Forsterite-rich accretionary rims around calcium-aluminum-rich inclusions from the reduced CV3 chondrite Efremovka

Abstract

It was suggested that multilayered accretionary rims composed of ferrous olivine, andradite, wollastonite, salite-hedenbergitic pyroxenes, nepheline, and Ni-rich sulfides around Allende calcium-aluminum-rich inclusions (CAIs) are aggregates of gas-solid condensates which reflect significant fluctuations in physico-chemical conditions in the slowly cooling solar nebula and grain/gas separation processes. In order to test this model, we studied the mineralogy of accretionary rims around one type A CAI (El04) and one type B CAI (E48) from the reduced CV3 chondrite Efremovka, which is less altered than Allende. In contrast to the Allende accretionary rims, those in Efremovka consist of coarse-grained (20-40 μm), anhedral forsterite (Fa18), Fe, Ni-metal nodules, amoeboid olivine aggregates (AOAs) and fine-grained CAIs composed of Al-diopside, anorthite, and spinel, ± forsterite. Although the fine-grained CAIs, AOAs and host CAls are virtually unaltered, a hibonite-spinel-perovskite CAI in the E48 accretionary rim experienced extensive alteration, which resulted in the formation of Fe-rich, Zn-bearing spinel, and a Ca, Al, Si-hydrous mineral. Forsterites in the accretionary rims typically show an aggregational nature and consist of small olivine grains with numerous pores and tiny inclusions of Al-rich minerals. No evidence for the replacement of forsterite by enstatite was found; no chondrule fragments were identified in the accretionary rims. We infer that accretionary rims in Efremovka are more primitive than those in Allende and formed by aggregation of high-temperature condensates around host CAIs in the CAI-forming regions. The rimmed CAIs were removed from these regions prior to condensation of enstatite and alkalies. The absence of andradite, wollastonite, and hedenbergite from the Efremovka rims may indicate that these rims sampled different nebular regions than the Allende rims. Alternatively, the Ca, Fe-rich silicates rimming Allende CAIs may have resulted from late-stage metasomatic alteration, under oxidizing conditions, of original Efremovka-like accretionary rims. The observed differences in O-isotope composition between forsterite and Ca, Fe-rich minerals in the Allende accretionary rims (Hiyagon, 1998) suggest that the oxidizing fluid had an 16O-poor oxygen isotopic composition.