IGNEOUS PETROGENESIS ON VESTA, IN THE LIGHT OF DAWN

Eucrite and diogenite meteorites are ancient basalts and ultramafic rocks, respectively, derived from the large, differentiated asteroid Vesta. Forty years ago, melting experiments by Edward Stolper showed that low-pressure partial melting of a primitive chondritic source could have produced eucrite parent magmas, and more advanced melting of that source may have produced liquids that fractionated to form diogenites. In contrast to serial magmatism, subsequent work has mostly attributed these rocks to crystallization of a global-scale magma ocean. In that scenario, eucrites are fractionated melts that formed the vestan crust and diogenites are mantle cumulates. However, neither equilibrium nor fractional crystallization models of a magma ocean can reproduce all the geochemical attributes of eucrites, and trace element data for diogenites cannot be reconciled with a single magma source. The very concept of magma oceans on asteroids has been questioned, based on modeling of magma ascent rates in low gravity.

Mapping of Vesta by Dawn spacecraft spectrometers has determined that the regolith is a varying mixture of eucrite and diogenite, similar to howardite meteorites. A chondritic bulk composition has been confirmed, and a massive core is consistent with the depletion of siderophile elements in eucrites. The deep Rheasilvia impact basin exposes no pervasive diogenite stratigraphy, as predicted by magma ocean models. However, magnesian harzburgite clasts in howardite breccias indicate that this impact excavated mantle material. The limited amounts of olivine-bearing diogenite in Rheasilvia suggest that olivine may be sequestered in the lower mantle. Gravity anomalies reveal the existence of dense quasi-circular structures within the vestan crust, interpreted as diogenite plutons. Although rapid heating by short-lived radionuclides likely produced pervasive melting and perhaps a shallow magma ocean, the Dawn data suggest that discrete basaltic magmas erupted or intruded the crust, at least during the later stages of Vesta’s magmatic history. It remains unclear whether these formed from subsequent partial melts or isolated batches of late-stage melt from a magma ocean. Petrogenesis on Vesta was apparently more complex and protracted than envisioned in previous models.