| Abstract: | 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. |
