replaced by the accreted matter. The amount of heat associated with the compression of the original crust is on the same order of magnitude as that from the sources acting in the accreted part of the hybrid crust.

Alteration of basaltic glass within the Surtsey hydrothermal system, Iceland – Implication to oceanic crust seawater interaction

Low-temperature hydrothermal alteration of basaltic glass on the seafloor has important implications on the chemical evolution of the oceanic crust and seawater composition. However, mass fluxes resulting from seawater-glass interaction in this type of environment remain poorly understood. This study presents new results on element mobilities for the palagonitization of basaltic glass and bulk rock hydrothermal alteration at Surtsey volcano, Iceland over a time period of ~50-years. Assessments of element mobilities were based on 1) immobile trace element isocon mass balance for palagonitization, 2) the assumption of large scale TiO2 immobility during bulk rock alteration and 3) chemical analyses of pore fluids and authigenic minerals.Element mobilities of glass alteration vary between initial palagonitization and subsequent palagonite maturation: Initially, palagonitization of sideromelane leaches SiO2, Al2O3, MgO, CaO, Na2O, K2O from the glass. Following this, the maturation of the palagonitized glass causes re-uptake of all of the abovementioned elements except for CaO, which shows either no change or slight uptake, and Na2O, which continues to be lost from the palagonitized glass. Among major elements TiO2 and FeO can be considered immobile during palagonitization, but not during the subsequent aging process of palagonitized glass. Among trace elements Hf, Ta, Zr, Nb, Y and REE were found to be immobile at all stages of the alteration. On the bulk rock scale, comprising both glass alteration and the formation of authigenic phases, early-stage alteration at Surtsey releases SiO2, Al, Mg and Ca to the fluid, whereas late-stage alteration poses a sink of SiO2, Al, Mg, Ca and Na. Net element budgets during early- and late-stage alteration indicate that chemical exchange between basaltic tuffs and seawater derived fluids at Surtsey acts as a long-term source of dissolved Ca, Al and SiO2 for seawater as well as a long-term sink for seawater Mg and Na.

Earth’s geodynamic evolution constrained by 182W in Archean seawater

Radiogenic isotope systems are important geochemical tools to unravel geodynamic processes on Earth. Applied to ancient marine chemical sediments such as banded iron formations, the short-lived 182Hf-182W isotope system can serve as key instrument to decipher Earth’s geodynamic evolution. Here we show high-precision 182W isotope data of the 2.7 Ga old banded iron formation from the Temagami Greenstone Belt, NE Canada, that reveal distinct 182W differences in alternating Si-rich (7.9 ppm enrichment) and Fe-rich (5.3 ppm enrichment) bands reflecting variable flux of W from continental and hydrothermal mantle sources into ambient seawater, respectively. Greater 182W excesses in Si-rich layers relative to associated shales (5.9 ppm enrichment), representing regional upper continental crust composition, suggest that the Si-rich bands record the global rather than the local seawater 182W signature. The distinct intra-band differences highlight the potential of 182W isotope signatures in banded iron formations to simultaneously track the evolution of crust and upper mantle through deep time.

Petrogenesis of Eocene Wangdui adakitic pluton in the western Gangdese belt, southern Tibet: implications for crustal thickening

AbstractLower-crust-derived adakitic rocks in the Gangdese belt provide important constraints on the timing of Tibetan crustal thickening and on the relative contributions of magmatic and tectonic processes. Here we present geochronological and geochemical data for the Wangdui porphyritic monzogranites in the western Gangdese belt. Zircon U–Pb dating yields emplacement ages of 46–44 Ma. All samples have high Sr (321–599 ppm), low Yb (0.76–1.33 ppm) and Y (10.6–18.3 ppm) contents, with high La/Yb (51.1–72.3) and Sr/Y (21.0–51.4) ratios, indicating adakitic affinities. The low MgO (0.97–1.76 wt %), Cr (7.49–53.6 ppm) and Ni (4.75–29.1 ppm) contents, as well as high 87Sr/86Sr(i) (0.7143–0.7145), low ϵNd(t) (−10.4 to −9.8) and zircon ϵHf(t) (−17.7 to 0.4) values, suggest that the Wangdui pluton most likely originated from partial melting of the thickened ancient lower crust. In combination with previously published data, despite the east–west-trending heterogeneity of crustal composition in the Gangdese belt, the La/Yb ratios of magmatic rocks reveal that both western and eastern segments experienced remarkable crustal thickening in the Eocene. However, in contrast to the thickened juvenile lower crust in the eastern segment formed by the underplating of mantle-derived magmas, tectonic shortening plays a more crucial role in thickening of the ancient basement in western Gangdese. In fact, such Eocene-thickened ancient lower-crust-derived adakitic rocks are widely distributed in the central Himalayan–Tibetan orogen. This, together with the extensive development of fold–thrust belts, suggests that tectonic shortening might be the main mechanism accounting for the crustal thickening associated with the India–Asia collision.

