Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.432s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 432
Other
Chondrites, Noble Gases, Shock, Solar Rare Gases
Scientific paper
A correlation has been observed between radiogenic 40Ar in chondrites and their shock stages and implications for different shock histories of L and H chondrites have been pointed out [1,2]. Here we extend this investigation to nonradiogenic solar noble gases, in particular, to solar 4He and 20Ne. Such a complementary study appeared worthwhile because solar noble gases in chondrites presumably date to very early in their history. These gases are prone to be affected by shock at any time in their history. Radiogenic gases, on the other hand, are rather insensitive sensors of shock events during the first 500 Ma or so in the life of meteorites. Measured concentrations of solar 4He in bulk samples (only the highest concentration of each meteorite is taken from [3]) are given in Fig. 1 for meteorites of different shock stages. Apparently, solar He is only present in chondrites of low shock metamorphism (S1 to S3). Either solar gases were lost during or after shock event (> S3) or impacts do not subject regolithic surface material to high stages of shock metamorphism. Figure 2 shows the distribution of radiogenic 4He in H and L chondrites. As expected, a general trend of loss of radiogenic 4He (and 40Ar) with increasing shock stage is seen. However, four meteorites of shock stages S1 and S2 also have low concentrations of radiogenic 4He. Apparently, there must be loss mechanisms for noble gases besides shock. These four chondrites (Ambapur Nagla, Densemore, Menow, and Sutton) also show a deficit of cosmogenic 3He, as seen by their low 3He/21Ne ratios. This loss of cosmogenic and radiogenic gas can be explained by solar heating during the flight as meteoroids and is not due to shock metamorphism. The ratio of H to L chondrites in S1 + S2 is about 4:1 while that in S5 + S6 is about 1:10. This different distribution of shock stages between chondrite groups is also evident in the concentrations of radiogenic gases confirming distinctly different collision histories of H and L chondrite parent bodies. References: [1] St"ffler D. et al. (1991) GCA, 55, 3831-3844. [2] St"ffler D. et al. (1992) Meteoritics, 24, 292-293. [3] Schultz L. and Kruse H. (1989) Meteoritics, 24, 155-172.
Schultz Ludolf
Stäffler Dieter
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