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Argon‐39‐argon‐40 “ages” and trapped argon in Martian shergottites, Chassigny, and Allan Hills 84001
Author(s) -
BOGARD Donald D.,
GARRISON Daniel H.
Publication year - 1999
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1999.tb01353.x
Subject(s) - martian , meteorite , radiogenic nuclide , isotopes of argon , geology , argon , atmosphere of mars , achondrite , meteoroid , astrobiology , mantle (geology) , mars exploration program , chemistry , chondrite , geochemistry , physics , organic chemistry
— Argon‐isotopic abundances were measured in neutron‐irradiated samples of Martian meteorites Chassigny, Allan Hills (ALH) 84001, ALH 77005, Elephant Moraine (EET) 79001, Yamato (Y) 793605, Shergotty, Zagami, and Queen Alexandra Range (QUE) 94201, and in unirradiated samples of ALH 77005. Chassigny gives a 39 Ar‐ 40 Ar age of 1.32 ± 0.07 Ga, which is similar to radiometric ages of the nakhlites. Argon‐39‐Argon‐40 data for ALH 84001 indicate ages between 3.9 and 4.3 Ga. A more precise definition of this age requires detailed characterization of the multiple trapped Ar components in ALH 84001 and of 39 Ar recoil distribution. All six shergottite samples show apparent 39 Ar‐ 40 Ar ages substantially older than the ∼165–200 Ma range in ages given by other isotope dating techniques. Shergottites appear to contain ubiquitous Ar components acquired from the Martian atmosphere, the Martian mantle, and commonly terrestrial atmospheric contamination. Zagami feldspar also suggests inherited radiogenic 40 Ar. These data analyses indicate that the recent Martian atmospheric component trapped in shergottites has a 40 Ar/ 36 Ar 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 40 Ar/ 36 Ar ratio for the Martian mantle component is probably <500 but is poorly constrained. The correlation between trapped 40 Ar/ 36 Ar and 129 Xe/ 132 Xe ratios in shergottite impact glasses and unirradiated samples of ALH 77005 shows considerable scatter and suggests that the 36 Ar/ 132 Xe ratio in the Martian components may vary. Resolution of Martian atmospheric 40 Ar/ 36 Ar ratio at different time periods ( i.e. , at ∼4.0 and 0.2 Ga) is also difficult without an understanding of the composition of various trapped components.