26Al-to-26Mg dating planetary formation and differentiation in the early Solar System
Magnesium has three isotopes (24, 25 & 26), one of which can be produced by the short-lived decay of 26Al (t1/2 = 0.73 Myr). Recent application of multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) to Mg isotope analysis has potentially opened up a new range of dating opportunities. However, in order take advantage of these opportunities it is necessary to resolve del26Mg* anomalies with a precision and accuracy of the Mg isotope measurements of ca. ±0.005‰ or better, and it is also crucial to investigate the initial distribution of 26Al and Mg isotopes in the early Solar System. This is especially of importance as it has been shown that there are significant nucleosynthetic variations in the neutron-rich isotope 54Cr in chondrites that are a magnitude greater than del26Mg* anomalies that have been reported for achondrites. If a homogeneous distribution of 26Al and Mg isotopes in the early Solar System can be demonstrated, small del26Mg* excesses or deficits in meteorites and their constituents that are now resolvable might allow isochron or model age dating of material that was previously impossible to date. We have measured Mg isotopes in nearly all major classes of meteorites including all groups of chondrites and most achondrites where small del26Mg* excesses (angrites, eucrites) and deficits (pallasite olivines, ureilites, aubrites) have previously been reported. We found the total range of del26Mg* anomalies in whole rock analyses of chondrites to be an order of magnitude lower than the variations reported for 54Cr and the anomalies generally correlate with Al/Mg ratios and are principally controlled by the presence of refractory inclusions in some carbonaceous chondrites, indicating an initial homogeneous distribution of 26Al throughout the planet forming region at the level currently resolvable. Very small del26Mg* deficits in pallasites, samples from the core-mantle boundaries of planetesimals, constrain mantle-core formation to the first million years of the Solar System. del26Mg* excesses in various basaltic meteorites date igneous crust formation of different planetesimals to 2.9 to 4.8 Myr after Solar System formation.