Calcium Isotope Evolution During Differentiation of Vesta and Calcium Isotopic Heterogeneities in the Inner Solar System

Abstract

We employed MC-ICP-MS to measure the mass-dependent Ca isotope compositions of Vesta-related meteorites. Eucrites and diogenites show distinct Ca isotope compositions, which is caused by crystallization of isotopically heavy orthopyroxene. The Ca isotope data support a model where the two lithologies are linked, where the diogenites, mainly composed of orthopyroxene crystallized from an eucritic melt. As normal eucrites are the main Ca reservoir on Vesta, their δ44/40Ca values (per mil 44Ca/40Ca ratios relative to NIST 915a) best represents that of bulk silicate Vesta (0.83 ± 0.04‰). This value is different from those of bulk Earth (0.94 ± 0.05‰) and Mars (1.04 ± 0.07‰), suggesting that there exists notable Ca isotope heterogeneity between inner solar system bodies. The δ44/40Ca difference between chondrules and these planets does not support the pebble accretion model as the main mechanism for planetary growth.