Origin and formation of a chondritic xenolith in Krymka ( LL3 .2, breccia): Indications for a late formation of the accretionary breccia

Abstract

An unusual chondritic xenolith was found in two sequentially prepared thin sections of a sample from the Krymka (LL3.2) chondrite. The xenolith has a rounded, slightly deformed shape of about 5 mm in apparent diameter and is partially surrounded by a double rim made of an inner fine‐grained silicate‐rich rim and an outer sulfide‐rich rim. The xenolithic inclusion is characterized by partially equilibrated mineral constituents, a recrystallized chondritic texture with relic chondrules, and a high abundance of CAIs (0.11 vol%). Within the core of the xenolith, olivine and low‐Ca pyroxene are the most abundant mineral phases, and randomly analyzed grains by grid analysis revealed mean compositions of Fa _9.8±5.5 and Fs _7.2±4.4 Wo _2.9±2.2 for olivine and low‐Ca pyroxene, respectively. Within the entire clast, a feldspar‐normative mesostasis is embedding all constituents, indicating partial melting of the xenolith, probably during impact metamorphism. Thus, the xenolithic clast is very likely an impact melt rock. Infrared (IR) spectroscopic studies revealed the dominance of olivine and low‐Ca pyroxene in the obtained spectra from the fine‐grained silicate‐rich rim of the xenolith. Oxygen isotope analyses by SIMS show that, in the three‐oxygen isotope diagram, most individual olivine grains from the xenolith plot within the field of bulk ordinary chondrites and their chondrules, except for three olivines: Two grains from the xenolith's core (Δ ¹⁷ O = −1.6 ± 0.5‰ and −2.4 ± 0.5‰) and one olivine from the rim (Δ ¹⁷ O = −6.5 ± 0.4‰) show significant ¹⁶ O enrichments. The chondritic impact melt rock studied here clearly demonstrates that this xenolithic clast formed prior to the Krymka parent body accretion within another pre‐existing chondritic parent body. While previous studies have discussed a potential late‐stage accretion of large Krymka constituents, the components within the apparent first‐generation parent body experienced thermal annealing, and, subsequently, the xenolith suffered partial melting due to a shock event that probably caused this fragment to be ejected from its first‐generation parent body.