Across and along-strike crustal structure variations of the western Afar margin and adjacent plateau: Insights from receiver functions analysis

We used teleseismic receiver function analysis to image the crustal structure beneath 24 broadband seismic stations densely deployed along two profiles traversing different structural units across the western Afar margin. Our high-resolution receiver function results image pronounced spatial variations in the crustal structure along the profiles and provide improved insights to understand how strain is partitioned in the crust during rifting. Beneath the western plateau next to northern Afar, the crust is likely felsic-to-intermediate in composition (average Vp/Vs 1.74), with a step like thinning of the crust from an average of 38 km beneath the western plateau to an average of 22 km beneath the marginal graben. Consistently thicker crust is observed beneath the southern profile (central Afar), showing four distinct regions of uniform crustal thickness: 1) an average crustal thickness of 42 km beneath the western plateau; 2) 34 km beneath the foothills area; 3) 28 km beneath the marginal graben and the wide extensional basin and 4) 21 km beneath the central rift axis. We use crustal thickness results to estimate a stretching factor β of 2.2 and 2.7 for central Afar and northern Afar respectively. Our estimated values are lower than β > 3.0 predicted from plate reconstructions, and we interpret that the variations are best explained by 2–5 km magmatic addition into the crust. The crustal composition beneath the southern profile is more complex with elevated Vp/Vs ratios ranging between 1.79 and 1.85 beneath the western plateau and marginal graben. This is consistent with a greater mafic component and best explained by crust altered by intrusions due to significant pre and syn-rift magmatic activity. Abnormally high Vp/Vs ratios of more than 1.90 are observed beneath the axial rift zone of central Afar, which most likely suggests the localization of partial melt within the crust.

Crustal Compositional Variations From Continental to Oceanic Domain: A VP/VS Ratio Study Across the Zhongsha Block, South China Sea

AbstractThe VP/VS ratio is an important property for understanding magmatic and tectonic processes at passive continental margins as it is an indicator of the crustal composition. To classify the dominant lithologies in the Zhongsha Block, South China Sea (SCS), we present a detailed VP/VS crustal model based on the independent tomographic inversion of P wave and S wave data. The average VP/VS in the crust of the Zhongsha Block is ∼1.77, indicating an overall felsic to intermediate composition lacking remnant magmatic intrusive rocks. The VP‐density relationship from gravity modeling suggests that the lower crust of the extended continental domain contains more greenschist and hence may have experienced metamorphism resulting from an elevated geotherm in the Northwest Sub‐basin either during the syn‐spreading or postspreading stage. The variability of the VP/VS ratio in the continental block is larger than that in the oceanic basin, showing distinct crustal properties. Several low VP/VS ratio anomalies (VP/VS < 1.7) were found near tectonic boundaries and are interpreted to either result from felsic metamorphism during an interval of rifting, or during the migration of magma along faults and cracks in the postrift period. VP/VS ratios occurring in concert with high VP anomalies in the continent‐ocean transition zone support a mafic composition of metapelitic granulite, which was either formed by magmatic intrusions or contact with mantle melting that stem from the upwelling of the asthenospheric mantle during the initial break‐up and onset of the seafloor spreading stage in the SCS.

Deciphering the State of the Lower Crust and Upper Mantle With Multi‐Physics Inversion

AbstractThe composition of the lower crust is a key factor in understanding tectonic activity and deformation within the Earth. In particular, the presence or absence of melt or fluids has strong control on tectonic evolution. Multi‐physics inversion results from the western United States indicate that tectonic inheritance plays a much stronger role in determining the location of melt in the lower crust than previously thought. Even in a currently active area such as the Yellowstone Hotspot, the results suggest that fluid dominated structures and fluid free regions are located directly next to each other. This is incompatible with the commonly used model of recent tectonic activity as a main controlling factor for the presence of fluids or melt. These results have global implications for how geophysical models are interpreted and how they can be related to geodynamic simulations.

