Insights into planetesimal evolution: Petrological investigations of regolithic howardites and carbonaceous chondrite impact melts

Asteroidal meteorites are the only available geologic samples from the early part of our solar system’s history. These meteorites contain evidence regarding how the earliest protoplanetary bodies formed and evolved. I use petrological and geochemical techniques to investigate the evolution of these early planetesimals, focusing on two meteorite types: Howardites, which are brecciated samples of a differentiated parent body (thought to be the asteroid 4 Vesta), and CV chondrites, which are primitive chondrites that have not undergone differentiation on their parent body.

Quantitative petrological analysis and characterization of paired regolithic (solar wind-rich) howardites indicate that this large sample of the surface regolith of Vesta is assembled from a range of diverse source materials ( > 18 basaltic and ultramafic lithologies). The abundance of plagioclase in these howardites is depleted relative to unbrecciated source lithologies, which provides evidence that plagioclase is preferentially comminuted by impact gardening on Vesta, as previous studies have suggested it is in lunar regolith. The extent of plagioclase depletion in solar wind-rich howardites may be an indicator of regolith maturity.

The regolithic howardites I studied contain fragments of Mg-rich olivine and pyroxene in low modal abundance ( < 1%); we present geochemical evidence that these mineral fragments are the first recognized mantle samples from Vesta. In general, mantle samples from differentiated meteorite parent bodies are lacking in meteorite collections, which presents an obstacle for understanding the differentiation of these early planetesimals. The geochemical signatures of these Mg-rich olivine and pyroxene fragments suggest that Vesta’s mantle may have only partially melted during its differentiation, rather than forming a whole-mantle magma ocean.

In a regolithic howardite and two CV chondrites, I found impact melt clasts derived from primitive chondrite precursors (CV and CM chondrites). Previously, it had been hypothesized that impact melts might not form from carbonaceous chondrites because their high volatile content might explosively disrupt the formation of a cohesive melt. These clasts provide evidence that impact melts of these precursors can form, that these melts experience some volatile loss, and that they retain some of the redox trends of their precursors.