  We model the heating of a primordial planetesimal by decay of the short-lived
radionuclides Al-26 and Fe-60 to determine (i) the timescale on which melting
will occur; (ii) the minimum size of a body that will produce silicate melt and
differentiate; (iii) the migration rate of molten material within the interior;
and (iv) 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-Pb and Hf-W 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 timescale 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 ~2.7 Myr of CAI formation.
