期刊,Chemical geology 2022年604卷期_国家学术搜索

期刊信息/Journal information

Chemical geology

Elsevier

主办单位：Elsevier

国际刊号：0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI

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Mercury (Hg) geochemistry of mid-ocean ridge sediments on the Central Indian Ridge: Chemical forms and isotopic composition

Jeong, DohyunKim, DongsungXu, ZhaokaiKim, Haryun...

11页

查看更多>>摘要：To geochemically characterize mercury (Hg) in the deep-sea ridge environment, the total concentration, chemical forms (sequential leaching extraction), and isotopic compositions of Hg in surface sediments from the middle portion of the Central Indian Ridge were investigated. Highly elevated Hg concentrations (up to 13,000 ng/g) in sediments near the hydrothermal vent are associated with intense hydrothermal activity driven by serpentinization. The hydrothermal impact on these sediments is also evident in the REECN fractionation pattern with a remarkably strong positive europium (Eu) anomaly. Most volcanic and hydrothermal Hg in the study area is preferentially precipitated with sulfides; in the hydrothermal vent area, however, scavenging by Fe-Mn hydroxides is another significant removal pathway of Hg. Thus, such precipitation and production of sulfides and hydroxides are a major cause of local enrichment of Hg around the mid-ocean ridge. Most sediments show limited or no mass-independent fractionation (Delta Hg-199 = +0.02 +/- 0.21 parts per thousand, 2 sigma, n = 15), indicating that syngenetic magmatic or mantle-derived materials are the dominant Hg source. However, the large variation in massdependent fractionation was observed, especially in the vent-distal sediments (delta Hg-202 = -1.10 +/- 0.80 parts per thousand, 2 sigma, n = 11), which occurred mainly during the formation of the sulfides and may be associated with preferential precipitation of lighter isotopes. Our study demonstrates that an off-axis high-temperature hydrothermal system driven by exothermic serpentinization of ultramafic mantle rocks may serve as a significant Hg source and provides further insights into grasping the behavior of hydrothermal and volcanogenic Hg in active deep-sea ridge systems.

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High molecular weight fractions of dissolved organic matter (DOM) determined the adsorption and electron transfer capacity of DOM on iron minerals

Li, YuanhangGong, XiaofengSun, YuhengShu, Yao...

10页

查看更多>>摘要：Dissolved organic matter (DOM) in the natural environment has a significant effect on the biogeochemical cycles of iron (Fe), such as adsorption and reduction reaction. However, the roles of the molecular weight (MW) fractions of DOM extracted from plants in adsorption onto Fe minerals coupled with the Fe reduction process are incompletely characterized. In this study, the effects of DOM (extracted from Carex cinerascens) with different MW fractions, labeled as low molecular DOM (L-DOM), medium molecular DOM (M-DOM), and high molecular DOM (H-DOM), respectively, on the adsorption onto Fe minerals and the electrons transfer capacity (ETC) during the Fe reduction process is investigated. The oxygen-rich and highly aromatic H-DOM exhibited stronger adsorption affinity onto Fe minerals, such as ferrihydrite, a poorly crystalline mineral, than other DOM fractions. The X-ray photoelectron spectra and the Fourier transform infrared spectra revealed that the functional groups of the oxygen-rich DOM, such as C???O bonds and O???C??????O bonds, preferentially adsorbed onto the Fe minerals. In addition, the cyclic voltammograms (CV) and the chronoamperometry (CA) measurements showed that H-DOM significantly enhanced the Fe reduction to its maximum extent, indicating that the efficiency of DOM on Fe reduction was strongly related to the ETC. Therefore, our findings confirm that H-DOM plays a key arole in the adsorption onto Fe minerals and Fe redox reaction. The results of this study may help evaluate the performance of the different MW of DOM from plants on the biogeochemical cycles of Fe in the soil environment.

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A possibility of O-18-depleted oceans in the Precambrian inferred from triple oxygen isotope of shales and oceanic crust

