Petrology and oxygen isotopic compositions of calcium‐aluminum‐rich inclusions in primitive CO3.0‐3.1 chondrites

Abstract

The petrologic and oxygen isotopic characteristics of calcium-aluminum-rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0-3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite-bearing inclusions (spinel-hibonite spherules, monomineralic grains, hibonite-pyroxene microspherules, and irregular/nodular objects), grossite-bearing inclusions (monomineralic grains, grossite-melilite microspherules, and irregular/nodular objects), melilite-rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel-pyroxene inclusions (fluffy objects resembling fine-grained spinel-rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene-anorthite inclusions. They are typically small (98.4 AE 54.4 µm, 1SD) and comprise 1.54 AE 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca-Alsilicate during terrestrial weathering and converted melilite-rich inclusions into spinelpyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM-like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0-3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (Δ 17 O: À25 to À2&) but left those in CO3.0-3.05 chondrites mostly unchanged (Δ 17 O: À25 to À20&), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.

Dust changes in Sakurai's Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 micron-sized melilite grains

“…review by Rubin & Ma 2017). CAI abundance in the most primitive (CO3) chondrites is ∼ 1.5 area per cent 1 with an average CAI size of 98.4 ± 54.4 𝜇m (Zhang et al 2020). Melilite-rich inclusions and spinel-pyroxene CAIs comprise 80-100 percent of the total CAIs for each chondrite; Zhang et al argued that the spinel-pyroxene CAIs in these and more processed meteorites are alteration products derived from melilites and found that a subset of fluffy melilites were composed of loosely aggregated < 15𝜇m-sized melilite grains.…”

“…Hibonite is less plausible because the fits are poorer, fewer than 4.8 per cent of CAIs in meteorites are hibonite (and grossite; ∼CaAl 4 O 7 )bearing (Zhang et al 2020), with the metamict form being relatively rare and other hibonites have weaker overtones which do not occur at 6.9 𝜇m, and hibonite bands are bands are 5-30 times weaker than the melilite feature (see Appendix A3 and Bowey & Hofmeister 2005).…”

Dust changes in Sakurai’s Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 μm-sized melilite grains

“…review by Rubin & Ma 2017). CAI abundance in the most primitive (CO3) chondrites is ∼1.5 area per cent 1 with an average CAI size of 98.4 ± 54.4 μm (Zhang et al 2020). Melilite-rich inclusions and spinel-pyroxene CAIs comprise 80-100 per cent of the total CAIs for each chondrite; Zhang et al argued that the spinel-pyroxene CAIs in these and more processed meteorites are alteration products derived from melilites and found that a subset of fluffy melilites were composed of loosely aggregated <15 μm-sized melilite grains.…”

“…Hibonite is less plausible because the fits are poorer, fewer than 4.8 per cent of CAIs in meteorites are hibonite (and grossite; ∼CaAl 4 O 7 )bearing (Zhang et al 2020), with the metamict form being relatively rare and other hibonites have weaker overtones which do not occur at 6.9 μm, and hibonite bands are bands are 5-30 times weaker than the melilite feature (see Appendix A3 and Bowey & Hofmeister 2005).…”