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

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 delta Ca-44/40 values (per mil Ca-44/Ca-40 ratios relative to NIST 915a) best represents that of bulk silicate Vesta (0.83 +/- 0.04 parts per thousand). This value is different from those of bulk Earth (0.94 +/- 0.05 parts per thousand) and Mars (1.04 +/- 0.07 parts per thousand), suggesting that there exists notable Ca isotope heterogeneity between inner solar system bodies. The delta Ca-44/40 difference between chondrules and these planets does not support the pebble accretion model as the main mechanism for planetary growth. Plain Language Summary Calcium is a major, refractory element in solar system, and its mass-dependent isotope fractionation effect is a robust proxy for probing planetary magmatic evolution and tracing the genetic relationships between solar system materials. We report high-precision Ca isotope data for the howardite-eucrite-diogenite and mesosiderite meteorites, which potentially derive from the asteroid 4 Vesta, to better understand the origin and differentiation of Vesta. Eucrites and diogenites have different mass-dependent Ca isotope compositions, which is caused by orthopyroxene crystallization from a magma ocean. We have modeled the Ca isotope evolution of this magma ocean and find that eucrites and diogenites can have formed from this melt. Eucrites show similar Ca stable isotope compositions to howardites and mesosiderites, consistent with a mixing model of eucrites and diogenites for howardites and the silicate portion of mesosiderites originating from Vesta. The Ca-rich eucrites can best represent the Ca isotope composition of bulk Vesta. It shows Earth, Mars, and Vesta do not share a common Ca isotope composition, suggesting their potentially different precursor material. All these planets and asteroids possess different Ca isotope composition from the chondrules formed in the inner solar system, which does not support a chondrule-rich model for accretion of terrestrial planets.