NOBLE GASES IN ALH 84001:  NOT JUST ANOTHER SNC.  T. D. Swindle, M. K. Burkland, and J. A. Grier, Lunar and Planetary Laboratory, University of Arizona, Tucson AZ 85721, USA.

Published in Meteoritics, 29, p. 538.

ALH 84001 is an orthopyroxenite that was recently found to be a relative of the SNC (shergottites, nakhlites, and Chassigny) meteorites, and hence probably comes from Mars [1]. 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 a 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 [2], of 16 ± 1 m.y., considerably longer than the 10-12-m.y. exposure age of the nakhlites and Chassigny or the <=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, as measured by [1], we calculate a K-Ar age of >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.

The heavy noble gases in most shergottites can be plausibly fit as a mixture of the gas in EETA 79001 (129Xe/132Xe = 2.4, 136Xe/132Xe = 0.36, 84Kr/132Xe = 25), thought to represent the martian atmosphere, and Chassigny (129Xe/132Xe = 1.0, 136Xe/132Xe = 0.30, 84Kr/132Xe = 1), perhaps representing the mantle. Nakhlites tend to have 129Xe/132Xe ratios of 1.5-2.0, but 84Kr/132Xe ratios of <=5, so they don’t fall on the shergottite mixing line, but could represent a mixture of Chassigny-like gas and elementally fractionated atmosphere [3].

The 129Xe/132Xe ratio in ALH 84001 is as high as 2.05 in some extractions. Among the SNCs, only Nakhla (up to 2.2) and EETA 79001 have higher values. The 84Kr/132Xe is rather low (~4), again similar to Nakhla. This would be consistent with a scenario in which the (isotopically) atmos-phericlike noble gas in nakhlites is carried by weathering products [4], since ALH 84001 contains what are probably martian weathering products, like the nakhlites. However, unlike the nakhlites, most of the temperature extractions do not fall on the Chassigny-EETA 79001 mixing line in Xe three-isotope plots such as 129Xe/132Xe vs. 136Xe/132Xe (Fig. 1).

References:  [1] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [2] Miura Y. N. et al. (1994) LPS XXV, 919-920. [3] Ott U. (1988) GCA, 52, 1937-1948. [4] Drake M. J. et al. (1993) LPS XXIV, 431-432.