Bindeman, Ilya N.Kanzaki, Yoshiki

15页

查看更多>>摘要：Oxygen-18/16 ratio recorded in geological media has been utilized to reconstruct surface temperature and discuss associated climatological forcing through Earth's history. The accuracy of the estimated paleo-temperature through the application of the delta O-18 record, however, is limited by our understanding of the oceanic O-18/O-16 ratio in the past. As a longstanding issue, sedimentary rocks in the Precambrian, which are generally depleted in O-18 relative to the modern sediments, can be interpreted either to have reflected higher surface temperature when assuming relatively constant oceanic delta O-18 values, or more O-18-depleted oceans relative to the present day with limited global temperature (climate) deviations. Additional use of a third isotope, O-17, can help constrain the isotopic composition of the porewater during the formation of a rock record as it provides an additional fractionation equation within the same isotopic system. This study utilizes reactive-transport models of triple oxygen isotope (O-16, O-17, O-18) for continental weathering and hydrothermal alteration of oceanic crust, built upon the delta O-18 modeling framework developed by Kanzaki, 2020a, Kanzaki, 2020b. Comprehensive oxygen isotope exchanges in dynamically moving media, both rocks and porewaters, are realized in these models, reflecting spatially-variable and process-dependent alteration conditions (e.g., spreading rate of midocean ridges, uplift rate of continents, porewater advection rate, and isotope exchange kinetics). The models were confirmed to be able to reproduce the signatures of triple oxygen isotope in Phanerozoic shales and oceanic crust. The validated models were then applied to Precambrian shales, which suggests that the Precambrian oceans could have been depleted in O-18, e.g., as low as -12 parts per thousand at >2.5 Ga and - 4 parts per thousand around 0.5 Ga, relative to the present-day seawater. General absence of positive and near-zero Delta'O-17 signatures in oceanic crust also suggests that the buffering of oceanic O-18 through hydrothermal alteration of the oceanic crust could have been weak. The O-18 flux from continental weathering thus could have played a significant role in controlling the O-18/O-16 ratio of ancient sea and together with climatic forcing could have enabled a secular transition of oceanic delta O-18 during the Precambrian.

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Elsevier

Barium stable isotopic composition of chondrites and its implication for the Earth

Fang, LinruMoynier, FredericHuang, FangDeng, Gengxin...

9页

查看更多>>摘要：Barium is a refractory and fluid mobile element that is highly incompatible in basaltic systems. The elemental and isotopic compositions of Ba are used to trace fluid processes or sediment recycling during subduction. However, the Ba isotopic composition of primitive meteorites (chondrites), which represents the possible building blocks of terrestrial planets, is presently poorly known. Here, we report high precision Ba isotopic compositions of major types of chondrites using a double-spike method. Chondritic meteorites without clear terrestrial weathering show relatively homogeneous Ba isotopic distributions and have an average delta Ba-137/134 (the permil deviation of the Ba-137/Ba-134 ratio from the SRM3104a standard) value of 0.09 +/- 0.09 parts per thousand (25D, N = 14). Given the refractory and lithophile behavior of Ba and the limited variations of Ba isotope composition between various chondrite groups, this value can represent the Ba isotopic composition of the bulk Earth and the Bulk Silicate Earth (BSE). It is actually consistent with the present estimate of BSE (0.05 +/- 0.06 parts per thousand, Li et al., 2020; Nan et al., 2022) based on the measurements of mid-ocean ridge basalts and carbonatites. Small Ba isotope variations are observed between carbonaceous chondrite groups where CO and CV show slightly heavier (by 0.09 parts per thousand) Ba isotopic compositions than CI and CM. Significant light stable Ba isotopes enrichment has previously been documented in calcium, aluminum-rich inclusions (CAIs) in the Allende CV chondrite, and the Ba isotopic differences between CO and CV might result from the various proportions of CAIs based on a mass balance calculation. However, the parent body of CI and CM might have experienced aqueous alteration and secondary carbonates are most likely responsible for their light Ba isotopic signatures.

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Chromium isotope systematics in three mantle-derived domains of Central European Variscides: Relationship between 853Cr values and progressive weathering of serpentinized ultramafic rocks

Novak, MartinAndronikov, Alexandre, VSebek, OndrejKotkova, Jana...

21页

查看更多>>摘要：Most chromium (Cr) entering the ocean originates from continental weathering of ultramafic rocks. Recent data indicate that (i) mantle-derived rocks are characterized by homogeneous 853Cr values of approximately -0.12%o; (ii) some serpentinized peridotites, serpentinites and weathered serpentinites contain isotopically heavier Cr than mantle protoliths; and (iii) Cr transported by rivers is isotopically variable, but positive 853Cr values predominate. There is a need to identify solid materials that, in sufficient volumes, store the complementary isotopically light Cr. We studied 853Cr systematics in three mantle domains belonging to the Paleozoic Central European orogenic belt. These domains are characterized by contrasting P-T histories. At two of six study sites, a 30-m deep drilling was carried out to assess spatial gradients in Cr concentrations ([Cr]) and 853Cr values toward the weathering front. The studied rocks, comprising serpentinized peridotites with preserved mantle signature, serpentinites, carbonatized serpentinites, and hornblendites, were characterized by a variable degree of weathering. Spinel in these rock types contained up to 33 wt% Cr but represented less than 1% of the rock volume; chromite was not detected in any of the samples. Silicates, such as pyroxene, serpentine, hornblende and chlorite, with an average [Cr] of 0.24 wt.% were likely the predominating potential Cr source for the environment. Despite the known higher weathering rates of Cr-silicates compared to Cr-spinelides, no systematic trends toward lower [Cr] and higher 853Cr values were detected upcore. The overall range of whole-rock 853Cr values was from -0.25 to 0.14%o (mean of -0.09 ?? 0.01%o). There was no relationship between whole-rock 853Cr values and indices of alteration/weathering, such as water content, loss on ignition, and leaching of alkalies. We did not observe progressive removal of isotopically light Cr during continental weathering of serpentinized mantle-derived rocks that had been previously reported from altered and hydrated ultramafic rocks at some other sites. Our data suggest that such Cr isotope fractionation, if present, may be associated with earlier hydrothermal alteration rather than with weathering. Mean 853Cr values of soils were indistinguishable from the underlying ultramafic rocks, indicating only small sensitivity of Cr isotope composition to pedogenesis.

