Differentiation of planetesimals and the thermal consequences of melt migration

We model the heating of a primordial planetesimal by decay of the short-lived radionuclides Al-26 and Fe-60 to determine (1) the time scale on which melting will occur, (2) the minimum size of a body that will produce silicate melt and differentiate, (3) the migration rate of molten material within the interior, and (4) the thermal consequences of the transport of Al-26 in partial melt. Our models incorporate results from previous studies of planetary differentiation and are constrained by petrologic (i.e., grain-size distributions), isotopic (e.g., Pb-207-Pb-206 and Hf-182-W-182 ages), and mineralogical properties of differentiated achondrites. We show that formation of a basaltic crust via melt percolation was limited by the formation time of the body, matrix grain size, and viscosity of the melt. We show that low viscosity (< 1 Pa . s) silicate melt can buoyantly migrate on a time scale comparable to the mean life of Al-26. The equilibrium partitioning of Al into silicate partial melt and the migration of that melt acts to dampen internal temperatures. However, subsequent heating from the decay of Fe-60 generated melt fractions in excess of 50%, thus completing differentiation for bodies that accreted within 2 Myr of CAI formation (i.e., the onset of isotopic decay). Migration and concentration of Al-26 into a crust results in remelting of that crust for accretion times less than 2 Myr and for bodies > 100 km in size. Differentiation would be most likely for planetesimals larger than 20 km in diameter that accreted within approximately 2.7 Myr of CAI formation.