A MAGMA OCEAN ON VESTA - CORE FORMATION AND PETROGENESIS OF EUCRITES AND DIOGENITES

Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED m antle indicate that the parent body may have been completely molten du ring its early history. During cooling of a chondritic composition mag ma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reach es a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded li quids are similar in composition to Main Group and Stannern trend eucr ites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Sub sequent fractional crystallization of a Main Group eucrite liquid, whi ch has been isolated as a body of magma, produces the Nuevo Laredo tre nd and the cumulate eucrites. The predicted cumulate mineral compositi ons are in close agreement with phase compositions analyzed in the cum ulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.