Large heterogeneous ²⁶Mg excesses in a hibonite from the Murchison meteorite

Abstract

The correlation of excess ²⁶Mg with Al/Mg ratio in coexisting phases of refractory inclusions from carbonaceous chondrites has been cited as evidence for the in situ decay of²⁶A1 in the early Solar System^1–8; many refractory inclusions indicate an initial ²⁶A1/²⁷A1 of ˜5 × 10⁻⁵. On the other hand, the short half life of ²⁶Al, and its apparently high activity in the interestellar medium as measured by γ-ray spectroscopy9, suggest that interstellar dust could be enriched in ²⁶Mg1^10,11. In this model, the ²⁶Mg excesses measured in refractory inclusions are the result of 'chemical memory' of the constituent material. Here we report measurements of ²⁶Mg excesses approaching 400% in a hibonite, grain 7-971, from the Murchison carbonaceous chondrite. The excesses are not homogeneous: the core is enriched by 355 ±6%, at the 95% confidence limit, whereas an analysis nearer the edge has a lower excess of 145±6%. The magnesium in 7-971 is also significantly mass-fractionated, with the heavy isotopes enriched relative to terrestrial compositions, and this is a possible signature of a distillation event. The magnesium isotope systematics of this hibonite are compatible with models incorporating either in situ²⁶Al decay and/or chemical memory of exotic Mg, but the Al–Mg system was probably disturbed in a later thermal event.