Carbonaceous Chondrite-Rich Howardites; The Potential for Hydrous Lithologies on the HED Parent

摘要

Howardites, eucrites, and diogenites, collectively referred to as the 'HED', are a clan of meteorites thought to represent three different lithologies from a common parent body. Collectively they are the most abundant type of achondrites in terrestrial collections. Eucrites are crustal basalts and gabbros, diogenites are mostly orthopyroxenites and are taken to represent lower crust or upper mantle materials, and howardites are mixed breccias containing both lithologies and are generally regarded as derived from the regolith or near-surface. The presence of exogenous chondritic material in howardite breccias has long been recognized. As a group, howardites exhibit divergence in bulk chemistry from what would be produced by mixing of diogenite and eucrite end-members exclusively, a phenomenon most evident in elevated concentrations of siderophile elements. Despite this chemical evidence for chondritic input in howardite breccias, chondritic clasts have only been identified in a minority of samples, and typically at levels of only a few percent. Three recent Antarctic howardite finds, the paired Mt. Pratt (PRA) 04401 and PRA 04402 and Scott Glacier (SCO) 06040, are notable for their high proportion of carbonaceous chondrite clasts. PRA 04401 is particularly well-endowed, with large chondritic clasts occupying more than half of the modal area of the sections we examined. Previously only a few percent chondritic clasts had been observed to occur in howardites. PRA 04401 is the most chondrite-rich howardite known