Discovery of nanophase iron particles and high pressure clinoenstatite in a heavily shocked ordinary chondrite:implications for the decomposition of pyroxene

1,2,3Zhuang Guo,1,2 YangLi,2Shen Liu,5Huifang Xu,1,4Shijie Li,1,4Xiongyao Li,6Yangting Lin,7Ian M.Coulson,1,4Mingming Zhang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.036]
1Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
2State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China
3University of Chinese Academy of Sciences, Beijing 100049, China
4Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, China
5Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706-1692, USA
6Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
7Solid Earth Studies Laboratory, Department of Geology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada
Copyright Elsevier

Although pure metallic iron (i.e. that with an Fe content of greater than 99%) commonly occurs in achondrites, and within the returned soil from asteroids or the Lunar surface, it is rarely found in ordinary chondrites meteorites. Abundant nanophase iron particles (np-Fe0) were identified in pyroxene glass, within the shock melt vein of Grove Mountains (GRV) 022115, which is an ordinary (L6) chondrite, with a shock stage determined as S5. The association of np-Fe0, highly defective high pressure clinoenstatite (HP-CEn), silica glass, as well as vesicles, embedded in a pyroxene glass selvage within the shock melt vein in this meteorite suggests that these phases formed as the result of decomposition of the host pyroxene grain, a process induced by the shock event that affected GRV 022115. The reaction to account for this mineral breakdown can be written as: FeSiO3 →Fe + SiO2 + 1/2O2 ↑ (MgSiO3 remain in the HP-CEn). The pressure and temperature condition attending this reaction are estimated at 20-23 GPa and over 1800 ℃, as indicated by the surrounded high-pressure mineral assemblage: ringwoodite, majorite, and magnesiowüstite. This study provides evidence to the formation of np-Fe0 derived from pyroxene, and HP-CEn quenched metastably in such shocked vein could preserve the metastable phase transitions history record.

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