Japan Geoscience Union Meeting 2015

Presentation information

International Session (Oral)

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

[S-IT04] Rheology of Earth's Interior

Thu. May 28, 2015 2:15 PM - 4:00 PM 106 (1F)

Convener:*Tomohiro Ohuchi(Geodynamics Research Center, Ehime University), Shun-ichiro Karato(Yale University, Department of Geology and Geophysics), Katsuyoshi Michibayashi(Institute of Geosciences, Shizuoka University), Chair:Noriyoshi Tsujino(Institute for Study of the Earth's Interior, Okayama University)

3:45 PM - 4:00 PM

[SIT04-12] Microstructure and growth-kinetics of forsterite reaction rim under high pressure

*Yu NISHIHARA1, Genta MARUYAMA2 (1.Geodynamics Research Center, Ehime University, 2.Earthquake Research Institute, University of Tokyo)

Keywords:Upper mantle, Element diffusion, Olivine, Forsterite, Grain-growth, Rheology

Since many transport properties, such as rheology, highly depend on grain-size of the constituent materials, knowledge of grain-growth is important for accurate understanding of dynamics in the Earth's mantle. Tasaka and Hiraga (2013) showed that grain-growth in forsterite-enstatite two-phase system is rate-limited by growth of secondary phase through Mg-O grain-boundary diffusion in primary phase. In this study, we have experimentally studied growth-kinetics of forsterite (Mg2SiO4) reaction rim at deep upper mantle conditions which is controlled by Mg-O grain-boundary diffusion in forsterite. Based on the experimental results, depth dependence of grain-growth in the upper mantle is discussed.
Experiments were carried out using multi-anvil apparatus installed at GRC, Ehime University, Japan. The starting materials, MgO (single crystal) and MgSiO3 (powder or aggregate), were packed in Pt capsule. The annealing experiments were conducted at pressure of 3.0-11.1 GPa and temperature of 1473-1873 K for duration of 0-780 min. Recovered samples were analyzed by SEM or FE-SEM for microstructural observation and by FT-IR to examine water content.
Water content in MgO single crystal in the recovered samples was relatively low and 6.9 wt ppm H2O at the maximum. The Pt marker, which was originally placed at MgO-MgSiO3 boundary, was always on MgO-Mg2SiO4 boundary indicating that Mg-O diffusion in Mg2SiO4 is the rate-limiting process in the rim growth. MgSiO3 inclusions were found in Mg2SiO4 grains suggesting the grain-boundary diffusion is rate-limiting. Based on the analysis using equation for reaction-rim growth rate-limited by grain-boundary diffusion (Gardes and Heinrich, 2011), the activation energy and the activation volume were determined to be 375 kJ/mol and -2.1 cm3/mol, respectively. Although reason for the small negative value of the activation volume is not quite clear, this may be due to successive structural change of grain-boundary. The results suggest that the grain-growth in the Earth's upper mantle is faster at deeper part.