HR: 0800h
AN: T41B-0578    [Abstracts]
TI: Effect of Brucite on the Frictional Strength of Serpentinite
AU: * Moore, D E
EM: dmoore@usgs.gov
AF: U. S. Geological Survey, MS 977 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: U. S. Geological Survey, MS 977 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Brucite is a common by-product of the serpentinization of olivine-rich ultramafic rocks. Brucite is considerably weaker than the serpentine mineral antigorite; in the T range 25-400°C, antigorite serpentinite has a coefficient of friction of 0.5-0.6 whereas that of brucite is 0.2-0.3. We investigated the possible weakening effect of brucite on serpentinite, because of the close association of serpentinite with faults in a variety of plate-tectonic settings. Frictional strength experiments were run on mixtures of brucite and an antigorite-rich serpentinite. Experiments were run at 100 MPa effective normal stress, comparing behavior at 25 and 300°C. Adding small amounts of brucite to serpentinite gouge dramatically lowers its residual frictional strength. At both temperatures, 5 wt% brucite reduces the coefficient of friction of serpentinite by about 25% towards that of the brucite end member, and serpentinite containing 50 wt% brucite has essentially the strength of pure brucite. However, at 300°C, gouges containing 65-85% brucite that were sheared at 0.01 μm/s axial displacement rate were stronger for a given amount of displacement and showed more pronounced stick-slip motion than gouges containing 50% and 100% brucite that were sheared at the same rate. Both the size of the stress drops and the shear strength decreased with increasing displacement for the gouges with 65-85% brucite, but strength was still decreasing when the experiments ended at about 3-mm axial displacement. At faster shearing rates of 0.1 and 1.0 μm/s, gouge containing 75% brucite reached its residual strength between 1 and 1.5 mm axial displacement and showed a reduced tendency for stick-slip behavior. The antigorite-rich samples slid stably at 300°C and 0.01 μm/s axial displacement rate. Antigorite and brucite both have platy habits, and shear of serpentinite gouge is highly localized along shears in which the platy grains are oriented parallel to the shear surfaces. SEM examination of gouge samples containing 5-50% brucite indicates that brucite is concentrated on the shear surfaces, particularly the R shears. The shears may therefore form in a manner that maximizes the proportion of the weaker mineral within them. The difference in strength between samples of gouge containing 75% brucite that were sheared at different rates at 300°C is consistent with the relative proportions of brucite incorporated in the shears, but the cause of this shear-rate dependent behavior is not understood. However, further study is warranted, because the gradually diminishing stick-slip behavior at the slow shearing rate resembles in some ways the tremor events that have been reported in subduction zones at depths where serpentinized ultramafic rock may be present.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting