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

We have determined Δ^′26Mg_DSM-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^′26Mg_DSM-3 that vary from −5 to 22 ppm. Our data define an array with a positive slope in a plot of Δ^′26Mg_DSM-3 against ²⁷Al/²⁴Mg, which can be used to determine (²⁶Al/²⁷Al)₀, i.e. initial ²⁶Al/²⁷Al, and (Δ^′26Mg_DSM-3)₀, i.e. initial Δ^′26Mg_DSM-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^′26Mg_DSM-3)₀ of −31.6 ± 5.7 ppm (2se) for ordinary and carbonaceous chondrites, other than CR chondrites, which have anomalously low Δ^′26Mg_DSM-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^′26Mg_DSM-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^′26Mg_DSM-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^′26Mg_DSM-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^′26Mg_DSM-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^′26Mg_DSM-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.