HR: 08:30h
AN: P31A-03    [Abstracts]
TI: Using Lunar Regolith Composition as a Geologic Tool
AU: * Korotev, R L
EM: korotev@wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, 1 Brookings Dr, Saint Louis, MO 63130, United States
AU: Jolliff, B L
EM: blj@levee.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, 1 Brookings Dr, Saint Louis, MO 63130, United States
AB: Despite that lunar surface rocks have been broken up, pulverized, and mixed to depths of 10's to 100's of meters by meteoroid impacts since the crust formed, the surface retains, both locally and regionally, a record of the geologic diversity of the crust. Mixing caused by the distribution of impact ejecta can be far reaching; witness the rays of Tycho. The process has nevertheless been inefficient. Compositional data obtained from orbit show a 10x range in iron concentration, and an even greater range in concentrations of incompatible elements, over the lunar surface. At all Apollo sites, any regolith sample contains a large variety of compositionally diverse rock types. Regolith compositions vary, e.g., from 80:20 to 20:80 across geologic boundaries (e.g., mare:highlands) on the scale of Apollo traverse distances (up to 10 km) because the relative proportions of the constituent rocks types change with lateral distance. Mature soils (long surface exposure) are rarely dominated by one lithology although immature soils (fresh ejecta) may be. Some Apollo regolith cores show distinct compositional variation with depth over decimeters because vertical mixing is inefficient. For all but the most volatile of lithophile elements mass-balance is preserved during impact melting and mixing (unlike many terrestrial processes that involve fluids). Thus, bulk compositions of lunar soils and breccias can be modeled as linear mixtures of the compositions of the target rocks or, inversely, constituent rock types can be inferred by mathematically unmixing regolith compositions, given what is known about lunar lithologies from sample studies. Over the past 25 years, 40-45 meteorites have been found on Earth that were blasted off the Moon from random locations by asteroidal impacts. Most of the meteorites are regolith or fragmental breccias from near the lunar surface. By comparing compositions of 100's of small (2-4 mm) lithic fragments from Apollo regoliths with those of the lunar meteorites, we can show that the Apollo collection is unrepresentative in containing a significant proportion of Th-rich rocks from the anomalous Procellarum KREEP Terrane and that Apollo rocks and regolith reflect large-scale mixing between the Procellarum KREEP Terrane and the Feldspathic Highlands Terrane as a result of the Imbrium and Serenitatis basin-forming impacts.
DE: 1026 Composition of the moon
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Major and trace element geochemistry
DE: 5464 Remote sensing
DE: 6250 Moon (1221)
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly