Water redistribution in experimentally deformed natural milky quartz single crystals-Implications for H2O-weakening processes

Natural quartz single crystals were experimentally deformed in two orientations: (1) ⊥ to one prism plane and (2) in O+ orientation at 900 and 1000°C, 1.0 and 1.5 GPa, and strain rates of ~1 × 10−6 s−1. In addition, hydrostatic and annealing experiments were performed. The starting material was milky quartz, which consisted of dry quartz with a large number of fluid inclusions of variable size up to several 100 µm. During pressurization fluid inclusions decrepitated producing much smaller fluid inclusions. Deformation on the sample scale is anisotropic due to dislocation glide on selected slip systems and inhomogeneous due to an inhomogeneous distribution of fluid inclusions. Dislocation glide is accompanied by minor dynamic recovery. Strongly deformed regions show a pointed broad absorption band in the ~3400 cm−1 region consisting of a superposition of bands of molecular H2O and three discrete absorption bands (at 3367, 3400, and 3434 cm−1). In addition, there is a discrete absorption band at 3585 cm−1, which only occurs in deformed regions and reduces or disappears after annealing, so that this band appears to be associated with dislocations. H2O weakening in inclusion-bearing natural quartz crystals is assigned to the H2O-assisted dislocation generation and multiplication. Processes in these crystals represent recycling of H2O between fluid inclusions, cracking and crack healing, incorporation of structurally bound H in dislocations, release of H2O from dislocations during recovery, and dislocation generation at very small fluid inclusions. The H2O weakening by this process is of disequilibrium nature because it depends on the amount of H2O available.The financial support of NF project NF200020.119878 is gratefully acknowledged

Nucleation and growth of myrmekite during ductile shear deformation in metagranites

Myrmekite is extensively developed along strain gradients of continuous, lower amphibolite facies shear zones in metagranites of the Gran Paradiso unit (Western Alps). To evaluate the role of stress, strain energy and fluid phase in the formation of myrmekite, we studied a sample suite consisting of weakly deformed porphyric granites (WDGs), foliated granites (FGs) representative of intermediate strains, and mylonitic granites (MGs). In the protolith, most K-feldspar is microcline with different sets of perthite lamellae and fractures. In the WDGs, abundant quartz-oligoclase myrmekite developed inside Kfeldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction. In the WDGs, stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between Kfeldspar and albite. In the FGs and MGs, K-feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine-grained recrystallized myrmekite and perthite aggregates. This was accompanied by a more pervasive fluid influx into the reaction surfaces, and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone. In the MGs, myrmekite forms complete rims along the outer boundary of the small K-feldspar porphyroclasts, which are almost completely free of internal reaction interfaces. Therefore, we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress. Mass balance calculations indicate that, in Al\u2013Si-conservative conditions, myrmekite growth was associated with a volume loss of 8.5%. This resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5-8.7×10-7s-1. The starting material is effectively dislocation-free so that all observed defects were intr

Normal grain growth of quartz by experiment and discussion on the effect of grain size reduction by deformation in natural conditions

International audienceGrain size is an important factor that contributes to rheology. Grain size increases or decreases by grain growth or grain size reduction, in which the latter includes fracturing, dynamic recrystallization, dissolution-precipitation, and reaction. These two opposite mechanisms would occur in nature. Quartz is a major constituent of the Earth's crust. However, little is known about its grain growth parameters. In this study, we experimentally determined grain growth laws of quartz aggregates. We performed experiments using a piston cylinder. We prepared two quartz samples; novaculite as a quartz aggregate whose grain size is ~3 µm and natural quartz powder whose grain size is ~2 µm. We enclosed the two samples with added water of up to 10 wt% in a platinum capsule. Experimental conditions were under pressure of 1.0-2.5 GPa, temperature of 800-1100 °C, and durations of 6-240 hours. Normal grain growth occurred in these two samples, but we did not see differences in grain growth due to differences in amounts of added water. The powder sample showed porosities of ~10 vol %, which caused a slightly slower grain growth rate than that of the novaculite. We obtained the grain size exponents of ~3 and water fugacity exponents of ~2 for the two samples, and activation energies of 50 and 60 kJ/mol for the powder and novaculite, respectively. We applied our grain growth laws to crustal conditions assuming different initial grain sizes. We calculated grain growth rates with time and discussed contribution of plastic strain given under different strain rates which could cause grain size reduction by dynamic recrystallization. The strain-time relationship shows that strain is negligible until ~1000 years when strain rate is -12/sec. This means that the contribution of dynamic recrystallization is negligible. In the meantime, grain size can increase to a few tens of µm under the middle crustal condition (at 400 °C where pressure and water fugacity are calculated with a geological temperature and pressure gradients of 25 °C/km and 27 MPa/km, respectively) and to ~a few hundred μm under the lower crustal condition (at 600 °C). Our results indicate that there can be natural conditions where grain growth can be dominant even plastic deformation is operating

