Aqueous alteration of porous microchondrules in Semarkona: Implications for hydration, oxidation and elemental exchange processes

Abstract : Porous, igneous glassy microchondrules are relatively common in Semarkona and other unequilibrated ordinary chondrites (UOC), occurring embedded within the fine-grained matrices and the chondrule rims. We have investigated the effects of aqueous alteration on two porous microchondrules and associated amorphous silicate materials in the fine-grained matrix of the Semarkona (LL3.00) ordinary chondrite. The iron valency was measured by synchrotron-based scanning transmission X-ray microscopy (STXM) and detailed compositional and mineralogical data were obtained by transmission electron microscopy (TEM). Our STXM data show that the iron in both microchondrules and the amorphous matrix materials is highly oxidized, with elevated ferric iron contents (up to Fe3+/PFe ratio 81%). The oxidation process appears to be the result of the interaction of aqueous fluids produced by melting of water ice that accreted into the LL chondrite parent body. The lack of secondary phases such as phyllosilicates or FeO-rich olivines in the microchondrules and the surrounding fine-grained matrix indicates that aqueous alteration was extremely limited. The distribution of major and minor elements indicates limited elemental exchange between the porous microchondrules and the adjacent fine-grained matrix, suggesting that the chemical composition of the two porous microchondrules was not extensively modified during the aqueous alteration. Therefore, these results strongly support the recent conclusions that porous and nonporous microchondrules have different precursor compositions, since their bulk compositions are different and have not been extensively modified by secondary processes.