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.

Siderophile and other geochemical constraints on mixing relationships among HED-meteoritic breccias

“…Taking solar noble gases as the most definitive test, the number of confirmed regolithic howardites is still small (4 or perhaps, including Malvern, 5); and our conclusions based on their populational characteristics are correspondingly tentative. Additional regolithic HEDs should be sought, by means of gas, petrographic, and siderophile measurements on howardites that are suggestively rich in glasses (e.g., Monticello: Olsen et al, 1987), or Ni (e.g., Muckera 001, LEW 85313, and perhaps even MAC 02666), or carbonaceous chondrite clasts (e.g., Y-793497: Gounelle et al, 2003). …”

Vesta, Vestoids, and the howardite, eucrite, diogenite group: Relationships and the origin of spectral differences

“…A few howardites have been found to have glass contents as high as -15-20 vol% (e.g., Desnoyers and Jerome, 1977;Olsen et al, 1987Olsen et al, , 1990. However, generally glass represents a very small proportion (<<1%) of the volume of known HED meteorites.…”

Chondrule‐like objects and brown glasses in howardites

“…3). Also plotted are the bulk compositions of the respective howardites (Mason et al, 1976;Vagi et al, 1978;Olsen et al, 1987) and the associated brown glass compositions. These data were plotted using the normalization of Stolper (1977).…”

“…The fact that a small number of howardites possess brown glass suggests a larger and different parent body. The body was probably not as large as 400 km radius because the abundance of glass is not great even in the most glass-rich howardites, Malvern and Monticello, 20% and 15%, respectively (Desnoyers and Jerome, 1977;Olsen et al, 1987). These percentages contrast sharply with the large number of glasses found in some lunar breccias (Housen et al, 1978) where there should be high retention of lofted material.…”