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Heterogeneous magnesium isotopic composition of the upper continental crust |
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| Authors: | Wang-Ye Li Fang-Zhen Teng Shan Ke Roberta L Rudnick Fu-Yuan Wu |
| Institution: | a CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China b Isotope Laboratory, Department of Geosciences & Arkansas Center for Space and Planetary Science, University of Arkansas, Fayetteville, AR 72701, USA c State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China d Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USA e State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China f State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China g State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China h ARC Key Center for the Geochemistry and Metallogeny of the Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia |
| Abstract: | High-precision Mg isotopic data are reported for ∼100 well-characterized samples (granites, loess, shales and upper crustal composites) that were previously used to estimate the upper continental crust composition. Magnesium isotopic compositions display limited variation in eight I-type granites from southeastern Australia (δ26Mg = −0.25 to −0.15) and in 15 granitoid composites from eastern China (δ26Mg = −0.35 to −0.16) and do not correlate with SiO2 contents, indicating the absence of significant Mg isotope fractionation during differentiation of granitic magma. Similarly, the two S-type granites, which represent the two end-members of the S-type granite spectrum from southeastern Australia, have Mg isotopic composition (δ26Mg = −0.23 and −0.14) within the range of their potential source rocks (δ26Mg = −0.20 and +0.15) and I-type granites, suggesting that Mg isotope fractionation during crustal anatexis is also insignificant. By contrast, δ26Mg varies significantly in 19 A-type granites from northeastern China (−0.28 to +0.34) and may reflect source heterogeneity.Compared to I-type and S-type granites, sedimentary rocks have highly heterogeneous and, in most cases, heavier Mg isotopic compositions, with δ26Mg ranging from −0.32 to +0.05 in nine loess from New Zealand and the USA, from −0.27 to +0.49 in 20 post-Archean Australian shales (PAAS), and from −0.52 to +0.92 in 20 sedimentary composites from eastern China. With increasing chemical weathering, as measured by the chemical index of alternation (CIA), δ26Mg values show a larger dispersion in shales than loess. Furthermore, δ26Mg correlates negatively with δ7Li in loess. These characteristics suggest that chemical weathering significantly fractionates Mg isotopes and plays an important role in producing the highly variable Mg isotopic composition of sedimentary rocks.Based on the estimated proportions of major rock units within the upper continental crust and their average MgO contents, a weighted average δ26Mg value of −0.22 is derived for the average upper continental crust. Our studies indicate that Mg isotopic composition of the upper crust is, on average, mantle-like but highly heterogeneous, with δ26Mg ranging from −0.52 to +0.92. Such large isotopic variation mainly results from chemical weathering, during which light Mg isotopes are lost to the hydrosphere, leaving weathered products (e.g., sedimentary rocks) with heavy Mg isotopes. |
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