The Lodranite Class: Asteroid Break-Up and Chemical Composition
Abstract
We continued our work on lodranites [1-3] and present here the cosmic ray produced noble gas abundances of EET84302, FRO90011, Gibson, and Y-74357. Additionally, we determined the target element abundances for the production of cosmogenic noble gases in aliquots of our samples by INAA and RNAA. We calculated production rates using the chemistry dependent equations for achondrites determined by [4]. Since the Mg/Si ratio in lodranites is similiar to that of H-chondrites, the shielding dependencies for the ^3He- and ^21Ne-production rates given by [5] were adopted. Because the contents of trapped He and Ne in lodranites are low we can derive reliable ^3He and ^21Ne cosmic-ray exposure ages. All lodranites fall within 30% of the average exposure age T(sub)21 of 4.6 Ma and within 20 % of the average exposure age T(sub)3 of 6.1 Ma, except MAC88177 with obvious diffusion loss of ^3He. Our exposure ages agree with those calculated by [6] based on ^26Al and ^10Be measurements that range from 3.8 to 6.4 Ma. The average exposure age T(sub)3 is about 30% higher than the average T(sub)21. Most lodranites show a high (^22Ne/^21Ne)c ratio, which is an indication for a small preatmospheric size. Perhaps the production rate ratio P(sub)3/P(sub)21 is underestimated for very small meteoroids. We conclude that one break-up event on the lodranite parent asteroid is responsible for the production of all eight lodranites dated till now. The chemical abundances presented here allow us to derive more reliable production rates than in our previous work. We, thus deduce that the common break-up occured 5 +- 1 Ma ago. Acknowledgements: We thank N. Takaoka, EUROMET , the NASA MWG, and the NIPR, Tokyo, for the meteorite samples and K. Nagao for sample preparation. This work was supported by the Swiss NSF. References: [1] Eugster O. and Weigel A. (1992) Meteoritics, 27, 219. [2] Eugster O. and Weigel A. (1993) LPS XXIV, 453. [3] Weigel A. and Eugster O. (1994) LPS XXV, 1479. [4] Eugster O. and Michel Th. (1994) GCA(in press). [5] Eugster O. (1988) GCA 52,1649-1662. [6] Xue S. et al. (1994) LPS XXV, 1523
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29Q.548W
- Keywords:
-
- Achondrites;
- Asteroids;
- Chemical Composition;
- Fragmentation;
- Abundance;
- Neutron Activation Analysis;
- Radioactive Age Determination;
- Rare Gases;
- Lunar and Planetary Exploration;
- COSMOGENIC ISOTOPES; EXPOSURE AGES; LODRANITE; LODRANITE CHEMISTRY; NOBLE GASES