PETROLOGY OF THE YAMATO METEORITES (j), (k), (l), AND (m) FROM ANTARCTICA*

Abstract

The Yamato (j), (k), (l), and (m) meteorites collected from near the Yamato Mountains in December, 1973, are respectively an H‐4 and L‐5 chondrite, a howardite, and an L‐5 chondrite. Yamato (l), the howardite, is a polymict breccia of diogenite and eucrite clasts. Olivine in the chondrites ranges in composition from Fo 75 to Fo 80 , whereas in the howardite, where it is rare, the composition is about Fo 60 . Pyroxenes in the chondrites are mostly orthopyroxenes and (En 83 ), while the pyroxenes in the howardite are more complex, comprising orthopyroxene, pigeonite, augite, and rare clinohypersthene (in the order of decreasing abundance), with the range from En 80 to En 37 . They form a definite trend, probably formed by the fractional crystallization of the parental magma of the achondrite, and later subjected to exsolution phenomena during the slow cooling. Plagioclase is high‐temperature oligoclase in the chondrites, and anorthite in the howardite. Maskelynitization is sometimes observed. Other shock effects are also observed. Opaque phases consist mostly of nickel‐iron, troilite, chromite, and rarely ilmenite. Intergrowth of these minerals are common. The accessory minerals comprise quartz, cristobalite, apatite, spinel, and rare uranium‐bearing minerals. The bulk composition and genetic significance are discussed.