Crystallographic preferred orientation of wadsleyite and ringwoodite:Effects of phase transformation and water on seismic anisotropy in the mantle transition zone

摘要

Simple-shear deformation experiments on wadsleyite and ringwoodite aggregates were performed at 15-18 GPa and 1473-1873 K to investigate the effect of water on the development of the crystallographic preferred orientation (CPO) of wadsleyite and ringwoodite. The [001] axes of wadsleyite are preferentially sub-parallel to the shear direction and the [010] axes of wadsleyite concentrate in the direction of the shear-plane normal for water content less than 9000 ppm H/Si (i.e.,~540 wt. ppm) in wadsleyite. At higher water content in wadsleyite (>9000 ppm H/Si), the concentration of the [100] axes of wadsleyite becomes stronger than that of the [010] axes in the direction of the shear-plane normal. The fabric strength of wadsleyite having low water content (<3000 ppm H/Si) was much stronger than that having water content higher than 3000 ppm H/Si. The magnitude of V_(SH)/V_(SV) (the ratio of horizontally and vertically polarized shear wave velocities) in the upper transition zone is well explained by the flow of wadsleyite aggregates having water content higher than 3000 ppm H/Si. The back transformation from ringwoodite to wadsleyite may help to suppress the increase in fabric strength of wadsleyite during the deformation. In contrast to wadsleyite, the fabric strength of ringwoodite CPOs was not sufficient to cause robust seismic anisotropy even though the deformation of ringwoodite was controlled by dislocation creep. Thus, the lower transition zone is expected to be largely isotropic.