Calcium isotope constraints on the origin of eucrites and diogenites: The role of magma ocean and magmatism

Abstract

Howardite-Eucrite-Diogenite (HED) meteorites represent a large suit of crustal and sub-crustal rocks from the Vesta. This work presents systematic examination of the Ca isotope data on multiple varieties of HED meteorites for a better understanding of the magmatic evolution of the Vesta. Falls and finds possess similar Ca isotope compositions, and no correlation is observed between δ44/40Ca and (Sr/Eu*)n, indicating that terrestrial weathering effect on Ca isotopes is insubstantial. According to the data in literature, the inner solar system may have a homogeneous δ44/40Ca and the average of inner solar system bodies (0.97±0.03‰) can approximate the composition of bulk silicate Vesta (BSV). Basaltic eucrites define a cluster in δ44/40Ca (0.95±0.07‰, 2SD, N=15) that is higher than the terrestrial mid-ocean ridge basalts (∼0.85‰). Combined with partial melting and magma ocean differentiation modeling, the Ca isotope signatures suggest that eucrites represent the residual melts evolved from a magma ocean formed by primordial Vesta's moderate-to-high degree melting (20-100%). Diogenites have distinguishingly higher δ44/40Ca (1.18±0.15‰, 2SD, N=7) than the basaltic eucrites, which displays a negative correlation with the 1000×Lu/Ti ratio and a positive correlation with 1/Ca. However, magma ocean crystallization can only explain diogenites with δ44/40Ca higher than 1.17‰, suggesting that diogenites have complicated petrogenesis and are not necessarily cogenetic with eucrites. Diogenites with δ44/40Ca<1.17‰ may result from magma-ocean-cumulate partial melts intruding the eucritic crust. Mixing models suggest that the eucritic component in these diogenites may be less than 10%. Two howardites have lower δ44/40Ca of 0.80±0.04‰ and 0.86±0.05‰ than eucrites and diogenites. This signature may reflect the addition of carbonaceous chondritic materials due to impact brecciation.