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Chemical geology
Elsevier
Chemical geology

Elsevier

0009-2541

Chemical geology/Journal Chemical geologySCIISTPAHCIEI
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    New insights into the distribution and speciation of nickel in a Myanmar laterite

    Sun, JingQin, HaiboYang, ShitongSanematsu, Kenzo...
    12页
    查看更多>>摘要：Because ferromanganese oxides can host significant amounts of Ni during the weathering, the Fe- and Mn-oxide are resultantly considered as the important Ni-bearing minerals. However, little is known about the role of Mn species to influence the Ni behavior during this process. In this study, we employed the techniques of ICP-MS, XRF, XRD, XAFS (XANES and EXAFS), and ??-XRF to interpret the Ni behavior along a laterite profile in Myanmar. Nickel concentrations in the saprolite developed on peridotites can reach up to 11 wt% Ni mainly because of Ni precipitation as fracture-fillings and thin coatings on joints, which produce a promisingly and importantly economic mineral resource of this territory. In the peridotites, Fe and Mn species predominantly consisted of olivine (96%) and biotite (74%), respectively. Nickel XANES spectra indicated that Ni was primarily hosted by olivine (86%) in the protolith under reduction condition, lizardite (90%) and Ni-bearing goethite (10%) in the lower garnierite vein under strong oxidation condition, and lizardite (37%) and Ni-bearing goethite (63%) in the upper saprolite under medium oxidation condition. Accordingly, minerals of Fe and Mn were progressively oxidized toward Fe3+ and Mn4+, which occurred primarily as Fe3+-bearing goethite and Mn4+bearing birnessite. The occurrence of Fe- and Mn-oxide plays an important role in Ni species and suggests that birnessite maybe facilitate the formation of Fe oxide precipitations by means of the oxidation of Fe2+ in the olivine to Fe3+ in the goethite. The results are of relevant importance, as it fills a gap in the knowledge of oreformation processes occurring in Myanmar
      原文链接:
    • NSTL
    • Elsevier

    Holocene microbialite geochemistry records > 6000 years of secular influence of terrigenous flux on water quality for the southern Great Barrier Reef

    Salas-Saavedra, MarcosWebb, Gregory E.Sanborn, Kelsey L.Zhao, Jian-xin...
    23页
    查看更多>>摘要：Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reefs. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the Great Barrier Reef (GBR), longer-term patterns of water quality are poorly understood. Here we present the first direct proxy-based Holocene water quality reconstruction for any reef. The unique geochronological framework provided by cores from Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8200 to 1800 years before present (BP) using centennially resolved microbialite-based geochemical proxies. Terrigenous sediment-sourced trace elements were measured in microbialites from a well dated succession of reef rock and in paleosol (ancient soil) formed at the Pleistocene-Holocene unconformity. Microbialite-hosted rare earth element and yttrium distributions (e.g., Nd/YbSN = 0.36; Y/Ho = 57) are consistent with precipitation from shallow oxygenated seawater but show a non-linear trend through the Holocene with distinct intervals of higher and lower terrigenous influence relative to average values. Immediately following reef initiation (>8300 years ago) our data suggest increasing terrigenous influence by 8000 ka. Surrounding reef seawater became less affected by terrigenous runoff from-7000 years ago, but showed marked mid-Holocene variability related to regional climatic factors. Major fluctuations between intervals of high and low relative terrigenous influence correlate well with particular regional and more global climate records. These include local relative sea level fluctuations, fluctuations in Indian-Australian Summer Monsoon (IASM) strength, and dampened El Nino Southern Oscillation (ENSO) frequency corresponding to greater terrigenous influence in the southern GBR at-7.0, 5.4, and 2.7 ka BP. Water quality then improved significantly after 3200 years BP. More broadly, it is well established that water quality has a major effect on reefs and reef communities, but for past reef history, inferences about water quality are commonly highly speculative. Reefal microbialite geochemistry provides an independent, high-quality proxy for ambient water quality that can be used to directly compare contemporaneous reef growth dynamics and ecological shifts to changing water quality. The high concentrations of trace elements in reefal microbialites, relative to other marine carbonates, provide a very robust, if time averaged, proxy for investigating ancient seawater chemistry, even in offshore reefs, such as Heron Reef. At the same time, the proxy provides a new independent data set for that may aid interpretation and model of climate change relevant to reef growth at centennial to millennial scales. As reefal microbialites are common in many global reef systems, where associated with high quality dating, they may provide useful proxies for investigating secular changes in water quality and associated climatic drivers at various temporal scales in other regions of the world.
      原文链接:
    • NSTL
    • Elsevier

