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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    Matrix effects during in situ U-Pb dating of perovskite with variable crystal structure: Evidence from the Tazheran Massif, Russia

    Sun, JingWu, Fu-YuanSklyarov, EugeneSarkar, Chiranjeeb...
    13页
    查看更多>>摘要：U-Pb geochronology of perovskite is one of the most reliable means of dating notoriously difficult to date and economically important kimberlites, carbonatites, and skarns. Matrix effects-isotopic variations due to chemical composition and crystal structure-are known to affect in situ U-Pb dating, but have not been documented in perovskite. Perovskite from the Tazheran Massif, Russia, has become a commonly used standard for U-Pb dating, so rigorously testing its general applicability is therefore critical. In this study, comparison between U-Pb ages generated by precise isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and in situ dating methods including both secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) for a series of perovskite crystals from the Tazheran Massif (Tazh-1, Tazh-2, Tazh-3, Tazh-7B1, and Tazh-7Br) suggests that the in situ ages of some perovskite crystals (Tazh-1, Tazh-7B1, and Tazh-7Br) are systematically offset from their corresponding ID-TIMS ages. Hence, not all Tazheran perovskite grains are suitable as standards and some previously published work may suffer from unrecognized inaccuracies. Various factors are investigated in an attempt to identify the origin(s) of the matrix effect biasing the in situ measurements. Although there are so far only a limited number of perovskite grains analyzed by all SIMS, LA-ICPMS, and TIMS methods, grains Tazh-1, Tazh-7B1, and Tazh-7Br all have extremely low FeO, Nb, and LREE contents compared with the other Tazh perovskite and well characterized reference materials (AFK-5 and Ice River), indicating that such chemical variability may cause analytical differences. Furthermore, and perhaps more importantly, the Tazh-1, Tazh-7Bl, and Tazh-7Br perovskite grains are cubic and thus have different crystal structure from most Tazh perovskite and their standards that are orthorhombic. Therefore, crystal structure could be the principal reason for the matrix effect, with secondary matrix effects due to differences in chemical composition, that need to be taken into account in future in situ U-Pb dating of perovskite.
      原文链接:
    • NSTL
    • Elsevier

    Silver complexation in chlorine- and sulfur-rich hydrothermal fluids: Insight from ab initio molecular dynamics simulations

    Lai, FengZou, ShaohaoXu, Deru
    17页
    查看更多>>摘要：Silver (Ag) is a precious metal and has been widely used in a variety of high-technology areas, and it is mainly mined from the various of hydrothermal deposits. To quantify changes of silver solubility in hydrothermal fluids, many previous studies were focused on the thermodynamic properties of silver complexation with single ligands under different P-T conditions. Recent studies have revealed that the mixed complexes of metals (including Au, Ag and Cu) are of great significance in improving their transportation efficiency. However, the behavior of mixed silver complexes is not substantially constrained by different ore formation conditions. In this study, eight ab initio molecular dynamics (MD) simulations were designed to investigate the stoichiometries and relative stabilities of AgCl(HS)(-) and Na[AgCl(HS)] complexes in solution boxes containing various Cl- and HS- concentrations within the temperature range 25 to 900 degrees C and under pressures above the water-saturation pressures. The results revealed that the AgCl(HS)(-) and Na[AgCl(HS)] complexes remained relatively stable at temperatures between 25 and 600 degrees C and 900 degrees C, respectively. The complexes were distorted linearly, and the degree of distortion increased with increasing the solution temperature. The thermodynamic properties of the AgCl(HS)(-) and Na[AgCl(HS)] complexes were obtained using ab initio thermodynamic integration under T-P conditions corresponding to those of MD simulations. The logK values for the formation of the AgCl(HS)(-) complex at 25-600 degrees C were obtained for the first time. Ag+ + Cl- + HS- = AgCl(HS)(-) The logK values for this reaction at 25 degrees C, 200 degrees C, 350 degrees C, and 600 degrees C were 13.12, 10.52, 10.46 and 10.13, respectively. The logK value for the formation of the Na[AgCl(HS)](-) complex at 900 degrees C was obtained to compare it with that of a the previous study. Ag+ + Cl- + HS- + Nat = Na[AgCl(HS)] The standard Gibbs energy for formation of Na[AgCl(HS)] complex in our study is highly consistent with that in a the previous report from Yin and Zajacz (2018). In addition, the newly obtained thermodynamic properties for the AgCl(HS) - complex were then used to build equilibrium models, which suggested that the stability of the AgCl(HS)(-) complex increases progressively with increasing temperature. Water-rock interaction modeling with and without the addition of this mixed-ligand complex indicated that the AgCl(HS)(-) complex plays at most a minor role in the transport of silver at temperatures below 200 degrees C but could be a predominant complex of silver in geofluids at elevated temperatures.
      原文链接:
    • NSTL
    • Elsevier

