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 EH_a3 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).

We infer that (i) PLAC-like inclusions formed in an isotopically heterogeneous reservoir in which ⁴⁸Ca and ⁵⁰Ti were coupled but both isotopes were decoupled from ⁴⁶Ti suggesting different carrier phases for ⁴⁸Ca + ⁵⁰Ti and ⁴⁶Ti. (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 Δ¹⁷O 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 Δ¹⁷O values, possibly inherited from the protosolar molecular cloud. (iv) The ²⁶Al-poor PLAC-like inclusions predate formation of ‘normal’ CAIs with the canonical (²⁶Al/²⁷Al)₀ supporting heterogeneous distribution of ²⁶Al in the CAI-forming region at the earliest stages of the protoplanetary disk evolution. This heterogeneity may have resulted from heterogeneous distribution of ²⁶Al in the protosolar molecular cloud or from thermal processing of presolar grains having different abundances of live ²⁶Al which were present in the molecular cloud with uniform distribution of ²⁶Al/²⁷Al ratio at the canonical level. We conclude that ²⁶Al-²⁶Mg systematics have a limited significance for the chronology of refractory inclusions.