Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.538s&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 538
Computer Science
3
Achondrites, Crystallization, Meteoritic Composition, Rare Gases, Weathering, Cosmic Rays, Mars (Planet), Radiation Effects, Radioactive Age Determination
Scientific paper
ALH 84001 is an orthopyroxene that was recently found to be a relative of the SNC (shergottites, nakhlites, and Chassigny) meteorites, and hence probably comes from Mars. We have measured noble gases in two samples of ALH 84001, obtaining values for all five gases in a 18.1-mg sample, and Kr and Xe in a stepwise heating of a 142-mg sample. We find that ALH 84001 has distinctly older cosmic-ray exposure age than any of the SNC meteorites and may have a significantly older crystallization age. In addition, we find that it, like the three nakhlites, appears to contain noble gases from the martian atmospheric reservoir, but it is not clear how those gases were acquired. We calculate a cosmic-ray exposure age, based on He, Ne, and Ar data from our work and Miura et al. (1994), of 16 +/- 1 m.y., considerably longer than the 10-12-m.y. exposure age of the nakhlites and Chassigny or the less than or equal to 3 m.y. of the shergottites. Thus, at least its most recent parent body was distinct from any of the previously known SNCs. Assuming a bulk-K abundance of 27 ppm, we calculate a K-Ar age of greater than 5 b.y. Unless our sample has more than an order of magnitude more K than Mittlefehldt's, this rock is the oldest martian meteorite yet identified. Both the cosmic ray exposure age and the K-Ar age suggest that this sample comes from a different martian crater than any of the SNCs.
Burkland Michael Kennedy
Grier Jennifer Ann
Swindle Timothy D.
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