Two isotopically distinct populations of refractory inclusions in the EHa3 chondrite Sahara 97072 – Significance for understanding the evolution of the CAI-formation region

The nature of isotopic differences between ‘normal’ Ca,Al-rich inclusions (CAIs) characterized by the canonical initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of ∼5 × 10 −5 and the anomalous refractory inclusions characterized by the significantly lower ( 26 Al/ 27 Al) 0 , < ∼3 × 10 −6 , which include PLACs (platy hibonite crystals), PLAC-like inclusions, and some corundum-, hibonite-, and grossite-rich CAIs, remains controversial. The 26 Al-poor inclusions may have formed earlier, prior to ‘normal’ CAIs, and recorded heterogeneous distribution of 26 Al in the CAI-forming region, or they may have formed after nearly complete decay of 26 Al, ∼ >4 Myr later than the canonical CAIs. Here we present the first high precision multi-isotopic (O, Mg, Ca, and Ti) study of refractory inclusions (RIs) in the EH a 3 enstatite chondrite Sahara 97072 using in situ SIMS measurements. Our study revealed the presence of two isotopically distinct populations of CAIs in this meteorite: ‘normal’ CAIs and PLAC-like inclusions. The ‘normal’ CAIs composed of spinel, Al,Ti-diopside, ±hibonite, and secondary minerals, most likely replacing melilite, have solar-like Δ 17 O of ∼−23 ‰, ∼ the canonical ( 26 Al/ 27 Al) 0 , and no resolvable nucleosynthetic isotope anomalies in Ca and Ti. The PLAC-like inclusions composed of hibonite, corundum, and ± Al,Ti-pyroxene have Δ 17 O of ∼−19 ‰, no resolvable excess of radiogenic 26 Mg, and large nucleosynthetic isotope anomalies in Ti and Ca: one inclusion has positive anomalies in 50 Ti (835ε) and 48 Ca (685ε), whereas another one has negative anomalies in 50 Ti (−116ε), 46 Ti (−112ε), 48 Ca (−284ε). We infer that ( i ) PLAC-like inclusions formed in an isotopically heterogeneous reservoir in which 48 Ca and 50 Ti were coupled but both isotopes were decoupled from 46 Ti suggesting different carrier phases for 48 Ca + 50 Ti and 46 Ti. ( ii ) ‘Normal’ CAIs formed in a reservoir with uniform distribution of Ca and Ti isotopes, possibly reflecting increasing homogenization of this region with time due to evaporation/condensation, mixing and aggregation of isotopically anomalous grains present in the protosolar molecular cloud. ( iii ) The observed differences in Δ 17 O of ‘normal’ and PLAC-like CAIs indicate their formation in nebular reservoirs with distinct O-isotope compositions, which could have resulted from evaporation of disk regions with different dust/gas ratios, assuming that dust and gas had different Δ 17 O values, possibly inherited from the protosolar molecular cloud. ( iv ) The 26 Al-poor PLAC-like inclusions predate formation of ‘normal’ CAIs with the canonical ( 26 Al/ 27 Al) 0 supporting heterogeneous distribution of 26 Al in the CAI-forming region at the earliest stages of the protoplanetary disk evolution. This heterogeneity may have resulted from heterogeneous distribution of 26 Al in the protosolar molecular cloud or from thermal processing of presolar grains having different abundances of live 26 Al which were present in the molecular cloud with uniform distribution of 26 Al/ 27 Al ratio at the canonical level. We conclude that 26 Al- 26 Mg systematics have a limited significance for the chronology of refractory inclusions.