Laser Microprobe Noble Gas Analysis of Chondrules in the Nwa

Introduction: Chondrules were formed by rapid cooling of silicate melt about 4.6 Ga, and have held their spherical shapes until to the present. Chondrules are free from trapped noble gases [e.g., 1], which is believed to be due to complete degassing during the chondrule formation. Owing to the absence of trapped noble gases, cosmogenic noble gases can be detected clearly. Cosmogenic noble gases were produced via irradiation of cosmic-rays on surface of the meteorite parent body and during transit to the earth of the mete-oroid. Noble gas analysis of chondrules is capable of revealing the exposure histories of the chondrules. We performed a laser microprobe noble gas analysis of the NWA 801 CR2 chondrite, which contains significant amounts of solar wind (SW) noble gases [2] and has large chondrules (0.5-2.5 mm in diameter). Chondrules in NWA 801 contain various concentrations of cosmogenic 3 He and 21 Ne, which reflects condition for cosmic-ray exposure on their parent body. In addition, we found SW-like noble gases in a chondrule. Laser microprobe noble gas analysis: We prepared a polished thick section (approximately 300 µm thick) of NWA 801 for noble gas analysis. The thick section was observed with a scanning electron microscope (SEM) so as to examine chondrule textures and major element compositions. Both sides of the thick section were examined by an optical microscope to confirm which cut surfaces of the chondrules were smaller. A Nd-YAG laser beam (50 µm in diameter, five pits) was shot toward the smaller cut surface of the chondrules to extract noble gases contained only in the chondrules, because matrix contains high concentrations of solar noble gases [2]. Noble gases in matrix portions surrounding chondrule were extracted by the same method for comparison. Extracted noble gases were analyzed with the modified VG5400 MS-II noble gas mass spectrometer at the University of Tokyo. We analyzed 5-18 sites for individual chondrules (named as CH1-CH8). After the noble gas analysis, we observed the back sides of each chondrule to confirm the diameters of pits (10-50 µm) and estimated fused volumes (2.