High-precision Mg isotopic systematics of bulk chondrites

Variations of the mass-independent abundance of 26Mg (δ26Mg*) and stable Mg (δ25Mg) isotope composition of chondrites are important because they constrain the homogeneity of 26Al and Mg isotopes in the proto-planetary disc and the validity of the short-lived 26Al-to-26Mg chronometer applied to meteorites. We present high-precision Mg isotope data and Al/Mg ratios of chondrites representing nearly all major chondrite classes, including a step-leaching experiment on the CM2 chondrite Murchison. δ26Mg* variations in leachates of Murchison representing acid soluble material are ≤ 30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in δ26Mg* (−0.002 to + 0.118‰). Very small variations in δ26Mg* anomalies in bulk chondrites (−0.006 to + 0.019‰) correlate with increasing 27Al/24Mg ratios and δ50Ti, reflecting the variable presence of calcium-aluminium-rich inclusions (CAIs) in some types of carbonaceous chondrites. Similarly, release of radiogenic 26Mg produced by 26Al decay from CAI material in the step-leaching of Murchison best explains the high δ26Mg* observed in the last, aggressive, leaching steps of this experiment. Overall, the observed variations in δ26Mg* are small and potential differences beyond that which result from the presence of CAI-like material cannot be detected within the analytical uncertainties of this study (± 0.004‰). The results do not allow radical heterogeneity of 26Al (≥±30%) or measurable Mg nucleosynthetic heterogeneity (≥±0.005‰) to have existed on a planetesimal scale in the proto-planetary disc. Combined with published δ26Mg* data for CAIs, the bulk chondrite data yield a precise initial (26Al/27Al)0 = (5.21 ± 0.06) × 10−5 and δ26Mg* = −0.0340 ± 0.0016‰ for the Solar System. However, it is not possible with the currently available data to determine with certainty whether CAIs and the material from which planetesimals accreted including chondrite parent bodies had precisely the same initial levels of 26Al, although planetesimals and planets appear to have accreted from material with a mean initial (26Al/27Al)0 in the range of 2.1 to 6.7 × 10− 5. The average stable Mg isotope composition of all analysed chondrites, with the exception of a chondrule from the CBa chondrite Gujba (δ25MgDSM-3 = −0.032 ± 0.035‰), is δ25MgDSM-3 = −0.152 ± 0.079‰ (2 sd) and is indistinguishable from that of the Earth's mantle.