Crystallization and cooling conditions for diogenite formation in the turbulent magma ocean of asteroid 4 Vesta

TL;DR

This study models the differentiation of asteroid 4 Vesta, focusing on magma ocean dynamics, crystal settling, and cooling, to explain diogenite formation and its mineral composition.

Contribution

It introduces a new differentiation model incorporating fluid dynamics, chemical equilibrium, and crystal separation, specifically analyzing the effects of crystal size and cooling conditions.

Findings

Large crystals (>1 cm) settle within ~20,000 years forming diogenite-like layers.

Smaller crystals create thinner cumulate layers.

Model results align with observed diogenite Mg# values.

Abstract

Despite its small size, the asteroid 4 Vesta has been completely differentiated to core and mantle. Its composition is similar to howardite--eucrite--diogenite (HED) meteorites of which the detailed petrology is known. Therefore, Vesta is a good target for understanding the differentiation of terrestrial planets. A new differentiation model for crust formation has been developed by taking magma ocean fluid dynamics, chemical equilibrium, the presence of $^{26}$Al, and cooling into consideration with a special focus on crystal separation. The role of crystal size, thickness of the conductive lid, and fO$_{2}$ are evaluated as parameters. The results show that large crystals of at least 1 cm settled and formed a kilometer-thick cumulate layer of orthopyroxene with Mg$\#$ of 0.70-0.90 in ~20 thousand years, which almost agrees with the Mg$\#$ of diogenites. Smaller grain sizes formed thinner layers.