Effects of phosphorus on partial melting of the Martian Mantle and compositions of the Martian Crust

Phosphorus is estimated to be ten times more enriched in the martian mantle compared to the terrestrial mantle. Yet, its effects on primary melt composition and melting phase relations in martian systems is unknown. We performed piston-cylinder experiments at a constant upper mantle pressure of 2 GPa and temperatures of 1210–1450 °C using a model martian primitive mantle composition with P2O5 content of 0 and 0.5 wt.%. All experiments produced an assemblage of olivine + orthopyroxene + melt ± pigeonite ± apatite ± spinel. Our experimental results, in combination with a previous study at similar P-T conditions and major element bulk composition but containing 0.2 wt.% bulk P2O5, show that the addition of phosphorus dramatically increases the abundance of more polymerized residual mineral such as orthopyroxene while decreasing the proportion of less polymerized residual phases such as olivine, especially at low extent of melting (∼11 wt.%). Such effects lead to lower SiO2 concentrations in the near-solidus melt by up to 10 wt.% for mantle P2O5 of 0.2 and 0.5 wt.%. Increasing bulk P2O5 to 0.5 wt.% also leads to slightly elevated CaO/Al2O3 ratio and increased FeO* concentration in mantle-derived melts with the latter likely due to formation of Fe-O-P complexes in the liquid. Our study suggests that elevated phosphorus in the martian mantle has important consequences regarding the composition and mineralogy of the crust, partly made with primary melts, and of the upper mantle. Because of elevated P, variably melt-depleted upper mantle of Mars is likely to be richer in orthopyroxene compared to the terrestrial mantle and the elevated P content is partly responsible for several geochemical attributes of martian basalts compared to those on Earth. Extrapolating our experimental results to a range of pressures, we suggest a depletion of P in the mantle through time, which likely contributed to major elemental compositional differences between ancient Gusev and Gale crater basalts and more recent martian meteorites.

Stable zirconium isotopic fractionation during alkaline magma differentiation: Implications for the differentiation of continental crust

Stable Zr isotopes have shown great potential in tracing magma differentiation. However, it remains unclear if different magmatic systems have similar Zr isotopic evolution patterns. Here we report high-precision Zr isotopic compositions of the well-defined Shidao alkaline complex (SAC), eastern China, which show small but important differences from the Hekla tholeiitic volcanic suite, Iceland. The SAC comprises monzodioritic, syenitic, and granitic intrusions with high alkali contents, representing a magmatic suite produced by various degrees of cognate magma differentiation. From mafic to felsic samples, the Zr concentrations and Zr/Hf ratios increase first and then decrease significantly at the inflection of ∼65 wt% SiO2, suggesting zircon saturation started at ∼65 wt% SiO2 during magma differentiation. The δ94Zr values (permil deviation of the 94Zr/90Zr ratio from IPGP-Zr standard) remain nearly constant at ca. +0.01‰ in samples with SiO2 < 65 wt% but then gradually increase with SiO2 in more felsic samples (from +0.05 to +0.63‰), indicating that the Zr isotope fractionation also occurs at ∼65 wt% SiO2. The Shidao alkaline and Hekla tholeiitic suites show zircon saturation at SiO2 ∼ 65 wt%, but their Zr isotopic evolution paths differ. The alkaline suite increases more slowly in δ94Zr compared to the tholeiitic one, with the former reaching the same δ94Zr value at SiO2 higher by ∼5 wt%. Our findings represent the first observation of differences in Zr isotopic evolution from different igneous systems (tholeiitic versus alkaline), which should be considered when interpreting Zr isotopic data of samples out of context, such as detrital zircons or sedimentary samples. Based on the more sluggish Zr isotopic enrichment in the alkaline series than the tholeiitic one, we re-interpret the Zr isotopic composition of the upper continental crust, which may simply be not felsic enough to be isotopically heavier.

