14:45 〜 15:00
[PPS06-17] 衝撃変成を受けたLコンドライト中のカンラン石新高圧相
キーワード：衝撃変成、高圧鉱物、カンラン石、相転移メカニズム、透過電子顕微鏡
A new high-pressure polymorph of Mg2SiO4 was discovered in the Tehnam L6 chondrite by transmission electron microscopy (TEM). Tenham is one of the renowned meteorite samples containing various types of high-pressure silicate minerals within and in the vicinity of shock veins [1]. Based on the occurrence of high-pressure mineral assemblages reported in Tenham and experimental phase equilibrium studies, the peak shock pressure was estimated to be ~25 GPa [2].
Euhedral and subhedral ringwoodite (Fo69) grains with sizes of 0.35 ± 0.2 mm occur as polycrystalline aggregates in a shock vein. These grains are crystallographically randomly oriented. Most of the ringwoodite grains exhibit pervasive planer defect on {110} planes. These defects have been reported in ringwoodite from many shocked ordinary chondrites. Despite the typical microtexture, the detailed analysis of the respective grains revealed novel crystallographical features. Selected area electron diffraction (SAED) patterns of most of the grains of the ringwoodite aggregates show strong diffraction spots that are consistent with a spinel structure. In addition, weak diffraction spots were observed. These diffraction spots could only be indexed by a new spineloid structure which was previously theoretically predicted as “ε*-phase” in Mg2SiO4 polymorph [3]. High-resolution TEM showed that some of planer defects in the ringwoodite correspond to the ε*-phase with a spacing below 10 nm. In addition, the SAED patterns of ringwoodite with ε*-phase lamellae show that both phases have a topotaxial relationship: (001)ε*//{001}Rwd and (100)ε*//{110}Rwd. The crystallographic relationship can be explained in terms of periodic arrangements of a basic unit of spinel and spineloid structures.
Olivine in the host rock of Tenham entrapped in the shock vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg2SiO4 polymorphs via a shear transformation mechanism [3]. Our discovery indicates that high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.
References: [1] Tomioka, N. and Miyahara, Meteorit. Planet. Sci., 52, 2017–2039 (2017). [2] Xie, Z., Sharp, T. G. and DeCarli, P. S., Geochim. Cosmochim. Acta 70, 504–515 (2006). [3] Madon, M. and Poirier, J. P., Phys. Earth Planet. Inter. 33, 31–44 (1983).
Euhedral and subhedral ringwoodite (Fo69) grains with sizes of 0.35 ± 0.2 mm occur as polycrystalline aggregates in a shock vein. These grains are crystallographically randomly oriented. Most of the ringwoodite grains exhibit pervasive planer defect on {110} planes. These defects have been reported in ringwoodite from many shocked ordinary chondrites. Despite the typical microtexture, the detailed analysis of the respective grains revealed novel crystallographical features. Selected area electron diffraction (SAED) patterns of most of the grains of the ringwoodite aggregates show strong diffraction spots that are consistent with a spinel structure. In addition, weak diffraction spots were observed. These diffraction spots could only be indexed by a new spineloid structure which was previously theoretically predicted as “ε*-phase” in Mg2SiO4 polymorph [3]. High-resolution TEM showed that some of planer defects in the ringwoodite correspond to the ε*-phase with a spacing below 10 nm. In addition, the SAED patterns of ringwoodite with ε*-phase lamellae show that both phases have a topotaxial relationship: (001)ε*//{001}Rwd and (100)ε*//{110}Rwd. The crystallographic relationship can be explained in terms of periodic arrangements of a basic unit of spinel and spineloid structures.
Olivine in the host rock of Tenham entrapped in the shock vein initially transformed into polycrystalline ringwoodite through a nucleation and growth mechanism. The ringwoodite grains then coherently converted into the ε*-phase by shear transformation during subsequent pressure release. This intermediate metastable phase can be formed by all Mg2SiO4 polymorphs via a shear transformation mechanism [3]. Our discovery indicates that high-pressure transformations of olivine that are enhanced by diffusionless processes, not only in shocked meteorites but also in thick and cold lithosphere subducting into the deep Earth.
References: [1] Tomioka, N. and Miyahara, Meteorit. Planet. Sci., 52, 2017–2039 (2017). [2] Xie, Z., Sharp, T. G. and DeCarli, P. S., Geochim. Cosmochim. Acta 70, 504–515 (2006). [3] Madon, M. and Poirier, J. P., Phys. Earth Planet. Inter. 33, 31–44 (1983).