Differentiation of Vesta and the parent bodies of other achondrites

Abstract

Numerical simulations have been performed to understand the planetary differentiation of Vesta and other differentiated asteroids. The short‐lived nuclides 26Al and 60Fe were used as the heat sources for the differentiation of asteroids of sizes 20–270 km. The essential aim was to simulate the two proposed differentiation scenarios associated with the formation of the basaltic achondrites. One of these scenarios deals with their origin by partial melting of silicates. The other scenario involves the origin of basalts from the residual melt left subsequent to crystallization in a convective and cooling magma ocean. The core‐mantle differentiation in both the scenarios was commenced subsequent to 40% silicate melting. The partial melt scenario seems to contradict the general trend in the chronological sequence of core‐mantle and mantle‐crust differentiations observed in various differentiated meteorites unless the irons meteorite parent bodies differentiated rapidly compared to prolonged accretion and differentiation of distinct parent bodies of achondrites. Even though the geochemical records of siderophile elements in eucrites favor the residual melt scenario, based on our simulations it is difficult to rapidly and globally cool the magma oceans to produce residual basaltic melts subsequent to crystallization within initial ∼10 million years. This is in contradiction with the chronological records of the basalts that suggest their early origin. In order to make this scenario viable by invoking rapid and global cooling of the magma ocean it would be essential to substantially remove the insulating crust of the planetesimal by continuous bombardment of small planetesimals in the early solar system.