Magnesium stable isotope composition of Earth's upper mantle

The mantle is Earth's largest reservoir of Mg containing >99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to >99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite +/- amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have delta(25)Mg(DSM-3) = -0.22 to -0.08 parts per thousand. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility <=+/- 0.07 parts per thousand [2 sd]). We find no systematic delta(25)Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier delta(25)Mg than coexisting olivine, any fractionation between mantle pyroxene and olivine is also within current analytical uncertainties with a mean Delta(25)Mg(pyr-ol) = +0.06 +/- 0.10 parts per thousand (2 sd: n = 5). Our average mantle olivine delta(25)Mg(DSM-3) = -0.14 +/- 0.07 parts per thousand and delta(26)Mg(DSM-3) = -0.27 +/- 0.14 parts per thousand (2 sd) are indistinguishable from the average of data previously reported for terrestrial basalts, confirming that basalts have stable Mg isotope compositions representative of the mantle. Olivine from five pallasite meteorites have delta(25)Mg(DSM-3)=-0.16 to -0.11 parts per thousand that are identical to terrestrial olivine and indistinguishable from the average delta(25)Mg previously reported for chondrites. These data provide no evidence for measurable heterogeneity in the stable Mg isotope composition of the source material in the proto-planetary disc from which Earth and chondrite and pallasite parent bodies accreted. (C) 2009 Elsevier B.V. All rights reserved.