Lodranites are a unique group of achondrites. We studied Lodran, Y-791491, LEW88280, and MAC88177. Mineralogical investigations of Lodran (Bild and Wasson, 1976) and MAC88177 (Prinz et al., 1991; Warren and Kallenmeyn, 1991; AMN, 1991) indicate varying Fe contents (8-30%) but a quite uniform composition of the silicate phase. The most remarkable result of our studies is the fact that all four lodranites yield the same ^21Ne exposure age of 2.6+-0.5 Ma (prelim. data). Solar noble gases are absent but planetary-type trapped ^36Ar ranges from 0.08-12 (all gas concentrations in this abstract are given in 10^-8cm^3STP/g). MAC88177 had been tentatively associated with ureilites (see ref. given above), which contain high concentrations of planetary-type trapped gases (^36Ar>20) in contrast to MAC88177 whose ^36Ar(sub)tr=0.08. The Fe phase of Lodran yields about 10 trapped ^36Ar (see also Zahringer, 1968). Relatively high ratios (^3He/^2lNe)(sub)c of about 11 indicate good ^3He retentivity, except for MAC88177, which has lost most of its ^3He. Low concentrations of ^40Ar(sub)rad (16-590) and ^4He(sub)rad (<500) are probably due to low K, Th, and U concentrations. Our data lead to the conclusion that the four lodranites originate from the same break-up event and, thus, presumably also from the same parent body. Shergottite LEW88516 is in many respects quite similar to the shergottites ALHA77005, Shergotty, and Zagami. We obtain a cosmic-ray exposure age of about 3 Ma. No solar noble gases were detected; planetary-type ^36Ar is about 0.9. Radiogenic ^4He and ^40Ar are <10 and about 750, respectively. Maralinga is a member of the CK chondrite group (Kallemeyn et al., 1991). We determined based on ^3He, ^21Ne, and ^38Ar a cosmic-ray exposure age of 6 Ma. The CK-chondrites dated till now fall into two groups: Maralinga and ALH82135 - 6 to 7 Ma, Karoonda, Ningqiang, and PCA82500 - 40 to 50 Ma. Thus, two break-up events are required to produce the CK-chondrites. Acknowledgements: We thank R. Hutchison of the British Museum of Natural History, the NASA Meteorite Working Group, and the Natl. Institut of Polar Research Tokyo for the meteorite samples. This work was supported by the Swiss NSF. References Antarctic Meteorite Newsletter (1991), 14, No. 2. Bild R.W. and Wasson J.T. (1976) Mineralogical Magazine 40, 721-735. Kallemeyn G.W., Rubin A.E. and Wasson J.T. (1991) Geochim. Cosmochim. Acta 55, 881-892. Prinz M., Chatterjee N., Weisberg M.K., Clayton R.N. and Mayeda T.K. (1991) Lunar Planet. Sci. (abstract) 22, 1099. Warren P.H. and Kallemeyn G.W. (1991) Lunar Planet. Sci. (abstract) 22, 1467. Zahringer J. (1968) Geochim. Cosmochim. Acta 32, 209-237.
- Pub Date:
- July 1992