Crustal Deformation Patterns in the Tibetan Plateau and Its Adjacent Regions as Revealed by Receiver Functions

ABSTRACT In the past decades, several mechanisms (e.g., the crustal flow and thin viscous sheet models) and combinations of them have been proposed to explain the uplift and outgrowth of the Tibetan plateau. These mechanisms, however, cannot account for all the features observed, and the issue is still highly debated. In this study, we computed receiver functions (RFs) at 254 permanent and portable stations in the Tibetan plateau and its adjacent regions to provide large-scale constraints for the crustal deformation. A harmonic analysis, which has been recently proposed to help distinguishing a dipping Moho from crustal anisotropy, was employed here to investigate the accurate crustal anisotropy. We further applied the H-κ stacking technique to determine the crustal composition using the RFs after harmonic correction or moveout correction. Our results showed the following: (1) Harmonic corrections on Ps phases led to average differences of 2.6 km and 0.04 in crustal thickness and in the VP/VS ratio, respectively; notably, the weighting factors of the Ps, PpPs, and PpSs + PsPs phases affected the uncertainties more obviously than the harmonic corrections. (2) The crustal deformation of the central–southern Tibetan plateau was controlled by the faulting and subduction of the Indian plate; meanwhile, faulting played a dominant role in the crustal deformation of the northern and eastern Tibetan plateau. (3) Widespread upper crustal or crustal shortening and thickening should have occurred beneath the entire study region, except for the Chuan–Dian block (which likely underwent tectonic extension). (4) Crustal partial melting might have enhanced the crustal anisotropy, and promoted the accumulation and release of strain, increasing the probability of fracture in the upper crust.

Crustal structure in and around the East Anatolian volcanic belt by using receiver functions stacking

This study aims to explore the crustal seismic properties across the East Anatolian volcanic belt. Teleseismic receiver functions obtained through deconvolution in the time domain are used to estimate crustal discontinuities and Moho interface. Further, slant stacked method, employed for the first time in the study region, is applied to identify the arrival times of P to S conversion and reverberations (Ps and PpPms) that give Vp/Vs ratio beneath each station. For slant stacking method, radial receiver functions are calculated using deconvolution based on frequency domain. Inversion results indicate that Moho depth varies from at most ∼40 km near the Bitlis-Zagros thrust zone to 45 km in the northeastward of Quaternary volcanic centers. The southwest of study region is characterized overall by a low average crustal Vp/Vs ratio (less than 1.71), implying a felsic crustal composition. A high average crustal Vp/Vs ratio of 1.82–1.87 is found near volcanic centers, characterized by predominantly a mafic crustal composition. The prominent crustal low-velocity zones especially around the Nemrut and Ararat volcanoes are identified below ∼20 km depth and likely imply the presence of partial melt zones or active magma chambers in the region. As a result, the findings of this study provide useful new constraints to the crustal thickness and composition in the study region.

Properties of the neutron star crust: Quantifying and correlating uncertainties with improved nuclear physics

A compressible liquid-drop model (CLDM) is used to correlate uncertainties associated with the properties of the neutron star (NS) crust with theoretical estimates of the uncertainties associated with the equation of state (EOS) of homogeneous neutron and nuclear matter. For the latter, we employ recent calculations based on Hamiltonians constructed using chiral effective-field theory ($\ensuremath{\chi}\mathrm{EFT}$). Fits to experimental nuclear masses are employed to constrain the CLDM further, and we find that they disfavor some of the $\ensuremath{\chi}\mathrm{EFT}$ Hamiltonians. The CLDM allows us to study the complex interplay between bulk, surface, curvature, and Coulomb contributions, and their impact on the NS crust. It also reveals how the curvature energy alters the correlation between the surface energy and the bulk symmetry energy. Our analysis quantifies how the uncertainties associated with the EOS of homogeneous matter implies significant uncertainties for the composition of the crust, its proton fraction, and the volume fraction occupied by nuclei. We find that the finite-size effects impact the crust composition but have a negligible effect on the net isospin asymmetry of matter. The isospin asymmetry is largely determined by the bulk properties and the isospin dependence of the surface energy. The most significant uncertainties associated with matter properties in the densest regions of the crust, the precise location of the crust-core transition, are found to be strongly correlated with uncertainties associated with the Hamiltonians. By adopting a unified model to describe the crust and the core of NSs, we tighten the correlation between their global properties such as their mass-radius relationship, moment of inertia, crust thickness, and tidal deformability with uncertainties associated with the nuclear Hamiltonians.