    Distribution patterns of dissolved trace metals (Fe, Ni, Cu, Zn, Cd, and Pb) in China marginal seas during the GEOTRACES GP06-CN cruise

    Zhang, RuifengRen, JinglingZhang, ZhaoruZhu, Zhu...
    13页
    查看更多>>摘要：Trace metals are both important micronutrients for life in the oceans, and can be tracers of various natural and anthropogenic processes. Metals including Fe, Ni, Zn, and Cu are known to be important nutrients for phytoplankton, elements including Pb are viewed primarily as anthropogenic contaminants, Cd is a biologically cycled element though it???s nutritive importance is unclear, and all of these metals are impacted by overlapping natural and anthropogenic processes in different parts of the oceans. China is bordered by four marginal seas, including the Bohai, Yellow Sea (YS), East China Sea (ECS), and South China Sea. Here we present the first systematic study of trace metals in the China marginal seas from the GEOTRACES GP06CN cruise in October 2015. The metals Fe, Ni, Cu, Zn, Cd, and Pb were sampled in the Yellow Sea and East China Sea at 69 surface locations using a towfish and with 8 depth profiles, along with auxiliary parameters including salinity, phosphate, and apparent oxygen utilization. We find that Ni, Cu and Cd are most strongly impacted by a combination of biological cycling and fluvial inputs, recent atmospheric deposition dominates surface ocean Pb concentrations, and Fe and Zn are impacted by a combination of multiple processes. Comparison with historical data shows that Cu concentrations have been increasing through time, presumably due to increasing anthropogenic inputs. Ni and Cd are also delivered in high concentrations from rivers, which can reflect the impact of anthropogenic inputs, and Pb is delivered from atmospheric deposition which may include anthropogenic sources, but these metals do not show a distinct temporal trend. The six metals studied here are thus impacted by their broad-scale oceanographic distributions, local coastal processes, and anthropogenic inputs, each in different ways and to different degrees. This study represents a unique contribution to the international GEOTRACES effort, by highlighting an ???end-member??? region with strong coastal and anthropogenic inputs.
      原文链接:
    • NSTL
    • Elsevier

    A reassessment on the timing and potential drivers of the major seawater Sr-87/Sr-86 drop in the Ordovician Period: New evidence from conodonts in China

    Liu, KangJiang, MaoshengHuang, TaiyuZhang, Liyu...
    13页
    查看更多>>摘要：The major drop in marine Sr-87/Sr-86 records across the Middle-Late Ordovician boundary (Darriwilian-Sandbian) was one of the most dramatic perturbations of the global Sr cycle in Phanerozoic. However, the precise timing of this isotopic event has not been well constrained, partially making it difficult to further clarify the possible drivers of this isotopic event. For this study, we present multi-continental conodont apatite-based Sr-isotope datasets from three well-studied Ordovician carbonate successions in China, i.e., the Huanghuachang section in South China, the Kalpin Shuinichang-Dawangou and the Nanyigou sections in Tarim Basin, northwestern China. Our new Sr-87/Sr-86 records generally accord with global seawater Sr-87/Sr-86 secular variation curve and are supposed to be a reflection of coeval primary seawater signatures. Constrained with detailed conodont biostratigraphic and high-resolution 6 13 C chemostratigraphic framework, the onset of the sharp decline in Sr-87/Sr-86 ratios was placed within the Histiodella holodentata zone-Eoplacognathus suecicus zone, and approximately synchronous with the initiation of the Middle Darriwilian Carbon Isotope Excursion (MDICE). Numerical modeling reveals that this major Sr-87/Sr-86 drop could have been generated by a sustainable increase of oceanic hydrothermal (mantle-derived) Sr flux and/or a progressive decrease of continental-derived Sr flux. Multi-proxy analyses further demonstrate that the impact of enhanced oceanic hydrothermal activities accompanying with the accelerating opening of the Rheic and Paleo-Tethys oceans, may have played a central role in driving the major Sr-87/Sr-86 drop during the Middle-Late Ordovician transition.
      原文链接:
    • NSTL
    • Elsevier

    Seasonal, weathering and water use controls of silicon cycling along the river flow in two contrasting basins of South India

