Japan Geoscience Union Meeting 2015

Presentation information


Symbol P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS22] Formation and evolution of planetary materials in the solar system

Thu. May 28, 2015 4:15 PM - 6:00 PM A02 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Shoichi Itoh(Graduate school of Science, Kyoto University), Tomohiro Usui(Department of Earth and Planetary Sciences,Tokyo Institute of Technology), Yusuke Seto(Graduate School of Science, Kobe University), Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University), Makoto Kimura(Faculty of Science, Ibaraki University), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Hitoshi Miura(Graduate School of Natural Sciences, Department of Information and Biological Sciences, Nagoya City University), Hikaru Yabuta(Osaka University, Department of Earth and Space Science), Chair:Tomohiro Usui(Department of Earth and Planetary Sciences,Tokyo Institute of Technology)

4:15 PM - 4:30 PM

[PPS22-20] High-pressure polymorphs in Gujba CB type carbonaceous chondrite.

*Masaaki MIYAHARA1, Eiji OHTANI2, Akira YAMAGUCHI3 (1.Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 2.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 3.National Institute of Polar Research)

Keywords:carbonaceous chondrite, high-pressure polymorph, shock metamorphism

One of the most unambiguous evidences for shock metamorphism is a dense polymorph, high-pressure polymorph in and around the shock-melt veins and/or melt-pockets of shocked meteorites. Now, the existences of high-pressure polymorphs have been reported from ordinary chondrite, enstatite chondrite, ureilite, eucrite, iron meteorite, lunar meteorite, Apollo sample and Martian meteorite (e.g., Ohtani et al., 2004; Miyahara et al., 2014). On the other hand, it is widely accepted that carbonaceous chondrites were less shocked than ordinary chondrites, implying that high-pressure polymorphs would not be included in carbonaceous chondrites. However, Hollister et al. (2014) identified ringwoodite and stishovite from Khatyrka CV type carbonaceous chondrite. Shock metamorphism in carbonaceous chondrites may be overlooked or underestimated. CB type carbonaceous chondrite is a unique grouplet because it consists mainly of metallic Fe-Ni and chondrtic fragment (Weisberg et al., 2001). Although the origin of CB type carbonaceous chondrite has been under debated, some previous studies propose that it may be formed through a planetesimal collision (Weisberg et al., 2010). In this study, we investigated a high-pressure polymorph in CB type carbonaceous chondrite, Gujba to clarify its shock metamorphism history and origin using FEG-SEM, EPMA and laser Raman spectroscope.
We prepared a polished Gujba sample for this study. Gujba studied here consists of metal and chondritic fragment. Shock-melt veins occur widely between the chondritic fragments and metals. The major constituent minerals of the chondritic fragments are low-Ca pyroxene (Fs1-5, En89-98, Wo0-6), Mg-rich olivine (Fa1-18, Fo82-99) and Ca pyroxene (Fs1-13, En41-66, Wo32-57) based on EPMA analysis. Many mineral fragments and fine-grained chondritic fragments are entrained in the shock-melt veins. We confirmed the existences of many kinds of high-pressure polymorphs from such fragments and chondritic fragments adjacent to the shock-melt veins. Raman spectroscopy analyses indicate that olivine entrained in the shock-melt veins transform into wadsleyite. A small amount of ringwoodite is accompanied with some wadsleyite. Low-Ca pyroxene is replaced with akimotoite, majorite or probably bridgmanite. Minor Fe-rich olivine (Fa39-40) and albitic feldspar (Ab82An13Or5) are entrained in the shock-melt veins. Lamellar ringwoodite occurs in the Fe-rich olivine. The albitic feldspar is replaced with jadeite, lingunite or maskelynite. We also clarified the distributions of these high-pressure polymorphs in Gujba studied here. High-pressure polymorphs occur ubiquitously in and around the shock-melt veins, indicating that the parent-body of Gujba was heavily shocked. The metals, which consist of kamacite and minor FeS, show evidence for melting. Alternatively, we can also infer that Gujba is a part of shock-met veins induced by a collision.

Miyahara et al. (2014) Discovery of coesite and stishovite in eucrite. Proceedings of the National Academy of Sciences U.S.A., doi: 10.1073/pnas.1404247111.
Ohtani et al. (2004) Formation of high-pressure minerals in shocked L6 chondrite Yamato 791384: constraints on shock conditions and parent body size. Earth and Planetary Science Letters 227, 505-515.
Hollister L.S., et al. (2014) Impact-induced shock and the formation of natural quasicrystals in the early solar system. Nature Communications, doi: 10.1038/ncomms5040.
Weisberg M.K. and Kimura M. (2010) Petrology and Raman spectroscopy of high pressure phases in the Gujba CB chondrite and the shock history of the CB parent body. Meteoritics & Planetary Science 45, 873-884.
Weisberg M.K. et al. (2001) A new metal-rich chondrite grouplet. Meteoritics & Planetary Science 36, 401-418.