39Ar–40Ar ages of martian nakhlites

Abstract

We report 39 Ar- 40 Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39 Ar recoil and variable amounts of trapped 40 Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2r errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40 Ar on weathered grain surfaces that was degassed along with the first several percent of 39 Ar. A total K-40 Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2r). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of $12 Ma is also similar to CRE ages for other nakhlites. Published by Elsevier Ltd.

Evidence for shock heating and constraints on Martian surface temperatures revealed by 40Ar/39Ar thermochronometry of Martian meteorites

“…The results have generally converged upon an age of 1.3-1.4 Ga (Podosek, 1973;Bogard and Husain, 1977;Swindle and Olson, 2004;Park et al, 2009). Rb-Sr, Sm-Nd, and U-Pb studies yield younger or equivalent ages, ranging from 1.24 to 1.36 Ga (Table S1; Gale et al, 1975;Papanastassiou and Wasserburg, 1974;Nakamura et al, 1982;Bouvier et al, 2007).…”

Ar–Ar and I–Xe ages and thermal histories of three unusual metal-rich meteorites

“…For PV-2 the isochron ages are slightly higher than, but within uncertainty of, the "plateau" age. Slightly higher ages and slightly negative intercepts for these isochrons suggest some isochron rotation as a consequence of the sloped age plateau (Park et al, 2009). For PV-1, the isochron ages show greater variation both above and below the plateau age, but the average of the four isochron ages (4478 ± 10 Ma) agrees with the preferred plateau age (4477 ± 11 Ma).…”

“…All isochron intercepts are much smaller than the values of individual plotted ratios and in three cases pass through the origin within their uncertainty. These characteristics indicate no significant presence of redistributed 40 Ar or recoiled 39 Ar (Park et al, 2009). The average of these four isochron ages is 4520 ± 10 Ma.…”

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

“…Such ( 40 Ar) weathering will tend to degas at lower extraction temperatures, and this can rotate counter-clockwise that part of the isochron giving higher 39 Ar/ 37 Ar ratios. Both 39 Ar recoil and 40 Ar incorporated into fine-grained phases or weathering products can increase isochron slope and apparent age, and both effects were observed and discussed for Martian nakhlites (Park et al 2009). …”

“…Isochron plots for Martian shergottites are particularly complex because of the possible and common presence of many argon components (e.g., Bogard and Garrison 1999;Swindle 2002;Walton et al 2007;Bogard and Park 2008 36 Ar cos concentrations for a given shergottite may have been measured in un-irradiated samples or may be estimated from other shergottite analyses, if their CRE ages are believed to be similar (Park et al 2009). In our data evaluations, we compare 36 Ar cos obtained in this way with expected concentrations.…”