ALBERT

Description: This study investigates the large strain rheological behaviour of pelitic rocks undergoing melting and subsequent crystallization during deformation. Constant strain rate ( =3x10 -4  s -1 ) torsion experiments were performed to achieve large strains ( γ max  = 15) on synthetic aggregates of quartz and muscovite at 300 MPa confining pressure and temperature of 750 °C. A set of hydrostatic experiments for equivalent times of the torsion experiments was also conducted at 300 MPa and 750 °C to evaluate the reaction kinetics and microstructures under static and under deforming conditions. Microstructures of the deformed samples reveal four distinct but gradational stages of crystal-melt interactions - a) solid state deformation, b) initiation and domination of partial melting, c) simultaneous partial melting and crystallization and d) domination of crystallization. These four microstructural stages are linked to the changes of the bulk mechanical response of the deforming samples. Partial melting starts at relatively low finite shear strains ( γ  = 1-3) and is associated with strong ( ca . 60%) strain softening. With further shearing ( γ  = 4-10) the partially molten bulk material shows a constant (“steady state”) flow at low stress. Further crystallization of new crystals at the expense of melt between γ  = 10 and 15 causes weak strain hardening until the material fails by developing brittle fractures. The stress exponent ( n ) values, calculated at three different shear strains ( γ  = 1, 5 and 10), increase from ∼ 3 to ∼ 43 with increasing deformation, pointing out a transition from power to power law break-down or exponential flow of the bulk system. These new experimental data establish that partially molten rock does not flow according to a constant strain rate dependent power law (“steady state”) rheology. The rheological transition from strain rate sensitive to strain rate insensitive flow is interpreted as a function of melt-crystal ratio, their mutual interactions and the evolution of microstructures in the partially molten rock.