Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection

Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this “Great Dunite Shortage” we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites. Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean DO value of 0.187 ± 0.016& (2r), which is fully resolved from the HED DO value of 0.246 ± 0.014 (2r) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for DO bimodality within the main-group pallasites, as suggested by a number of previous studies. Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have DO values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to DO, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation. Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in howardites may have a similar origin. http://dx.doi.org/10.1016/j.gca.2015.07.023 0016-7037/ 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). ⇑ Corresponding author. Tel.: +44 1908 654107. E-mail address: [email protected] (R.C. Greenwood). www.elsevier.com/locate/gca Available online at www.sciencedirect.com