Dynamic Recrystallization and Development of Olivine LPO at Water-rich Condition

Recent study by Jung and Karato (2001) showed that water can drastically modify the olivine lattice preferred orientation (LPO) and hence the nature of seismic anisotropy for a given flow geometry. However, the application of such results to Earth's upper mantle requires detailed understanding of underlying physical mechanisms, because large extrapolation is needed in various parameters particularly stress and temperature. In addition to the change in the dominant slip systems (Karato, 1995; Jung et al., 2002; Kaminski, 2002), water appears to affect microstructures through its influence on dynamic recrystallization. Unlike A-type LPO where the [100] maximum has two peaks one sub-parallel to the shear direction another at a high angle to the shear direction, the [001] maximum of both B-type and C-type is inclined to the shear direction with angle of 20-40 degree even at the shear strain of ~4 and there is no strong peak parallel to the shear direction. In addition, dislocation density is more heterogeneous than in dry samples (particularly in B-type samples). A comparison of these results with our previous work on A-type samples (Lee et al., 2002) suggests an important role of enhanced grain-boundary migration under water-rich conditions and less pronounced role of grain-size reduction. Detailed microstructural studies are underway using SEM including an automated orientation mapping and the measurements of dislocation density as a function of orientation.