    Nitrogen isotope evidence for stratified ocean redox structure during late Ediacaran to Cambrian Age 3 in the Yangtze Block of South China

    Wu, YaowenTian, HuiJia, WangluLi, Jie...
    17页
    查看更多>>摘要：The Ediacaran-Cambrian (E-C) transition witnessed profound biological and oceanic changes, among which the appearance of explosive radiation of skeletonized animals is considered as the most remarkable one. Although the prominent rise of oxygen concentration in atmosphere and ocean is considered to be the major trigger of the "Cambrian Explosion", the links between the oxygenated process of deep ocean and the diversification of Cambrian metazoans are still under debate. In this study, delta N-15 and delta C-13(org) values, TOC and TN contents, as well as trace elements concentrations of fresh shale core samples were analyzed to explore the redox condition and nitrogen cycle of deep oceans and their relationships with biological evolution. The Mo concentrations, Mo-U covariations and Th/U ratios of our samples indicate suboxic/anoxic waters in the mid-depth ocean during the late Ediacaran to Cambrian Fortunian age, widespread anoxic/euxinic waters with temporary sulfidic waters during Cambrian Age 2 to early Age 3, and suboxic waters again during the middle to late Age 3. The positive delta N-1(5) values from late Ediacaran to Cambrian Age 2 indicate a large NO3- reservoir with a deep chemocline. In contrast, the low delta N-15 values (<0%) in the deepwater slope regions suggest a dominant biological N 2 -fixation, and/or partial NH4+-assimilation in the photic zone during Cambrian Age 3. Integrated delta N-1(5) values of this study and early publications suggest a continuous redox stratified paleo-ocean with anoxic/suboxic deep waters from late Ediacaran to Cambrian Age 3. It is also worthy to note that although the deep ocean may not experience substantial oxygenation during the main phase of Cambrian diversification, the expanding oxic surface waters and supply of bioavailable fixed nitrogen may satisfy complex high O-2-demanding animals, and thus at least have promoted the Cambrian biodiversity in shallow waters.
      原文链接:
    • NSTL
    • Elsevier

    Magma evolution of the South China Sea basin from continental-margin rifting to oceanic crustal spreading: Constraints from In-situ trace elements and Sr isotope of minerals

    Chen, Shuang-ShuangChen, JingGuo, ZhengfuWu, Tao...
    33页
    查看更多>>摘要：The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulkrocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E-MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in Sr-87/Sr-86 ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopymxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt-rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies.
      原文链接:
    • NSTL
    • Elsevier

    The use of Laser Induced Breakdown Spectroscopy for the mineral chemistry of chromite, orthopyroxene and plagioclase from Merensky Reef and UG-2 chromitite, Bushveld Complex, South Africa

    Meima, Jeannet A.Rammlmair, DieterJunge, Malte
    15页
    查看更多>>摘要：A new approach using scanning Laser Induced Breakdown Spectroscopy (LIBS) was developed to unravel cryptic variations in the mineral chemistry of chromite, orthopyroxene, and plagioclase in drill core from Merensky Reef and UG-2. The LIBS method provides continuous spatially resolved chemical data, elucidating variations in mineral chemistry and in element partitioning, taking into account textural features of the rock itself, of individual minerals, and of immediate neighbourhoods. Since the method is relatively fast and easy to apply to numerous core sections, it is of special interest when comparing the lateral variability of diagnostic horizons such as Merensky Reef and UG-2. This approach for measuring Mg/Fe, Cr/Al and Ca/Na ratios is based on the strength of LIBS in detecting chemical variations using intensity ratios within a single matrix, which in this application is one single particular type of mineral phase. Supervised classification followed by k-means clustering was applied to identify those spectra that consist of pure chromite, orthopyroxene, and plagioclase. Measurements and data processing strategies were focused on minimizing the standard deviation in the LIBS-based ratios to be able to detect relatively small variations in mineral chemistry. The LIBS-based Cr/Al and Mg/Fe ratios were successfully validated with respect to electron probe microanalysis on multiple polished sections. Distinct trends could indeed be extracted from a continuous LIBS measurement of 6 m drill core through the Merensky Reef. The data correlate with whole-rock geochemistry and hydrous phases such as phlogopite. The LIBS-based approach opens up unique insights into chemical and mineralogical expressions of large- and local-scale processes. This allows unique and detailed interpretations relevant for the genesis of mineralizations and metal distributions, based on a comprehensive dataset.
      原文链接:
    • NSTL
    • Elsevier

