The histories of ordinary chondrite parent bodies: U,Th-He age distributions |
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Authors: | John T Wasson Sichao Wang |
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Abstract: | Abstract— Age patterns observed in meteorite groups reflect the different thermal or impact histories experienced by their parent bodies. To assess the number of ordinary chondrite (OC) parent bodies rare-gas data in the Schultz and Kruse (1989) data base were used to calculate U,Th-He gas-retention ages. Most H- and LL-chondrite ages are high; ~81% are >2.2 Ga. In contrast, most L-chondrite ages are low; ~69% are ≤2.2 Ga, and ~35% are ≤0.9 Ga. The latter fraction is substantially lower than the value of 44% given by Heymann (1967). The difference is attributed to the preferential inclusion of shocked L chondrites in early studies. Broad age peaks in the H and LL groups near 3.4 Ga probably reflect thermal loss during metamorphism, but in the H distribution there is a hint of minor outgassing “events” near 1 Ga. The L/LL chondrites have chemical properties intermediate between and unresolvable from L and LL chondrites. The high ages of most L/LL chondrites are evidence against these originating on the L parent body; the L/LL age distribution is consistent with an origin on the LL parent body or on an independent body. |
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