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Heterogeneous mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

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Acta Geochimica 2018年第1期37卷 1-18页

作者： Haoran Ma Yihe Xu Kangjun Huang Yuanlin Sun Shan Ke Yang Peng Xianguo Lang Zhen Yan Bing Shen Key Laboratory of Orogenic Belts and Crustal Evolution MOE School of Earth and Space Sciences Peking University State Key Laboratory of Geological Processes and Mineral Resources China University of Geosciences

Carbonate precipitation and hydrothermal reaction are the two major processes that remove mg from *** isotopes are significantly(up to 5%)fractionated during carbonate precipitation by preferential incorporation of ^(24)mg,while hydrothermal reactions are associated with negligible mg isotope fractionation by preferential sequestration of^( 26)***,the marine mg cycle could be reflected by seawater mg isotopic composition(δ^(26)mg_(sw)),which might be recorded in marine ***,carbonates are both texturally and compositionally heterogeneous,and it is unclear which carbonate component is the most reliable for reconstructing δ^(26)mg_(sw).In this study,we measured mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South *** on petrographic studies,four carbonate components were recognized:micrite,marine cement,brachiopod shell,and *** four components had distinct δ^(26)mg:(1)micrite samples ranged from -2.86% to -2.97%;(2)pure marine cements varied from -3.40% to -3.54%,while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range(-3.27% to-3.75%);(3)values for the mixture component were-3.17% and -3.49%;and (4)brachiopod shells ranged from -2.20% to -3.07%,with the thickened hinge area enriched in ^( 24)*** to having multiple carbonate sources,neither the micrite nor the mixture component could be used to reconstruct δ^(26)mg_(sw).In addition,the marine cement was homogenous in mg isotopes,but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ^(26)mg_(sw).Furthermore,brachiopod shells had heterogeneous C and mg isotopes,suggesting a significant vital effect during ***,the heterogeneous δ^(26)mg of the Huangjin limestone makes it difficult to reconstruct δ^(26)mg_(sw)using bulk carbonate/calcareous ***,δ^(26)mg_(sw)was only slightly affected by

关键词： seawater mg isotopic composition Limestone Fossil Micrite Cement

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