[P-CG22] Shock responses of planetary materials elucidated from meteorites and dynamic compression experiments

[PCG22-02] Lattice-shear-induced metastable formation of high-pressure silicates in shocked meteorites

★Invited Papers

*Naotaka Tomioka¹, Takuo Okuchi², Masaaki Miyahara³ (1.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 2.Institute for Planetary Materials, Okayama University, 3.Department of Earth and Planetary Systems Science, Hiroshima University)

Keywords:shock metamorphism, meteorites, transformation mechanisms, high-pressure minerals

Heavily shocked meteorites contain various types of high-pressure polymorphs of silicate minerals. These high-pressure minerals are micron to submicron sized and occurred within and in the vicinity of shock-induced melt in stony meteorites. Their occurrence suggests two types of formation mechanisms: (1) solid-state high-pressure transformation of the host-rock minerals into monomineralic polycrystalline aggregates, and (2) crystallization of chondritic or monomineralic melts under high pressure [1]. In some cases of the former process, high-pressure minerals occur as coherent lamellae within grains of the host low-pressure minerals. A typical example is the (Mg,Fe)SiO₃pyroxene-akimotoite intergrowth in the Tenham meteorite [2]. The akimotoite has relatively high Fe content [Fe/(Mg+Fe)=0.21] but does not have its stability field at any P-T conditions [3]. Therefore, the akimotoite would have been formed metastably due to a rapidly-changing P-T history in the shock event. As another example, a new high-pressure polymorph of olivine, which has been defined as “ε-phase” with a spinelloid structure, was recently discovered as nano-scale lamellae in ringwoodite [(Mg,Fe)₂SiO₄-spinel] and wadsleyite [(Mg,Fe)₂SiO₄-spinelloid] grains in Tenham and Miami meteorites, respectively [4]. The phase has never been observed in previous phase equilibrium studies. Both pyroxene-akimotoite and ringwoodite/wadsleyite-ε-phase intergrowths have topotaxy with preserving close-packed oxygen layers for their respective structures. The crystallographic relationships between these host and the product phases suggest that these polymorphic phase transformations are promoted by shear mechanism [5]. This process is achieved by shear deformation of oxygen sublattice associating movements of interstitial cations within single unit-cell dimension. Therefore, shear mechanism is considered to be a favorable mechanism under high differential stress, or under relatively low-temperatures, where atomic diffusion is kinetically hindered. Future experimental studies, which include ultrafast in-situ X-ray diffraction under laser shock compression, and high-resolution electron microscopy on recovered specimens of static high-pressure experiments, would contribute to better understanding of detailed structural changes in naturally shocked samples.

References: [1] Tomioka & Miyahara (2017) Meteorit. Planet. Sci.,52, 2017–2039. [2]Tomioka & Fujino (1999) Amer. Mineral., 84, 267–271. [3] e.g. Ito & Yamada (1982) inHigh-Pres. Res. Geophys., 405–419.[4] e.g. Tomioka & Okuchi (2017) Sci. Rep., 7, 17351. [5] Tomioka (2007) J. Mineral. Petrol. Sci.,102, 226–232.