Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v21d..04b&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V21D-04
Mathematics
Logic
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1065 Trace Elements (3670), 6250 Moon (1221)
Scientific paper
Significant gaps remain in our understanding of the early impact history of the Earth and Moon, and their influence on geologic and biologic development. Outstanding controversies include whether or not the Moon, and by inference the early Earth, was hit by an unusually heavy "terminal cataclysm" of collisions during the period 3.8 to 4.0 Ga, and the number of large impact events represented by lunar samples. Coincidentally or not, the large nearside lunar basins are almost identical in age with the oldest terrestrial rocks, and are therefore relevant for consideration of the possible role of impacts in shaping the terrestrial continents and early life environments. To improve our understanding of the impact history of the Moon and to provide new information about the types of planetesimals that were involved in the early impact history of the inner Solar System, we measured the concentrations of highly siderophile elements (HSE: Re, Ir, Pt, Pd, Ru) in a suite of Apollo 17 impact melt breccias using high precision ID-ICPMS methods. These breccias all have poikilitic textures, relatively mafic bulk compositions, and high levels of incompatible trace elements and all likely represent ejecta from the Serenitatis basin. Ar-Ar ages are consistent with the formation of these breccias in a single impact event at 3893 +/- 9 Ma. HSE from 11 representative samples have W-shaped patterns on CI-normalized diagrams, with enrichments in Re, Ru and Pd relative to Ir and Pt, and absolute abundances ranging from \sim0.5 to 4% of CI reference values. Stronger depletions of Ir and Pt relative to Re, Ru, and Pd are correlated with decreasing HSE concentrations. The samples with the highest HSE concentrations have patterns that are identical to those of EH chondrites, but the patterns become increasingly less diagnostic of meteorite group with decreasing concentrations. The systematic variation of HSE patterns with decreasing concentration suggests that simple chemical fingerprints of impactor type using individual samples can potentially give misleading results and consideration of impact-related fractionation using high precision data for suites of impact breccias is required. An additional sample (77035) has a distinctly different HSE pattern and is more consistent with an impactor of ordinary or EL-type chondrite composition. It likely represents a discrete impact event that was sampled at the Apollo 17 site. The clear recognition of specific and different types of meteoritic impactors indicates a diverse population of planetesimals bombarded the Moon, and that the lunar crust was not heavily contaminated with HSE prior to 3.8-4.0 Ga, a feature more consistent with a late cataclysm than a smoothly declining accretionary flux.
Bennett Victor
Norman Michael
Ryder Graham
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