The Isheyevo meteorite: Mineralogy, petrology, bulk chemistry, oxygen, nitrogen, carbon isotopic compositions, and 40Ar-39Ar ages

Abstract

Isheyevo is a metal-rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7-90 vol%). The metal-rich lithologies with 5060 vol% of Fe,Ni metal are dominant. The metal-rich and metal-poor lithologies are most similar to the CBb and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar componentsFe,Ni metal, chondrules, refractory inclusions (Ca, Al-rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clastsbut show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non-porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15N (δ15N = 1122). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non-magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CBb chondrites, ~20% of Fe,Ni-metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni-rich (>20 wt% Ni). In contrast to CBb and CH chondrites, most metal grains are thermally decomposed into Ni-rich and Ni-poor phases. Similar to CH chondrites, chondrules have porphyritic and non-porphyritic textures and ferromagnesian (type I and II), silica-rich, and aluminum-rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite-, grossite-, and melilite-rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico-chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine-grained dust. Such a scenario is consistent with the presence of solar windimplanted noble gases in Isheyevo and with its comparatively old K-Ar age. We cannot exclude that the K-Ar system was affected by a later collisional event. The cosmic-ray exposure (CRE) age of Isheyevo determined by cosmogenic 38Ar is ~34 Ma, similar to that of the Bencubbin (CBa) meteorite.