The nature of asteroidal differentiation processes - Implications for primordial heat sources

A review of the geochemical data on basaltic achondrite group meteorites, here defined as diogenites, eucrites, howardites, mesosiderites, main-group pallasites and IIIAB irons, suggests that the following processes and properties were important in determining the compositions of igneous materials on asteroidal sized bodies: (1) inhomogeneities in source rock compositions, (2) variable degrees of partial melting from 5-100%, (3) multiple melt genesis from a single source region, (4) fractional crystallization, and (5) remelting of earlier formed igneous rocks. All of this activity started during or shortly after the major asteroidal accretion phase as suggested by the numerous 4.6 Gyr ages for eucrites and may have lasted for at least 200 m.y. The above properties imply an early, intense heat source that was both spatially and temporally variable. A review of asteroid thermal modeling for various heat sources shows that heating by short-lived Al-26 alone appears to be incapable of supplying the observed geochemical variations.