Premium
Isotopic composition of trapped and cosmogenic noble gases in several Martian meteorites
Author(s) -
GARRISON DANIEL H.,
BOGARD DONALD D.
Publication year - 1998
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.1945-5100.1998.tb01678.x
Subject(s) - meteorite , martian , astrobiology , noble gas , geology , earth science , mars exploration program , physics , atomic physics
— Isotopic abundances of the noble gases were measured in the following Martian meteorites: two shock glass inclusions from Elephant Moraine (EET) 79001, shock vein glass from Shergotty and Yamato (Y) 793605, and whole‐rock samples of Allan Hills (ALH) 84001 and Queen Alexandra Range (QUE) 94201. These glass samples, when combined with literature data on a separate single glass inclusion from EET 79001 and a glass vein from Zagami, permit examination in greater detail of the isotopic composition of Ne, Ar, Kr, and Xe trapped from the Martian atmosphere. The isotopic composition of Martian Ne, if actually present in these glasses, remains poorly defined. The 40 Ar/ 36 Ar ratio of trapped Martian atmospheric Ar is probably considerably lower than the nominal ratio of 3000 measured by Viking , and data on impact glasses suggest a value of ∼1900. The atmospheric 36 Ar/ 38 Ar ratio is ≤4.0. Martian atmospheric Kr may be enriched in lighter isotopes by ∼0.5%/amu compared to both solar‐wind Kr and to the Martian composition previously reported. The isotopic composition of Xe in these glasses agrees with that previously reported in the literature. The Martian atmospheric 36 Ar/ 132 Xe and 84 Kr/ 132 Xe elemental ratios are higher than those reported by Viking by factors of ∼2.5–1.6 (depending on the 40 Ar/ 36 Ar ratio adopted) and ∼1.8, respectively, and are discussed in a separate paper. Cosmogenic gases indicate space exposure ages of 2.7 ± 0.6 Ma for QUE 94201 and Shergotty and 14 ± 1 Ma for ALH 84001. Small amounts of 21 Ne produced by energetic solar protons may be present in QUE 94201 but are not present in ALH 84001 or Y‐793605. The space exposure age for Y‐793605 is 4.9 ± 0.6 Ma and appears to be distinctly older than the ages for basaltic shergottites. However, uncertainties in cosmogenic production rates still makes somewhat uncertain the number of Martian impact events required to produce the exposure ages of Martian meteorites.