Microstructures of experimentally deformed blueschist

Abstract

We conducted a series of high-P deformation experiments under 400？500 °C and 1？2.5 GPa for deciphering the brittle-ductile transition of blueschist. Microstructures of lawsonite and titanite in the experimentally deformed samples are brittle at 1？2 GPa, however ductile at 2.5 GPa. J-indices for glaucophane systematically decrease with increasing shear strain and confining pressure while those for lawsonite are random, indicating strain during experiments is efficiently localized into glaucophane. Angular change of (100) peak with flow direction at >2 GPa is similar to that of a strain ellipsoid and the various orientations of fine grains in a selected area electron diffraction image suggest the brittle-ductile transition of glaucophane at 2 GPa. Considering a strong initial fabric in the starting material and a dry experimental condition, the brittle-ducting transition of glaucophane will be much shallower than 2 GPa. Therefore this study indirectly demonstrates the dehydration embrittlement of subducting oceanic crust (glaucophane) as an important contributor of intraplate intermediate-depth earthquake.