Other
Scientific paper
Jan 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000lpico.997...67p&link_type=abstract
Workshop on Extraterrestrial Materials from Cold and Hot Deserts, p. 67
Other
Weathering, Cosmic Rays, Exposure, Enstatite, Chondrites, Cosmology, Spectra, Feldspars, Decomposition
Scientific paper
The cosmic ray record and exposure ages (CREA's) of 62 Enstatite chondrites have been reported recently. As one of the main results, an age spectrum has been given ranging between 1 and 60 Ma. A similar spectrum is known for the ordinary chondrites. One surprising aspect, however, that has not been observed for ordinary chondrites, is a significant difference between CREA's calculated from cosmogenic Ne-21 and Ar-38, respectively (Fig. 1). Since Ne-21-exposure ages are believed to be the most reliable, not concordant Ar-ages need to be explained. In some Enstatite chondrites (EC) with high concentrations of trapped Ar, high Ar-38-ages compared to Ne-21-ages are determined. This discrepancy is probably caused by insufficient correction for trapped Ar-38 as the trapped Ar-36/Ar-38 ratio is not well known. Other EC with lower abundances of trapped Ar show - again in comparison with the Ne-21-ages - considerably lower Ar-38-ages. In those cases we suggested that weathering effects may be responsible for the difference. Most notably Ca, one main target element of cosmogenic Ar-38, partly resides in minerals readily affected by terrestrial alteration. The comparatively abundant metal phase (Kamazite) represents another target nuclide. The dissolution or decomposition of both metal and Ca-bearing minerals, like oldhamite (CaS) as well as plagioclase, presumably results in a loss of cosmogenic Ar-38. As a consequence, the calculated Ar-38-exposure ages are too small.
Patzer Andrea
Schultz Ludolf
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