Cosmic-ray exposure ages of enstatite chondrites

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Eleven enstatite (E-) chondrites were analyzed for abundances and isotopic ratios of noble gases by total melting. Light noble gases, He, Ne and Ar are discussed. We obtained cosmic-ray exposure ages (T^<21>) based on cosmogenic ^<21>Ne that are divided into two groups, below 15 Myr and above 40 Myr. Both ranges of T^<21> contain various petrologic-types of E-chondrites. Isotopic ratios and concentration of Ne for Yamato (Y)-8414 and -86004 are almost the same, which suggests that they are from a common fragment. Identical exposure age T^<21> of 10 Myr for Y-8414 and -86004 supports their pairing. The pairing for Y-8414 and -86004 based on noble-gas signatures and T^<21> is consistent with their petrologic similarities. Pairing for Y-792959 and -793161 is also suggested by their similarities in mineral compositions, their close sampling location and the same petrologic-type. However, there are discrepancies in He and Ar contents between the two samples, which fails to support their pairing. Including literature data, the exposure ages show no systematic correlation with petrologic type. Additionally, both EH-and EL-chondrites are found in both two peaks of T^<21>. The exposure age distribution has no implications about the structure of E-chondrite parent body. The parent body of E-chondrites has possibly lost the original structure after many collisions in the early solar system evolution.
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