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Discovery of High-Pressure Minerals in HED Meteorites

Tuesday, July 15, 2014

Assistant Prof. Masaaki Miyahara | Hiroshima University |

Fig. 1: Electron microscopy image of high-pressure polymorph of silica (SiO2) coesite, discovered in Béréba meteorite

A research group discovered the high-pressure polymorphs*1 of silica (SiO2), coesite, and stishovite, in a Howardite–eucrite–diogenite (HED) meteorite, which probably originated from the asteroid 4 Vesta. The group was led by Masaaki Miyahara (associate professor) of the Graduate School of Science, Hiroshima University, Eiji Ohtani (professor) and Shin Ozawa (assistant professor) of the Graduate School of Science, Tohoku University, and Akira Yamaguchi (Assistant Professor) of the National Institute of Polar Research. NASA’s spacecraft Dawn revealed that there are many impact basins on 4 Vesta, indicating that 4 Vesta experienced heavy impact events. However, high-pressure polymorphs had not been discovered in HED meteorites although they should be formed under ultrahigh pressure and temperature conditions induced by an impact event. The research group succeeded in discovering high-pressure polymorphs of silica (SiO2) in one of HED meteorites, Béréba (Fig. 1) using nanoscale analysis techniques such as electron microscopy and a focused ion beam machining device*2.

Conventionally, it has been assumed that giant impact basins were formed on 4 Vesta by an impact event that occurred approximately one billion years ago, and that the surface materials of 4 Vesta were blown off and fell to the Earth. Considering the features of the high-pressure polymorphs of silica (SiO2) and radiometric ages, however, the impact event recorded in Béréba meteorite occurred approximately 4.1 billion years ago. This is inconsistent with the predicted formation age of the giant impact basins on 4 Vesta and indicates the necessity of reconsidering the origin of HED meteorites and their process of falling to the Earth. These achievements were published online in Proceedings of the National Academy of Sciences of the United States of America on 15 July 2014.

Masaaki Miyahara1, Eiji Ohtani, Akira Yamaguchi, Shin Ozawa, Takeshi Sakai and Naohisa Hirao
1 First author, Corresponding author
"Discovery of coesite and stishovite in eucrite"
Proceedings of the National Academy of Sciences U.S.A.


*1 High-pressure polymorphs Minerals are naturally occurring solid substances that are uniform in chemical composition and crystal structure. Their crystal structure changes depending on the surrounding conditions such as pressure and temperature. Minerals that are stable at a pressure higher than the pressure at the earth’s surface (1 atmosphere) are called “high-pressure polymorphs.”

*2 Focused ion beam machining device A device in which a finely focused ion beam is scanned over a sample for the observation or micrometer-scale machining of the sample surface. In this study, such a device was used for cutting out a part of the samples and preparing thin films for electron microscopy.