The influence of dolomite on the plastic flow of calcite: Rheological, microstructural and chemical evolution during large strain torsion experiments

Aggregates composed of calcite and dolomite have been deformed and annealed at high temperature (700-800 DGC) and high pressure (300 MPa) to investigate the mutual effects of deformation and chemical reaction, strain partitioning, fabric development and phase segregation in a polyphase rock. Samples deformed in torsion displayed an almost Newtonian creep with stress exponent of 1.7 - 0.3. Specimens were deformed up to a maximum shear strain of g = 10 and developed a clear texture documented by electron back-scatter diffraction measurements in both calcite and dolomite. Both the texture and the foliation created during the large strain experiments display a monoclinic symmetry closely reproducing that of naturally deformed carbonate marbles, with the preferred orientation of the c-axes inclined against the sense of shear. Detailed microstructural investigation shows that grains are internally strain-free and that grain boundaries are preferentially aligned in straight segments, implying that deformation occurred through a combination of grain boundary sliding and diffusion processes accommodated by dislocation activity. Chemical investigations performed on both statically annealed and torsion samples indicate that deformation may have an important influence on the phase equilibrium of a reacting system. Calcite grains deformed with excess of dolomite with respect to the equilibrium were able to incorporate higher amounts of Mg within the calcite lattice than the static samples, as documented by electron probe microanalysis. Moreover, the above analysis documented that reaction rate was faster in torsion than in static experiments at the same conditions of pressure and temperature.