The iron isotope composition of enstatite meteorites: Implications for their origin and the metal/sulfide Fe isotopic fractionation factor

摘要

Despite their unusual chemical composition, it is often proposed that the enstatite chondrites represent a significant component of Earth's building materials, based on their terrestrial similarity for numerous isotope systems. In order to investigate a possible genetic relationship between the Fe isotope composition of enstatite chondrites and the Earth, we have analyzed 22 samples from different subgroups of the enstatite meteorites, including EH and EL chondrites, aubrites (main group and Shallowater) and the Happy Canyon impact melt. We have also analyzed the Fe isotopic compositions of separated (magnetic and non-magnetic) phases from both enstatite chondrites and achondrites. On average, EH3-5 chondrites (delta Fe-56 = 0.003 +/- 0.042 parts per thousand; 2 standard deviation; n = 9; including previous literature data) as well as EL3 chondrites (delta(56) Fe = 0.030 +/- 0.038 parts per thousand; 2 SD; n = 2) have identical and homogeneous Fe isotopic compositions, indistinguishable from those of the carbonaceous chondrites and average terrestrial peridotite. In contrast, EL6 chondrites display a larger range of isotopic compositions (-0.180 parts per thousand< delta Fe-56 < 0.181 parts per thousand; n = 11), a result of mixing between isotopically distinct mineral phases (metal, sulfide and silicate). The large Fe isotopic heterogeneity of EL6 is best explained by chemical/mineralogical fragmentation and brecciation during the complex impact history of the EL parent body. Enstatite achondrites (aubrites) also exhibit a relatively large range of Fe isotope compositions: all main group aubrites are enriched in the light Fe isotopes (delta Fe-56 = -0.170 +/- 0.189 parts per thousand; 2 SD; n = 6), while Shallowater is, isotopically, relatively heavy (delta Fe-56 = 0.045 +/- 0.101 parts per thousand; 2 SD; n = 4; number of chips). We take this variation to suggest that the main group aubrite parent body formed a discreet heavy Fe isotope-enriched core, whilst the Shallowater meteorite is most likely from a different parent body where core and silicate material remixed. This could be due to intensive impact-induced shearing stress, or the ultimate destruction of the Shallowater parent body. Analysis of separated enstatite meteorite mineral phases show that the magnetic phase (Fe metal) is systematically enriched in the heavier Fe isotopes when compared to non-magnetic phases (Fe hosted in troilite), which agrees with previous experimental observations and theoretical calculations. The difference between magnetic and non-magnetic phases from enstatite achondrites provides an equilibrium metal-sulfide Fe isotopic fractionation factor of Delta Fe-56(metal-troilite) - delta Fe-56(metal)-delta Fe-56(troilite) of 0.129 +/- 0.060 parts per thousand (2 SE) at 1060 +/- 80 K, which confirms the predictions of previous theoretical calculations. (C) 2014 Elsevier Ltd. All rights reserved.