71st Meeting of the Meteoritical Society: Abstracts A53 5157 MINERALOGY OF VESICULAR OLIVINE IN THREE CK4–6 CHONDRITES: RELATIONSHIP TO SILICATE DARKENING

Introduction: A unique feature of CK chondrites is that their chondrules and matrices show silicate darkening. Most previous workers suggested that the silicate darkening resulted from shock metamorphism [e.g., 1], but the true cause has long been unknown. Recent studies of CK4 chondrites [2–4] revealed that they contain abundant unusual olivine in their matrices, and this contains numerous micron to submicron-size vesicles and inclusions of magnetite, pentlandite, and a variety of other minerals. The authors suggested that the vesicular olivine resulted from recrystallization of partially melted olivine by shock, and this is the principal cause of the silicate darkening. Here we present the results of our mineralogical and petrological investigation of LEW 86258 (CK4), EET 87507 (CK5), and EET 87860 (CK5/6). Our purpose was to examine whether vesicular olivine is the cause of the silicate darkening in these chondrites and to determine its origin and relationship to the silicate darkening. Results: The three CK4–6 chondrites show a similar degree of strong silicate darkening in their matrices and chondrule mesostases. Backscattered electron images of highly darkened regions in the matrices of the three chondrites show that olivine with numerous vesicles (<0.1–3 μm in diameter) fills interstices of nonvesicular olivine crystals, exhibiting a complex network of veinlets. The vesicular olivine contains numerous spherical grains (<0.1– 5 μm) of magnetite and pentlandite as well as grains of plagioclase, low-Ca pyroxene and diopside, although the relative abundance of these inclusions differs between the chondrites. There is a tendency that a region having a higher volume proportion of vesicular olivine to exhibit a darker and dustier appearance in transmitted light. Discussion: Vesicular olivine occurs pervasively in the matrices in LEW 86258, EET 87507, and EET 87860. The mineralogical characteristics of the vesicular olivine in all the chondrites closely resemble those in the Kobe and Karoonda CK4 chondrites [2–4]. From these observations, we conclude that microinclusion-rich vesicular olivine is the principal cause of the silicate darkening in the LEW 86258, EET 87507, and EET 87860 chondrites. The internal texture of the vesicular olivine resembles that of shockinduced local melts in the matrices of carbonaceous chondrites [e.g., 5]. Thus we suggest that the vesicular olivine formed from melts that were produced from fine-grained olivine in the matrix by shock. During melting, numerous small vesicles were produced in the melts, and the melts trapped numerous droplets of melted magnetite and pentlandite as well as fragmented grains of other minerals. References: [1] Rubin A. E. 1992. Geochimica et Cosmochimica Acta 56:1705–1714. [2] Tomeoka K. et al. 2001. Meteoritics & Planetary Science 36:1535–1545. [3] Tomeoka K. et al. 2005. Journal of Mineralogical and Petrological Sciences 100:116?125. [4] Ohnishi I. et al. 2007. Journal of Mineralogical and Petrological Sciences 102:346–351. [5] Tomeoka K. et al. 1999. Geochimica et Cosmochimica Acta 63:3683–3703. 5267 ARGON, KRYPTON, AND XENON ABUNDANCES IN THE SOLAR WIND MEASURED IN SILICON FROM THE GENESIS MISSION V. S. Heber1, H. Baur1, R. Wieler1, R. C. Wiens2, and D. S. Burnett3. 1IGMR, NW C, ETH, 8092 Zurich, Switzerland. E-mail: [email protected]. 2LANL, Space and Atmospheric Science, Los Alamos, NM 87544, USA. 3CalTech, JPL, Pasadena, CA 91109, USA.