Bulk chondrite variability in mass independent magnesium isotope compositions – Implications for initial solar system 26Al/27Al and the timing of terrestrial accretion

Abstract

We have determined Δ′26MgDSM-3, the mass-independent variations in 26Mg/24Mg, of primitive, bulk meteorites to precisions better than ±3 ppm (2se). Our measurements of samples from 10 different chondrite groups show Δ′26MgDSM-3 that vary from −5 to 22 ppm. Our data define an array with a positive slope in a plot of Δ′26MgDSM-3 against 27Al/24Mg, which can be used to determine (26Al/27Al)0, i.e. initial 26Al/27Al, and (Δ′26MgDSM-3)0, i.e. initial Δ′26MgDSM-3. On such an isochron plot, the best fit of our new measurements combined with literature data implies (26Al/27Al)0 of (4.67±0.78)×10−5 and (Δ′26MgDSM-3)0 of −31.6 ± 5.7 ppm (2se) for ordinary and carbonaceous chondrites, other than CR chondrites, which have anomalously low Δ′26MgDSM-3. These parameters are within uncertainty of those defined by previous measurements of bulk calcium-, aluminium-rich inclusions (CAIs) that set canonical (26Al/27Al)∼05×10−5. The most straightforward interpretation of all these observations is that differences in the Al/Mg of bulk ordinary and carbonaceous chondrites are dominantly controlled by variable contributions of early-formed refractory and major silicate components derived from a common, canonical reservoir. The Δ′26MgDSM-3 of enstatite chondrites are slightly more radiogenic (∼3 ppm) at similar Al/Mg to the ordinary chondrites. We speculate that this is related to the timing of removal of a refractory component from the source reservoirs of these different meteorite groups; the higher Δ′26MgDSM-3 of the enstatite chondrites suggests later (∼0.5 Ma post CAIs) condensation and loss of this refractory component. Despite inferred consistency of (26Al/27Al)0 and (Δ′26MgDSM-3)0 across most chondrite groups, some nebular heterogeneity is required to account for the compositions of CR chondrites. Our preferred interpretation is that the CR source region has lower (Δ′26MgDSM-3)0. As the most appropriate isotopic reference for the Earth, our new mean enstatite chondrite composition allows us to assess possible ingrowth of 26Mg from live 26Al during accretion of the Earth. The Earth has Δ′26MgDSM-3 within uncertainty of enstatite chondrites, despite its higher Al/Mg. This requires that the terrestrial increase in Al/Mg, which we attribute to vapour loss during accretion, must have happened >1.5 Ma post CAI formation, in an instantaneous fractionation model.