Rare earth element geochemistry in soils along arid and semiarid grasslands in northern China

BackgroundRare earth elements (REE) are a group of trace elements that behave geochemically coherently. REE fractionation patterns normalized to reference materials provide a powerful tool for documenting pedogenesis. In-soil processes are particularly difficult to illustrate with respect to contemporary and past climate conditions. In this study, we characterize the rare earth element (REE) contents in bulk soils and respective geochemical fractions (e.g., exchangeable, carbonate-bound, reducible, and oxidizable fractions) and to decipher the relationships between REE geochemistry components and climatic factors across a large-scale northern China transect (NCT).ResultsAcross the NCT, bulk REE concentrations ranged from 55.2 to 241.1 μg g−1 with a main portion in the residual fraction (49–79%), followed by oxidizable fraction (2–40%), reducible fraction (3–22%), carbonate-bound fraction (0.1–16%), and negligible exchangeable fraction. The REE contents of geochemical components (carbonate-bound, reducible, and oxidizable) in topsoils correlated to climate factors (mean annual precipitation, mean annual temperature, potential evaporation, and aridity index (AI)). The normalized abundances to the upper continental crust (UCC) composition show that the middle REE was generally enriched than the light REE and heavy REE in topsoils along the transect. The overall UCC-normalized bulk REE patterns in topsoils and subsoils were similar, characterized by weak negative Ce anomalies and positive Eu anomalies.ConclusionsOur data in topsoils and depth profiles collectively suggest that cycling of REE was primarily regulated by abiotic processes in area with AI < 0.2, while the biological effect on REE circulation in soil played a more effective role in area with AI > 0.3. The similar UCC normalized patterns in topsoils suggest that the REE was originated from a common source with limited influences from other sources (e.g., atmospheric dusts and anthropogenic contributions). Our results to some extent provide evidence for climatic influence REE distribution patterns both in topsoils and subsoils across the continental-scale transect. Our investigation gives insights into future studies on vertical REE mobility and its associated biogeochemical pathways.

Rare earth elements and yttrium as tracers of waste/rock-groundwater interactions

Increasing concentrations of Rare Earth Elements (REE) plus yttrium (REY) are entering the environment due to human activities. The similar chemical behaviour across the whole REY, i.e. the lanthanide series (lanthanum to lutetium) and yttrium, allows their use as tracers, fingerprinting rock-forming processes and fluid-rock interactions in earth science systems. However, their use in fingerprinting waste and particularly low-level radioactive waste has not received much attention, despite the direct use of REE in the nuclear industry and the traditional use of REE as proxies to understand the environmental mobility of the actinide series (actinium to lawrencium). The highly instrumented low-level radioactive waste site at Little Forest (Australia) allows a detailed REY study, investigating interactions with local strata, neighbouring waste forms and shallow groundwater flows. Groundwater samples and solids from cored materials were recovered from 2007 to 2012 from the study site and regional baseline sites in the same geological materials. The REY in water samples were analysed by automated chelation pre-concentration (SeaFast, ESI) followed by ICP-MS determination, while solid samples were analysed using Neutron Activation Analysis (NAA) and X-ray fluorescence scanning (ITRAX). Solid rocks showed no REY departed from typical Upper Crust compositions in either Little Forest or regional background sites. Shallow groundwater from ~4–5 m, at or slightly below waste trench levels, showed water-waste interaction as a marked enrichment, relative to shale-normalised patterns, in samarium, europium and gadolinium, with depleted yttrium. Leachate samples from the neighbouring urban landfill show different REY normalised patterns. REY distribution changes with depth through increased interaction with shales and sandstones. Variations in pH and redox conditions lead to widespread precipitation of Fe-hydroxides, which scavenge REY with differential uptake by precipitating solids, resulting in increases in Y and higher Y/Ho ratio in the groundwater along the flow path. Our study revealed that the Little Forest low-level radioactive waste has a REY fingerprint different to that of groundwater in surrounding land uses. REY can be used to fingerprint diverse waste sources, assess the mobility of lanthanides inferring the mobility of selected actinides, and to trace the fate of REY during groundwater recharge. The approach presented can refine source allocation and trace pollutant mobility in current and legacy urban, mixed and radioactive waste sites around the world.