    Magmatic-hydrothermal zircons in syenite: A record of Nb-Ta mineralization processes in the Emeishan large igneous province, SW China

    Zeng, ZhiyaoLiu, Yan
    21页
    查看更多>>摘要：Zircon is a common accessory mineral in igneous rocks and can provide a record of radiation damage and hydrothermal alteration through changes in oxygen isotopic values, crystal structures, and geochemical and petrographic characteristics. Syenites with NbTa mineralization have been discovered recently in the Panxi region, which is part of the ca. 260 Ma Emeishan large igneous province in SW China. Four types of zircon were identified in samples of altered syenite from the Baicao and Luku deposits in the Panxi region. Type I zircons are fresh, similar to magmatic zircons from the Baima syenite. Type II zircons crystallized later than those of Type I and underwent weak hydrothermal alteration. Type III zircons were strongly affected by ore-forming fluids. Type IV zircons were precipitated from ore-forming fluids and occur as dipyramidal zircons or rims on Type III zircons. Type III zircons have higher Th, Hf, rare earth element (REE), Y, Pb, Nb, and Ta contents (24,261, 11978, 5597, 9080, 476, 1511, and 160 ppm, respectively) than Type I zircons (267, 918, 1344, 16.2, 5.66, and 2.16 ppm, respectively), Type II zircons (570, 8768, 1808, 3122, 42.7, 42.3, and 15.9 ppm, respectively), and Type IV zircons (2115, 9304, 3123, 5580, 61.5, 153, and 19.4 ppm, respectively). The measure D-a(T) is used to quantify the radiation damage undergone by zircon. A comparison of D-a(T) values, trace-element contents, and Raman characteristic peaks of the four types of zircon shows that chemical incorporation of trace elements into the zircon lattice has a greater impact than radiation damage on lattice distortion. Radiation damage and fluid alteration has affected the UPb isotopic system of the Type III zircons, meaning that the UPb ages of these zircons are unreliable. Similarly, U-Pb ages for the Type II and Type IV zircons are unreliable, due to their high U and high common Pb contents, respectively. Therefore, only the ages of Type I zircons are robust. U-Pb dating of Type I zircons yielded similar ages for mineralized and unaltered syenites (257 +/- 0.8 and 259 +/- 0.7 Ma, respectively). The Nb-Ta mineralization of the Baicao and Luku deposits can be divided into four stages on the basis of the morphologies, element contents, and oxygen isotope values of the four types of zircon. Type I zircons represent the initial magmatic state of the syenites (stage I); Type II zircons represent the gradual evolution of magma and initial hydrothermal fluid activity (stage II); Type III zircons represent a fluid stage in which NbTa was enriched in large quantities but without mineralization (stage III); and Type IV zircons represent the mineralization stage, during which pyrochlores crystallized (stage IV). The geochemical characteristics of the Type III and IV zircons suggest that highly evolved ore-forming fluids became gradually more enriched in F, Ca, REE, Y, Th, Zr, Hf, Nb, Ta, and Pb. Furthermore, the lower delta O-18 values of Type II, III, and IV zircons compared with those of Type I show that meteoric water was added to the ore-forming fluid. Our findings may be more widely applicable to the development of other syenite-related deposits and their hydrothermal systems.
      原文链接:
    • NSTL
    • Elsevier

    Arc magmatism and porphyry-type ore deposition are primarily controlled by chlorine from seawater

