Magnesium isotopic compositions of the Mesoproterozoic dolostones: Implications for Mg isotopic systematics of marine carbonates

作者：Kang‐Jun Huang, Bing Shen, Xian-Guo Lang, Wenbo Tang, Yang Peng, Shan Ke, Alan J. Kaufman, Hao-Ran Ma, Fang-Bing Li · 发表于：Geochimica et Cosmochimica Acta · 年份：2015 · DOI：10.1016/j.gca.2015.05.002 · 被引用次数：104 · 研究领域：Paleontology and Stratigraphy of Fossils、Geology and Paleoclimatology Research、Geochemistry and Elemental Analysis

Available Mg isotope data indicate that dolostones of different ages have overlapping range of Mg isotopic composition (δ 26 Mg) and there is no systematic difference among different types of dolomites. To further explore the Mg isotopic systematics of dolomite formation, we measured Mg isotopic compositions of Mesoproterozoic dolostones from the Wumishan Formation in North China Block, because dolomite formation in Mesoproterozoic might have been fundamentally different from the younger counterparts. Based on petrographic observations, three texturally-different dolomite phases (dolomicrite, subhedral dolomite and anhedral dolomite) are recognized in the Wumishan dolostones. Nevertheless, these three types of dolomites have similar δ 26 Mg values, ranging from −1.35‰ to −1.72‰, which are indistinguishable from Neoproterozoic and Phanerozoic dolostones. To explain δ 26 Mg values of dolostones, we simulate the Mg isotopic system during dolomite formation by applying the one-dimensional Diffusion–Advection–Reaction (1D-DAR) model, assuming that the contemporaneous seawater is the Mg source of dolostone. The 1D-DAR modeling results indicate that the degree of dolomitization is controlled by sedimentation rate , seawater Mg concentration, temperature, and reaction rate of dolomite formation, whereas Mg isotopic composition of dolostone is not only dependent on these factors, but also affected by δ 26 Mg of seawater and isotope fractionation during dolomite formation. Moreover, th...