Strain dependent variation of microstructure and texture in naturally deformed Carrara marble

This study investigates the microstructure and texture variations across a mm-scale shear zone in Carrara marble of the Alpi Apuane (Italy). The microstructures have been investigated for grain size, texture, and shape fabrics. Textures have been measured with Computer-Integrated Polarization Microscopy (CIP) and Electron Back Scattered Diffraction (EBSD) separating porphyroclast and recrystallized grains. The deformation, which post-dates an earlier deformation phase and subsequent annealing, is strongly localized. The microstructures and textures change across the shear strain gradient and are interpreted to preserve a time sequence of progressive stages of deformation. The bulk shear strain rate is estimated to be about 1011 sec1 at deformation temperatures of approximately 325 C 30 C. The protomylonite is characterized by a core mantle structure with a bimodal grain size distribution which changes gradually to a completely dynamically recrystallized microstructure with a unimodal grain size distribution in the mylonitic center of the shear zone. Core-mantlestructures are produced by dominant rotation recrystallization accompanied by some grain boundary migration. The microstructural transition from protomylonite to mylonite coincides with a change in texture. With increasing strain the single c-axis maximum of an earlier inherited texture in the protomylonite is replaced by a similar texture in a different orientation (maximum normal to the shear plane) which is consistent with dominant basal hai and r h2201i slip. The microstructural and textural variations depend on the proportion of recrystallized grains. As dynamic recrystallization progresses with finite strain the texture development is finite strain-dependent. The comparison of the microstructures and textures to other natural and to experimental examples explains the progressive change of the texture and demonstrates the texture evolution produced by dynamic recrystallization

Strain dependent variation of microstructure and texture in naturally deformed Carrara marble

This study investigates the microstructure and texture variations across a mm-scale shear zone in Carrara marble of the Alpi Apuane (Italy). The microstructures have been investigated for grain size, texture, and shape fabrics. Textures have been measured with Computer-Integrated Polarization Microscopy (CIP) and Electron Back Scattered Diffraction (EBSD) separating porphyroclast and recrystallized grains. The deformation, which post-dates an earlier deformation phase and subsequent annealing, is strongly localized. The microstructures and textures change across the shear strain gradient and are interpreted to preserve a time sequence of progressive stages of deformation. The bulk shear strain rate is estimated to be about 10-11 sec-1 at deformation temperatures of approximately 325 °C ± 30 °C. The protomylonite is characterized by a core mantle structure with a bimodal grain size distribution which changes gradually to a completely dynamically recrystallized microstructure with a unimodal grain size distribution in the mylonitic center of the shear zone. Core-mantle-structures are produced by dominant rotation recrystallization accompanied by some grain boundary migration. The microstructural transition from protomylonite to mylonite coincides with a change in texture. With increasing strain the single c-axis maximum of an earlier inherited texture in the protomylonite is replaced by a similar texture in a different orientation (maximum normal to the shear plane) which is consistent with dominant basal 〈a〉 and r 〈-2201〉 slip. The microstructural and textural variations depend on the proportion of recrystallized grains. As dynamic recrystallization progresses with finite strain the texture development is finite strain-dependent. The comparison of the microstructures and textures to other natural and to experimental examples explains the progressive change of the texture and demonstrates the texture evolution produced by dynamic recrystallization

