HR: 17:30h
AN: MR34A-07    [Abstracts]
TI: Measurement of Activation Volume of Dry Olivine at High Pressure
AU: * Durham, W B
EM: wbdurham@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, United States
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, United States
AU: Mei, S
EM: meixx002@umn.edu
AF: University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, United States
AU: Dixon, D A
EM: dixonn@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, United States
AU: Wang, L
EM: liping.wang@sunysb.edu
AF: Stony Brook University, John S. Toll Road, Stony Brook, NY 11794, United States
AB: Despite considerable effort to measure the activation volume V* of creep of olivine using a new generation of high-pressure deformation machines, namely the Rotational Drickamer Apparatus (RDA) and the Deformation- DIA (D-DIA) in conjunction with synchrotron x-ray sources, progress has been marred by a combination of an apparently weak signal (i.e., low V*) and measurement noise. The latter has a broad spectrum of causes, from inherent limitations of measurement systems (of stress, primarily) to limited time available for creep experiments at synchrotron sources to investigator-induced variations of sample state during measurement. We report here measurements following a significant advance in regulation of sample state: deforming polycrystalline olivine in a dry state under more uniform mechanical conditions gives us our first clear signal of a positive activation volume. The advance has been made possible by a choice of assembly materials that assures anhydrous conditions around the sample, and a technique for testing in the absence of a thermocouple within the sample assembly. The key to the anhydrous assembly is a self-gasketing D-DIA cube of hybrid composition: a sphere of mullite embedded in a cube of unfired pyrophyllite, where the diameter of the sphere matches the edge length of the cube (6-mm in our case). Unfired pyrophyllite is an excellent gasketing material, being soft and (in notable contrast with mullite) non-friable. Additionally, as a cube-filling "web" around the mullite sphere, the pyrophyllite is ideally configured for D-DIA self gasketing: maximum volume at the cube corners, minimum (zero) volume at cube faces. Thus the configuration under pressure is a bone-dry mullite pressure medium gasketed by pyrophyllite. Removal of the thermocouple from the deformation piston results in a demonstrably more symmetric deformation column within the assembly for the entire duration of the experiment, and the absence of the thermocouple is reliably compensated by off-line calibration of furnace power vs temperature. Results of creep experiments in the new cell indicate a value of V*/n (where n is the stress exponent) of 3 ± 1 × 10^-6 m³/mol between 3 and 6 GPa. The value of n will be better resolved in future experiments, but if the deformation is in the dislocation creep regime (n ~ 3.5), then V* ~ 10 × 10^-6 m³/mol.
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 5120 Plasticity, diffusion, and creep
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting