Effects of pressure on high-temperature dislocation creep in olivine

Effects of pressure on high-temperature, dislocation creep in olivine ((Mg, Fe)(2)SiO4) aggregates have been determined under both water-poor ('dry') and water-saturated ('wet') conditions. New experimental data were obtained at pressures of 1-2 GPa under 'dry' and 'wet' conditions using a newly developed high-resolution dislocation density measurement technique to estimate the creep strength. These data are compared with previous data at lower and higher pressures to determine the pressure dependence of high-temperature dislocation creep in olivine aggregates. We find that the creep strength or under 'dry' conditions increases monotonically with increasing pressure, whereas the creep strength under 'wet' conditions changes with pressure in a non-monotonic fashion: it first decreases rapidly with increasing pressure and then becomes less sensitive to pressure at above I GPa. Such behaviour can be described by the following formula: <(ε) over dot>(d,w) = A(d,w)f(H2O)(r) (T,P) exp (-(E-d,w(*) + PVd,w*)/RT) sigma(n), where the subscripts d and w refer to parameters for 'dry' and 'wet' conditions respectively. The present study gives A(d) = 10(6.1) (+/- 0.2) s(-1) (MPa)(-n), E-d(*) = 510 +/- 30 kJ mol(-1), V-d(*) = 14 +/- 2 cm(3) mol(-1) and r = 0 for 'dry' d conditions, and A(w) = 10(2.9 +/-) (0.1) s(-1) (MPa)(-n-r), E-w(*) 470 +/- 40 kJ mol(-1), V-w(*) = 24 +/- 3 cm(3) mol(-1) and r = 1.20 +/- 0.05 for 'wet' conditions (n = 3.0 +/- 0.1 for both 'dry' and 'wet' conditions). The large activation volume for 'wet' conditions can be interpreted as due to the additional contribution from the activation volume for dissolution of OH in the olivine structure (V-OH(*) approximate to 11cm(3) mol(-1)). The value of r(approximate to 1.2) is consistent with a model in which creep in olivine is rate controlled by the motion of positively charged jogs through the diffusion of silicon via an interstitial mechanism.