Other
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999m%26ps...34..451b&link_type=abstract
Meteoritics & Planetary Science, vol. 34, no. 3, pp. 451-473 (1999).
Other
36
Scientific paper
Argon isotopic abundances were measured in neutron-irradiated samples of Martian meteorites Chassigny, ALH84001, ALH77005, EET79001, Y793605, Shergotty, Zagami, and QUE94201, and in unirradiated samples of ALH77005. Chassigny gives a 39Ar-40Ar age of 1.32 (0.07 Gyr, similar to radiometric ages of the nakhlites. 39Ar-40Ar data for ALH84001 indicate ages between 3.9 and 4.3 Gyr. A more precise definition of this age requires detailed characterization of the multiple trapped Ar components in ALH84001 and of 39Ar recoil distribution. All six shergottite samples show apparent 39Ar-40Ar ages substantially older than the ~165-200 Myr range in ages given by other isotope dating techniques. Shergottites appear to contain ubiquitous argon components acquired from the Martian atmosphere, the Martian mantle, and commonly terrestrial atmospheric contamination. Zagami feldspar also suggests inherited radiogenic 40Ar. These data analyses indicate that the recent Martian atmospheric component trapped in shergottites has a 40Ar/36Ar ratio possibly as low as ~1750, and no greater than ~1900. These ratios are less than the value of 3000 (500 reported by Viking. The 40Ar/36Ar for the Martian mantle component is probably <500, but is poorly constrained. The correlation between trapped 40Ar/36Ar and 129Xe/132Xe in shergottite impact glasses and unirradiated samples of ALH77005 show considerable scatter and suggests that the 36Ar/132Xe ratio in the Martian components may vary. Resolution of Martian atmospheric 40Ar/36Ar at different time periods (i.e., at ~4.0 and 0.2 Gyr) also is difficult without an understanding of the composition of various trapped components.
Bogard Donald D.
Garrison Daniel H.
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