Stress induced transformation of pigeonites from achondritic meteorites

A transmission electron microscopic (TEM) study was conducted to investigate the stress-induced transformation mechanisms of pigeonites from two achondrites. In pigeonites from the Allan Hills 77257 ureilite, abundant lamellae of orthopyroxene-pigeonite intergrowth accompanied by minor amounts of blebby augite precipitates were observed. In pigeonites from the Juvinas eucrite, some stacking faults running through the (001) augite lamellae were observed although most of the stacking faults terminated at the host-lamella interface. High resolution TEM observation shows the fringe offset produced by the stacking fault in the Juvinas pigeonites. The magnitude of the mismatch parallel to [001] is approximately c/4, which is related to the formation of the partial edge dislocation near the host-lamella interface. The partial dislocation is imaged as an edge dislocation with an (002) extra-plane. Three partial dislocation mechanisms with distinct displacement vectors are proposed in order to explain the stress-induced transformation textures observed in the achondritic pyroxenes. Further study will be required to determine unequivocally the mechanism of atomic displacements during stress-induced transformation. However, regardless of the actual mechanism of transformation, it is not difficult to convert orthopyroxene to clinopyroxene or vice versa by this transformation from the structural point of view. Low-Ca pyroxenes are deformed without a stress-induced transformation by slip at high temperature and geologically reasonable strain rates. However, meteoritic pyroxenes can be deformed by stress-induced transformation even at high temperature because preterrestrial impact processes will produce a deformation condition with high strain rates. Deformation at high strain rates and high temperature is very important when we interpret the microtexture of meteoritic pyroxenes.     

Strain analysis of a shear zone in a granodiorite

A ductile shear zone in a late Precambrian granodiorite, from the Rouergue (southwest part of the French Massif Central) has been studied.A single episode of deformation is responsible for the formation of a foliation and a well-defined lineation which are localized into an elongated zone, a few decimeters wide.The strain features can be attributed to a simple-shear mechanism (Ramsay and Graham, 1970), so that the main parameters of the deformation are defined.At stages of increasing deformation, the quartz isotropic sub-fabric of the undeformed host rock is progressively transformed into an anisotropic fabric composed of a single oblique girdle while the subgrain size progressively decrease and the dislocation density remains constant. It is suggested that the gliding planes of quartz are the basal plane (0001) and a predominant prismatic plane 101̄0 the slip directions may be a for both glide-planes.The results obtained in this investigation provide a basis for a high voltage electron microscope (H.V.E.M.) study which shows that the fabrics development may be related to dislocation processes. The difference of strain rates in the host rock and in the shear zone is calculated from the dislocation microstructures.     

雅鲁藏布江蛇绿岩中橄榄石的位错构造及上地幔流变状态

本文通过对雅鲁藏布江蛇绿岩中橄榄石的透射电子显微分析（TEM）和变形参数测算,划分出3个应变带,即强应变带、过渡型应变带和弱应变带,并详细地研究了它们的岩石结构、位错构造、变形机制和流变状态。镁铁质糜棱岩组成的强应变带中橄榄石发育亚颗粒和位错网络,具有低自由位错密度(10⁷～10⁸cm^-2),是在温度为850～950℃、压力10～13kbar、差异应力2～3MPa、应变速率为10^-16～10^-17s^-1、粘度10²²～10²³Pa.s,大致相当于35～40km的岩石圈深部由位错机制控制的深层剪切流变作用形成的。     

The transition between deformation regimes dominated by intercrystalline diffusion and intracrystalline creep evaluated by oxygen isotope thermometry

The dominant flow mechanism in tectonic processes depends on the rheological properties of geological materials and the physical conditions prevailing during deformation. We have evaluated the relative importance of intercrystalline diffusion and intracrystalline creep in crustal deformation in terms of temperature and grain size.Oxygen isotope thermometry has been used to elucidate the thermal environment obtaining during deformation and contemporaneous metamorphism of Dalradian rocks from Southwest Scotland. The temperature and grain size data, applied in conjunction with microstructural criteria for evaluating independent mechanisms of steady-state flow, allow recognition of a low-temperature deformation regime dominated by intercrystalline diffusion, and a high-temperature regime dominated by dislocation processes.The transition between the fields of intercrystalline diffusion and dislocation creep for quartz and calcite of 100 Mm grain size occurs at about 450° C and about 300° C, respectively. These empirically derived results are consistent with the temperature intervals over which intercrystalline diffusion and dislocation creep, respectively, are predicted to be dominant at geologically reasonable strain rates, as derived from theoretically formulated deformation mechanism maps for quartz and calcite.Grain growth may play an important role in delimiting the higher-temperature boundary of the intercrystalline diffusion field. Intercrystalline diffusion is the only deformation mechanism that involves mass transfer over distances that are large in relation to the grain size. This result has important consequences for geochemical transport phenomena.     

