NOBLE GASES IN THE BELLS (C2) AND SHARPS (H3) CHONDRITES

Bells and Sharps have some mineralogical and chemical peculiarities that make their classification uncertain. For Bells, the⁴⁰ Ar content at 890 × 10^?8 cm³ STP/g is greater than the highest C1 chondrite value (Ivuna; 640), but close to the mean value for C2's (880). The²¹ Ne-exposure age of 0.38 ± 0.07 Ma is very short, and coincides with the distinctive cluster of five C2's (0.17 to 0.76 Ma). Very likely Bells belongs to the same cluster, in which case it comes from the C2 parent body. Hence the C2 parent body seems to contain transitional C1-C2 material, like Bells, within a few km of the region of C2 chondrites proper. Thus the radiogenic and especially the cosmogenic gases link Bells to the C2 group. For Sharps, the elemental concentrations of primordial Ar, Kr and Xe (127, 0.76 and 0.59, all 10^?8 cc/g) are ～ 3 x higher than for any other H3 chondrite. While Sharps is classified as 3.4 based on five indicators of metamorphism, the very high concentration of remaining two parameters of the Sears et al. (1980) scheme — C and primordial³⁶ Ar — (together with the high concentration of the volatile trace elements Bi, In and Tl) implies a classification of 3.0 for its volatile element content. The²¹ Ne-exposure age is 25.5 ± 2.5 Ma, placing Sharps in the second largest peak of the H-chondrite distribution. The nominal K-Ar and U, Th-He ages are 4.6 ± 0.2 and 4.2 ± 1.5 Ga, suggesting that Sharps has remained at low temperatures since its beginnings.