Monticello meteorite, Monticello, Wayne Co., Kentucky, USAi

Regional Level Types
Monticello meteorite	Meteorite Fall Location
Monticello	- not defined -
Wayne Co.	County
Kentucky	State
USA	Country

Howardite (HED achondrite)
1982 Find, 210 g

A small stone found on the shore of Lake Cumberland was determined to be a relatively unweathered Howardite in spite of the fact that no remnant of any original fusion crust was observed. Monticello is a Howardite, a meteorite group defined by the presence of significant amounts of both Eucritic pyroxene-plagioclase rich basaltic intergrowths and Diogenitic plutonic orthopyroxene. However, Monticello is not a normal Howardite. Many Howardites are, in the main, mechanical mixtures of Eucritic and Diogenitic material with perhaps some additional features due to post-impact annealing. Monticello, however, contains some geochemically anomalous components which are not found in Eucrites and Howardites. Most significant is its wide compositional range for Ca-poor pyroxenes (Fs 16-53). At the time of discovery only Monticello and another unusual Howardite, Kapoeta, were known to contain such Mg-rich pyroxene. Monticello plagioclases include both bytownite and anorthite grains — including nearly pure anorthite (An >99 mol%) which is likewise rare in any of the HED clan (Howardites, Eucrites and Diogenites). A second unusual feature of Monticello is the texture of its dominant comminuted pyroxene, plagioclase, and glass. While small individual clasts are present, the predominant textures suggest a meteorite aggregated from diverse igneous rocks which were battered, broken, and dispersed — and then battered, broken, and dispersed again and perhaps again. In the parlance of lithological discourse, Monticello is a regolithic breccia par excellence. The prominence of Monticello glass (~15% in thin sections) only underscores this conclusion. For Howardites, breccias by definition, glass is a frequent and, indeed, expected component. However, it is usually a very minor constituent. Howardites like Monticello and Malvern with glass in the ~10% range are quite unusual. Other constituents included in first reports include trace amount of chromite, ilmenite, and troilite as well as tiny silica and silica-rich grains too small to characterize mineralogically. A few exotics are also present.

Oxygen isotope and FeO/MnO ratios are fully consistent with normal HED trends. Currently, reappraisal of the HED family membership is required by new very high resolution oxygen isotope studies which have shown that not all HED meteorites are derived from the same parent body. However, nothing noted here appears to be inconsistent with Monticello’s membership in the yet-to-be defined ‘main group’ of HED meteorites almost certainly formed on the asteroid 4 Vesta.

References

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