Nebular history of an ultrarefractory phase bearing CAI from a reduced type CV chondrite

Abstract

Ultrarefractory (UR) phases in calcium-aluminum-rich inclusions (CAIs) could have formed at higher temperature compared to common CAI minerals and thus they potentially provide constraints on very high temperature processes in the solar nebula. We report a detailed characterization of the mineralogy, petrology and oxygen isotopic composition of an UR phase davisite (CaScAlSiO 6 ) bearing CAI from a reduced type CV chondrite. The CAI is an irregular-shaped, compound inclusion that consists of five units that are composed of melilite + spinel + Al,Ti-rich pyroxene ± perovskite with various modal abundances of minerals and lithologies, and surrounded by the Wark-Lovering (WL) rim. Davisite occurs only in one lithological unit that consists of three chemically and isotopically distinct parts: i) 16 O-poor (−20‰ ≤ δ 18 O ≤ 0‰) regions with reversely-zoned melilite and davisite; ii) 16 Orich (−50‰ ≤ δ 18 O ≤ −40‰) regions consisting of unzoned, gehlenitic melilite, Al,Ti-rich diopside and spinel; and iii) spinel framboids composed of 16 O-rich spinel and 16 O-poor melilite. Absence of secondary iron-and/or alkali-rich phases, occurrence of low-iron, manganese-enriched (LIME) olivine, and random distribution of the oxygen isotopic heterogeneity indicate that primitive chemical and isotopic compositions are preserved in the inclusion. The occurrence of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates formation of the inclusion by an aggregation of mineral assemblages formed and processed * Corresponding author. separately at different time and/or space in the solar nebula. Although isotope exchange between 16 O-rich solids and 16 O-poor gases prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of oxygen isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct 16 O-rich and 16 O-poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the inclusion (i.e., the WL rim formation event) was short (e.g., 10 3 hours at 1400 K; 10 5 hours at 1100 K), which can be achieved by rapid outward transport of the CAI. High Ti 3+ /Ti tot ratios of pyroxene from CAI interior and the rim and LIME composition of the olivine rim document that the entire CAI formation process took place under highly reducing conditions.

Mineralogical and oxygen isotopic study of a new ultrarefractory inclusion in the Northwest Africa 3118 CV3 chondrite

“…Most pristine CAIs in most unmetamorphosed carbonaceous chondrites are usually uniformly 16 O-rich (D 17 O~-22&; Yurimoto Krot et al 2012) and should have formed in the 16 O-rich gaseous reservoirs (Yurimoto et al 2008). Meanwhile, numerous investigations suggested that 16 Opoor gaseous reservoirs might also exist in the CAI formation regions (Yurimoto et al 1998;Simon et al 2011;Katayama et al 2012;Kawasaki et al 2012Kawasaki et al , 2017Park et al 2012;Zhang et al 2015;Yoshizaki et al 2019;Krot et al 2019aKrot et al , 2019b. The presence of 16 Opoor gaseous reservoirs might have led to the formation of uniformly 16 O-poor CAIs and CAIs with heterogeneous oxygen isotopic compositions.…”