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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    Geochemistry of mafic-ultramafic rocks of the 3.33 Ga Kromberg type-section, Barberton greenstone belt, South Africa: Implications for early Earth geodynamic processes

    Grosch, E. G.Ndlela, S.Murphy, D.McLoughlin, N....
    11页
    查看更多>>摘要：In conjunction with major, trace and rare earth element data, this study presents the first whole-rock Lu???Hf isotope analyses of mafic-ultramafic rocks of the c. 3.3 Ga Kromberg type-section in the Barberton greenstone belt of South Africa. Three compositionally distinct volcanic rock types are identified namely Group 1 metabasalts, Group 2 metabasalts and komatiitic metabasalts. Group 1 metabasalts are Fe???Mg tholeiitic with compositions that plot close to the Primitive Mantle composition, have isochron intercept ??Hf(t) value of 0.01 ?? 0.4 and Eoarchean Hf model ages. Group 2 metabasalts have very high Al2O3/TiO2 ratios (c. 50), small negative Nb anomalies, LREE-depleted slightly concave up REE patterns, and negative ??Hf(t) values. The komatiitic basalts variably record small negative Nb anomalies in a primitive mantle normalized plot. Group 1 metabasalts and cumulate peridotites (wehrlites) yield a Lu???Hf isochron age of 3.340 ?? 0.190 Ga (95% confidence level). The highly variable major and trace element geochemistry of the rocks in the sequence implies highly dynamic and variable mantle melting environments. The Group 2 metabasalts have geochemical characteristics similar to that of boninites and are interpreted to have formed from a shallow, refractory mantle source. Group 2 metabasalts are argued to have formed in a proto-plate tectonic back-arc basin involving crustal contamination, whereas Group 1 metabasalts likely formed from within a deep mantle plume.
      原文链接:
    • NSTL
    • Elsevier

    The zinc isotope composition of late Holocene open-ocean marine sediments

    Dickson, Alexander J.
    9页
    查看更多>>摘要：The controls on Zn burial fluxes into marine sediments are not well constrained by existing datasets. To address this problem, new Zn-isotope data have been generated from a globally distributed array of late-Holocene age sediments that accumulated in open ocean settings underneath a diverse range of depositional conditions (Namibian margin, West African margin, Arabian margin, Californian margin, Peruvian margin, Southern Ocean, Indian Ocean, North and South Atlantic Oceans, South China Sea). The mean non-detrital isotopic composition of Zn in the whole dataset (??66/64Zn = 0.37 ?? 0.25 %o, 2. S.D., n = 33) is only slightly lower than the modern deepocean composition (0.45 ?? 0.14 %o). Locations with total organic carbon >1 wt% have greater non-detrital enrichments of Zn and higher Zn isotope compositions than organic-poor sites, although the uncertainties overlap. Zinc associated with organic matter burial (using Zn/C ratios) and/or oxyhydroxide burial (using Zn/Fe and Zn/Mn ratios) account for a combined total of -16???73% of the total non-detrital Zn content of the investigated sediments, therefore requiring an additional burial flux of Zn to balance the sedimentary budget. This flux is probably related to ZnS formation although Zn incorporation into authigenic clays is also possible. The new data indicate that open-ocean marine sediments track the deep-ocean Zn isotope composition to within 0.1???0.2 %o, except in areas where isotopically distinct Zn fractions dominate the non-detrital Zn budget. The fraction of Zn removed into different sedimentary components is used to calculate Zn residence times ranging from -2.7 kyr to 13.5 kyr that are within the range of previous estimates based on input fluxes to the ocean.
      原文链接:
    • NSTL
    • Elsevier

    Carbon stable isotope constraints on CO 2 degassing models of ridge, hotspot and arc magmas