基于细观变形理论的盐岩塑性本构模型研究

唯象学本构模型不能很好解决诸如夹杂含量、温度、应变速率等对盐岩力学性质的影响,更难以解释盐岩变形机制。盐岩为石岩晶体组成,其变形机制主要由多晶结构所控制,故基于固体位错理论研究方法建立的盐岩塑性本构模型更能反映盐岩的变形机制。研究表明,盐岩的塑性-蠕变交互作用机制是（亚）晶粒内部位错的滑移与（亚）晶界及其干涉面内位错的攀移运动之间的耦合。基于此,可确定亚晶（或晶粒）平均尺寸与流动应力之间的关系、（亚）晶内的位错平均密度;建立微观参量（位错、亚晶直径、亚晶界宽度等）演化模式;根据Orowan定律建立盐岩微观-宏观变形联系,从而导出盐岩塑性本构方程。导出的本构方程体现了盐岩塑性-蠕变变形的物理机制,相对于传统的塑性本构方程具有更好的物理意义。     

Grainsize reduction, grain boundary sliding and the flow strength of mylonites

Most mylonitic rocks are characterized by a fine grainsize and it is now widely accepted that the grainsize reduction commonly takes place by dynamic recrystallization. This reduction in grainsize will favour a number of deformation mechanisms whose strain rate depends inversely on grainsize (e.g., diffusion flow, grain boundary sliding — GBS). We have constructed deformation maps to illustrate the effect of these mechanisms, especially GBS, on flow strength. It is shown that, if the recrystallized grainsize equilibrates with the steady state dislocation flow stress, the grainsize reduction is insufficient to cause extensive strain softening. In general, a monomineralic rock that has undergone dynamic recrystallization by dislocation creep will continue to deform by the same basic mechanism with limited strainrate acceleration (1–10 times) due to GBS. Extensive diffusion flow will only take place if the grainsize is held below the equilibrium value by some means (e.g., impurity pinning of grain boundaries).Several microstructural characteristics of mylonites are found to be consistent with flow by dislocation creep-accommodated GBS. It is suggested that the strain softening implied by the geometry of mylonite zones is the result of a variety of phenomena that are generally only indirectly related to the reduction in grainsize.     

Rheological evolution of a Mediterranean subduction complex

We use field and microstructural observations, coupled to previously published P-T-time histories, to track the rheological evolution of an intracontinental subduction complex exposed in the Betic Cordillera in the western Mediterranean region. The body of rock we focus on, known as the Nevado-Filabride Complex (NFC), was originally part of the upper crust of the Iberian margin. It was subducted into hot asthenospheric mantle, then exhumed back toward the surface in two stages: an early stage of fast exhumation along the top of the subducting slab in a subduction channel, and a late stage of slower exhumation resulting from capture by a low-angle detachment fault rooted at the brittle-ductile transition. Each stage of deformation in the NFC was punctuated by changes in the dominant deformation mechanism. Deformation during initial subduction of the complex was accommodated by pressure-solution creep in the presence of a fluid phase – the grain sizes, stress magnitudes, and estimated strain rates for this stage are most consistent with a thin-film model for pressure solution in which the diffusion length scale is controlled by the grain size. During the early stages of exhumation within the subduction channel, deformation transitioned from pressure solution to dislocation creep due to increases in temperature, which resulted in increases in both water fugacity and grain size, each of which favor the dislocation creep mechanism. Differential stress magnitudes for this stage were ∼10 MPa, and are consistent with simple models of buoyancy-driven channel flow. With continuing subduction-channel exhumation, deformation remained within the dislocation creep field because sequestration of free water into hydrous, retrogressive minerals suppressed the pressure-solution mechanism. Differential stresses progressively increased to ∼100 MPa near the mouth of the channel during cooling as the rocks moved into mid-crustal levels. During the final, core-complex stage of exhumation, deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid-crustal detachment fault. Localization was promoted by a transition from dislocation creep to dislocation-creep-accommodated grain boundary sliding at temperatures of ∼350–380 °C, grain sizes of ∼4 μm and differential stress magnitudes of ∼200 MPa. Peak differential stress magnitudes of ∼200 MPa recorded just below the brittle-ductile transition are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures. Overall, the distribution of stress with temperature, coupled to independent constraints on strain rate from field observations and geochronology, indicate that the naturally calibrated Hirth et al. (2001) flow law for wet quartzite accurately predicts the rheological behavior of mid-crustal rocks deforming by dislocation creep.     