Normal grain growth of quartz by experiment and discussion on the effect of grain size reduction by deformation in natural conditions

International audienceGrain size is an important factor that contributes to rheology. Grain size increases or decreases by grain growth or grain size reduction, in which the latter includes fracturing, dynamic recrystallization, dissolution-precipitation, and reaction. These two opposite mechanisms would occur in nature. Quartz is a major constituent of the Earth's crust. However, little is known about its grain growth parameters. In this study, we experimentally determined grain growth laws of quartz aggregates. We performed experiments using a piston cylinder. We prepared two quartz samples; novaculite as a quartz aggregate whose grain size is ~3 µm and natural quartz powder whose grain size is ~2 µm. We enclosed the two samples with added water of up to 10 wt% in a platinum capsule. Experimental conditions were under pressure of 1.0-2.5 GPa, temperature of 800-1100 °C, and durations of 6-240 hours. Normal grain growth occurred in these two samples, but we did not see differences in grain growth due to differences in amounts of added water. The powder sample showed porosities of ~10 vol %, which caused a slightly slower grain growth rate than that of the novaculite. We obtained the grain size exponents of ~3 and water fugacity exponents of ~2 for the two samples, and activation energies of 50 and 60 kJ/mol for the powder and novaculite, respectively. We applied our grain growth laws to crustal conditions assuming different initial grain sizes. We calculated grain growth rates with time and discussed contribution of plastic strain given under different strain rates which could cause grain size reduction by dynamic recrystallization. The strain-time relationship shows that strain is negligible until ~1000 years when strain rate is -12/sec. This means that the contribution of dynamic recrystallization is negligible. In the meantime, grain size can increase to a few tens of µm under the middle crustal condition (at 400 °C where pressure and water fugacity are calculated with a geological temperature and pressure gradients of 25 °C/km and 27 MPa/km, respectively) and to ~a few hundred μm under the lower crustal condition (at 600 °C). Our results indicate that there can be natural conditions where grain growth can be dominant even plastic deformation is operating

Water pumping in mantle shear zones: From field observations to experimental evidence

International audienceWater plays an important role in geological processes. Providing constraints on what may influence the distribution of aqueous fluids is thus crucial to understanding how H2O impacts Earth's geodynamics. In a deep-seated environment, viscous shear zones have been identified as sites of massive fluid circulation, with many implications for ores deposits and rock rheology. However, although seismic pumping, fluid permeation and/or creep cavitation have been proposed as important processes, the source mechanism of such a fluid concentration remains unresolved. In this contribution based on both field and experimental data, we demonstrate that viscous flow exerts a dynamic control on H2O-rich fluid circulation in mantle shear zones. Using the distributions of amphibole and olivine dislocation slip-systems, we first highlight H2O accumulation around fine-grained shear zones in the Ronda peridotite massif (Spain). These observations give rise to a long-term and continuous process of fluid pumping during ductile deformation, which strongly suggest creep cavitation as the driving mechanism. Secondly, we used secondary ion mass spectrometry to document the H2O content of fine-grained olivine across an experimental shear zone. The latter developed with grain size reduction during a H2O-saturated shear experiment at 1.2 GPa and 900 °C. Through data interpolation, the olivine matrix reveals high H2O concentrations where shear strain is localized. These concentrations far exceed the predicted amount of H2O that grain boundaries can contain, excluding diffusive fluid permeation as a unique source of water storage. We also show that the H2O content increases per unit of grain boundary across the shear zone, highlighting an excess volume of H2O that depends on strain and/or strain rate. Based on tensile experiments in metals, we propose that a larger pore volume is produced with increasing strain rate due to competition between creep cavitation and "healing" processes, which include phase nucleation. Altogether, our findings therefore support creep cavitation to occur in mantle shear zones, providing a dynamic process for H2O to be infiltrated and stored in the deep lithosphere