    Aubaud, Cyril
    27页
    查看更多>>摘要：Carbon dioxide emissions from volcanoes are important parameters to constrain in order to fully understand the Earth system, especially the effect of volcanic forcing on climate. The characterization of carbon concentration in magmas has been used to constrain volcanic fluxes. Because of low CO2 solubility in silicate melts, however, CO2 is significantly degassed from magmas due to decompression during transfer to the surface. The measurement of the carbon stable isotope ratio (13C/12C expressed as delta 13C-values) in natural submarine glasses has been a helpful geochemical tool to study magma degassing. Carbon stable isotope fractionation at magmatic temperature between CO2 in vesicles and carbonate ions dissolved in the melt is still large enough to cause variations in delta 13Cvalues. This variability can be used to deduce the mode of degassing (open vs. closed system, equilibrium vs. kinetic) operating in a given magmatic system. In this study, I present a review of the existing carbon isotope data for magmas of three different settings (ridge, hotspot and arc). This review allows to (1) investigate the diversity of degassing modes operating within a given setting and (2) compare the prevailing degassing mode between these three settings. Except for rare undersaturated samples and for volatile-rich, vesicular popping rocks, Mid-Ocean Ridge Basalts (MORBs) are predominantly extensively degassed and supersaturated in CO2 reflecting incomplete degassing during their last degassing step. Such a behavior is also reflected in their vesicle-dissolved carbon isotopic fractionations that are generally smaller than equilibrium values stemming from kinetic/diffusive effects. By contrast, the CO2 + H2O gas phase in hotspot and arc magmas is predominantly in chemical equilibrium with the melt because of volatile-rich initial conditions (and thus larger vesicularity) enhancing vapor-melt chemical and isotopic equilibrium. This larger initial volatile content is responsible for the extensive open-system (Rayleigh distillation) degassing generally observed in hotspot and arc magmas. This review highlights the fact that one single degassing model cannot explain the geochemical evolution of all magmas. Also, no single model can be assigned to one specific type of magma (i.e., a significant diversity of degassing mode exists within a given type of magma, notably in MORBs). It is important to take into account these observations when degassing corrections are applied on the basis of noble gas (He/Ne or He/Ar) ratios. It appears helpful to characterize the carbon concentration and stable isotope ratios in vesicles and dissolved in the glass in order to (1) identify the mode of degassing operating in a given magmatic system and (2) apply the most appropriate degassing correction to these magmas. It is only after this critical correcting step has been achieved that an assessment of initial magmatic CO2 contents can be reasonably undertaken.
      原文链接:
    • NSTL
    • Elsevier

    Extremely light molybdenum isotope signature of sediments in the Mariana Trench

    Chen, ShunPeng, Xiao-TongLi, JieLin, Zhiyong...
    9页
    查看更多>>摘要：Molybdenum (Mo) isotope signature (Mo-898/95) of marine sediments is a powerful proxy for constraining marine redox conditions and early diagenetic processes. Oxic pelagic sediments have been increasingly acknowledged as an important sink for Mo in its global cycling, however, the mechanisms controlling Mo enrichment and isotope fractionation during early diagenesis in this environment are not fully understood. In this study, four sedimentary cores were sampled by full-ocean-depth lander system in the Mariana Trench, aiming to explore the processes controlling Mo partitioning in the sediments of the deepest ocean in the world. Sediments from four locations exhibit a wide range of authigenic Mo contents (Moauth, relative to continental crust), varying from 1.98 to 43 mu g/g. A positive relation between Mo and Mn contents was identified for these sediments, which suggests that Mo are mainly hosted in Fe-Mn (oxyhydr)oxides. In addition, negative correlations between Mo-898/95 values and Mo/Mn ratios are found at each site. The Mo-898/95 values of the sediments vary from-1.71 +/- 0.05 to -0.15 +/- 0.04%0, revealing similar trends towards lighter values with burial depth from three sites. However, throughout the sediment core TY41 obtained from Mariana Trench, the Mo-898/95 values of sediment remain nearly constant around -0.55%0. Supported by the abundant Fe (oxyhydr)oxides throughout this site, it is likely the sediment is affected by enhanced fluid exchange across sediment-seawater interface, and the Mo-898/95 values of dissolved Mo in pore water resemble seawater signature. Therefore, the Mo isotopic offset between dissolved Mo and sediment is similar to the expected fractionation during the adsorption to Mn oxides (similar to+ 3.0%0) and hematite (similar to+ 2.2%0), which suggests that most of the dissolved Mo is scavenged by Mn oxides and/or hematite. The extremely light Mo-898/95 values of sediments in deeper parts of the cores coincide with enhanced Mo accumulation. This indicates an even lighter isotope signature of dissolved Mo in pore water during the re-precipitation of Fe-Mn (oxyhydr)oxides upon an isotopic equilibrium for Mo adsorption to metal oxides. Both the patterns of both dissolved oxygen and nitrate suggest that the study sites are located within the nitrate reduction zone and Mn reduction occur at deeper depths below the studied interval. Therefore, the source of the isotopically light dissolved Mo in pore water was explained by the preferential release of isotopically lighter Mo from Fe-Mn (oxyhydr)oxides during the enhanced dissolution at deeper depth. In contrast, at the shallower depths of these cores (expect for core TY41), the higher Mo-898/95 values towards seawater-sediment interface suggest the dissolved Mo is more impacted by the seawater exchange. This study provides a deeper insight into the diagenetic Mo cycling in trench environments, with Fe-Mn (oxyhydr)oxides rich sediments as an important sink for Mo.
      原文链接:
    • NSTL
    • Elsevier

