Differentiation of Vesta: Implications for a shallow magma ocean

Abstract

The Dawn mission confirms earlier predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and supports the conjecture of Vesta being the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid. We have expanded the thermo-chemical evolution model of Neumann et al. (2012) that includes accretion, compaction, melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources, advective heat transport, and differentiation by porous flow, to further consider convection and the associated effective cooling in a potential magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity. Our results show that partitioning of 26Al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean. Due to the enrichment of 26Al in the liquid phase and its accumulation in the sub-surface (for formation times t0<1.5 Mat0<1.5 Ma), a thin shallow magma ocean with a thickness of 1 to a few tens of km forms – its thickness depends on the viscosity of silicate melt. The lifetime of the shallow magma ocean is O(104)O(104)–O(106)O(106) years and convection in this layer is accompanied by the extrusion of 26Al at the surface, resulting in the formation of a basaltic crust. The interior differentiates from the outside inwards with a mantle that is depleted in 26Al and core formation is completed within ∼0.3 Ma∼0.3 Ma. The lower mantle experiences a maximal melt fraction of 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of non-cumulate eucrites by the extrusion of early partial melt while cumulate eucrites and diogenites may form from the crystallising shallow magma ocean. Silicate melt is present in the mantle for up to 150 Ma, and convection in a crystallising core proceeds for approximately 100 Ma, supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some HED meteorites.