Ar-39-Ar-40 ages of martian nakhlites

作者：Park Jisun; Garrison Daniel H; Bogard Donald D^*

来源：Geochimica et Cosmochimica Acta, 2009, 73(7): 2177-2189.

DOI：10.1016/j.gca.2008.12.027

We report Ar-39-Ar-40 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 Ar-39 recoil and variable amounts of trapped Ar-40 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, (2 sigma 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 Ar-40 on weathered grain surfaces that was degassed along with the first several percent of Ar-39. A total K-Ar-40 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 (2 sigma). 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 similar to 12 Ma is also similar to CRE ages for other nakhlites.

• 出版日期2009-4-1