Showing papers by "David L. Kohlstedt published in 2006"

The Role of Water in High-Temperature Rock Deformation

Abstract: The first report of water/hydrolytic weakening of silicate minerals was based on the observation that the strength of quartz decreases significantly in the presence of the water released from talc, the confining medium used in some of high-pressure experiments (Blacic and Griggs 1965). Subsequently, a number of researchers have studied this important phenomenon in several different minerals. Publications examining the influence of water or protons on the creep behavior of nominally anhydrous silicate minerals are listed in Table 1⇓. Initial studies treated the water-weakening phenomenon as an on-off process; that is, minerals and rocks are weak under hydrous conditions but strong under anhydrous conditions. Further investigations, however, demonstrated that the strengths of nominally anhydrous minerals (NAMs) and rocks decrease systematically with increasing hydrogen concentration (Kronenberg and Tullis 1984; Kohlstedt et al. 1995; Post et al. 1996; Mei and Kohlstedt 2000a,b; Karato and Jung 2003). View this table: Table 1. Investigations of the effect of water/protons on the strength of nominally anhydrous silicate minerals. Two quite different approaches have been used in analyzing the effect of water or protons on the strength of nominally anhydrous silicate minerals. In the first model, a mechanism is envisioned in which water hydrolyzes strong Si-O bonds via the reaction Si-O-Si + H2O → Si-OH·OH-Si (Griggs 1967), thus the term water or hydrolytic weakening. As a result, the glide of dislocations becomes easier in wet quartz than in dry quartz since Si-O bonds do not need to be broken if water is present. In this case of water/hydrolytic weakening, the rate limiting step is the propagation of kinks along dislocations, which is facilitated by diffusion of HOH along dislocation cores. The resulting dislocation velocity is assumed to be proportional to the HOH concentration (Griggs 1974). In effect, this analysis implies …

Water weakening of clinopyroxene in the dislocation creep regime

Abstract: 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. Samples were deformed under water-saturated conditions at temperatures between 1373 and 1473 K, confining pressures of 150 and 300 MPa, and differential stresses from 34 to 261 MPa. Strain rates were in the range 10 -7 to 10 -5 s -1 . Water fugacity was controlled at either 140 or 280 MPa. The creep results yield a stress exponent of 2.7 ± 0.3 and an activation energy of 670 ± 40 kJ/mol. Compared to dry clinopyroxene, wet samples creep over 100 times faster at a given temperature, confining pressure, water fugacity, and differential stress. The creep rate of clinopyroxene is proportional to the water fugacity to the 3.0 ± 0.6 power, with an activation volume of 0 m 3 /mol. One possible water-weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions. Compared to other major minerals in Earth's lower crust, specifically olivine and plagioclase, the water-weakening effect is most significant for clinopyroxene. Under hydrous conditions the strengths of clinopyroxene and anorthite are comparable over the investigated stress range, and both phases are weaker than olivine. Since the mineral assemblages in Earth's lower continental crust are often dominated by plagioclase and pyroxene, in places where a wet flow law applies, the mechanical behavior of clinopyroxene will have a substantial effect on creep strength.

Metal-silicate segregation in deforming dunitic rocks

Abstract: We report the results of an investigation of the microstructural evolution of rocks composed of a silicate plus a molten metal sulfide phase deformed plastically in simple shear. Deformation experiments on samples of San Carlos olivine (Fo90) + 3, 5, or 9 vol% iron sulfide reveal the segregation of iron sulfide melt from the solid polycrystalline olivine matrix into regions enriched in metallic melt separated by regions depleted in the metallic melt. Previously published experimental studies on core-composition metals in silicate rocks, performed under hydrostatic pressure conditions, suggested that metallic melts could not segregate from silicates by porous flow below a critical melt fraction of ∼0.05. In contrast, we demonstrate that metallic melts can segregate from solid silicates by grain boundary percolation assisted by deformation, down to a pinch-off melt fraction of ∼0.01. The segregated metallic melt can accumulate into bands containing melt fractions exceeding 0.3. Using a model network of melt-rich sheets with melt fractions of 0.3, calculated Earth-scale permeabilities are as high as 10−9 m2. Segregation velocities for these sulfide melts can be up to 150 km/yr, fast enough for formation of Earth's core to proceed by deformation-induced segregation well within the geochemically determined timescale of 30 million years. Therefore plastic deformation can facilitate removal of large enough quantities of metallic liquid from a solid silicate mantle to satisfy the geochemical constraints of the inefficient core formation theory without invoking the presence of a magma ocean.

Role of dynamic grain boundary wetting in fluid circulation beneath volcanic arcs

Abstract: [1] We report the influence of shear deformation on the microstructure of water-saturated olivine and clinopyroxene aggregates. Prior to deformation, the aqueous fluid was isolated in pockets along grain corners in the olivine-water aggregates, while it was interconnected by a network of grain-edge tubules in the clinopyroxene-water aggregate. During deformation of both types of aggregates, the aqueous fluid phase segregates into grain boundaries at an angle of ∼16° to the shear direction, inclined in a sense antithetic to the applied shear. Fluid-rich planes formed by such dynamic wetting of grain boundaries lead to the formation of high permeability paths in the matrix. In a deforming mantle, such high permeability paths will enhance the efficiency of aqueous fluid extraction from subducting slabs.

Effect of water on the rheology of iron rich olivine

Abstract: We have undertaken an experimental investigation of the effect of water on the viscosity of iron rich olivine aggregates. Tri-axial compressive creep experiments were performed on hydrous fine-grained olivine samples of Fo_(50),Fo_(70)and Fo_(90).Two drops of water were added for hot-pressing using a telescoping can to prevent the water from escaping during the hot-press.Samples were deformed in a gas-medium apparatus at a confining pressure of 300MPa,a temperature between 1323 and 1473K,a differential stress 10～300MPa,a strain rate 10~(-7)～10(~-4)s~(-1),and the maximum accumulative strain 20% for each sample.The samples were deformed both in the diffusion and the dislocation creep regime.From three dimensional global non-linear fitting of the test data,the stress exponent for wet iron rich olivine aggregates Fo_(50),Fo_(70)and Fo_(90)were determined as 3.8,3.7 and 3.6 in dislocation creep regime, respectively,and the activation energy for wet Fo_(50),Fo_(70)and Fo_(90)as 444kJ/mol,479kJ/mol and 514kJ/mol,respectively.These hydrous samples creep at a rate over a factor of 10 faster than those anhydrous samples with the same iron content and under the same deformation conditions.Therefore,this paper gives a preliminary test result of effect of water on the viscosity of iron rich olivine aggregates.