Magnesium isotopic composition of the Earth and chondrites

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 parts per thousand on delta Mg-25 and +/-0.07 parts per thousand on delta Mg-26, 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 delta Mg-25 = -0.13 +/- 0.05 (2SD) and delta(26)mg = -0.26 +/- 0.07 (2SD) for global oceanic basalts (n = 110) and delta Mg-25 = 0.13 +/- 0.03 (2SD) and delta Mg-26 = -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 delta Mg-25 = 0.15 +/- 0.04 (2SD) and delta Mg-26 = 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 delta Mg-25 and 0.25 +/- 0.07 for delta Mg-26 (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. (C) 2010 Elsevier Ltd. All rights reserved'.