    Mangalaa, K. R.Cardinal, D.Gurumurthy, G. P.Dapoigny, A....
    13页
    查看更多>>摘要：We present the first study of river water silicon isotopic composition from two contrasting basins in South India, the east flowing Kaveri river and the west flowing Netravathi river. Both rivers originate from the Western Ghats. River water samples were collected from mainstream, tributaries and reservoirs at different locations along the river flow during dry and wet (monsoon) seasons, with an additional post-monsoon sampling in Netravathi. High rainfall in Netravathi basin and upper reaches of Kaveri induce intense weathering in the region, with superposing contribution from anthropogenic controls in downstream Kaveri. The delta Si-30(DSi) values range from +0.42 to +1.65 parts per thousand for Netravathi river basin and + 0.32 to +2.85 parts per thousand for Kaveri river basin. Silicate weathering index (R-e) shows intense weathering associated with monosiallitization (kaolinite-gibbsite formation) in Netravathi basin and relatively moderate weathering with bisiallitization (smectite-kaolinite formation) in Kaveri basin. The seasonal changes in delta Si-30(DSi) and R-e in each basin shows similar patterns, with a likely higher and heavier contribution of soil water from deeper soil profiles closer to weathering front and bedrock, associated with significant secondary mineral formation during dry season and leaching of superficial soil profiles during the high discharge periods of monsoon. We provide a theoretical framework to estimate relative contribution of silicate weathering vs. anthropogenic processes on riverine delta Si-30(DSi). R-e is broadly correlated with delta Si-30(DSi) and an isotopic mass balance involving the whole rock composition (excluding the stable quartz and sericite) shows that the delta Si-30 in river water are well explained by silicate weathering and gibbsite-kaolinite formation in the Netravathi and upper upper reaches of Kaveri. However, silicate weathering explains only partially the heavier delta Si-30(DSi) signatures in the middle and lower reaches of the Kaveri, and the additional enrichment of about +1.06 parts per thousand can be attributed to uptake of silicon and Si-depleted return flow through irrigated agriculture in the basin. This study confirms the major control of pedoclimatic conditions on the delta Si-30 of rivers and provides for the first time an estimation of the impact of human activities on the silicon isotopic signature of rivers.
      原文链接:
    • NSTL
    • Elsevier

    Spatial distribution and geochemical characterization of Icelandic mantle end-members: Implications for plume geometry and melting processes

    Halldorsson, Saemundur A.Haroardottir, SunnaMatthews, SimonJackson, Matthew G....
    15页
    查看更多>>摘要：We compile a new geochemical database (Icelandic Volcanic rocks Isotopic Database, IVID) which includes previously reported Sr-87/Sr-86, Nd-143/Nd-144, Hf-176/Hf-177, Pb-206/Pb-204, Pb-207/Pb-204, Pb-208/Pb-204, Os-187/Os-188 and He-3/He-4 data and major and trace element concentrations measured in Icelandic volcanic rocks. Isotopic compositions were evaluated and filtered to identify the highest quality data and data most likely to represent mantle-derived compositions. The carefully filtered, comprehensive geochemical database is an important contribution to the geochemical community and can be used to put further constraints on the generation of geochemical heterogeneity in Iceland. We use the compiled database to examine the spatial distribution of geochemical components in the Icelandic mantle, and test whether melting processes control how source heterogeneity from the deep mantle is extracted on the surface. The PRIMELT software (which combines an inverse model for crystallization in the crust with a forward model of mantle melting to estimate the primary mantle-derived magma composition) is used to estimate mantle potential temperatures and melt fractions in Iceland. The results of these calculations and forward melting models demonstrate that mantle potential temperatures have a largely insignificant role, and that lithospheric thickness plays a key role in generating geochemical heterogeneity in Icelandic volcanic rocks. Volcanic rocks from the off-rift volcanic zones are generally produced by lower melt fractions, consistent with preferential sampling of geochemically enriched and fertile "blobs" at lower degrees of melting, where the lithosphere is thicker; more geochemically depleted volcanic rocks from the axial rift zones, where the lithosphere is thinner, show higher degrees of melting, consistent with dilution of enriched blob melts by melts of more depleted and refractory mantle.
      原文链接:
    • NSTL
    • Elsevier

    Cl isotope fractionation in magmatic and hydrothermal eudialyte, sodalite and tugtupite (Il?maussaq intrusion, South Greenland)

