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

Zhuang Guo, Yang Li, Shen Liu, Huifang Xu, Shijie Li, Xiongyao Li, Yangting Lin, Ian M.Coulson, Mingming Zhang

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 4 November 2019


“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.”