Investigation of Orthopyroxene Diversity in Howardite Meteorites

摘要

The howardite, eucrite and diogenite (HED) family of meteorites is considered to originate from the asteroid 4-Vesta (1). Howardites are polymict breccias made mostly of diogenitic and eucritic debris (2), and have recently been divided into two types: regolithic and fragmental (3). Regolithic howardites have higher noble gas contents due to solar wind exposure, have a greater abundance of impact-produced glass, are richer in siderophile elements, e.g. Ni, and may preferentially have a mixing ratio of eucrite to diogenite of approx.2:1 (3). The hypothesis is that these characteristics are a result of originating from an ancient, well-mixed regolith (3). Fragmental howardites, by contract, show less evidence of regolithic processing and are suggested to have originated in more recently formed impact ejecta (3). Our work aims to evaluate this hypothesis. We have examined the compositional variations of orthopyroxene (diogenite) clasts within eight howardites. We posited that because regolithic howardites sampled a wider range of the asteroid surface, they would contain orthopyroxene fragments with wider ranges in incompatible element contents than would fragmental howardites that sampled fewer diogenitic source rocks. One purpose of developing an additional method to differentiate regolithic and fragmental howardites is to aid in interpretation of data expected from the Dawn mission to 4-Vesta. The Dawn analyses will be of the regolith layers, making an understanding of regolithic meteorites and the processes by which they were formed an important constraint on understanding Dawn data.