Dislocation Creep of Diopside Aggregates Under Unsaturated Hydrous Conditions at a Confining Pressure of 300 MPa

Abstract

To understand the dislocation creep behavior of clinopyroxene in the upper mantle, hot-pressed diopside aggregates without predrying treatment were triaxially deformed under water-unsaturated conditions at a confining pressure of 300 MPa, temperatures of 1323–1523 K, and strain rates of 10⁻⁶–10⁻⁴ s⁻¹, using a Paterson gas-medium apparatus. Fourier transform infrared measurements of the water contents of the samples before and after deformation revealed that water diffusion loss occurred during the deformation process. A simple diffusion model based on Fick's law was established to predict the variation in the water content with respect to time during deformation. Fitting the mechanical data with a power flow law yielded a stress exponent n of 4.3 ± 0.3, an activation energy Q of 427 ± 31 kJ/mol and a water content exponent r of 1.2 ± 0.2 for the dislocation creep of the diopside aggregates under water-unsaturated conditions. When the flow law was extrapolated to anhydrous and water-saturated conditions, the calculated flow strengths of the diopside aggregates were generally in agreement with the strengths determined directly by deformation experiments, but there also existed contribution from grain boundary water under water-saturated conditions. The results of our study indicate that the strength of diopside or upper mantle clinopyroxene is comparable to the strength of olivine under anhydrous conditions but weaker than that of olivine under water-saturated conditions in the dislocation creep regime. Therefore, diopside might dominate the rheological behavior in some clinopyroxene-enriched and hydrous regions of the upper mantle.