Laser Microprobe Noble Gas Analysis of Chondrules in the Moorabie L3.8 Chondrite

Introduction: Trapped noble gases were discovered in chon-drules by laser microprobe noble gas analysis [1-3]. The observed noble gases were explained by solar gas acquisition in dust grains before the chondrule formation [1-3]. These results show importance of trapped noble gases in chondrites to understand environment and formation processes of chondrules. We performed the laser microprobe noble gas analysis on chondrules of the Moorabie L3.8 chondrite, which characterized by solar free and short cosmic-ray exposure age [4]. Experimental method: A polished thick section (~350 μm thick) was made from the Moorabie chondrite, and then observed by scanning electron microscope (SEM). Noble gases in five chondrules and matrix of the section were extracted by melting small portions with a Nd-YAG laser (50 μm in diameter). Fused mass was approximately 12 μg for one measurement (5-6 fused spots) and the extracted noble gases were analyzed on a modified VG5400 (MS-II) at the University of Tokyo. We analyzed 4 – 13 sites for individual chondrules depending on their sizes. Results and discussion: All the five chondrules show por-phyritic texture and consist of Fe-rich olivine, Fe-rich pyroxene, and Ca-plagioclase. He and Ne in these chondrules are composed of cosmogenic component and radiogenic 4 He. Average concentrations of cosmogenic 3 He and 21 Ne are constant among the five chondrules and matrix