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

Seann J Mckibbin,Stepan M Chernonozhkin,Christopher Hamann,Christina Makarona,Philippe Claeys,Lidia Pittarello,Lutz Hecht,Steven Goderis

doi:10.1111/maps.13392

Abstract

AbstractMain 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.

Highlights

Delivered samples of differentiated planetesimals, formed in the first few million years of the solar system’s history, are available to us as nonchondritic meteorites
Processes of differentiation leading to separate planetesimal cores and mantles are poorly recorded by meteorites, the final stages of core–mantle separation are preserved in a few planetary sources, the various pallasite parent bodies
Aside from the lack of phosphate-bearing phases in primitive metal, there is no obvious relationship among the PMG between Ir-Au metal systematics and the other correlations that we have identified in this study, namely olivine texture, olivine composition, and type of accessory phases (Al-free or Al-bearing chromite, the various phosphates, and phosphoran olivine)

Summary

INTRODUCTION

Delivered samples of differentiated planetesimals, formed in the first few million years of the solar system’s history, are available to us as nonchondritic meteorites. A piece has been identified measuring ~5 9 5 cm (RBINS sample M226) with approximately half containing relatively abundant large rounded olivine, with most ~4–5 mm and up to ~6 mm diameter, while the rest contains finer angular and fragmental olivine with a wide range of grain sizes that is more typical of this meteorite. This petrographic arrangement has not been reported previously in Brahin and the section was mapped by microscale X-ray fluorescence (micro-XRF) using a Bruker M4. These are the same instrumental settings as previously reported in Chernonozhkin et al (2017)

Background

DISCUSSION

CONCLUSIONS