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Magnesium isotopic composition of the Earth and chondrites |
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| Authors: | Fang-Zhen Teng Wang-Ye Li Shan Ke Bernard Marty Shichun Huang Ali Pourmand |
| Institution: | a Isotope Laboratory, Department of Geosciences & Arkansas Center for Space and Planetary Science, University of Arkansas, Fayetteville, AR 72701, USA b 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 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 Centre de Recherches Pétrographiques et Géochimiques, CNRS, 15 rue Notre-Dame-des-Pauvres, 54501 Vandoeuvre-lès-Nancy, France e Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Ave., Chicago, IL 60637, USA f Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA g State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China h Department of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA |
| Abstract: | To constrain further the Mg isotopic composition of the Earth and chondrites, and investigate the behavior of Mg isotopes during planetary formation and magmatic processes, we report high-precision (±0.06‰ on δ25Mg and ±0.07‰ on δ26Mg, 2SD) analyses of Mg isotopes for (1) 47 mid-ocean ridge basalts covering global major ridge segments and spanning a broad range in latitudes, geochemical and radiogenic isotopic compositions; (2) 63 ocean island basalts from Hawaii (Kilauea, Koolau and Loihi) and French Polynesia (Society Island and Cook-Austral chain); (3) 29 peridotite xenoliths from Australia, China, France, Tanzania and USA; and (4) 38 carbonaceous, ordinary and enstatite chondrites including 9 chondrite groups (CI, CM, CO, CV, L, LL, H, EH and EL).Oceanic basalts and peridotite xenoliths have similar Mg isotopic compositions, with average values of δ25Mg = −0.13 ± 0.05 (2SD) and δ26Mg = −0.26 ± 0.07 (2SD) for global oceanic basalts (n = 110) and δ25Mg = −0.13 ± 0.03 (2SD) and δ26Mg = −0.25 ± 0.04 (2SD) for global peridotite xenoliths (n = 29). The identical Mg isotopic compositions in oceanic basalts and peridotites suggest that equilibrium Mg isotope fractionation during partial melting of peridotite mantle and magmatic differentiation of basaltic magma is negligible. Thirty-eight chondrites have indistinguishable Mg isotopic compositions, with δ25Mg = −0.15 ± 0.04 (2SD) and δ26Mg = −0.28 ± 0.06 (2SD). The constancy of Mg isotopic compositions in all major types of chondrites suggest that primary and secondary processes that affected the chemical and oxygen isotopic compositions of chondrites did not significantly fractionate Mg isotopes.Collectively, the Mg isotopic composition of the Earth’s mantle, based on oceanic basalts and peridotites, is estimated to be −0.13 ± 0.04 for δ25Mg and −0.25 ± 0.07 for δ26Mg (2SD, n = 139). The Mg isotopic composition of the Earth, as represented by the mantle, is similar to chondrites. The chondritic composition of the Earth implies that Mg isotopes were well mixed during accretion of the inner solar system. |
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