    A multi-proxy (delta Ca-44/40, Sr/Ca, and Delta(47)) study of fish otoliths for determination of seawater temperature

    Mondal, SurajitChakrabarti, RamanandaGhosh, Prosenjit
    9页
    查看更多>>摘要：Fish otoliths are valuable archives for paleotemperature studies. In this study, we have measured delta Ca-44/40, Sr/Ca, Mg/Ca, and Ba/Ca ratios in six fish otolith samples of four different species from water with different annual average temperatures ranging from 2 degrees C to 25 degrees C. We demonstrate that the delta Ca-44/40 values of fish otoliths can be used as a temperature proxy. The temperature sensitivity of delta Ca-44/40 in fish otoliths is 0.036 parts per thousand/degrees C which suggests that with the current analytical uncertainty of +/- 0.08 parts per thousand for delta Ca-44/40 measurements, we can resolve temperature changes of +/- 2 degrees C. The Sr/Ca ratio of fish otoliths also shows a negative correlation with water temperature. In addition, both delta Ca-44/40 values and Sr/Ca of fish otoliths display linear correlations with published delta O-18 and Delta(47) measured in the same otolith powder samples. This study further demonstrates that simultaneous measurements of delta Ca-44/40, Sr/Ca, and Delta(47) in fish otoliths can be used for robust paleotemperature reconstruction. Using a multi-regression approach, a multi proxy calibration equation is proposed (T(K) = 5.43 x Sr/Ca - 16.8 x delta Ca-44/40 - 484.6 x Delta(47) + 580.9) by combining the delta Ca-44/40, Sr/Ca, and Delta(47) values in the fish otolith samples, which results in a low uncertainty of temperature estimation of +/- 0.9 degrees C. However, the temperature sensitivity of Mg/Ca and Ba/Ca ratios in fish otoliths is not statistically significant and hence, not a reliable proxy for paleotemperature reconstruction.
      原文链接:
    • NSTL
    • Elsevier

    Magnetite geochemistry and iron isotope signature of disseminated and massive mineralization in the Kalatongke magmatic Cu-Ni sulfide deposit, northwest China

    Tang, DongmeiQin, KezhangMao, YajingEvans, Noreen J....
    17页
    查看更多>>摘要：Magnetite is a common accessory mineral and an indicator to magma evolution and sulfide fractional crystallization in magmatic Ni???Cu sulfide deposits. The Kalatongke magmatic sulfide deposit at the southern margin of the Central Asian Orogenic Belt is composed of 11 mafic intrusions. The sulfide orebodies (Y1, Y1 deep, and Y2) are hosted by three small mafic intrusions, and magnetite is common in all mineralized samples. Based on ilmenite exsolution texture, composition, and type of mineralization, three types of magnetite are identified, (1) primary Fe-rich magma-related magnetite (type 1), (2) sulfide liquid-related magnetite (type 2), and (3) hydrothermal magnetite (type 3). We speculate that Fe isotope equilibrium fractionation between early or cocrystallized light 54Fe-enriched sulfide and magnetite may account for the ultra-high magnetite 856Fe value (0.70%o to 1.35 %o) in the magmatic magnetite (types 1 and 2). Enriched compatible elements in type 1 magnetite (TiO2 = 1.52 wt% to 7.89 wt%, V = 775 ppm to 2420 ppm, Cr = 2697 ppm to 14,658 ppm, and Ni contents = 478 ppm to 1256 ppm), reflect the characteristics of primary Fe-rich magmatic magnetite crystallized at relatively high temperature and low oxygen fugacity. Type 2 magnetite in massive ores has Ni (762 ppm to 1769 ppm) and 856Fe (0.89 %o to 1.10 %o) values similar to type 1 magnetite, but significantly lower TiO2, V and Cr contents (TiO2 = 0.07 to 3.72 wt%, V = 61 to 2314 ppm, Cr = 5 to 2937 ppm), indicating co-crystallization with an evolved sulfide liquid across a wide span of temperature and oxygen fugacity. Type 3 magnetite is of hydrothermal fluid origin, with low 856Fe (0.29 %o to 0.41 %o) and Ni (177 ppm to 377 ppm). Magnetite composition and Fe isotope signatures at the Kalatongke magmatic sulfide deposit can reveal the genesis of magnetite and the evolution of the melt and sulfide. The Y1 orebody was completely crystallized over a short period of time, while the Y1 deep and Y2 east orebodies underwent a protracted evolution during which magnetite co-crystallized from primitive Fe-rich melt, Fe-rich evolved melt and Cu-rich intermediate sulfide solution. Magnetite and sulfide in the Y2 west massive orebody likely formed last. The compatibility of both lithophile and chalcophile elements, combined with Fe isotopes in magnetite, makes magnetite a good indicator of silicate and sulfide melt evolution.
      原文链接:
    • NSTL
    • Elsevier

