Mineralogy and Petrology of Amoeboid Olivine Inclusions in CO3 Chondrites: Relationship to Parent-Body Aqueous Alteration

摘要

Petrographic and mineralogic studies of amoeboid olivine inclusions (AOI) in CO3 carbonaceous chondrites reveal that they are sensitive indicators of parent-body aqueous and thermal alteration. As the petrologic subtype increases from 3.0 to 3.8, forsteritic olivine Fa(sub 0-1) is systematically converted into ferroan olivine Fa(sub 60-75). We infer that the Fe, Si and O entered the assemblage along grain boundaries, forming ferroan olivine that filled fractures and voids. As temperatures increased, Fe(sup +2) from the new olivine exchanged with Mg(sup +2) from the original AOI to form diffusive haloes around low-FeO cores. Cations of Mn(sup +2), Ca(sup +2) and Cr(sup +3) were also mobilized.