Grossite-bearing refractory inclusions from reduced CV chondrites: Mineralogical and oxygen isotopic constraints on the parent body alteration history

We report the results of coordinated mineralogical, microstructural, and oxygen isotopic analyses of grossite-bearing refractory inclusions from reduced CV (Vigarano type) chondrites to obtain a more complete picture of secondary parent body alteration processes and conditions. Grossite (CaAl₄O₇) occurs in cores of nodules in fine-grained Ca,Al-rich inclusions (CAIs) that likely represent aggregates of nebular condensates. In many occurrences, grossite has been partially replaced by hercynite [(Fe,Mg,Zn)Al₂O₄], which displays complex microstructures and compositions, and magnetite nanoparticles. The alteration of grossite was a crystallographically-controlled, fluid-driven process that occurred via partial dissolution of grossite and subsequent precipitation of hercynite and magnetite during short-lived and low-temperature metasomatic alteration on the CV chondrite parent body. The constituent phases of grossite-bearing CAIs show heterogeneous oxygen isotopic compositions, with grossite and perovskite displaying systematically ¹⁶O-depleted compositions (Δ¹⁷O= − 12 ‰ to − 1 ‰) relative to uniformly ¹⁶O-rich hibonite and spinel (Δ¹⁷O= − 25 ‰ to − 21 ‰). Melilite is variably ¹⁶O-depleted (Δ¹⁷O= − 25 ‰ to − 2 ‰). The observed oxygen isotopic distribution is interpreted as a result of mineralogically controlled oxygen isotopic exchange with an ¹⁶O-poor fluid on the CV chondrite parent body. Collectively, the presence of limited fluids played an important role in preferential alteration of grossite to hercynite and magnetite and various degrees of ¹⁶O depletion in grossite, perovskite, and melilite during thermal metamorphism. We conclude that, among refractory phases in the inclusions, grossite was the most susceptible to metasomatic reactions with Fe-rich fluids and the second most susceptible, after perovskite, to oxygen isotopic exchange with an ¹⁶O-poor fluid during the thermal history of the CV chondrite parent asteroid.