Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmmr13b1679s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #MR13B-1679
Other
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5499] Planetary Sciences: Solid Surface Planets / General Or Miscellaneous, [6235] Planetary Sciences: Solar System Objects / Mercury, [6240] Planetary Sciences: Solar System Objects / Meteorites And Tektites
Scientific paper
Ancient surfaces of solid planets show that impact cratering is a major process in planet formation. Understanding origin and influence of impactors on the chemical composition of planets (core, mantle and crust) it is important to know the relative abundances of highly siderophile elements (Os, Ir, Ru, Pt, Rh, Pd) in the silicate mantle and crust of planets and meteorites. Refractory highly siderophile elements, such as Os and Ir, are abundant in most meteorites but depleted in crustal rocks (low target/meteorite ratios) and thus the most reliable elements for projectile identification. However, target/meteorite ratios are high if target rocks consist of mantle rocks. In such cases elements are enriched in impactites due to relatively high abundances (ng/g level) in target rocks to make the identification of projectile types difficult (e.g., Gardnos impact structure in Norway). The Ru/Ir ratio is the most reliable key ratio that rules out Earth primitive upper mantle (PUM) derived refractory highly siderophile element components in impactites. The well established Ru/Ir ratio of the Earth mantle of 2.0 ± 0.1 (e.g. Schmidt and Kratz 2004) is significantly above the chondritic ratios varying from 1.4 to 1.6. On Earth Rh/Ir, Ru/Ir, Pd/Ir, and Pt/Os derived from PUM match the ratios of group IV irons with fractionated trace element patterns. The question raise if HSE in mantle rocks are added to the accreting Earth by a late bombardment of pre-differentiated objects or the cores of these objects (magmatic iron meteorites as remnants of the first planetesimals, e.g. Kleine et al. 2009) or some unsampled inner solar system materials from the Mercury-Venus formation region, not sampled through meteorite collections (Schmidt 2009). The PGE and Ni systematics of the upper continental crust (UCC) closely resembles group IIIAB iron meteorites with highly fractionated refractory trace element patterns, pallasites, and the evolved suite of Martian meteorites (representing Martian crust). To date, about 20 iron meteorites and about 20 chondrites have been identified as projectiles of the 176 known impact craters on Earth. The projectiles for the other 136 impact craters are still unknown. Recently, non-magmatic irons (i.e., irons with unfractionated trace element patterns) have been suggested as projectiles for Rochechouart and Sääksjärvi impact craters (Tagle et al. 2009), in contradiction to previous studies (Janssens et al 1977; Schmidt et al. 1997). Melt rocks from many impact craters on Earth (e.g., Rochechouart, Sääksjärvi, Dellen, Mien, Boltysh) are depleted in Os relatively to Ir and Ru. Subchondritic Os/Ir (about 3 times lower than Os/Ir in the metal phase of non-magmatic irons), fractionated Ru/Ir and Rh/Os ratios are strong arguments to conclude that about 50% of the currently identified iron projectiles from terrestrial impact craters are related to magmatic irons. References: Janssens et al. (1977) Journal of Geophysical Research 82, 750-758; Kleine et al. (2009) Geochimica et Cosmochimica Acta (GCA) 73, 5150-5188; Schmidt et al. (1997) GCA 61, 2977-2987; Schmidt (2009) Workshop on Planet Formation and Evolution, Tübingen; Schmidt and Kratz (2004) Lunar and Planetary Institute Contribution, 9017; Tagle et al. (2009) GCA 73, 4891-4906.
Kratz K.
Schmidt Georg
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