K‐Ar dating of rocks on Mars: Requirements from Martian meteorite analyses and isochron modeling

Abstract

40 Ar could be difficult, and degassing cosmogenic Ar from mafic phases even more so. Considering all these factors, robotic K-Ar dating of Martian rocks may be achievable, but will be challenging.

Variability of diffusion of argon in albite, pyroxene, and olivine in shocked and unshocked samples

“…Surprisingly, unshocked high-Ca Px has an intermediate E of $100 kcal/mol, nearly the same as the pyroxene in ALH 84001 (Cassata et al, 2010). All of the pyroxene Ar E values are intermediate to those of pyroxene separates in Shergottites and Nakhlites (Bogard, 2009). We found Ar diffusion in olivine to have an E of $45 kcal/mol, higher than that measured via Ar ion implantation (Futagami et al, 1993), and intermediate to those of previous determinations of the He E (Trull and Kurz, 1993;Shuster et al, 2004).…”

“…Therefore, we will not discuss the Thomas et al (2008) results further. Bogard (2009) used step-heating on Martian meteorites and determined the Ar E of three shergottite pyroxene separates to be $65 kcal/mol, and that of two nakhlite pyroxene separates to be $160 kcal/mol. Cassata et al (2010) found the pyroxene in ALH 84001 to have an Ar E of $97 kcal/mol, and Cassata et al (2011) found gem-quality pyroxene to have an Ar E of 91 ± 1 kcal/mol.…”

39Ar‐40Ar “ages” and origin of excess 40Ar in Martian shergottites

“…17 is unlikely to be Martian atmosphere that was shock-implanted or adsorbed on weathered grain surfaces. Given the different degrees of shock heating of shergottites (Fritz et al 2005) and different Ar diffusivities in Plag and Pyx (Bogard 2009), it also is very unlikely that this trapped 40 Ar is a diffusion residue from shergottites that formed 4 Ga ago, as suggested by Bouvier et al (2005Bouvier et al ( , 2008. (Also see Bogard and Park 2008 for a more detailed discussion of this conclusion.…”

39Ar–40Ar ages of martian nakhlites

“…We conclude, therefore, that 39 Ar recoil probably has occurred in these nakhlite analyses but cannot be the full explanation of all the observed sloped age spectra. It also is unlikely that differences in shock levels generated this difference in Ar diffusion, and we point out that Ar diffusion in shergottites, with higher shock levels than nakhlites, is well behaved (Bogard, 2009).…”