    Cobalt hydroxide-cobalt carbonate competitive growth on carbonate surfaces

    Riechers, Shawn L.Ilton, Eugene S.Qafoku, OdetaDu, Yingge...
    11页
    查看更多>>摘要：The presence of mineral surfaces can affect the outcome of geochemical reactions by providing alternative nucleation pathways, but not in ways that can yet be reliably quantified. In this work, the reaction of Co(II) with calcite (CaCO3) and magnesite (MgCO3) powders at room temperature was used to quantify the effects of the nature of the mineral substrate and of solution chemistry on the competitive heterogeneous growth between cobalt carbonate (CoCO3) and cobalt hydroxide (Co(OH)2). Experiments were first performed to determine the appropriate solubility product constants of CoCO3 and Co(OH)2, for which several values have been reported in the literature. X-ray photoelectron spectroscopy measurements were then performed to quantify the relative proportion of each phase in surface precipitates as a function of the nature of the substrate, initial saturation level, and pH. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were also used to characterize the morphology and composition of surface precipitates. On calcite powders, Co(OH)2 formed predominantly despite the initial solutions being more supersaturated with respect to CoCO3 than to Co(OH)2, indicating that the kinetics of heterogenous growth were faster for Co(OH)2 than for CoCO3. In contrast, magnesite powders were much more favorable to the growth of CoCO3 because of the low lattice mismatch between the two phases, which allowed for heteroepitaxial growth. However, the proportion of CoCO3 in surface precipitates decreased with increasing initial supersaturation, likely due to the resulting decrease of the free energy barrier for Co(OH)2 nucleation and the rapid kinetics of Co(OH)2 growth. Lowering pH increased the proportion of CoCO3 on both substrates. These findings highlight how the interplay between lattice mismatch, heterogeneous nucleation barriers, and rates of heterogenous growth can influence competitive heterogeneous growth and dramatically affect the outcome of geochemical reactions.
      原文链接:
    • NSTL
    • Elsevier

    Zinc isotope constraints on carbonated mantle sources for rejuvenated-stage lavas from Kaua?i, Hawai?i

    Yao, JunhuaHuang, JianZhang, Guoliang
    14页
    查看更多>>摘要：Sedimentary carbonates could be transported into Earth's mantle by plate subduction, but the ultimate fate of these carbonates remains unclear. Zinc isotopes can allow the tracing of recycled isotopically heavy Zn-rich sedimentary carbonates (??66Zn up to 0.91%o) within the mantle. This study presents new whole-rock majorand trace-element geochemical and Sr???Nd???Pb???Hf???Zn isotopic data for a complete series of rejuvenated-stage alkaline samples from Kaua??i, Hawai??i. These samples have high MgO contents (MgO 8.6 wt%) and show a wide range of whole-rock geochemical compositions, with lower SiO2 contents, higher alkali element contents, higher CaO/Al2O3 values at a given MgO content, more depleted Sr???Nd???Hf isotopic compositions, and more radiogenic Pb isotopic compositions relative to shield tholeiites. The trace-element compositions of these samples have some similarities to carbonatites, including large ion lithophile element enrichments and K, Pb, and Zr???Hf depletions. These samples also have systematically higher ??66Zn values (0.34???0.43%o) than shield tholeiites from the Kaua??i (0.27 ?? 0.04%o) and K????lauea Iki lava lake (0.26???0.30%o) and normal mantle (0.18 ?? 0.05%o). Modeling the elemental and isotopic behavior of Zn indicates that the heavy Zn isotopic compositions of these samples reflect the nature of their mantle source rather than any processes associated with mantle melting and fractional crystallization. The low 87Sr/86Sr and heavy Zn isotopic compositions of these alkaline samples can be replicated by mixing depleted mantle with minor amounts of sedimentary Mg???carbonates (<5%). A decrease in whole-rock SiO2 contents from the alkaline basalts to the melilitites in this region correlates with increases in the geochemical signature of sedimentary carbonate-bearing enriched components (e.g., high ??66Zn and low ??Nd and ??Hf values). The correlation between the ??66Zn and 206Pb/204Pb values of the samples is in concert with the role of recycled carbonates that could elevate the U/Pb ratio of mantle component caused by the carbonated melt. This implies that the studied samples formed from magmas derived from a mixed mantle source at least containing depleted peridotite and sedimentary carbonate-bearing components. The addition of recycled sedimentary carbonates may be a key factor for the high ??66Zn and 206Pb/204Pb values of the Kaua??i rejuvenated-stage lavas. These findings suggest that the subducted sedimentary carbonates could accompany the whole circle of recycling of oceanic slab and experience a recycle deep-to-lower mantle in the Hawaiian mantle plume.
      原文链接:
    • NSTL
    • Elsevier

