Evolution in H2O contents during deformation of polycrystalline quartz: An experimental study

Accepted manuscript version, licensed CC BY-NC-ND 4.0. Published version available at https://doi.org/10.1016/j.jsg.2018.05.021.Shear experiments were performed in a Griggs-type apparatus at 800 °C and 1.5 GPa, at a strain rate of 2.1 × 10−5s−1 using different starting materials: (i) Powder (grain size 6–10 μm) of dry Brazil quartz with 0.15 wt% added H2O, (ii) “dry” Brazil quartz porphyroclasts (grain size ∼100–200 μm), devoid of fluid inclusions embedded in the same fine grained powder, and (iii) “wet” porphyroclasts (grain size ∼100–200 μm), containing initially a high density of μm-scale fluid inclusions embedded in the same powder. After hot pressing, samples were deformed to large shear strains (γ∼3 to 4.5), in order for the microstructures and H2O distribution to approach some state of “equilibrium”. The H2O content and speciation in quartz were analyzed by Fourier Transform Infra-Red (FTIR) spectroscopy before and after the experiments. Mechanical peak strength is generally lower in experiments with 100% hydrated matrix, intermediate in experiments incorporating wet porphyroclasts (with a proportion of 30 or 70%) and highest in those with dry porphyroclasts. All experiments with porphyroclasts show pronounced strain weakening, and the strengths of most samples converge to similar values at large strain. Wet porphyroclasts are pervasively recrystallized during deformation, while dry porphyroclasts recrystallize only at their rims and remain weakly deformed. Recrystallization of the initially fluid-inclusion-rich porphyroclasts results in a decrease in inclusion abundance and total H2O content, while H2O content of initially dry clasts increases during deformation. H2O contents of all high strain samples converge to similar values for matrix and recrystallized grains. In samples with wet porphyroclasts, shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free-fluid phase. These high porosity sites are sinks for collecting H2O in excess of the storage capacity of the grain boundary network of the recrystallized aggregate. The H2O storage capacity of the grain boundary network is determined as a H2O-boundary-film of ∼0.7 nm thickness

Brittle-to-viscous behaviour of quartz gouge in shear experiments

International audienceIn order to study the microstructure development across the brittle-viscous transition and to derive the corresponding flow laws, we performed shear experiments on quartz gouge in a Griggs-type deformation apparatus. The starting material is a crushed quartz single crystal (sieved grain size 650 ° C. For T < 700 ° C, the friction coefficient decreases slightly with increasing temperature, from 700-1000 ° C it shows a strong temperature dependence. Between 650 ° C and 700 ° C at shear strain rates of ∼2.5 x 10-5 s-1 a change in the deformation process occurs from one dominated by cataclastic flow to one dominated by crystal plasticity. The microstructure reveals a less abrupt transition in terms of operating processes, because brittle and viscous processes are equally active around 650 ° C. With increasing temperature the volume fraction of recrystallised grains increases, and at 900 ° C - 1000 ° C recrystallisation is nearly complete at strains of γ ∼ 3. The crystallographic preferred orientation of the c-axis evolves from a random distribution at low temperatures towards two peripheral maxima at intermediate temperatures. At high temperatures the c-axis show a single Y-maximum. At high temperature, the stress exponent is n = 2.1 ± 0.2. The activation energy Q is 193 ± 12 kJ/mol at strain rates of 10-5 s-1, at faster strain rates the activation energy drops down to Q = 119 ± 12 kJ/mol. This small stress exponent at high temperatures indicates a combination of deformation processes (diffusion in very fine grained material and dislocation creep in coarser grained material). At lower temperatures the n-value is significantly higher (n ∼ 8) indicating the beginning of the power-law-breakdown. Interestingly, the mechanical data indicate a sharp transition between brittle- and viscous-dominated regimes around 650 ° C to 700 ° C, while the microstructure reveals a smoother transition over a wider temperature range. The relatively low stress exponent of n ∼ 2 clearly suggests the activation of diffusion creep deformation processes

Brittle-to-viscous behaviour of quartz gouge in shear experiments

International audienceIn order to study the microstructure development across the brittle-viscous transition and to derive the corresponding flow laws, we performed shear experiments on quartz gouge in a Griggs-type deformation apparatus. The starting material is a crushed quartz single crystal (sieved grain size 650 ° C. For T < 700 ° C, the friction coefficient decreases slightly with increasing temperature, from 700-1000 ° C it shows a strong temperature dependence. Between 650 ° C and 700 ° C at shear strain rates of ∼2.5 x 10-5 s-1 a change in the deformation process occurs from one dominated by cataclastic flow to one dominated by crystal plasticity. The microstructure reveals a less abrupt transition in terms of operating processes, because brittle and viscous processes are equally active around 650 ° C. With increasing temperature the volume fraction of recrystallised grains increases, and at 900 ° C - 1000 ° C recrystallisation is nearly complete at strains of γ ∼ 3. The crystallographic preferred orientation of the c-axis evolves from a random distribution at low temperatures towards two peripheral maxima at intermediate temperatures. At high temperatures the c-axis show a single Y-maximum. At high temperature, the stress exponent is n = 2.1 ± 0.2. The activation energy Q is 193 ± 12 kJ/mol at strain rates of 10-5 s-1, at faster strain rates the activation energy drops down to Q = 119 ± 12 kJ/mol. This small stress exponent at high temperatures indicates a combination of deformation processes (diffusion in very fine grained material and dislocation creep in coarser grained material). At lower temperatures the n-value is significantly higher (n ∼ 8) indicating the beginning of the power-law-breakdown. Interestingly, the mechanical data indicate a sharp transition between brittle- and viscous-dominated regimes around 650 ° C to 700 ° C, while the microstructure reveals a smoother transition over a wider temperature range. The relatively low stress exponent of n ∼ 2 clearly suggests the activation of diffusion creep deformation processes

