Melt migration in rubble-pile planetesimals: Implications for the formation of primitive achondrites

Abstract

Primitive achondrites represent residual mantle material of planetesimals from which up to 20% partial melts were extracted. Melting experiments on chondritic compositions suggest that melts produced by ≲20% partial melting are rich in silica and alkali elements. Such melts are highly viscous (≳103Pa⋅s), and percolation models predict that they would only migrate negligible distances over timescales of 1–10Myr. After these timescales, a planetesimal would either be melted into a magma ocean by radiogenic heating from Al26, if it formed early; or it would cool below solidus, if it formed relatively late. However, melt migration is also controlled by permeability, which could be high for aggregates of rock boulders (compared to those of mineral grains). Specifically, the theory of planet formation suggests that collisions occurred frequently between planetesimals in the early solar system. These collisions may have shattered the planetesimals into fragments with sizes of meters to tens of meters, which would have accreted gravitationally into one or more daughter bodies. We develop a model to investigate melt migration in “rubble-pile” planetesimals; in particular, the melt exchange between partially molten rock boulders and the void space between them. The results suggest that, with typical properties of primitive achondrite materials at the conditions of low-degree partial melting, melts may have been squeezed into the voids between boulders, and migrated rapidly through these channels. Therefore, primitive achondrites may record melt migration in rubble-pile bodies reaccreted from fragments of partially molten planetesimals.