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

Abstract

Meteorites with significantly sub-chondritic Al/Mg that formed in the first 2million years of the Solar System should be characterised by deficits in the abundance of 26Mg (δ26Mg∗) due to the absence of in-growth of 26Mg from the decay of short-lived 26Al (t1/2=0.73Myr). However, these 26Mg deficits will be small (δ26Mg∗ >−0.037‰) even for material that formed at the same time as the Solar System’s oldest solids – calcium–aluminium-rich inclusions – and thus measurement of these deficits is analytically challenging.Here, we report on a search for 26Mg deficits in three types of ultramafic meteorites (pallasites, ureilites and aubrites) by multiple-collector inductively coupled plasma mass spectrometry. A range of analytical tests were carried out including analysis of: (1) a range of synthetic Mg solution standards; (2) Mg gravimetrically doped with a high purity 26Mg spike; (3) Mg cuts collected sequentially from cation exchange separation columns with fractionated stable Mg isotope compositions; (4) Mg separated from samples that was bracketed by analyses of both DSM-3 and Mg separated from a natural olivine sample subjected to the same chemical processing as the samples. These tests confirm it is possible to resolve differences in δ26Mg∗ from the terrestrial materials that are ⩽0.005‰. However, if Mg yields from chemical separation are low or an inappropriate equilibrium-isotopically fractionated standard is used this will generate analytical artefacts on δ26Mg∗ when this is calculated with the kinetic/exponential mass fractionation law as is the case when correcting for instrumental mass bias during mass spectrometric analysis.Olivine from four different main group pallasites and four bulk ureilites have small deficits in the abundance of 26Mg with δ26MgDSM-3∗=-0.0120±0.0018‰ and δ26MgDSM-3∗=-0.0062±0.0023‰, respectively, relative to terrestrial olivine (δ26MgDSM-3∗=+0.0029±0.0028‰). Six aubrites have δ26MgDSM-3∗=+0.0015±0.0020‰, which is identical to terrestrial olivine.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 >2.9Myr 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.