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Abstract-We performed high-resolution 40 Ar-39 Ar dating of mineral separates and whole-rock samples from the desert meteorites Dhofar 300, Dhofar 007, and Northwest Africa (NWA) 011. The chronological information of all samples is dominated by plagioclase of varying grain size. The last total reset age of the eucrites Dhofar 300 and Dhofar 007 is 3.9 ± 0.1 Ga, coeval with the intense cratering period on the Moon. Some large plagioclase grains of Dhofar 007 possibly inherited Ar from a 4.5 Ga event characteristic for other cumulate eucrites. Due to disturbances of the age spectrum of NWA 011, only an estimate of 3.2-3.9 Ga can be given for its last total reset age. Secondary events causing partial 40 Ar loss ≤3.4 Ga ago are indicated by all age spectra. Furthermore, Ar extractions from distinct low temperature phases define apparent isochrons for all samples. These isochron ages are chronologically irrelevant and most probably caused by desert alterations, in which radiogenic 40 Ar and K from the meteorite and occasionally K induced by weathering are mixed, accompanied by incorporation of atmospheric Ar. Additional uptake of atmospheric Ar by the alteration phase(s) was observed during mineral separation (i.e., crushing and cleaning in ultrasonic baths). Consistent cosmic-ray exposure ages were obtained from plagioclase and pyroxene exposure age spectra of Dhofar 300 (25 ± 1 Ma) and Dhofar 007 (13 ± 1 Ma) using the mineral's specific target element chemistry and corresponding 38 Ar production rates.

Abstract-We present 40 Ar- 39 Ar dating results of handpicked mineral separates and wholerock 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 1.4 Ga rather than 1.3 Ga 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 1.1 Ga, 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 3 He, 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.