FAYALITE-SILICA ASSOCIATION IN UNEQUILIBRATED ORDINARY CHONDRITES - EVIDENCE FOR AQUEOUS ALTERATION ON A PARENT BODY

We report ten occurrences of high-fayalite (Fa56-99 mol%; four with Fa > 82 mol%) olivine in association with silica in type-3 ordinary chon drites. Pyroxene with high Fs contents is much less common; Fs content s do not exceed 66 mol%, and most maxima are < 50 mol%. In those cases where the amount of fayalite is minor relative to that of silica, the fayalite forms a layer on the silica, and shows textural evidence of formation by reaction of silica with oxidized Fe; the latter seems to have resulted from reaction of metallic Fe-Ni with an oxidant, most li kely H2O vapor. The fayalite is generally in contact with pyroxene (an d, in one case, olivine) having much lower Fe/(Fe + Mg) ratios, indica ting that lattice diffusion has been minimal. Formation of fayalite fr om SiO2 explains the low Mg content of this olivine; the Mg was seques tered inside the lattices of mafic minerals and was thus inaccessible. In contrast, the moderately high Mn contents of the fayalite indicate that an appreciable fraction of the Mn in the precursor assemblage wa s accessible; it was probably sited in the matrix in the form of tiny, poorly crystallized oxide grains produced by nebular condensation at temperatures too low to permit diffusion into forsterite or enstatite. The reaction of SiO2 with FeO produced by oxidation (during metamorph ism) of Fe-Ni can also account for fayalitic olivine associated with S iO2 microspherules in the fine-grained matrices of type-3 ordinary cho ndrites and, because matrix is SiO2 normative, for other occurrences o f fayalite in matrix. The presence of Mn in the fayalitic rims on the olivine of carbonaceous chondrites does not require a nebular origin.