MONTICELLO: A GLASS‐RICH HOWARDITE

Abstract

Monticello is a new howardite similar to Malvern in that it contains abundant (15%) glass fragments. These fragments show a range of compositions from olivine‐normative to quartz‐normative. Like Kapoeta, it contains pyroxene grains that range up to highly magnesian compositions, Fs16. Because their pyroxenes are more magnesian than those occurring in diogenites, Monticello and Kapoeta are exceptions to the simple two‐component mixing model in which howardites are considered to be mechanical mixtures of fragmented eucrites and diogenites. Monticello also contains clasts of what appear to be a cumulate eucrite and a non‐cumulate eucrite, as well as a radiating pyroxene chondrule from a chondrite. Monticello is a regolith breccia containing more evolved components than are usually considered in eucrite‐diogenite genesis models. As such, it supports those models that involve reworking of a complex parent body crust rather than straightforward partial melting of primitive chondritic parent material.