3:45 PM - 4:00 PM
[SIT04-12] Microstructure and growth-kinetics of forsterite reaction rim under high pressure
Keywords:Upper mantle, Element diffusion, Olivine, Forsterite, Grain-growth, Rheology
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.