Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry

1,5,6Seann J. MCKIBBIN,1,7Lidia PITTARELLO,1Christina MAKARONA,2,3
Christopher HAMANN,2,3Lutz HECHT,4,8Stepan M. CHERNONOZHKIN,1Steven GODERIS,1Philippe CLAEYS
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13392]
1Analytical, Environmental and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels 1050, Belgium
2Museum fur Naturkunde, Leibniz-Institut fur Evolutions- und Biodiversitatsforschung, Invalidenstraße 43,
10115 Berlin, Germany
3Institut fur Geologische Wissenschaften, Freie Universitat Berlin, Malteserstraße 74-100, 12249 Berlin, Germany
4GeoRessources, Faculte des Sciences et Technologies, Universite de Lorraine, Rue Jacques Callot, BP 70239, 54506,
Vandoeuvre-les-Nancy CEDEX, France
5Present address: Institut f€ur Erd- und Umweltwissenschaften, Universitat Potsdam, Haus 27, Karl-Liebknecht-Straße 24-25,
Potsdam-Golm 14476, Germany
6Present address: Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universitat Göttingen,
Goldschmidtstraße 1, Göttingen 37073, Germany
7Present address: Department of Lithospheric Research, Universit€at Wien, UZA 2, Althanstraße 14, Vienna A-1090,Austria
8Department of Chemistry, Universiteit Gent, Krijgslaan 281-S12, Ghent 9000, BelgiumPresent address: Department of Chemistry, Universiteit Gent, Krijgslaan 281-S12, Ghent 9000, Belgium
Published by arrangement with John Wiley & Sons

Main group pallasite meteorites are samples of a single early magmatic planetesimal, dominated by metal and olivine but containing accessory chromite, sulfide, phosphide, phosphates, and rare phosphoran olivine. They represent mixtures of core and mantle materials, but the environment of formation is poorly understood, with a quiescent core–mantle boundary, violent core–mantle mixture, or surface mixture all recently suggested. Here, we review main group pallasite data sets and petrologic characteristics, and present new observations on the low‐MnO pallasite Brahin that contains abundant fragmental olivine, but also rounded and angular olivine and potential evidence of sulfide–phosphide liquid immiscibility. A reassessment of the literature shows that low‐MnO and high‐FeO subgroups preferentially host rounded olivine and low‐temperature P2O5‐rich phases such as the Mg‐phosphate farringtonite and phosphoran olivine. These phases form after metal and silicate reservoirs back‐react during decreasing temperature after initial separation, resulting in oxidation of phosphorus and chromium. Farringtonite and phosphoran olivine have not been found in the common subgroup PMG, which are mechanical mixtures of olivine, chromite with moderate Al2O3 contents, primitive solid metal, and evolved liquid metal. Lower concentrations of Mn in olivine of the low‐MnO PMG subgroup, and high concentrations of Mn in low‐Al2O3 chromites, trace the development and escape of sulfide‐rich melt in pallasites and the partially chalcophile behavior for Mn in this environment. Pallasites with rounded olivine indicate that the core–mantle boundary of their planetesimal may not be a simple interface but rather a volume in which interactions between metal, silicate, and other components occur.

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