Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.524r&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 524-525
Mathematics
Logic
Asteroids, Astronomical Models, Chondrites, Cooling, Meteoritic Microstructures, Metallography, Metamorphism (Geology), Petrology, Radioactive Age Determination, Radioactive Isotopes, Shock Heating
Scientific paper
A popular model for the internal structure of ordinary chondrite (OC) parent bodies is the onion shell, wherein the metamorphic grade of material increases with depth. Chondrites derived from such asteroids should exhibit a negative correlation between cooling rate and petrologic type. Deeply buried, metamorphosed material should remain hot longer than shallow, less metamorphosed material, hence isotopic closure should have occurred later in highly metamorphosed samples. Using the Pu-244 fission track method, Pellas and Storzer determined cooling rates of six H chondrites and found a negative correlation between petrologic type and cooling rate. However, the more extensive metallographic cooling rate data of Taylor et al. show no such correlation. The less-extensive datasets for metallographic and fission track cooling rates also do not support onion shells for the L and LL bodies. Goepel et al. recently determined the Pb-207/Pb-206 model ages of phosphates in seven H chondrites and found a negative correlation between age and petrologic type. Although these data seem consistent with an onion-shell structure, there is no concordancy between Pb/Pb systematics and different radiochronometers. Goepel et al. also determined the internal U/Pb systematics of H3.4 Sharps and obtained a Pb/Pb age that is younger than that of H6 chondrites, inconsistent with an onion-shell structure for the H asteroid. A large OC body with an onion-shell structure should exhibit an inverse correlation between petrologic type and Ar-39/Ar-40 age. Pellas and Fieni found no such correlation in H chondrites: 11 of 12 H chondrites in their study have Ar-Ar ages within the same narrow range. The evidence for onion-shell structures in OC parent bodies is weak.
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