科研之友
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摘要
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* %26gt; -0.037 parts per thousand) even for material that formed at the same time as the Solar System%26apos;s oldest solids - calcium-aluminium-rich inclusions - and thus measurement of these deficits is analytically challenging. %26lt;br%26gt;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 %26lt;= 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. %26lt;br%26gt;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. %26lt;br%26gt;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 %26gt;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.
- 出版日期2012-1-15