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The system KCl-CaCO3-MgCO3 at 6 GPa: A link between saline and carbonatitic diamond-forming fluids

V. Arefiev, AntonBekhtenova, AltynaLitasov, Konstantin D.Shatskiy, Anton...

13页

查看更多>>摘要：Alkaline chlorides and carbonates are abundant in melt inclusions in diamonds, mantle xenoliths, and igneous minerals from kimberlites. Despite this, the phase relationships in chloride-carbonate systems remain poorly constrained. Here we studied phase equilibria in the system KCl-CaCO3-MgCO3 in multianvil experiments at 6 GPa and 1000-1600 degrees C. It was found that at 1000 degrees C, subsolidus assemblage consists of KCl, magnesite, and aragonite. At higher temperatures, the stabilization of dolomite splits the system into two partial ternaries: KCl + dolomite + aragonite and KCl + magnesite + dolomite. Both ternaries start to melt near 1200 degrees C. The melting of the first ternary is controlled by the KCl-dolomite-aragonite eutectic situated at 30 mol% (24 wt%) KCl and Ca# 92, where Ca# = 100 center dot Ca/(Ca + Mg). The melting of the second ternary is controlled by the peritectic, KCl + dolomite = magnesite + liquid, producing chloride-carbonate melt with 33 mol% (27 wt%) KCl and Ca# 69. The established melting temperature of the KCl + Ca-Mg carbonate assemblages is 200-500 degrees C higher than the subduction geotherms. This indicates that chlorides and carbonates can survive subduction to a depth of 200 km. However, the stagnation and warming of slabs to the ambient mantle temperature must result in partial melting of KCl + Ca-Mg carbonate assemblages producing a KCl-rich low-Mg carbonate melt. Percolation of this melt into the subcontinental lithospheric mantle should yield crystallization of KCl and Ca-Mg carbonates. In the presence of water, KCl will form hydrous saline melt or fluid, while Ca and Mg carbonates, which are poorly soluble in water, will remain solid. The obtained results imply that the hydrous saline fluids (brines) found as inclusions in diamonds are a lower temperature derivative of mantle carbonatite melts and do not support the hypothesis of chloride melt generation owing to the chloride-carbonate liquid immiscibility since no such immiscibility was established.

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Mixing, fluid infiltration, leaching, and deformation (MILD) processes on the slab-mantle wedge interface at high T and P conditions: Records from the Dalrymple Amphibolite, Philippines

Valera, Gabriel Theophilus V.Kawakami, TetsuoPayot, Betchaida D.

27页

查看更多>>摘要：The whole rock compositions of the blocks and the surrounding matrix of the Dalrymple Amphibolite are investigated in this study to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite. Similarities in their rare earth and trace element patterns indicate the genetic relationship between the mafic lavas of the Palawan Ophiolite and the metamafic blocks of the Dalrymple Amphibolite. This confirms that the metamafic blocks represent the basalt to gabbro section of the oceanic lithosphere of the subducting slab. The matrix surrounding the blocks exhibit highly variable phase assemblages. In order to determine its petrogenesis, we distinguished groups of components/elements which behave similarly (Group 1 TiO2, Al2O3, Zr, Th and the light rare earth elements; Group 2 Cr, Ni and MgO) based on statistical (correlation coefficient) analyses. These groups indicate mixing of metasedimentary (Group 1) and metamafic (Group 2) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO2 and Al2O3) in the matrix samples were fitted against a modelled concentration by changing the end-member and their relative proportions. The end-members and mixing ratio with the highest regression value (r2) was selected to obtain the modelled composition of the matrix. The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO2, Al2O3, Cr, Nd, Zr, and Hf are immobile. This assumption is supported by the prevalence of kyanite, ilmenite and zircon in the matrix mineral assemblage. This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. In particular, the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones. Mass balance calculations further revealed that an early fluid infiltration event likely occurred following the mixing process. This preferentially leached out elements which are either fluid-mobile (e.g. CaO and SiO2) or are not incorporated into the growing minerals in the matrix. The strong control of mineral assemblage of the matrix in its chemistry is exhibited by a number of samples which showed variable degrees of losses and gains in elements traditionally interpreted to be fluid immobile (e.g. heavy rare earth elements and Y). A later hydration event linked to retrograde metamorphic stage imprinted gains of K2O, Rb, and Ba in the matrix samples with the growth of replacement minerals (e.g. muscovite on kyanite). This later fluid infiltration event possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction.

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Elsevier