Noble gases in the Martian meteorite Northwest Africa 2737: A new chassignite signature

Abstract

We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic 3He, 21Ne, and 38Ar around 10-11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U-Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm-Nd signature comparable tothat of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 doesnot contain, in detectable amount, the solar-like xenon found in Chassigny and thought to characterizethe Martian mantle nor apparently fission xenon from 244Pu, which is abundant in Chassigny andsome of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped inamounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noblegases, together with the trapping of Martian atmospheric gases, could have occurred duringassimilation of Martian surface components, or more likely during shock metamorphism, which isrecorded in the petrology of this meteorite.