The effect of ductile deformation on the kinetics and mechanisms of the aragonite-calcite transformation

Abstract The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330° C and 360° C) and 900-1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10^-6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry-stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry-stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.     

Strain and Crystallographic Fabric in Mesoscopic Ductile Shear Zones of Garhwal Himalaya

The MCT Zone of Bhagirathi valley of Garhwal Himalaya is characterized by numerous mesoscopic ductile shear zones. These shear zones are developed in response to nearly NNE-SSW maximum horizontal compression and provide an opportunity to study the variation in strain and crystallographic fabrics within the ductile shear zones.The grain shape and orientation of quartz under microscope reflect that strain is higher in the center and it progressively decreases towards the shear zone boundary. The preferred orientation of quartz c-axes across the shear zone suggests that the single girdle of the quartz c-axes are probably first developed at the shear zone boundary and become prominent in the center of shear zone with increase in the intensity of deformation. The strong crystallographic preferred orientation normal to foliation suggests that the internal deformation of the quartz might have taken place by dislocation creep mechanism exhibiting a non-coaxial deformation history.     

Experimental deformation of partially melted granitic aggregates

Abstract The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10^-4 to 10^-6/s) where melt-free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz-albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed. The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ? 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM-scale evidence of dissolution-precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt-enhanced diffusion creep. At natural strain rates melt-enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse-grained granites. The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ? 5 and 10% melt, deformation at 10^–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10^-5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.     

Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle

It is now admitted that the high strength of the subcontinental uppermost mantle controls the first order strength of the lithosphere. An incipient narrow continental rift therefore requires an important weakening in the subcontinental mantle to promote lithosphere-scale strain localisation and subsequent continental break-up. Based on the classical rheological layering of the continental lithosphere, the origin of a lithospheric mantle shear/fault zone has been attributed to the existence of a brittle uppermost mantle. However, the lack of mantle earthquakes and the absence of field occurrences in the mantle fault zone led to the idea of a ductile-related weakening mechanism, instead of brittle-related, for the incipient mantle strain localisation. In order to provide evidence for this mechanism, we investigated the microstructures and lattice preferred orientations of mantle rocks in a kilometre-scale ductile strain gradient in the Ronda Peridotites (Betics cordillera, Spain). Two main features were shown: 1) grain size reduction by dynamic recrystallisation is found to be the only relevant weakening mechanism responsible for strain localisation and 2), with increasing strain, grain size reduction is coeval with both the scattering of orthopyroxene neoblasts and the decrease of the olivine fabric strength (LPO). These features allow us to propose that grain boundary sliding (GBS) partly accommodates dynamic recrystallisation and subsequent grain size reduction.A new GBS-related experimental deformation mechanism, called dry-GBS creep, has been shown to accommodate grain size reduction during dynamic recrystallisation and to induce significant weakening at low temperatures (T < 800 °C). The present microstructural study demonstrates the occurrence of the grain size sensitive dry-GBS creep in natural continental peridotites and allows us to propose a new rheological model for the subcontinental mantle. During dynamic recrystallisation, the accommodation of grain size reduction by three competing deformation mechanisms, i.e., dislocation, diffusion and dry-GBS creeps, involves a grain size reduction controlled by the sole dislocation creep at high temperatures (> 800 °C), whereas dislocation creep and dry-GBS creep, are the accommodating mechanisms at low temperatures (< 800 °C). Consequently, weakening is very limited if the grain size reduction occurs at temperatures higher than 800 °C, whereas a large weakening is expected in lower temperatures. This large weakening related to GBS creep would occur at depths lower than 60 km and therefore provides an explanation for ductile strain localisation in the uppermost continental mantle, thus providing an alternative to the brittle mantle.     