    Castillo, Paterno R.
    18页
    查看更多>>摘要：A better understanding of magmatism and ore metal deposition at convergent plate margins is essential to the safety and benefit of society. Despite their spatial and, to a certain extent, temporal association, a direct connection between these two geologic processes is enigmatic. A detailed evaluation of available data reveals that seawater is key to the connection between these processes as it is the source of the ubiquitous, hot aqueous fluid from the subducting slab that is central to the generation of primary arc magmas and transfer or recycling of crustal materials at convergent margins. Particularly regarding the transfer of fluid-mobile elements or the so-called subduction component from the slab to arc lavas, however, the slab-derived hydrothermal fluid is perceived as weak and, thus, other transfer agents are proposed. Most petrogenetic studies also do not connect the source and transfer of metals in the spatially associated ore deposits to the same fluid. Chlorine from seawater can provide the crucial link between the two processes. The presence of Cl makes the slab-derived fluid a potent solvent not only to transfer but also to solubilize both the subduction component and ore trace metals from the slab, mantle wedge and lower-middle arc crust to arc lavas and ore deposits, respectively. Moreover, the dissolution and mobility of many elements in the Cl-rich fluid are primarily dependent upon their respective ionic potential (i.p.) values. In general, elements with cations that have low (soft cations) and high (hard cations) i.p. are mobile whereas those with in-between (intermediate cations) values are immobile in the Cl-rich fluid. Subducted sediment provides the bulk of subduction component and ore metals. These results led to the creation of a unified conceptual model claiming that the mobility, or immobility, of almost all elements in hydrothermal fluids at or near the Earth's surface is primarily dependent upon the amount of Cl in the fluids.
      原文链接:
    • NSTL
    • Elsevier

    The impact of outgassing of CO2 and prior calcium precipitation to the isotope composition of calcite precipitated on stalagmites. Implications for reconstructing climate information from proxies

    Dreybrodt, WolfgangFohlmeister, Jens
    13页
    查看更多>>摘要：Degassing of CO2 and precipitation of calcite to the surface of stalagmites can strongly impact isotope signals imprinted into the calcite of these speleothems. Here, we show that in all the variety of conditions occurring in nature only two distinct types of degassing exist. First, when a thin film of calcareous solution comes in contact to cave air, which has a lower p(CO2) value than that of the aqueous CO2 in the water, molecular CO2 escapes by physical diffusion in several seconds. In a next step lasting several ten seconds, pH and DIC in the solution achieve chemical equilibrium with respect to the CO2 in the cave atmosphere. This solution becomes supersaturated with respect to calcite. During precipitation for each unit CaCO3 deposited one molecule of CO2 is generated and escapes from the solution. This precipitation driven degassing is active during precipitation only. We show that all variations of out gassing proposed in the literature are either diffusive outgassing or precipitation driven degassing and that diffusive outgassing has no influence on the isotope composition of the HCO3- pool and consequently on that of calcite. Its isotope imprint is determined solely by precipitation driven degassing in contrast to most explanations in the literature. We present a theoretical model of delta C-13 and delta O-18 that explains the contributions of various parameters such as changes in temperature, changes of p(CO2) in the cave atmosphere, and changes in the drip intervals to the isotope composition of calcite precipitated to the apex of the stalagmite. We use this model to calculate quantitatively changes of delta C-13 and delta O-18 observed in field experiments (Carlson et al., 2020) in agreement to their experimental data. We also apply our model to prior calcite precipitation (PCP) in the field as reported by Mickler et al. (2019). We discuss how PCP influences isotope composition signals. In summary, we present a transparent method based on few commonly accepted equations that allows calculation of the isotope composition delta C-13 and delta O-18 of CaCO3 under various temperatures, p(CO2) in the cave air, degrees of PCP, and concentrations c(Ca)(o) of the water entering the cave.
      原文链接:
    • NSTL
    • Elsevier

    Evidence from gas-rich ultramafic xenoliths for Superplume-derived recycled volatiles in the East African sub-continental mantle

