Effect of varying enstatite content on the deformation behavior of fine-grained synthetic peridotite under wet conditions

The effect of varying enstatite content on the deformation behavior of synt hetic, fine-grained (1 to 2 mu m) forsterite-enstatite rock with similar to 0.5wt % added water was investigated at temperatures of 900 degrees to 100 0 degrees C, strain rates between 10(-7) and 10(-5) s(-1), and a confining pressure of similar to 600 MPa. The samples exhibited approached steady sta te flow at stresses less than or equal to 60 MPa. The results show that, at constant strain rate, increasing enstatite content is associated with a sh arp decrease in flow strength in the range 0-2.5% vol % enstatite, with lit tle further change up to 20 vol %. The observed power law n value of simila r to 1.7 and microstructures are similar to those obtained in previous work on material with 2.5 wt % enstatite and are consistent with a water-enhanc ed grain boundary sliding (GBS) dominated deformation mechanism. Significan tly, a negative correlation was found between grain size and enstatite cont ent, indicating that enstatite content played a role in controlling the gra in size of the starting materials. Moreover, a high correlation between mea sured flow strength and grain size was found, consistent with a grain size exponent of -3 in a conventional grain size sensitive now equation. A water -enhanced deformation mechanism involving GBS accommodated probably by grai n boundary diffusion and/or dislocation activity is therefore implied, with the effect of enstatite content on flow strength being an indirect physica l effect caused by grain size control. Other effects of second-phase conten t, such as weakening caused by interphase boundary diffusion and/or migrati on processes, seem to be unimportant under the conditions investigated.