The isotopic composition of magnesium in the inner Solar System

We report high-precision MC-ICPMS measurements of (25)mg/(24)mg and (26)mg/(24)mg isotope ratios for 14 meteorites, both primitive and differentiated, 5 lunar basalts and breccia samples, 5 terrestrial basalts, 1 dunite and 3 peridotites. We estimate the delta(26)Mg value of the bulk silicate Earth (BSE) at -0.54 +/- 0.04 (2SE) w.r.t. DSM3. Carbonaceous and ordinary chondrites show an average delta(26)Mg of -0.52 +/- 0.04 (2SE), martian meteorites show a narrow range in delta(26)Mg with an average delta(26)Mg of -0.57 +/- 0.02 (2SE), average delta(26)Mg of pallasite olivines is -0.54 +/- 0.04 (2SE) whereas that of lunar samples, including ilmenite basalts, olivine basalt and one crystalline breccia is -0.51 +/- 0.03 (2SE). Our data demonstrate that the overall stable Mg isotopic composition of the inner Solar System (average delta(25)Mg = -0.273, delta(26)Mg = -0.535) is homogeneous to within +/- 0.04 parts per thousand for (26)mg/(24)Mg. This value is indistinguishable from that of the BSE and lunar samples demonstrating that the Mg isotopic composition of the Earth-Moon system is chondritic. Accurate determination of the Mg isotopic composition of the rocky planets is important for Mg isotope studies of surface processes on Earth as well as for using Mg isotope data to understand evaporation and condensation effects in the solar nebula. Consistent with a recent report we find that clinopyroxenes have higher delta(26)Mg than co-existing olivines and orthopyroxenes in mantle peridotites, although the magnitude of this difference varies (0.08 to 0.2 parts per thousand). Such small mineral fractionations along with differences between components in primitive meteorites (CAls, chondrules, and matrix) preclude the resolution of mass-dependent fractionation differences smaller than about 0.05 parts per thousand. between Solar System objects. We demonstrate that our results do not support the recent claims that: (i) the Earth-Moon system has a unique Mg isotope composition different from chondrites, or (ii) the higher delta(26)Mg values (up to 0.0) reported for chondrites, achondrites, terrestrial basalts and peridotites. To address the issue of inter-laboratory discrepancies on the Mg isotopic composition of silicate rocks, we have prepared 3 pure Mg standards with wide ranging delta(26)Mg (-3.75 to -0.69 with respect to DSM3) as well as a large volume of homogenized olivine from the San Carlos peridotite, which will be available to the scientific community, for future comparative studies. (C) 2010 Elsevier BM. All rights reserved.