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

Abstract

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.

New constraints on the formation of main group pallasites derived from in situ trace element analysis and 2D mapping of olivine and phosphate

“…However, the cooling rates determined for pallasite metal and olivine are distinct, suggesting a complex formational history. Earlier suggestions of faster cooling of silicate at high temperature and slower cooling at lower temperatures following the metal-olivine mixing event (Tomiyama and Huss 2005;Yang et al 2010) are supported by Mn-Cr isotope ratio data (McKibbin et al, 2016), although detailed interpretations of pallasite formation histories remain controversial (McKibbin et al, 2019). The exact relationship between PMG and IIIAB iron meteorites (Yang et al, 2010) and how pallasite silicates formed (McKibbin et al, 2013) remain outstanding 4 questions.…”

“…The PMG exhibit a limited range of trace element compositions (Wasson and Choi, 2003) and oxygen isotope ratios that are closely related and only different from one another as the result of mass-dependent fractionation processes (Greenwood et al, 2006;Greenwood et al, 2015). The PMG may be further sub-divided into i) a common subgroup, which may represent mechanical mixtures of olivine, chromite, primitive solid metal and evolved metal, and ii) low-MnO and high-FeO subgroups, originating from the core-mantle interface of their planetesimal, where interactions between metal, silicate and other components took place (McKibbin et al, 2019). The trace element systematics of PMG metal (Wasson and Choi, 2003) and preliminary chromite oxygen isotope ratio results (Franchi et al, 2013) indicate a possible link with the IIIAB iron meteorite parent body.…”

A Time‐Resolved Paleomagnetic Record of Main Group Pallasites: Evidence for a Large‐Cored, Thin‐Mantled Parent Body

“…The pallasites have an enigmatic formation history. They have commonly been interpreted to represent the core–mantle boundary, but this has been questioned given their paleomagnetic record of the dynamo, which suggests that they must have formed at a much lower temperature than the molten core (Bryson et al., 2015; Mckibbin et al., 2019; Nichols et al., 2016; Tarduno et al., 2012). In addition, their varied cooling rates have been used to argue that they formed at a variety of depths in a disorganized asteroid reassembled after impact (Yang et al., 2010).…”

Olivines in main-group pallasites: magma-ocean cumulates or partial melting residues?

“…While pallasites are recognised to be mixtures of mantle and core materials, their formation is still debated (Boesenberg et al, 2012;McKibbin et al, 2019;and references therein). Since the 1960s, they are usually seen as samples of the core-mantle transition zone of a differentiated body (e.g., Anders, 1964;Wasson and Choi, 2003), but this hypothesis has been repeatedly challenged.…”