26Al-26Mg deficit dating ultramafic meteorites and silicate planetesimal differentiation in the early Solar System?

Model ages from these deficits can be calculated by assuming that 26Al was homogeneously distributed in the planetesimal-forming regions of the proto-planetary disc at the same level as calcium-aluminium-rich inclusions (CAIs). The absence of 26Mg deficits in aubrites, means these can only be constrained to have formed relatively late >2.9 Myr after CAI formation. Model ages calculated from pallasite olivine deficits would suggest that pallasite olivine crystallised and was diffusively isolated on its parent body 1.24-0.28+0.40Myr after formation of CAIs. Similarly, ureilites would have experienced silicate partial melting and lowering of their bulk Al/Mg ratios 1.9-0.7+2.2Myr after CAI formation. The model ages for silicate differentiation on the main group pallasite parent body are intermediate between those for metal-silicate fractionation for core formation obtained from magmatic iron meteorites and those for asteroidal silicate magmatism obtained from basaltic meteorites.