A heterogeneous solar nebula as sampled by CM hibonite grains

The magnesium isotopic compositions of 43 hibonite-bearing inclusions extracted from CM2 meteorites Murchion and Paris were measured with a Cameca 1280 HR2 ion microprobe. The results are broadly consistent with the literature data: most Spinel–HIBonite spherules (SHIBs) bear evidence for in situ decay of 26Al at the canonical level, whereas PLAty–hibonite Crystals (PLACs) either lack resolvable 26Mg excesses or are characterized by apparent 26Mg deficits by up to ~ 5‰ relative to the terrestrial composition. With higher precisions achieved in this study, the 26Al/27Al scatter ranging from ~ 6 × 10− 6 to ~ 6 × 10− 5 previously seen in lower precision SHIB data can be clearly resolved. A statistical treatment to all the data reveals three distinctive peaks at 26Al/27Al = 4.9 × 10− 5, 3.5 × 10− 5 and 6.5 × 10− 6, and two minor (barely resolved) components at 26Al/27Al = 6 × 10− 5 and 2.5 × 10− 5. Possible explanations for the observed 26Al/27Al scatter in SHIBs include (1) isotopic disturbance, (2) protracted SHIB formation, (3) early formation before complete homogenization of 26Al in the solar nebula, and (4) initial Mg isotopic heterogeneity during SHIB formation. We argue that the early formation of SHIBs was the most plausible scenario. The PLACs could have formed in a reservoir characterized by 26Al/27Al similar to the average galactic background value, or in an 26Al-free environment. In either case, it is almost certain that the Mg isotope distribution in the PLAC-forming region was heterogeneous. Combining the hydrodynamic timescales of isotope mixing in the solar nebula, one could infer that the duration for PLAC and SHIB formation should only be around tens of thousands of years.