Japan Geoscience Union Meeting 2015

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT06] Early Earth - from accumulation to formation -

Mon. May 25, 2015 9:00 AM - 10:45 AM 303 (3F)

Convener:*Tatsuya Sakamaki(Department of Earth Science, Tohoku University), Akio Suzuki(Department of Earth and Planetary Materials Science, Faculty of Science, Tohoku University), Seiji Kamada(Graduate school of Science, Tohoku University), Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington), Chair:Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington)

9:00 AM - 9:15 AM

[SIT06-20] High-pressure polymorphs of silica in shocked meteorites and their implications

*Masaaki MIYAHARA1 (1.Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University)

Keywords:silica, high-pressure polymorph, meteorite

A high-pressure polymorph of silica is one of most unambiguous evidences for an impact event. Natural coesite and stishovite were discovered from impact craters on the Earth for the first time. Some meteorites are heavily shocked. Recent our studies reveal that high-pressure polymorphs of silica occur in many kinds of meteorites. Stishovite and coesite were identified from a lunar meteorite, Asuka 881757 for the first time (Ohtani et al., 2011). Subsequently, α-PbO2 type silica, seifertite along coesite and stishovite were found from a lunar meteorite, NWA 4734 (Miyahara et al., 2013). In addition to lunar meteorites, we also identified stishovite from a lunar return sample, Apollo 15299 breccia (Kaneko et al., 2014). Considering radio-isotope chronology, the high-pressure polymorphs of silica are closely related with the late heavy bombardment and subsequent meteoroid impacts on the Moon. Miyahara et al. (2014) identified coesite and stishovite from eucrite which was expected to originate from 4 Vesta, which raised an objection about howardite-eucrite-diogenite delivery model to the Earth. Coesite and stishovite are also found from enstatite and carbonaceous chondrites (Weisberg et al. 2010; Kimura et al., 2014) although their parent-bodies are expected to be less shocked. Now the existences of coesite and stishovite in shocked meteorites appear to be ubiquitous. Therefore, high-pressure polymorphs of silica, which were overlooked, will become a new clue for clarifying a dynamic event in the solar system. On the other hand, the pervasive existence of coesite in shocked meteorites is enigmatic. The phase transition from quartz to coesite is not easily achieved in a transient high-pressure condition due to a high kinetic barrier (e.g., Mosenfelder and Bohlen, 1997). Coesite occurs in a silica grain entrained in a shock-melt vein or melt-pocket. Most coesite are un-oriented fine-grained assemblages accompanying silica glass. Coesite in shocked meteorites may crystallize from silica-glass or melt subsequent to amorphization or melting. Coesite probably has a high nucleation rate in silica-glass or melt.

Kaneko S. et al. (2014) Discovery of stishovite in an Apollo 15 sample and impact record on the Moon. Japan Geoscience Union Meeting 2014, PPS22-10.
Kimura M. et al. (2014) Shock vein in an enstatite chondrite, Asuka 10164. 77th Annual Meteoritical Society Meeting, 5100pdf.
Mosenfelder J.L. and Bohlen S.R. (1997) Kinetics of the coesite to quartz transformation. Earth and Planetary Science Letters 153, 133-147.
Miyahara M. et al. (2013) Discovery of seifertite in a shocked lunar meteorite. Nature Communications, doi: 10.1038/ncomms2733.
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 E. et al. (2011) Coesite and stishovite in a shocked lunar meteorite, Asuka-881757, and impact events in lunar surface. Proceedings of the National Academy of Sciences U.S.A., 108, 463-466.
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.