Lithologic Structure of the Anninghe Fault Zone: Constraints From High-Pressure Wave Velocity Experiments

The P-wave velocities of typical rocks in the Anninghe fault zone under pressures of 50–600 MPa were systematically measured. The P-wave velocities of the felsic, intermediate, and mafic rock types under atmospheric pressure were 5.86, 6.06, and 6.50 km/s, respectively, with pressure coefficients of 2.19 × 10–, 3.80 × 10–4, and 4.03 × 10–4 km/s/MPa, respectively. The results were combined with deep-imaging seismic data to establish crustal rock composition models at different depths in the study area. The composition of the Anninghe crust is very different in the horizontal and vertical directions. The most notable feature in the vertical direction is that the lithologic compositional change is gradual rather than abrupt with increasing depth. In the middle and upper crust, shallower than 25 km, the lithologic difference between the southern and northern sections of the Anninghe fault zone is primarily that the rocks in the southern section (Xichang) are more felsic than those in the northern section (Shimian).

Tectonic burial of sedimentary rocks drives the building of juvenile crust of magmatic arc

AbstractContinental arcs grow primarily by addition of mantle-derived magmas, thus forming juvenile crust, although geophysical evidence, alongside field investigation of exhumed terranes, show that supracrustal rocks are common components of the lower portions of continental arcs. The mechanisms by which metasedimentary rocks are transported to the deep arc crusts and their contributions to the juvenile arc crusts are ambiguous. Here, we conduct a systematic petrological, geochronological, and geochemical study of pelitic migmatites within Late Cretaceous meta-gabbros from the lower crust of the eastern Gangdese arc, southern Tibet. Our results show that the pelitic migmatites were derived from the Late Carboniferous sedimentary rocks of the upper arc crust, have significantly enriched Sr-Nd-Hf-O isotopic compositions, and underwent Late Cretaceous (95–80 Ma) high-pressure granulite-facies metamorphism and partial melting at ~850 °C and 15 kbar to generate voluminous granitic melts. This indicates that the eastern Gangdese arc underwent significant crustal thickening and chemical differentiation during the final stages of subduction of the Neo-Tethys. We suggest that the metasedimentary rocks were transported into the lower crust of the Gangdese arc by underthrusting of arc crust and accretion of mantle-derived magma, and the deeply buried supracrustal rocks altered the petrological constitution and chemical compositions of juvenile lower arc crust, and assimilated the mantle- and juvenile crust-derived melts. We propose that tectonic burial of sedimentary rocks is a key mechanism driving the building and reworking of juvenile crust of magmatic arcs throughout most of geological time.

Deep Crustal Structure Beneath the Pamir–Tibetan Plateau: Insights From the Moho Depth and Vp/Vs Ratio Variation

The Cenozoic convergence between India and Asia has created Earth’s thickest crust in the Pamir–Tibetan plateau, leading to broadly distributed deformation and extensive crustal shortening; however, the crustal deformation of the high plateau is still poorly constrained. The variation of the Moho topography and crustal composition beneath the Pamir–Tibetan plateau has an important correlation with the major tectonic units. In this study, the results of the receiver functions have been reviewed and analyzed to observe variations in the Moho depth and crustal Vp/Vs ratio beneath the Pamir–Tibetan plateau. We found a notable SE–NW-oriented deep Moho interface that starts from the southeast of the Tibetan plateau and continues to the eastern Pamir with a northward dipping direction, which may indicate the northern frontier of the decoupled lower crust of northward underthrusting of the Indian plate. In contrast, the deepest Moho beneath the Pamir plateau has a southward dipping direction indicating the southward underthrusting Asian plate. In general, the average crustal Vp/Vs ratio is relatively low beneath the South-Central Pamir (∼1.70), while it is relatively higher (∼1.75) under the Himalaya–Lhasa terrane, suggesting more felsic to intermediate rock composition with locally high values indicating a low-velocity zone, possibly caused by partial melting. Elevated Vp/Vs ratios are observed beneath the northern Pamir (&amp;gt;1.77) and Qiangtang and Songpan–Ganze terranes (&amp;gt;1.80), which can be related to the high mafic rock content and upwelling hot materials from the upper mantle. The Vp/Vs ratio beneath the Pamir–Tibetan plateau presents complex north–south variations with a relatively low crustal Vp/Vs ratio in the south, while it gradually increases toward the north of the Pamir and central-northern Tibet, which is probably caused by the joint effects of the northward underthrusting Indian lower crust and southward subduction of the Asian plate, the low-velocity zones within the mid-upper crust, and substantial crustal shortening and thickening. The low to average crustal Vp/Vs ratio throughout the plateau (except the central Tibet) indicates a limited amount of hot materials to support the low crustal channel flow model, instead suggesting that crustal thickening and shortening is the main uplifting mechanism of the Pamir–Tibetan plateau.