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

Meteorites with significantly sub-chondritic Al/Mg that formed in the first 2 million years of the Solar System should be characterised by deficits in the abundance of Mg-26 (delta Mg-26*) due to the absence of in-growth of Mg-26 from the decay of short-lived Al-26 (t(1/2) = 0.73 Myr). However, these Mg-26 deficits will be small (delta Mg-26* > -0.037 parts per thousand) 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 Mg-26 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 Mg-26 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 delta Mg-26* from the terrestrial materials that are <= 0.005 parts per thousand. However, if Mg yields from chemical separation are low or an inappropriate equilibrium-isotopically fractionated standard is used this will generate analytical artefacts on delta Mg-26* 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 Mg-26 with delta Mg-26*(DSM-3) = -0.0120 +/- 0.0018 parts per thousand and delta Mg-26*(DSM-3) = -0.0062 +/- 0.0023 parts per thousand, respectively, relative to terrestrial olivine (delta Mg-26*(DSM-3) = -0.0029 +/- 0.0028 parts per thousand). Six aubrites have delta Mg-26*(DSM-3) = +0.0015 +/- 0.0020 parts per thousand, which is identical to terrestrial olivine. Model ages from these deficits can be calculated by assuming that Al-26 was homogeneously distributed in the planetesimalforming regions of the proto-planetary disc at the same level as calcium-aluminium-rich inclusions (CAIs). The absence of Mg-26 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)(+0.40) Myr after formation of CAIs. Similarly, ureilites would have experienced silicate partial melting and lowering of their bulk Al/Mg ratios 1.9(-0.7)(+2.2) Myr 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. (C) 2011 Elsevier Ltd. All rights reserved.