    Abebe, TsegeyeHopp, JensBarry, Peter H.Castillo, Paterno R....
    19页
    查看更多>>摘要：Volatile-rich fluids are believed to play a key role in the metasomatic enrichment of the East African subcontinental lithospheric mantle (SCLM), but limited data is available on key volatile tracers such as CO2 and N-2. We report new CO2 and N-2 isotope and relative abundance data in exceptionally gas-rich mantle peridotite and pyroxenite xenoliths from nine localities throughout the eastern branch of the East African Rift System (EARS). Importantly for our study, the xenolith localities straddle the transition between continental and oceanic lithosphere. In addition, we report major and trace element contents together with oxygen isotopes on host crystals and bulk multiple sulfur isotope data on select pyroxene crystals from the same xenolith suite. These new data are interpreted alongside previously published He, Ne and Ar isotope and relative abundance data. Pyroxenite xenoliths formed from the infiltration and crystallization of mafic melts in the lithospheric mantle. This was followed by fluids fracturing through the pyroxenites leaving gas-rich fluid inclusions. The elemental and isotope systematics of the fluid inclusion-hosted volatile species (He, Ar, N, CO2) within the mantle xenoliths are consistent with the enrichment of the EARS SCLM by CO2-rich mantle fluids from subducted carbonate-rich material. Such CO2 enrichments (CO2/He-3 > 7 x 10(9), delta C-13 similar to 0 parts per thousand) are also associated with positive delta N-15 values (as high as +3.4 parts per thousand), reinforcing the link between the metasomatic fluids and subduction of hydrothermally altered oceanic crust. Recycled signatures are also consistent with oxygen and sulfur isotope compositions of host crystals, which are distinct from compositions typical for the depleted upper MORB-source mantle. The occurrence of recycled volatile signatures in the EARS SCLM is widespread and must therefore be associated with a large-scale mantle process currently supplying material to EARS magmas. Notably, mantle xenoliths with high He-3/He-4 from the Ethiopian Rift are also associated with delta N-15 and delta C-13 values similar to the composition of sediments. This suggest that recycled materials have been entrained within the deep-seated African Superplume, which supplies the EARS with both primordial and recycled volatiles.
      原文链接:
    • NSTL
    • Elsevier

    Chemical isolation and isotopic analysis of terrigenous sediments with emphasis on effective removal of contaminating marine phases including barite

    Wilson, Paul A.Jewell, Amy M.Cooper, Matthew J.Milton, J. Andrew...
    12页
    查看更多>>摘要：The radiogenic isotope composition (e.g. Sr-87/Sr-86 and epsilon(Nd)) of the terrigenous fraction is commonly used to determine the provenance and transport history of sediments. In ocean sediments, the terrigenous fraction must be isolated prior to analysis to avoid contamination by marine phases. While the removal of calcium carbonate, organic matter, biogenic opal, authigenic coatings and adsorbed cations is widely advised, steps to remove all these phases are not consistently employed, and there is no standard procedure for the effective removal of marine barite. Marine barite is found throughout the world's oceans, particularly in regions of high export production, is highly enriched in strontium (Sr) and therefore has the potential to bias measurements of terrigenous Sr-87/Sr-86 towards seawater. Here we present results from sequential leaching experiments on marine sediments from the Atlantic and the Pacific Oceans to quantitatively determine the contaminating effect on Sr-87/Sr-86 and epsilon(Nd) exerted by the presence of barite and develop a protocol to successfully isolate the terrigenous fraction. Our results show that marine barite can skew measurements of terrigenous Sr-87/Sr-86 by >0.01 towards the Sr-87/Sr-86 value of seawater, and we highlight the importance of assessing the location-specific impact of barite contamination on terrigenous Sr-87/Sr-86. Our results show that marine barite can severely contaminate terrigenous Sr-87/Sr-86, even where marine barite accumulation rates are modest (< similar to 5 mg/m(3)/yr and <similar to 20 g C/m(2)/yr, respectively). Barite contains epsilon(Nd) in very low concentrations so barite contamination of terrigenous cm is not a major concern but an important aspect of our protocol is that it has minimal effect on terrigenous epsilon(Nd), allowing Sr-87/Sr-86 and cm to be analysed on identically cleaned aliquots. Our systematic graduated approach will help standardise Sr-87/Sr-86 data, allow for robust comparisons between sites, and will contribute to an improved understanding of the provenance, transport and weathering histories of windblown continental dust and riverine sediments.
      原文链接:
    • NSTL
    • Elsevier