Walton, Erin L.; Kelley, Simon P. and Spray, John G.
Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe 40Ar/39Ar investigation.
Geochimica et Cosmochimica Acta, 71(2),
Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a similar to 50 mu m diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, Ar-40/Ar-39 ratios and K2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb Ar-40) than shock veins and matrix minerals (0.3-3 ppb Ar-40). The Ar-40/Ar-36 ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenie Ar-36, is: Los Angeles (similar to 1852), Zagami (similar to 1744) and NWA 1068 (similar to 1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (Ar-40/Ar-16: 480-490) in matrix minerals. Average apparent Ar-40/Ar-39 ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These Ar-40/Ar-39 apparent ages are substantially older than the similar to 170-0474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic Ar-16 and Ar-38 production. Los Angeles (3.1 +/- 0.2 Ma), Zagami (2.9 +/- 0.4 Ma) and NWA 1068 (2.0 +/- 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 +/- 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.
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