An experimental study of the role of shear deformation on partial melting of a synthetic metapelite

Experiments on a synthetic, foliated, quartz-muscovite rock were undertaken to evaluate and explain the influence of shear deformation on partial melting at 750 °C and 300 MPa. Torsion experiments were conducted at a constant strain rate of 3 × 10−4 s−1 with the initial foliation parallel and orthogonal to the direction of applied angular displacement. The non-equilibrium melting reaction forms melt, sillimanite, biotite, and K-feldspar at the expense of muscovite and quartz. In both static and torsion experiments, for the first 1.5 h the rate of melting is identical, thereafter melting accelerates in the torsion experiments. The rate of melting is highest in torsion experiments with the sample foliation orthogonal to the shear plane. First order models for shear heating, surface and strain energy, and mean stress effects suggest that none of these effects explain the difference in melting rate between static and torsion experiments. We conclude that the most probable explanation for the correlation between reaction rate and shear deformation is that shear deformation lowers the effective viscosity of the rock matrix. The reduction in effective viscosity facilitates the elimination of the grain-scale mean-stress perturbations caused by the volume change of the melting reaction. As these perturbations inhibit reaction, their elimination by ductile dilational deformation accelerates the melting process.

Research Highlights
► We perform both, hydrostatic and torsion experiments in quartz-muscovite system.
► Deformation enhances the muscovite melting reaction rate.
► Surface and strain energies were rejected from possible explanations.
► A lowering of effective viscosity is the most plausible reason of the additional melting.