^1,2Seann J. McKibbin,³Lutz Hecht,¹Christina Makarona,⁴Matthew Huber,³Hermann Terryn, ¹Philippe Claeys
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14147]
¹Analytical-, Environmental-, and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
²Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität Göttingen, Göttingen, Germany
³Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany
⁴Department of Earth Science, University of the Western Cape, Bellville, South Africa
⁵Research Group of Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Brussels, Belgium
Published by arrangement with John Wiley & Sons

The occurrence of forsteritic olivine in EH enstatite chondrites is indicative of bulk disequilibrium. In MgO-rich magmatic systems, forsterite can either crystallize as a liquidus phase or be produced during peritectic melting of enstatite. Because diffusion of divalent cations through forsterite is relatively rapid, it records peak melting (i.e., chondrule-forming events) and is also sensitive to subsequent metamorphism in the EH chondrite parent body. Here, we report the major and minor element geochemistry of olivine in EH chondrites across petrologic types 3 and 4. In all cases, olivine meets the technical definition of forsterite (>90 mole% Mg2SiO4). For unequilibrated EH chondrites, minor elements identify CaO-Al2O3-TiO2-rich (refractory forsterite), MnO-rich (“LIME” forsterite), and FeO-bearing (forsteritic olivine) endmember components, the latter with Cr2O3-rich and Cr2O3-poor varieties. At higher petrologic type, minor element concentrations become restricted and compositions approach pure forsterite, while grain sizes reduce strongly with peak metamorphic temperatures. These changes reflect diffusive equilibration with enstatitic groundmass and dissolution reaction with free silica. The global geochemical distribution of forsteritic olivine in EH chondrites is, perhaps unexpectedly, more similar to those in low-FeO type I chondrules and associated objects in carbonaceous chondrites (CCs), rather than equivalent objects in ordinary (H, L, LL), low-FeO (or HH), or Kakangari (K) chondrites. Among achondrites, there is similarity between pure forsterite in aubrites and EH4 chondrites arising due to subsolidus equilibration in both settings, while Cr2O3-poor forsteritic olivine in EH3 and CCs is similar to magnesian xenocrystic olivine in angrites. This might reflect CaO-rich and SiO2-poor magmatic sources across multiple early solar system reservoirs.

^1,2Robert W. Nicklas,³Kathryn G. Gardner-Vandy,¹James M. D. Day
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14142]
¹Scripps Institution of Oceanography, UniRobert W. Nicklas, Department of Earth and Environmental Sciences,
²Boston College, Chestnut Hill, MA 02467, USA.versity of California San Diego, La Jolla, California, USA
³Aviation and Space, Oklahoma State University, Stillwater, Oklahoma, USA
Published by arrangement with John Wiley & Sons

Highly siderophile elements (HSE) strongly partition into metal phases over silicate minerals and so offer important constraints on nebular and core formation processes acting on early planetesimals. Abundances of the HSE are also an important tool for constraining relationships between metal-rich meteorites. The first bulk rock and in situ HSE abundance and 187Re-187Os data are reported for the ungrouped metal-rich achondrite Tafassasset to examine models of its petrogenesis and origin. Bulk rock and metal grain HSE abundances are elevated at ~2 and ~15 times CI chondrite abundances, respectively, and are largely unfractionated from one another. Metal within Tafassasset is therefore likely to have quenched shortly after partial melting without significant fractional crystallization. Metal grain HSE abundances can be used to calculate a metal fraction of 14 ± 4 wt%, overlapping with the parent bodies of CC iron meteorites, which have also been related to Tafassasset using nucleosynthetic isotope anomalies. Despite such similarities, HSE systematics of bulk rock Tafassasset are not equivalent to any known chondrites, and metal grains do not overlap with iron meteorites or chondrite metal grains, precluding a direct genetic relationship.