Jan Tullis

Dislocation creep regimes in quartz aggregates

Abstract Using optical and TEM microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates, depending on the relative rates of grain boundary migration, dislocation climb and dislocation production. Within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization. At lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion-controlled dislocation climb to be an effective recovery mechanism. In this regime recovery is accommodated by strain-induced grain boundary migration recrystallization. With an increase in temperature or decrease in strain rate, the rate of dislocation climb becomes sufficiently rapid to accommodate recovery. In this regime dynamic recrystallization occurs by progressive subgrain rotation. With a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery. However, in this regime grain boundary migration is rapid, thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation. The identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers. In addition, the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred.

A flow law for dislocation creep of quartz aggregates determined with the molten salt cell

We have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing ∼ 0.15 wt. % water. Co-axial deformation experiments were conducted at constant piston displacement rates, approximating constant strain rates at low strain. The strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because, unlike solid confining media, molten salt does not contribute to the strength of the sample; it reduces the friction on the moving piston, and it allows clear identification of the ‘hit’ point. We have limited the experimental conditions to those required for dislocation creep, and have used only steady-state flow stresses measured during climb-accommodated dislocation creep to calculate the flow law parameters. Two flow laws were determined, one for samples containing minor amounts of melt (1–2%) and one for melt-free samples. In both cases, the power law stress exponent, n, is 4.0 ± 0.9, which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media. The activation energy, Q, is 137 ± 34 kJ mol−1 for samples with melt and 223 ± 56 kJ mol−1 for those without, within the range of previously determined values for quartz aggregates containing ∼ 0.1 wt.% water. The pre-exponential term, A, is 1.1 × 10 (−4 ± 2) MPa−ns−1 for samples without melt and 1.8 × 10(−8 ± 2) MPa−ns−1 for those with melt. The lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct. Extrapolation of the flow law for melt-free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws. However, accurate extrapolation requires determining the dependence of flow stress on fH2O and / or aH+.

Microstructures and Preferred Orientations of Experimentally Deformed Quartzites

An experimental study of the plastic deformation of quartzite has produced microstructures and preferred orientations similar to those found in many natural rocks, and has identified the operative orienting mechanisms in most cases. The microstructures vary widely with conditions and presumably are related to the deformation mechanisms. Below 850°C at 10−5/sec (or 650°C at 10−7/sec), no recrystallization occurs; the deformation of the original grains is very inhomogeneous and deformation lamellae of many orientations are observed. At higher temperatures or slower strain rates, grain boundary recrystallization is present; the original grains are continuously flattened with increasing strain and only basal and prismatic deformation lamellae are observed. Above 800°C at 10−7/sec, recrystallization is complete after low strain. Below 800°C at 10−5/sec (or 600°C at 10−7/sec), a maximum of c axes develops parallel to the compression direction (σ1), while at higher temperatures and slower strain rates, a small-circle girdle of c axes develops about σ1. The opening half-angle of this girdle ranges from 20° to 45° and increases with increasing temperature and decreasing strain rate. Super-imposed on both of these c axis patterns is a tendency for the poles to positive trigonal forms, and the pole to the second order prism to be aligned parallel to σ1. The preferred orientations of the c axes and the prisms are consistent with external rotations produced by the observed intragranular glide. The difference in the preferred orientations of the positive and negative forms is due to mechanical Dauphine twinning. Strong evidence exists that these same orienting mechanisms have operated in many naturally deformed rocks.

Dynamic recrystallization of feldspar: A mechanism for ductile shear zone formation

Feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations. Recrystallizing aggregates have a lower strength because of the cyclic production of small, strain-free grains, and they develop a strong preferred orientation, consistent with that observed in mylonites. Thus, recrystallization-accommodated dislocation creep may be responsible for the grain-size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones.

Transition from cataclastic flow to dislocation creep of feldspar: Mechanisms and microstructures

Feldspar aggregates exhibit cataclastic flow over a wide range of conditions between low-temperature faulting and high-temperature dislocation creep; this is due to the ease of cracking on the two good cleavages and the difficulty of dislocation motion. Albite aggregates experimentally deformed at moderate to high pressures in the cataclastic flow regime are macroscopically ductile; their optical microstructures show little evidence of crushing and resemble those expected for dislocation creep. However, transmission electron microscopy (TEM) shows no dislocations or very limited dislocation mobility, but does show abundant distributed microcracks and microcrush zones that contain <0.1-µm-diameter grains. Cataclastic flow is likely to be an important deformation mechanism in nature, but it may have been overlooked because its optical microstructures have been misinterpreted and because the extreme grain size reduction facilitates transitions to other phases and deformation mechanisms.

