Japan Geoscience Union Meeting 2016

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT07] Structure and dynamics of Earth and Planetary deep interiors

Sun. May 22, 2016 10:45 AM - 12:15 PM 201B (2F)

Convener:*Takashi Yoshino(Institute for Study of the Earth's Interior, Okayama University), Dapeng Zhao(Department of Geophysics, Tohoku University), Takashi Nakagawa(JAMSTEC/MAT), Chair:Takashi Yoshino(Institute for Study of the Earth's Interior, Okayama University), Koichiro Umemoto(Earth-Life Science Institute, Tokyo Institute of Technology)

11:15 AM - 11:30 AM

[SIT07-09] Messengers from the deep: Fossil wadsleyite-chromite microstructures from the Mantle Transition Zone

*Takako Satsukawa1, William L Griffin2, Sandra Piazolo2, Suzanne Y O'Reilly2 (1.Department of Geophysics, Division of Earth and Planetary Sciences, Kyoto University, 2.CCFS&GEMOC, Department of Earth and Planetary Sciences, Macquarie University)

Keywords:Mantle transition zone, chromitite, olivine, CPO, EBSD, microstructure

Investigations of the Mantle Transition Zone (MTZ; 410-660 km deep) by deformation experiments and geophysical methods suggest that the MTZ has distinct rheological properties, but their exact cause is still unclear due to the lack of natural samples. Here we present the first direct evidence for crystal-plastic deformation by dislocation creep in the MTZ using a chromitite from the Luobusa peridotite (E. Tibet). Chromite grains show exsolution of diopside and SiO2, suggesting previous equilibration in the MTZ. Electron backscattered diffraction (EBSD) analysis reveals that olivine grains co-existing with exsolved phases inside chromite grains and occurring on chromite grain boundaries have a single pronounced crystallographic preferred orientation (CPO). This suggests that olivine preserves the CPO of a high-pressure polymorph (wadsleyite) before the high-pressure polymorph of chromite began to invert and exsolve. Chromite also shows a significant CPO. Thus, the fine-grained high-pressure phases were deformed by dislocation creep in the MTZ. Grain growth in inverted chromite produced an equilibrated microstructure during exhumation to the surface, masking at first sight its MTZ deformation history. These unique observations provide a window into the deep Earth, and constraints for interpreting geophysical signals and their geodynamic implications in a geologically robust context.