~(40)Ar-~(39)Ar and cosmic-ray exposure ages of nakhlites-Nakhla, Lafayette, Governador Valadares-and Chassigny

摘要

We present ~(40)Ar-~(39)Ar dating results of handpicked mineral separates and whole-rock samples of Nakhla, Lafayette, and Chassigny. Our data on Nakhla and Lafayette and recently reported ages for some nakhlites and Chassigny (Misawa et al. 2006; Park et al. 2009) point to formation ages of approximately 1.4Ga rather than 1.3Ga that is consistent with previous suggestions of close-in-time formation of nakhlites and Chassigny. In Lafayette mesostasis, we detected a secondary degassing event at approximately 1.1Ga, which is not related to iddingsite formation. It may have been caused by a medium-grade thermal event resetting the mesostasis age but not influencing the K-Ar system of magmatic inclusions and the original igneous texture of this rock. Cosmic-ray exposure ages for these meteorites and for Governador Valadares were calculated from bulk rock concentrations of cosmogenic nuclides ~3He, ~(21)Ne, and ~(38)Ar. Individual results are similar to literature data. The considerable scatter of T_3, T_(21), and T_(38) ages is due to systematic uncertainties related to bulk rock and target element chemistry, production rates, and shielding effects. This hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and Chassigny from a confined Martian surface terrain (Eugster 2003; Garrison and Bogard 2005). Cosmic-ray exposure ages from stepwise release age spectra using ~(38)Ar and neutron induced ~(37)Ar from Ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates, although not from shielding conditions.