Flow laws of polyphase aggregates from end-member flow laws

An incompressible finite element model has been used to study the plane strain deformation of two-phase aggregates deformed by dislocation creep. Input for the model includes the power law flow laws of the two end-member phases and their volume fractions and configuration. The model calculates the overall flow law of the aggregate as well as the stress and strain rate variations within it. The input flow laws were experimentally determined for monomineralic aggregates of clinopyroxene and plagioclase. Results were calculated for a temperature of 1000°C, strain rates from 10−4 to 10−12S−1, and stresses of 1–1000 MPa. For these conditions, the end-member flow laws intersect on a log stress versus log strain rate plot at 10−8S−1. Some runs were made on finite element grids fit to an actual diabase texture (∼64% pyroxene, ∼ 36% plagioclase.) Other runs were made on idealized geometries to test the effects of varying the volume fraction of two phases, shape of inclusions, and relative strengths of inclusion and matrix. Important results include the following: (1) The model results satisfy the requirement that the aggregate strength must lie between the bounds set by the end-member flow laws and those set by assumptions of uniform stress and uniform strain rate. (2) The calculated diabase flow law matches well with that experimentally determined. (3) The aggregate strength within the uniform stress and uniform strain rate bounds is primarily affected by volume fraction, although certain phase geometries can also affect the strength. (4) Although the flow law for an aggregate of power law phases need not be a simple power law, we find it to be a good approximation. We have developed two simple methods of estimating the strength of an aggregate, given the end-member flow law parameters and volume fractions; both give results that agree with the finite element model calculations. (1) One method takes into account the phase geometry and gives a strength for the aggregate at any strain rate. (2) The other method can be used even if the phase geometry is unknown and gives expressions for the aggregate flow law parameters.

The brittle‐plastic transition in experimentally deformed quartz aggregates

Deformation experiments have been conducted to provide constraints on the processes responsible for the brittle-plastic transition in quartz aggregates. A correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle-plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and/or pressure. First there is a transition from cataclastic faulting to semibrittle faulting; microstructural observations indicate that this transition occurs due to the activation of dislocations. In addition, faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide. Second, there is a transition from semibrittle faulting to semibrittle flow; this transition corresponds to a change from localized to distributed deformation. Microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime. We conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries. Third, there is a transition from semibrittle flow to dislocation creep. Microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature. In addition, microstructural observations indicate that a transition from dominantly mode I (axial) to mode II (shear) microcracking occurs with an increase in confining pressure from 0.4 to 0.8 GPa, regardless of temperature. The differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions. However, a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the Earth. The results of this study provide further evidence to indicate that the brittle-plastic transition in the continental crust occurs over a relatively wide range of conditions.

Hydrolytic weakening of experimentally deformed Westerly granite and Hale albite rock

In order to determine the effect of water on deformation in the brittle-ductile transition region of crustal rocks, experiments have been conducted on Westerly granite and a polycrystalline albite rock, comparing samples dried at 160°C for 12 h (‘dry’) and samples with about 0.2 wt% water added (‘wet’). The deformation mechanisms and style of deformation of the wet and dry samples, determined using optical and transmission electron microscopy, have been found to depend on temperature, pressure, strain rate, and strain. At 15 kb and 10−6, the added water reduces the temperature of the transition between microcracking and dislocation glide and climb by about 150–200°C for both quartz and feldspar. However, the penetration of ‘water’ into the grains is slow compared with the time of the experiments and many of the wet samples show evidence of initial microcracking and later dislocation creep. Wet samples deformed at 10 kb show less hydrolytic weakening than wet samples deformed at 15 kb. Because the deformation mechanism and strength of silicates depend so sensitively on trace amounts of water, and because the water content of experimental samples varies with temperature and pressure and thus with time, flow laws for any samples are only meaningful if the water content has been carefully controlled or characterized.

The recrystallized grain size piezometer for quartz

D=1 0 3.56±0.27 * s 1.26 ±0.13 , with no change in slope at the regime 2–3 transition and no effect of temperature or a/b stability field. Another experimental piezometer relation for regime 1 of Hirth and Tullis [1992] differs in slope, suggesting that different recrystallization mechanisms require different piezometer calibrations. INDEX TERMS: 3902 Mineral Physics: Creep and deformation; 5120 Physical Properties of Rocks: Plasticity, diffusion, and creep; 8030 Structural Geology: Microstructures; 8159 Tectonophysics: Rheology—crust and lithosphere; 8164 Tectonophysics: Stresses—crust and lithosphere. Citation: Stipp, M., and J. Tullis, The recrystallized grain size piezometer for quartz, Geophys. Res. Lett., 30(21), 2088, doi:10.1029/2003GL018444, 2003.

Grain Growth Kinetics of Quartz and Calcite Aggregates

Grain growth rates for novaculite, flint and jasper have been determined between 1000° and 800°C and 15 to 2 kb water pressure. A pre-treatment established an equilibrium microstructure and a nearly uniform mean grain diameter (