Relationship between microstructures and resistance in mafic assemblages that deform and transform

Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new phases) may lead to grain size reduction, producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size-sensitive deformation processes. In order to study the effect of deformation–reaction feedback(s) on sample strength, we performed rock deformation experiments on “wet” assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 ∘C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single throughgoing high-strain zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress, and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition

Locally Resolved Stress-State in Samples During Experimental Deformation: Insights Into the Effect of Stress on Mineral Reactions

Understanding conditions in the Earth's interior requires data derived from laboratory experiments. Such experiments provide important insights into the conditions under which mineral reactions take place as well as processes that control the localization of deformation in the deep Earth. We performed Griggs-type general shear experiments in combination with numerical models, based on continuum mechanics, to quantify the effect of evolving sample geometry of the experimental assembly. The investigated system is constituted by CaCO3 and the experimental conditions are near the calcite-aragonite phase transition. All experimental samples show a heterogeneous distribution of the two CaCO3 polymorphs after deformation. This distribution is interpreted to result from local stress variations. These variations are in agreement with the observed phase-transition patterns and grain-size gradients across the experimental sample. The comparison of the mechanical models with the sample provides insights into the distribution of local mechanical parameters during deformation. Our results show that, despite the use of homogeneous sample material (here calcite), stress variations develop due to the experimental geometry. The comparison of experiments and numerical models indicates that aragonite formation is primarily controlled by the spatial distribution of mechanical parameters. Furthermore, we monitor the maximum pressure and σ1 that is experienced in every part of our model domain for a given amount of time. We document that local pressure (mean stress) values are responsible for the transformation. Therefore, if the role of stress as a thermodynamic potential is investigated in similar experiments, an accurate description of the state of stress is required

The effect of garnet and muscovite on the recrystallized grain size of quartz from general shear experiments

International audienceTo investigate the role of strong and weak secondary phases on the recrystallized grain size of quartz, we performed grain size analyses on quenched samples from general shear experiments on quartz-garnet and quartz-muscovite mixtures. Six general shear experiments were conducted in the Griggs apparatus; three with mixtures of quartz-garnet (vol.% garnet 5, 15, 30) and three with mixtures of quartz-muscovite (vol.% muscovite 5, 10, 25). The starting powders for both set of experiments were synthetic mixtures of quartz-muscovite or quartz-garnet with 0.1 wt.% water added. The quartz-garnet experiments were conducted at 900°C, a pressure of 1.2 GPa, and a shear strain rate of ~10-5 s-1, while the quartz-muscovite experiments were conducted at 800°C, a pressure of 1.5 GPa, and a shear strain rate of ~10-5 s-1. At these deformation conditions quartz is stronger than muscovite and weaker than garnet. We observed that the bulk strength of the aggregate decreases with a greater volume percent of muscovite and increases with a greater volume percent of garnet. Garnet at these conditions does not deform plastically. The presence of secondary phases within the deforming aggregate causes stress concentrations and partitioning of strain rate between the different phases relative to the measured bulk stress and strain rate. The degree of partitioning is primarily related to the rheology and volume percent of the phases. Due to the piezometric relationship between recrystallized grain size and stress, we can use the quartz recrystallized grain size to determine the local stress of quartz in the experiments and compare it to the measured bulk stress. The results from these analyses will provide new insight into the effect of strain partitioning in general and of strong and weak secondary phases on quartz rheology

Dissolution precipitation creep as a process for the strain localisation in gabbro

International audienceStrain localisation and fabric development in the lower crust is controlled by the active deformation mechanisms. Understanding the driving forces of such deformation aids in quantifying the stresses and rates of the deformation processes. Here we show that diffusion creep plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C (depths of ∼26-34 km). The Kågen gabbro on south Arnøya is made up of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and textural data indicate that preferential crystal growth of amphibole grains in the extension direction has produced the deformation microstructure and the CPO. Dissolution precipitation creep is inferred to be the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: amphibole, garnet and zoisite. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. Deformation and metamorphic reaction were both important transformation processes during diffusion creep deformation of the margins of the gabbro lenses. The weakening is directly connected to a transformation process that facilitates diffusion creep deformation of strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses

The role of deformation-reaction interactions to localize strain in polymineralic rocks: Insights from experimentally deformed plagioclase-pyroxene assemblages

In order to study the mutual effect of deformation and mineral reactions, we have conducted shear experiments on fine-grained plagioclase-pyroxene assemblages in a Griggs-type solid-medium deformation apparatus. Experiments were performed at a constant shear strain rate of 10−5 s−1, a confining pressure of 1 GPa and temperatures of 800, 850 and 900 °C. While the peak stress of plagioclase + orthopyroxene assemblages reaches values between those of the end-member phases, the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase (plagioclase). This weakening correlates with the coeval development of high-strain shear zones where new phases are preferentially produced, including new pyroxene, plagioclase and amphibole. The reaction products mostly occur as intimately mixed phases within fine-grained and interconnected shear bands, together with different compositions with respect to the starting material. This indicates that deformation significantly enhances the kinetics of mineral reactions, which in turn strongly weaken the deforming sample, here attributed to a switch to grain-size-sensitive diffusion creep through phase nucleation and grain size reduction. Such an interplay between deformation and mineral reactions may have strong implications for the initiation, development, and durability of shear zones in the lower crust