Experimental Investigation of Glaucophane Rheology Through General Shear Deformation Experiments

Glaucophane is a major constituent mineral associated with subducted mafic oceanic crust at blueschist facies conditions. Viscous deformation of glaucophane has been documented in natural blueschists; however, no experimental study has characterized the specific deformation mechanisms that occur in glaucophane nor the flow law parameters. We are conducting a suite of general shear deformation experiments in a Griggs apparatus to investigate crystal-plastic deformation mechanisms and microstructures of deformed glaucophane over a range of experimental conditions. Experimental samples consist of glaucophane powder separated from natural MORB blueschists  from Syros Island, Greece. Our experimental suite thus far includes temperatures and pressures ranging from 650° to 750°C and 1.0 to 1.5 GPa, strain rates ranging from ~3x10^-6/s to ~8x10^-5/s (both constant-rate and strain-rate stepping), and different grain size populations from 75-90 µm, 63-125 µm , and 63-355 µm. The lowest temperature and the strain-rate-stepping experiments exhibit evidence for combined frictional-viscous deformation and provide constraints on the brittle-ductile transition in glaucophane at laboratory conditions. The constant-rate experiments conducted at higher temperatures show greater evidence for viscous deformation by dislocation creep, including kinked grains, deformation lamellae, undulose extinction, and bulging via bulge recrystallisation. Mechanical data from the strain-rate stepping experiments allow us to interpret what parameters have the largest effect on peak stress. When comparing experiments conducted at 1 GPa and initial powder grain sizes of 63-355 µm, we find temperature having the largest effect on peak stresses. The 700°C experiment with an initial deformation speed 5 times faster (LH038) than another 700°C strain-rate stepping experiment (LH042) has a ~90 MPa higher peak shear stress, whereas the 750°C strain-rate stepping experiment with an initial deformation speed 4 times faster than LH042 has a ~115 MPa lower peak shear stress. At the time of abstract submission, further constant-rate experiments are planned at slower strain-rates to continue exploring the laboratory conditions necessary to activate glaucophane crystal-plastic deformation mechanisms. These data will be used with further strain-rate stepping experiments to develop flow law parameters from mechanical data.