63rd Annual Meeting 5070.pdf

NATURAL OCCURRENCE OF HIGH-PRESSURE PHASES, JADEITE, HOLLANDITE, AND -PYROPE GARNET, IN AN H , Y75100. M. Kimura1, A. Suzuki2, T. Kondo2, E. Ohtani2, and A. El Goresy3, 1Faculty of Science, Ibaraki University, Mito 310-8512, Japan (ki- [email protected]), 2Faculty of Science, Tohoku University, Sendai 980-8578, Japan, 3Max-Planck- Institut für Chemie, 55128 Mainz, Germany.

Introduction: High-pressure minerals, such as grains near the vein in the host were identified as glass ringwoodite, wadsleyite, (Mg,Fe)SiO3-ilmenite (aki- (). motoite) and -perovskite, majorite-pyrope garnet, Discussion: Survival of high-pressure phases in magnesiowüstite, and hollandite, have been found in shocked L indicates the veins were the shock melt veins of many L chondrites, which in- quenched under high-pressures [1]. On the other hand, dicated shock-induced melting and crystallization un- H chondrites hardly contain these high-pressure der the pressure and long duration of high-pressures phases. Therefore, the of H chondrites and -temperatures [1,2]. However, no high-pressure might have experienced different shock events from phases have been encountered in the shock veins of H- that of L chondrites. chondrites [3], although some of them also show evi- Jadeite may have been formed through retrograde dence for high degrees of shock-induced melting [4]. transformation of hollandite under moderate post- Kimura et al. [3] suggested that the petrologic setting shock pressure and high temperature conditions. The in the shocked H chondrites is indicative of shock-melt assemblage of wadsleyite plus majorite-pyrope garnet after pressure release. Following our previous work in the shock vein of Y75100 constrains the pressure [3], we conducted mineralogical study, with laser mi- and temperature to be ~15GPa and ~1800°C, respec- cro Raman investigations, of heavily shocked H- tively [6,7], which is much lower than the pressures chondrites, Y791524 (H4), ALH78108 (H5), A880993 estimated for the shocked L chondrites [1,2]. We as- (H6), Y75100 (H6) and Y75277 (H6). The purpose of sume that the duration of shock-induced high-pressure this study is to explore the shock history of H chon- in the parent bodies of H-chondrite could be much drites, in comparison with the case of L chondrites. shorter than that of L-chondrites. Petrography and Mineralogy: All the H chon- References: [1] Chen M. et al. (1996) Science, drites studied here consist of highly deformed chon- 271, 1570–1573. [2] Gillet P. et al. (2000) Science, dritic hosts with shock-induced melt veins. The melt 287, 1633–1602. [3] Kimura M. et al. (1999) Antarctic veins mainly comprise two lithologies: (1) coarse- XXIV, 67–68. [4] Stöffler D. et al. (1991) grained rounded to irregular silicate grains (~10 to GCA, 55, 3845–3867. [5] El Goresy A. et al., this vol- ~100 µm), and (2) fine-grained (below ~5 µm) silicate ume. [6] Zhang J. and Herzberg C. (1989) JGR, 99, aggregate. Our Raman spectroscopic investigations 17729–17742. [7] Agee C.B. et al. (1995) JGR, 100, indicate that the veins in all H-chondrites including 17725–17740. three samples by Kimura et al. [3], except Y75100, contain no any high-pressure polymorphs of silicate phases. On the other hand, a vein in Y75100 contains some high-pressure phases both in two lithologies. Coarse- grains of Fa17–20 are mixture of wadsleyite and . All coarse grains of plagioclase composition (An13Ab83Or4) are evidently jadeite. This is the first occurrence of jadeite in H-chondrites, along with re- cent discovery of jadeite from an L-chondrite [5]. We noticed the strongest line of hollandite in some spectra of jadeite-rich grains. Such spectra display no evidence for high-pressure polymorphs of silica. However, we can not exclude the possibility that the strongest line of coesite may overlap the line of jadeite at 520cm–1. Coarse-grains with average composition of En82Fs16Wo2 were identified as low-Ca . On the other hand, fine-grained lithology in the vein of Y75100 is a mixture of olivine, wadsleyite, low-Ca pyroxene and majorite-pyrope garnet. Plagioclase