Pressure and Timing of the Shock Events Recorded in L6 Chondrites

Introduction: Collision of planetesimals is a fundamental process of formation and evolution of planets. Shocked meteorites are the unique and important samples that provide us some information about collision histories in the solar system. Based on the shock me-tamorphic features, several studies estimated shock conditions of meteorites such as pressure, temperature and shock duration [e.g., 1-4]. Other works reported impact ages of shocked meteorites using radiometric methods [e.g., 5-8]. However, there are few studies to estimate both of the shock conditions and the impact ages for the same meteorites. In order to understand the impact histories in the solar system, we need both of the two information, i.e., what is the magnitude of the impact and when the impact occurred. In this study, we estimated the shock pressure of two L6 chondrites (Sahara 98222 and Yamato 74445) based on the mineralogy of shock melt veins (SMVs) and the stability field of observed high-pressure phases determined by static high-pressure and –temperature experiments. At the same time, we also estimate the impact age of Sa-hara 98222 based on U-Pb dating of phosphates in and around the SMVs. Analytical Procedures: Several petrographic thin sections of Samples (Sahara 98222 and Yamato 74445) were investigated, which contain SMVs. We identified mineral phases in the SMVs using micro-Raman spectroscopy. Fine textures of minerals were observed by using a FE-SEM. Chemical compositions of minerals were analyzed by using an EPMA-WDS. For U-Pb dating, we first identified phosphate minerals in or around the SMVs of Sahara 98222 by conducting chemical mappings of P and Ca. After the phase identifications and chemical composition analyses of the phosphates, we measured the concentrations and isotopic ratios of U and Pb in the phosphates by using a Sensitive High-Resolution Ion Microprobe (SHRIMP II). Results and Discussions: Both of the samples consist of chondritic host rock and black SMVs. Major constituent minerals in the samples are olivine, ensta-tite, diopside, plagioclase, Fe-Ni-S metal and minor amounts of phosphates. There were two lithology in the SMVs: one is a coarse-grained polymineralic fragments of host rock entrained into the SMVs and another is fine-grained silicates or metallic phases that fill the inside of the SMVs. In this study, we mainly examined the coarse-grained lithology: