Experimental Study on the Crystal Preferred Orientation (CPO) of Minerals in Lawsonite Blueschist and Implications for Seismic Property in Cold Subduction Zones

Seismic anisotropy and low velocity layers have been observed in the subducting slabs in cold subduction zones where lawsonite blueschist is considered to be widely stable under high pressure and low temperature conditions. To understand deformation microstructures (i.e., CPO) of minerals (glaucophane and lawsonite) in lawsonite blueschist and the resultant seismic properties, we conducted deformation experiments of lawsonite blueschist in simple shear using a modified Griggs apparatus. Lawsonite blueschist was deformed under the pressures (P = 1 - 2.5 GPa), temperatures (T = 230 - 400℃), shear strain (γ = 1 - 4) and shear strain rates (10-6 - 10-4 s-1). Two types of CPOs of glaucophane and lawsonite in lawsonite blueschist were developed with increasing shear strain. The CPOs found in this study are similar to those of natural lawsonite blueschists. Based on seismic properties calculated using the CPO data, glaucophane showed a high P-wave anisotropy (7.7 - 16.9%) and a relatively low maximum S-wave anisotropy (4.4 - 9.2%). In contrast, lawsonite showed much higher maximum S-wave anisotropy (8.3 - 20.7%) than glaucophane, but showed a low P-wave anisotropy in the range of 4.7 - 10.3%. Both P-wave and maximum S-wave anisotropies of whole rocks were calculated as 4.1 - 7.6%, 3.0 - 8.6% showing lower values than those of individual glaucophane and lawsonite aggregates. Furthermore, whole rock seismic velocities of lawsonite blueschist are in the range of Vp = 7.70 - 7.85 km/s and Vs = 4.35 - 4.36 km/s which are lower than those of other surrounding mantle peridotites, indicating that the low velocity layers observed at the top of cold subducting slabs may be attributed to the CPOs of glaucophane and lawsonite in lawsonite blueschist.