Petrologic, Geochemical, and Spectral Characteristics of Oxidized Planetary Differentiation

Abstract

Meteorites provide evidence that planetary formation occurred across a wide range of oxidation environments in the early Solar System. While the process of differentiation for many reduced, oxygen-poor assemblages has been thoroughly explored, significantly less is known about how differentiation occurred in more oxidized regions of the Solar System. Results from petrologic and geochemical investigations of oxidized chondrites (Rumurutiites) and primitive achondrites (brachinites) reveal that significant mineralogic differences occur with increasing degrees of oxidation. As a consequence, the differentiation pathways of oxidized and reduced assemblages diverge during the earliest stages of partial melting. While reduced materials differentiate to form a basaltic crust, magnesian peridotite mantle, and metallic core, oxidized materials may instead form felsic crusts, ferroan peridotite mantles, and sulfide-dominated cores. These pathways are evident in distinct siderophile trace element systematics for oxidized and reduced endmembers of the brachinite meteorite family. The compositions of olivine between oxidation endmembers are resolvable using remote sensing techniques that are applicable to asteroids. Most olivine-dominated asteroids examined in this work are consistent with having formed in oxidized environments, similar to R chondrites and brachinites, or in even more oxidizing environments not recorded among the meteoritic record. This provides strong evidence that environments capable of supporting oxidized, sulfide-dominated core formation are widespread among asteroidal materials. Several of these asteroids are likely mantle restites, based on their olivine composition and the estimated abundances of pyroxene. The predominance of oxidized over reduced environments among olivine-dominated asteroids is likely related to their respective petrogenetic histories: reduced assemblages must reach and sustain much higher temperatures to fully melt and segregate their pyroxene contents from olivine, which requires larger and earlier-accreted parent bodies. Consequently, sampling reduced mantle restites without significant pyroxene contamination would require catastrophic parent body destruction without mixing crustal and mantle materials. Oxidized materials, in contrast, have much higher initial olivine/pyroxene ratios, and thus are much more prone to producing asteroids dominated by olivine.