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

It was suggested that multilayered accretionary rims composed of ferrous ol ivine, 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 fluctuat ions 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 (E104) 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 Efrem ovka consist of coarse-grained (20-40 mum), anhedral forsterite (Fa(1-8)), Fe,Ni-metal nodules, amoeboid olivine aggregates (AOAs) and fine-grained CA Is composed of Al-diopside, anorthite, and spinel, +/- forsterite. Although the fine-grained CAIs, AOAs and host CAIs are virtually unaltered, a hibon ite-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 typ ically show an aggregational nature and consist of small olivine grains wit h numerous pores and tiny inclusions of Al-rich minerals. No evidence for t he 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 th ese regions prior to condensation of enstatite and alkalies. The absence of andradite, wollastonite, and hedenbergite from the Efremovka rims may indi cate that these rims sampled different nebular regions than the Allende rim s. Alternatively, the Ca, Fe-rich silicates rimming Allende CAIs may have r esulted 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 Al lende accretionary rims (Hiyagon, 1998) suggest that the oxidizing fluid ha d an O-16-poor oxygen isotopic composition.