Iron isotope fractionation during planetary differentiation

The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements (delta(56) Fe approximate to +/- 0.04 parts per thousand, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation. Fractionation during planetary core formation is confined to <0.1 parts per thousand for delta(56)Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (approximate to 0.5 parts per thousand) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest (delta(56)Fe approximate to -0.25 parts per thousand) and schreibersite the heaviest (delta(56)Fe approximate to +0.2 parts per thousand) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate-metal separation. Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings (delta(56)Fe=+0.015+/- 0.018 parts per thousand), which is significantly lighter than the mean of terrestrial basalts (delta(56)Fe = +0.076 +/- 0.029 parts per thousand). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples (delta(56)Fe=+0.073 +/- 0.019 parts per thousand), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here (delta(56)Fe approximate to +0.2 parts per thousand). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts. Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine + metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average delta(56)Fe is very close to that of the IRMM-014 standard. (C) 2005 Elsevier B.V. All rights reserved.