Noble Gases in the Nwa 852/801 Cr2 Chondrites

Introduction: North West Africa (NWA) 852 and 801 are classified as CR2 chondrites [1-2]. The two meteorites contain relatively large chondrules (1-2 mm in diameter), FeNi metal, and dark inclusions composed of serpentine and olivine. We measured noble gases in the two meteorites with stepwise heating, which facilitates decomposition of noble gas components with different origins and trapping sites. Laser extraction noble gas analysis was also performed so as to observe microdistribution of noble gases in matrices and chondrules. We found through the noble gas analyses that the both meteorites contain high concentrations of solar wind noble gases (NWA 801 has been found to be a solar gas rich meteorite [3]). This result and petrological similarities suggest that NWA 852 and 801 are the paired meteorites. Here we focus noble gases in the bulk samples and in matrices and discuss the exposure history of the two meteorites. Results of noble gas analysis of the chondrules in NWA 852 and 801 are reported in [4-5]. Stepwise heating noble gas analysis: Bulk samples of NWA 852 (66.3 mg) and 801 (85.2 mg) were heated at from 300 to 1300 °C with 100 °C interval and finally degassed completely at 1800 °C. The extracted noble gases were purified with Ti-Zr getters and separated into four fractions (He-Ne, Ar, Kr, and Xe) with a charcoal trap and a cryogenically cooled trap. The separated four fractions were analyzed with a noble gas mass spectrometer at University of Tokyo (MS-II). Laser extraction noble gas analysis: Polished sections of NWA 852 and 801 (~ 300 µm thick) were prepared for noble gas analysis. The two polished sections were observed with SEMs (with EDSs) at University of Tokyo so as to search for phases to be analyzed. A Q-switched Nd-YAG laser (1064 nm, 2 kHz pulse) was used for laser ablation of the NWA 852 polished section. The laser diameter was 30 µm (assuming the density of fused matrix is ~ 3 g/cm 3 , the fused mass is estimated as ~ 0.6 µg). A Nd-YAG pulse laser without Q-switch was used for laser ablation of the NWA 801 polished section. The laser diameter was 50 µm (in case of 3 g/cm 3 , the fused mass is ~ 1.8 µg). Since it is not clear if one fused spot releases detectable amounts of noble gases, five fused spots were