The chemical compositions and textures of matrices and chondrule rims of unequilibrated ordinary chondrites-II. Their constituents and the implications for the formation of matrix olivine

The micron-size constituents of fine-grained matrices and chondrule rims in ten unequilibrated ordinary chondrites, Semarkona (LL3), Krymka (L3), Sharps (H3), Chainpur (LL3), Tieschitz (H3), Mezo-Madaras (L3), ALH-764 (LL3), ALH-77214 (L3), ALH-77216 (L3), Yamato-790448 (LL3), have been investigated in detail. Special emphasis is put on the mode of occurrence and mineral chemistry of micron-sized matrix olivine in six primitive type-3 ordinary chondrites having lower petrologic subtypes (Krymka, Chainpur, Sharps, Tieschitz, ALH-764 and Yamato-790448). Textural evidences concerning occurrence of matrix ferrous olivine observed in matrices of chondrite samples studied here seem to indicate that some of matrix intermediate to Fe-rich olivine would have formed through solid-solid reactions between matrix enstatite, silica-rich spherules and metallic Fe-Ni. It is shown that the MnO content increases remarkably with increasing FeO content, showing a strong positive correlation of Mn with Fe in matrix ferrous olivine. The ratios of MnO to FeO appear to be distributed around solar ratio and lower than those of chondrule olivines. Although the solid-solid reaction hypothesis explains a wide variation of Fa mole % of matrix olivine, it is not clear whether reactions between matrix enstatite, sillica-rich spherules and metallic Fe-Ni can explain the positive correlation of Mn with Fe in matrix ferrous olivine. Direct vapour⟶solid condensation is suggested as an alternative mechanism for the formation of matrix ferrous olivine