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

Abstract

The nature of isotopic differences between ‘normal’ Ca,Al-rich inclusions (CAIs) characterized by the canonical initial 26Al/27Al ratio [(26Al/27Al)0] of ∼5 × 10−5 and the anomalous refractory inclusions characterized by the significantly lower (26Al/27Al)0, < ∼3 × 10−6, which include PLACs (platy hibonite crystals), PLAC-like inclusions, and some corundum-, hibonite-, and grossite-rich CAIs, remains controversial. The 26Al-poor inclusions may have formed earlier, prior to ‘normal’ CAIs, and recorded heterogeneous distribution of 26Al in the CAI-forming region, or they may have formed after nearly complete decay of 26Al, ∼ >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 EHa3 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 Δ17O of ∼−23 ‰, ∼ the canonical (26Al/27Al)0, and no resolvable nucleosynthetic isotope anomalies in Ca and Ti. The PLAC-like inclusions composed of hibonite, corundum, and ± Al,Ti-pyroxene have Δ17O of ∼−19 ‰, no resolvable excess of radiogenic 26Mg, and large nucleosynthetic isotope anomalies in Ti and Ca: one inclusion has positive anomalies in 50Ti (835ε) and 48Ca (685ε), whereas another one has negative anomalies in 50Ti (−116ε), 46Ti (−112ε), 48Ca (−284ε).