HR: 0800h
AN: MR41A-0900    [Abstracts]
TI: Water-weakening of a Crustal Clinopyroxenite: A Few Protons go a Long Way
AU: * Chen, S
EM: chen0705@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE Suite 108, Minneapolis, MN 55455 United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE Suite 108, Minneapolis, MN 55455 United States
AB: We performed a series of triaxial compressive creep experiments at two different water fugacities to investigate the effect of water on the creep strength of a natural clinopyroxenite of lower crustal origin. Samples were deformed under water-saturated conditions at temperatures from 1373 to 1473 K and confining pressures of 150 and 300 MPa. Under these thermodynamic conditions, water fugacities were either ~140 or ~280 MPa. For differential stresses of 34 to 261 MPa, strain rates were in the range 10^-7 to 10^-5s^-1. The creep results yield a stress exponent of 2.6±0.3 and an activation energy of 644±56 kJ/mol. Compared to dry clinopyroxenite samples from the same formation, wet samples creep over 100 times faster at a given temperature, confining pressure, water fugacity, and differential stress. In addition, the creep rate is proportional to the water fugacity to the 2.6 power, assuming an activation volume of 0 m³/mol. One possible water weakening mechanism is an enhancement of the rate of dislocation climb associated with an increase in the concentration of jogs and in the diffusivity of silicon ions associated with the introduction of protons as point defects into the clinopyroxene structure. Compared to the other major mineral in Earth's lower crust, plagioclase, the water-weakening effect is more significant for clinopyroxene. Under hydrous conditions, the strengths of clinopyroxene and anorthite are comparable over the investigated stress range. Hence, in places in the lower continental crust where a wet flow law applies, the mechanical behavior of clinopyroxene will have a substantial effect on creep strength.
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 3924 High-pressure behavior
DE: 5120 Plasticity, diffusion, and creep
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005