Structures and rheology of crystal-rich silicic magmas deformed in torsion at high pressure: Experimental constraints
Abstract
Structures such as Mineral fabrics and mechanical anisotropies together with the rheological properties are key parameters to constraint the flow history of crystallising magmas. In particular, a rheological transition from an heterogeneous granular dilatant suspension to a solid material is expected at solid fractions 40%<Φ<80%. We present preliminary results of high pressure torsion experiments performed to investigate the rheological and structural behaviour of silicic magma suspensions crystallised in this solid fraction range. The starting materials are plagioclase + melt suspensions. These were obtained by crystallizing synthetic haplotonalite glasses + 3-4% wt H2O at T < 850°C, 300 MPa during 7 days, following an initial homogenisation step at 1000°C. Suspensions with 50-60% vol of randomly distributed and oriented, 20 µm long plagioclases were studied. Torsion experiments were performed using a Paterson HP-HT apparatus, at a confining pressure of 300 MPa, temperatures of 500°C, 750°C, 825°C and 850°C and a shear strain rate from 3.10-4 s-1 to 5.8x10-4 s-1. For a constant T=750°C and constant strain rate 5.8x10-4 s-1, the apparent viscosity is 109.84 Pa.s. At Γ=3.5, a strong Crystal Preferred Orientation (CPO) is developed with an average orientation Α≈47° anticlockwise with respect to the shear direction (SD). This mineral foliation is locally crosscut by a 40 Μm thick shear zone oriented at 8° clockwise from SD and bordered by shear gradients indicating normal shear sense. These structures, usually developed during the late emplacement stage of highly crystallised volcanic rocks, are regarded as evidence of shear thickening behaviour.