    Strontium isotope stratigraphic insights on the end-Permian mass extinction and the Permian-Triassic boundary in the Dolomites (Italy)

    Garbelli, ClaudioCipriani, AnnaBrand, UweLugli, Federico...
    14页
    查看更多>>摘要：The Dolomites (Southern Alps, Italy) is a most significant region to investigate the evolution of shallow-marine ecosystems during the end-Permian mass extinction (EPME). Shallow-marine ecosystems are complex places from an oceanographic viewpoint and combine high biological productivity and ecological diversity. Therefore, establishing the timing and correlation of globally recognisable events in this region are fundamental to interpreting the biological crisis that followed. We took advantage of the Strontium Isotope Stratigraphy (SIS) concept to correlate our stratigraphic succession, ranging from the upper Bellerophon (Bulla Member, upper Changhsingian) to the lower Werfen (Tesero Member, upper Changhsingian - lower Induan) Formations of the Dolomites, to the Meishan (Zhejiang Province, China), the Global Stratotype Section and Point (GSSP) for the Permian-Triassic boundary. We integrated new and previously published 87Sr/86Sr data from well preserved brachiopods and show that the topmost part of the Bellerophon Formation (the Bulla Member) correlates with beds 24 e-f of the GSSP section, while the Tesero Member, where the Permian-Triassic boundary is located, is characterised by more radiogenic Sr isotope ratios than expected. Differences in Sr isotope ratios may be due to small-scale variations in local paleoenvironmental and paleogeographic conditions/settings. Continental weathering of sedimentary or igneous rocks, distance from terrestrial environment and nearby occurrence of radiogenic rocks might also produce locally variable Sr isotope signatures especially in shallow water environments complicating the application of SIS principles.
      原文链接:
    • NSTL
    • Elsevier

    Photodegradation of pyrogenic dissolved organic matter increases bioavailability: Novel insight into bioalteration, microbial community succession, and C and N dynamics

    Chen, YalanSun, KeSun, HaoranYang, Yan...
    14页
    查看更多>>摘要：Photodegradation and biodegradation of pyrogenic dissolved organic matter (pyDOM) play crucial roles in regulating greenhouse gas emissions and stabilizing organic matter. In this study, we conducted a 56-day microbial laboratory incubation to investigate the biodegradation patterns of pristine and photobleached pyDOM leached from a thermal series of chars (300, 450, and 600 ???C) based on FT-ICR-MS, gas analysis, N speciation, and 16S rDNA sequencing. As the pyrolysis temperature increased, the among of biomineralized carbon of pyDOM decreased, while the molecular diversity and aromaticity increased. Photobleaching increased the among of biomineralized carbon of pyDOM from 40%???70% to 60%???80%, but decreased the molecular diversity, molecular weight, and aromaticity, with a major removal of lignin-like compounds. Microbial incubation lowered the molecular diversity but increased the molecular weight and aromaticity of both pristine and photobleached samples. Tannin-like structures, carbohydrates, and aliphatic/protein were preferentially biodegraded, while lipid, lignin-like structures, unsaturated hydrocarbons, and condensed aromatics were more likely to be preserved or newly produced. The photobio-resistant components were mainly comprised of lignin-like and tanninlike structures. Photobleached pyDOM samples released more CO2 but less N2O, as they may possess high denitrification rate due to their high carbon reactivity. The bacterial richness and diversity in pyDOM were lower than the original inoculum. In addition, distinct differences were noted between the bacterial community structure of the original inoculum and that of pristine and photobleached pyDOM samples following various incubation times. Specifically, proteobacteria was increased and remained the predominant phylum in all pyDOM solutions compared to the original inoculum, while Actinobacteriota, Bdellovibrionota, Firmicutes, or Nitrospirota phyla were increased in several samples either after 7 or 56 days of incubation. In particular, photobleached pyDOM pyrolyzed at 600 ???C exhibited the strongest filtration effects on the bacterial microflora. These results help to elucidate the biogeochemical cycling and turnover of pyDOM in sunlit and dark aquatic environments such as fluvial and groundwater networks, lakes, and the ocean.
      原文链接:
    • NSTL
    • Elsevier