Olivine Microporphyry in the St. Mesmin Chondrite

Olivine microporphyry, of metal‐ and troilite‐poor LL‐group composition and composed chiefly of olivine (58 vol. %), pyroxenes (18%, including orthopyroxene, pigeonite, and subcalcic augite), and quartz‐oligoclase glass (22%), occurs in and locally intrudes clasts in the St. Mesmin chondrite (LL‐group breccia). Its crystallization history resembles that of many Apollo 12 lunar basalts, modified somewhat by a lower bulk Fe/Mg ratio and a higher (Fe+Mg)/Ca ratio. The microporphyry crystallized from a melt which was emplaced during a period of brecciation or between two episodes of brecciation, but after the thermal metamorphism recorded in other components of the breccia. This melt originated by complete melting of normal LL‐group material with loss of Fe, Ni, and S, or, less likely, by fusion of such material (LL‐7?) which had lost these elements during prior anatexis. Comparison of the St. Mesmin microporphyry with similar material in the Parnallee chondrite (Binns, 1968) suggests that the latter was derived from a different parent, perhaps of C‐1 chondritic composition.