    Eggenkamp, H. G. M.Marks, M. A. W.Bonifacie, M.Barboux, G....
    12页
    查看更多>>摘要：We determined the Cl stable isotope composition of Cl-rich minerals (eudialyte, sodalite and tugtupite) from evolved peralkaline rocks of the Il??maussaq complex (South Greenland) to determine the effects of orthomagmatic and hydrothermal processes on Cl isotope variations and to estimate the Cl isotopic composition of the lithospheric mantle source for the complex. The total variation of ??37Cl values is between +0.1 and + 0.4%o for sodalites (N = 16), 0 and + 0.3%o for tugtupites (N = 6) and + 0.2 and + 1.7%o for eudialytes (N = 31). This data set allowed for estimating Cl isotope fractionation between minerals, melts and fluids in a well-defined natural system. The Cl isotope fractionation between magmatic sodalite and melt is slightly negative (about -0.05%o), decreasing to about -0.1%o during hydrothermal conditions; Cl isotope fractionation between tugtupite and fluid (about -0.2%o) is slightly larger. Magmatic eudialyte has ??37Cl values that are approximately 0.35%o higher than magmatic sodalite, indicating Cl isotope fractionation between eudialyte and melt of about +0.3%o. The Cl isotopic composition of the melt was estimated to a ??37Cl of about +0.3%o, which may be representative for the lithospheric mantle below South Greenland. This value is within the range of ??37Cl values of various mantle reservoirs (varying between -3%o and + 3%o) including a recently suggested value of +0.9%o derived from Mexican mantle-derived deep geothermal waters, and the generally accepted average mantle value of -0.2%o. The data indicate that large Cl isotope heterogeneities exist within the mantle that question the validity of using a general average Cl isotope ratio when Cl isotope mantle values are discussed, and that it is important to better understand the global variability of Cl isotopes in the mantle and Cl isotope fractionation between the fluid and crystalline phases. Cl isotopes in sodalite and tugtupite samples are compared to previously published Br isotope values. The range in Cl isotope values is less than the range of Br isotope values, and different sample groups are better constraint with Br isotopes. Br isotope fractionation is larger due to competition of Br with Cl which better fits at mineral site locations in the crystals. This results in better isotope separation between different sample groups using Br isotopes. Due to low Br concentrations in most samples, it is only rarely possible to measure Br isotope compositions so that generally it is not possible to apply this comparison.
      原文链接:
    • NSTL
    • Elsevier

    Nitrogen isotope evidence for oxygenated upper ocean during the Cryogenian interglacial period

    Li, TingtingWang, PengjuZhu, GuangyouZhao, Kun...
    18页
    查看更多>>摘要：The Cryogenian interglacial period have witnessed dramatic changes in dim ate, oceanic environment and biological evolution. The nitrogen isotopic composition, as an important biogeochemical proxy, has the potential to track both the nutrient cycling and redox conditions in the past. However, nitrogen isotopic data during this interglacial time is rather limited. Here, we present integrated data for nitrogen isotopes (delta N-15), as well as organic carbon isotopes (delta(13)Corg), iron (Fe) speciation, pyrite morphology and trace elements from the Cryogenian interglacial Datangpo Formation derived from a drill core from South China to figure out the nitrogen cycling and coeval redox states. The results reveal a gradual oxygenation bottom waters along with deepening O-2-H2S chemocline from euxinic black shale in the lower unit (LU), to oxic siltstone in the upper unit (UU), which is consistent with the redox variation inferred in previous studies. Sedimentary delta N-15 values constantly remain positive (mean value> + 4 parts per thousand) throughout the Datangpo Formation and display an increase from the LU to the UU. We interpreted the positive delta N-15 values in the LU as a result of denitrification related to the development of euxinic conditions. The further increase towards the UU associated with limited nutrient supply may indicate the existence of a stable nitrate reservoir in the upper water column, which was consistent with an increased oxygenation during the interglacial time. The elevated O-2 levels and bioavailable nitrogen in seawater may have further contributed to the appearance of the earliest metazoan. In addition, our data lay good foundation for further global delta N-15 correlations among other interglacial successions.
      原文链接:
    • NSTL
    • Elsevier

    The role of organic matter diversity on the Re-Os systematics of organic-rich sedimentary units: Insights into the controls of isochron age determinations from the lacustrine Green River Formation

