Heterogeneous Noble Gas Distribution in the Nwa 2737

Introduction: NWA 2737 is the second chassignite, whose brown colored appearance different from Chassigny would be due to shock metamorphism. Noble gas compositions so far reported for the NWA 2737 [1, 2] are largely different from those of Chassigny. High Xe/Xe up to ~2 observed in NWA 2737 might be shock-implanted Martian atmospheric Xe. Common ejection event at ∼11 Myr was suggested for NWA 2737, Chasigny and nakhlites [2]. Strong degassing occurred at ~160-170 Gyr was suggested by Ar-Ar dating [3]. Experimental method: We measured noble gases several times for NWA 2737 by total melting on 27.9 mg and 26.3 mg, and stepwise heating on 221.9 mg (13 steps), 109.3 mg (7 steps) and 210.7 mg (17 steps). Extraction temperatures were from 400 to 1850°C. The samples were taken from a small chip of NWA 2737 weighing 1.0 g. Results and discussion: Appreciable amounts of noble gases were released at >1600°C, suggesting high retentivity for noble gases in olivine crystals of this meteorite. Calculated cosmic-ray exposure ages showed relatively wide variation from 9 to 15 Myr. Fig. 1 is a plot of Xe/Xe versus Ar/Ar corrected for cosmogenic Ar. All the samples show high Xre/Xe (∼2) and Ar/Ar (∼1500) at high temperatures, which might be shockimplanted Martian atmosphere. This is clearly different from the low Xe/Xe (<1.1) observed for Chassigny as indicated in Fig. 1. The observation is consistent with the reported ones [1, 2]. The Ar/Ar ratios at low temperatures, however, are variable from 700 to 3300, implying heterogeneous distribution of K among the samples, or variable concentrations of Ar derived from Martian interior as pointed out by [3].