Chemical zoning and homogenization of pasamonte‐type pyroxene and their bearing on thermal metamorphism of a Howardite parent body

Abstract

We studied thermal metamorphism in the surface layer of a eucrite parent body by examining pyroxene zoning. Ca‐Mg‐Fe zoning of pyroxenes in the Pasamonte eucrite can be classified into three types: type 1 with a core composition of Ca5Mg62Fe32 and rim of Ca41Mg20Fe4o; type 2 with core of Ca5Mg62Fe32 and rim of Ca25Mg34Fe41; and type 3 with core of Ca5Mg66Fe29 and rim of Ca7Mg32Fe61. Type 1 typically occurs adjacent to a mesostasis, type 2 typically occurs adjacent to a surrounding plagioclase, and type 3 is common in pyroxenes in the matrix. Pasamonte is less metamorphosed than previously believed because of the wide range of composition of zoned pyroxenes. On the basis of Ca‐Mg‐Fe variation data, we have calculated the degree of homogenization of zoned pyroxenes by numerically solving the diffusion equation. A Pasamonte‐type pyroxene may yield a Juvinas‐type pyroxene by thermal metamorphism. The homogenization of Juvinas pyroxene could not have taken place during initial cooling and may be related to reheating events, although our calculation rests upon some assumptions.