    Pietras, Jeffrey T.Dennett, AbbySelby, DavidBirdwell, Justin E....
    14页
    查看更多>>摘要：The range of Re-187/Os-188 values measured from samples of five organic-rich lacustrine mudstones units in the Eocene Green River Formation in the easternmost Uinta Basin covaries with organic matter diversity driven by changing water column conditions. A set of samples from the Douglas Creek Member has the highest pristane/phytane ratio and lowest beta-carotane/n-C-30 ratio compared to overlying units, indicating deposition in an oxicanoxic environment with low salinity that would have allowed for the accumulation of a diverse assemblage of aquatic organisms. These samples define the broadest Re-187/Os-188 range of 1504. In contrast, samples from the R6 and Mahogany zones possess lower pristane/phytane ratios and higher beta-carotane/n-C-30 ratios indicating deposition in a more restricted lacustrine environment with elevated salinities and alkalinities that would have limited aquatic organic matter diversity. The R6 and Mahogany zones have the narrowest range of Re-187/Os-188 values measured in this study of 254.9 and 154.6, respectively. As noted by previous workers, these results suggest that organic matter diversity plays a primary role in determining the range of Re-187/Os-188 ratios in a sample set, and in turn the uncertainty of Re-Os age determinations from organic-rich sedimentary rocks. The Re-Os data from the R3 zone and R6 zone yield ages of 49.7 +/- 3.4 Ma and 42.0 +/- 18 Ma, respectively, which are statistically indistinguishable based on 2 sigma uncertainty from three previously reported Re-Os age determinations and those provided by Ar-40/Ar-39 geochronology of interbedded volcanic ash beds. Although the age uncertainty is high, these findings further highlight the importance of Re-Os geochronology in lacustrine basins, particularly those with thick mudstone successions that lack volcanic ash layers, reliable biostratigraphy, or magnetostratigraphic control. In these cases, even ages with large uncertainties can be useful to constrain burial history and thermal history models. Together, the initial Os-18(7)/Os-188 ratios of five sets of samples analyzed from the Uinta Basin define the largest Os isotope stratigraphic record from any lacustrine basin compiled to date and record a shift from a value of 1.40 to 1.48 between the R3 and R4 zones in the lower part of the Parachute Creek Member. This small shift may signify a change in the chemical weathering products that entered the lake preserved 20 to 50 m above the contact between the Douglas Creek and the lower Parachute Creek members during a period when the basin transitioned from a shallow lake with mostly open hydrology to an alkaline lake with more frequent basin restrictions.
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    • NSTL
    • Elsevier

    The role of iron-bearing minerals for the deep weathering of a hydrothermally altered plutonic rock in semi-arid climate (Chilean Coastal Cordillera)

    Hampl, Ferdinand J.Schiperski, FerryByrne, James M.Schwerdhelm, Christopher...
    15页
    查看更多>>摘要：Weathering is a fundamental process that controls the development of Earth's surface by the formation of erodible material and the release of mineral-bound nutrients. Weathering at depth has been predominantly studied in humid environments, where fluid flow sets mineralogical transformations, whereas (semi-)arid regions were barely investigated in this respect. In many studies the weathering of iron-bearing minerals was shown to play a major role for the advance of weathering at depth. Weathering of iron-bearing minerals is often superimposed on factors that precondition rocks for weathering such as hydrothermal alteration and fracturing by tectonic stress. The low degree of water-rock interactions in (semi-)arid climates offers the opportunity to single out the preconditioning factors and explore the role of iron-bearing minerals for weathering in detail. To this end the interactions of tectonic fracturing, hydrothermal alteration and weathering were investigated in a deep, weakly weathered profile of plutonic rock in a semi-arid region (Santa Gracia, Northern Chile). The profile was recovered by an 87-m-deep drill core and a 2-m-deep soil pit which were used to investigate the mineral composition, magnetic susceptibility, geochemical gradients, and Fe redox-state. Non-cemented fractures of tectonic origin were identified as pathways for the transport of fluids and reagents to depth that are fuelling the initial weathering reactions. Previous hydrothermal fluids mainly migrated along the same fractures and altered the properties of the original bedrock. Therefore, the weathering features along fractures depend on the preconditioning by hydrothermal alteration and tectonic stress which affected the vicinity of these fractures. From the fracture surfaces, water and O-2 diffuse into the rock and initiate oxidation and dissolution-reprecipitation reactions. Our study shows that the transformation of iron-bearing silicates (especially biotite) to Fe(III) (oxyhydr)oxides and clay minerals is the dominant weathering-promoting process as it produces stress which leads to micro-fracturing of the rock and finally to the formation of saprolite. Deciphering the links between tectonic fracturing, hydrothermal alteration, and weathering-induced fracturing by iron-bearing silicates in semi-arid settings provides unique insights into the controls of deep weathering in regions with limited fluid flow.
      原文链接:
    • NSTL
    • Elsevier