  Rocky planetesimals in the early solar system melted internally and evolved
chemically due to radiogenic heating from Al-26. Here we quantify the
parametric controls on magma genesis and transport using a coupled petrological
and fluid mechanical model of reactive two-phase flow. We find the mean grain
size of silicate minerals to be a key control on magma ascent. For grain sizes
larger than $\approx$ 1 mm, melt segregation produces distinct radial structure
and chemical stratification. This stratification is most pronounced for bodies
formed at around 1 Myr after formation of Ca,Al-rich inclusions. These findings
suggest a link between the time and orbital location of planetesimal formation
and their subsequent structural and chemical evolution. According to our
models, the evolution of partially molten planetesimal interiors falls into two
categories. In the magma ocean scenario, the whole interior of a planetesimal
experiences nearly complete melting, which would result in turbulent convection
and core-mantle differentiation by the rainfall mechanism. In the magma sill
scenario, segregating melts gradually deplete the deep interior of the
radiogenic heat source. In this case, magma may form melt-rich layers beneath a
cool and stable lid, while core formation would proceed by percolation. Our
findings suggest that grain sizes prevalent during the internal heating stage
governed magma ascent in planetesimals. Regardless of whether evolution
progresses toward a magma ocean or magma sill structure, our models predict
that temperature inversions due to rapid Al-26 redistribution are limited to
bodies formed earlier than $\approx$ 1 Myr after CAIs. We find that if grain
size was smaller than $\approx$ 1 mm during peak internal melting, only
elevated solid-melt density contrasts (such as found for enstatite chondrite
compositions) would allow substantial melt segregation to occur.
