Misha Bystricky

Extreme dynamic weakening of faults during dehydration by coseismic shear heating

The dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release. High velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0.6 to 0.15 as the slip velocity reaches 1.1 m/s at normal stresses up to 24.5 MPa. The extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding. The rapid heating at asperities causes serpentine dehydration. In impermeable fault zones in nature, serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening. This dynamic fault-weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the San Andreas.

Creep of dry clinopyroxene aggregates

We have determined diffusional and dislocation creep rheologies for clinopyroxenite Ca1.0Mg0.8Fe0.2Si2O6 under dry conditions by deforming natural and hot-pressed samples at confining pressures of 300–430 MPa and temperatures of 1100°–1250°C with the oxygen fugacity buffered by either nickel-nickel oxide or iron-wustite powders. The coarse-grained natural Sleaford Bay clinopyroxenite yielded a stress exponent of n = 4.7 ± 0.2 and an activation energy for creep of Q = 760 ± 40 kJ mol−1, consistent with deformation in the dislocation creep regime. The strength of the natural clinopyroxenite is consistent with previous high-temperature measurements of dislocation creep behavior of Sleaford Bay clinopyroxenite by Kirby and Kronenberg [1984] and Boland and Tullis [1986]. Fine-grained clinopyroxenite was prepared from ground powders of the natural clinopyroxenite. Hot-pressed samples were deformed under similar conditions to the natural samples. Mixed-mode deformation behavior was observed, with diffusional creep (n = 1) at lower differential stresses and dislocation creep (with n and Q similar to those of the natural samples) at higher differential stresses. Within the dislocation creep field the predried hot-pressed samples generally yielded creep rates that were about an order of magnitude faster than the natural samples. Thus, even at the highest differential stresses, a component of strain accommodation by grain boundary diffusion was present in the hot-pressed samples. Optical and electron microscope investigations of the deformation microstructures of the natural and hot-pressed samples show evidence for mechanical twinning and activation of dislocation slip systems. When extrapolated to geological conditions expected in the deep crust and upper mantle on Earth and other terrestrial planets, the strength of dry single-phase clinopyroxene aggregates is very high, exceeding that of dry olivine-rich rocks.

Microstructures and lattice preferred orientations in experimentally deformed clinopyroxene aggregates

Microstructures and lattice preferred orientations (LPO) were analysed on experimentally deformed natural and hot-pressed clinopyroxene aggregates in order to understand the relationship between deformation processes and evolving microstructures. The LPO was measured using electron backscatter diffraction techniques in the scanning electron microscope (SEM). Microstructures were observed by polarized light microscopy and by orientation contrast in the SEM. Natural samples (Sleaford Bay pyroxenite) were deformed in axial compression stepping tests up to 16% shortening. These samples deformed mainly by twinning and dislocation glide with very little recrystallization. No clear LPO evolution apart from the initial LPO could be attributed to deformation. Synthetic clinopyroxenite samples were hot-pressed from powders of the same material with three different grain size ranges, and deformed in compression stepping experiments up to 28% shortening. In the samples with coarse (30 μm) and intermediate (20 μm) grain sizes, deformation was dominated by dislocation creep accommodated by subgrain rotation recrystallization. The recrystallized grains have sizes up to 8 μm and represent 15–25% of the sampled area. The sample with the finest initial grain size (5 μm) deformed dominantly by diffusion creep accompanied by grain boundary migration. All of the hot-pressed samples deformed in compression have a similar texture, consisting of a girdle of c[001] axes normal to compression and a point maximum of b[010] axes and a*(100) poles parallel to the compression direction. The LPO of recrystallized grains was separated on spatially resolved orientation maps and shows an S-type fabric with b[010] parallel to the compression axis and a random a*(100) distribution. The bulk fabric was largely present in the hot-pressed starting material and is interpreted as the result of compaction during cold-pressing. At moderate strains in axial compression, the initial compaction fabric has been weakly overprinted by the recrystallization fabric. One hot-pressed sample (coarse grain size) was deformed in torsion to a shear strain of γ=0.1. The microstructure indicates dominant dislocation creep and contains about 20% recrystallized grains. The texture evolved from the rotationally symmetric compaction LPO to an oblique fabric.

Microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure

The relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high-temperature high-pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0% to 76% solid fraction (alumina grains). Torsion experiments were conducted at 300 MPa confining pressure, temperatures ranging from 475°C to 1000°C and shear strain rates ranging from 2.0 × 10 −5 to 2.1 × 10 −3 s −1 up to total strains of 21.3. Flow is Newtonian for a solid fraction of s = 0.0–0.16 with a log dynamic viscosity η = 10.3 Pa s (T = 500°C). A deviation from Newtonian behavior is observed for s > 0.16 with an increase in apparent viscosity of about 1 order of magnitude between s = 0.16 and 0.54. The shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction. Both shape fabric and deviation from Newtonian behavior originate from the increase in the number of particle clusters in the suspension. The apparent viscosity increases by 1.5 orders of magnitude between s = 0.54 and 0.65, and extrapolation of the data suggests a very sharp increase in apparent viscosity for s ≥ 0.65. At T ≥ 550°C and s = 0.54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains. At T ≤ 550°C and s = 0.65, intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles. The resulting bulk extensional fabric is almost parallel to the shortening direction.

High-Temperature Deformation of Enstatite Aggregates †

Synthesized polycrystalline enstatite samples were deformed in a Paterson gas-medium apparatus at 1200–1300°C, oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 MPa (protoenstatite field) or 450 MPa (orthoenstatite field). At both confining pressures, the mechanical data display a progressive increase of the stress exponent from n = 1 to n~3 with increasing differential stress, suggesting a transition from diffusional to dislocation creep. Nonlinear least squares fits to the high-stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720 kJ/mol for orthoenstatite and protoenstatite, respectively. Deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy. Microstructures show undulatory extinction and kink bands, evidence of dislocation processes. Crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on (100)[001]. Most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae. While many lamellae may have formed during quenching from run conditions, those in samples deformed in the orthoenstatite field are often bordered by partial [001] dislocations, suggesting transformation due to glide of partial [001] dislocations in (100) planes. Comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions. However, as orthoenstatite has a higher activation energy and smaller stress exponent than olivine, this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle.

Dense fine-grained aggregates prepared by spark plasma sintering (SPS), an original technique in experimental petrology

We have fabricated dense polyphase mixtures (silicate and oxide minerals, metal, and silicate melt) by Spark Plasma Sintering (SPS), a technique that has only recently been applied to silicates. We describe the SPS method and some of the characteristics of sintered specimens of forsterite, forsterite + MgO, forsterite + metal (Ni or Fe) and forsterite + metal + silicate melt. We show that SPS is a very efficient method to quickly prepare dense (>99 %) polyphase aggregates with homogeneous microstructures (well distributed phases, narrow grain size distributions, polygonal grain boundaries. . .) and small grain sizes with little or no grain growth during sintering. We observe the development of slight Shape Preferred Orientations (SPO), especially for the metal phases, due to the very simple setup we used here. Overall, SPS offers substantial advantages over traditional sintering techniques, making it perfectly suitable for experimental studies in Earth sciences, in particular those involving kinetic processes.

Use of the spark plasma sintering technique for the synthesis of dense mineral aggregates suitable for high-pressure experiments

In this paper, we present a novel sintering technique, called spark plasma sintering, that we apply for the synthesis of mineral aggregates used for experimentation. We show how dense polycrystalline aggregates of forsterite Mg2SiO4 and MgO mixed in various proportions can be obtained within minutes and with limited grain growth. Finally, we briefly mention the high-pressure experiments we performed with such samples.