Fractal analysis of magnetite grains — implications for interpreting deformation mechanism

In the present study, the grain size (d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension (D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension (D) of the former. Assuming a natural strain rate ranging between 10⁻¹⁰ s⁻¹ and 10⁻¹⁴ s⁻¹, the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks.     

In situ TEM observations of plastic deformation in quartz crystals

With in situ nanocompression experiments in a transmission electron microscope, we investigated plastic deformation in natural quartz crystals and observed both dislocation plasticity as well as mechanical twinning. Through this experimental method, we are able to provide direct evidence of Dauphiné twin nucleation and could measure the intrinsic twinning stress. The twinning phenomena appear to include a memory effect, where the same twin can reappear upon successive loading and unloading events. The data provide insight into this twin generation mechanism and can be used as a benchmark for the use of twins in quartz for paleopiezometry. Together, the observation of room-temperature dislocation plasticity and reversible twinning adds new insight into the extensive field of quartz plasticity and demonstrates the usefulness of small-scale testing techniques for mineral physics.     

Grain-size sensitive deformation of a stretched conglomerate from Plymouth, Vermont

The finite strain of clasts (maximum aspect ratio varying from 2 to 40) in a deformed conglomerate from Dry Hill, Plymouth, Vermont, correlates inversely with the average grain size (300-150 μm) in the clast, suggesting that the operative deformation mechanism was grain-size sensitive. In a general way, the average quartz grain size appeared to be smaller in those clasts with higher volume of minerals other than quartz. Dislocation densities varied by as much as a factor of 10 from grain to grain within a clast, but the average dislocation density was relatively constant from clast to clast. If grain-size sensitivity of strength is accepted as a working hypothesis, other elements of the microstructure, such as grain flattening, grain morphology, and dislocation structure can be reconciled as happening either through a late, low strain, high stress pulse—if the current palaeostress indicators are correct to within a factor of 10 or as happening concurrently with the grain-size sensitive mechanism if the current palaeostress estimates are in error. The evidence from this study agrees with several previously published suggestions that grain-size sensitive deformation occurs in the crust for quartzose rocks with grain size of 100 to 300 μm at temperatures of 350 to 420°C.     

石英结晶学优选与应用

石英集合体的结晶学优选可由位错滑移、双晶滑移、定向成核与生长等形成,其中位错滑移是塑性变形岩石中石英结晶学优选产生的最重要的机制。影响变形石英结晶学优选的因素有温度、应变速率、应变、差应力、水、复矿物岩石中各种矿物间的相互作用、初始结晶学方向等。系统总结了石英晶体变形与滑移系,结晶学优选的测量与表达,多种条件下石英的结晶学优选,以及在判断剪切方向、计算运动学涡度、判定变形温度、分析变形历史等方面的应用,并认为应用石英组构作运动学和动力学解析时需与其它微观、宏观现象相结合。     

Weakening and strain localization produced by syn-deformational reaction of plagioclase

There are many observations in naturally deformed rocks on the effects of mineral reactions on deformation, but few experimental data. In order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase + H₂OM more albitic plagioclase + zoisite + kyanite + quartz. We utilized fine-grained (2-6 µm) plagioclase aggregates of two compositions (An54 and An60), both dried and with 0.1-0.4 wt% H₂O present, in shear deformation experiments at two sets of conditions: 900 °C, 1.0 GPa (in the plagioclase stability field) and 750 °C, 1.5 GPa (in the zoisite stability field). Dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep, but samples with 0.1 to 0.4 wt% water deformed in the zoisite stability field showed extreme strain localization into very narrow (~1-3 µm) shear bands after low shear strain. In these samples the microstructures of reaction products in the matrix differ from those in the shear bands. In the matrix, large (up to 400 µm) zoisite crystals grew in the direction of finite extension, and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic. In the shear bands, the reaction products albitic plagioclase, zoisite, white mica, and traces of kyanite form polyphase aggregates of very fine-grained (<0.1 µm) dislocation-free grains. Most of the sample strain after % ~2 has occurred within the shear bands, within which the dominant deformation mechanism is inferred to be diffusion-accommodated grain boundary sliding (DAGBS). The switch from dislocation creep in dry samples deformed without reaction to DAGBS in reacted samples is associated with a decrease in flow stress from ~800 to <200 MPa. These experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine-grained polyphase assemblage, leading to a switch in deformation mechanism and significant weakening. Thus, localization of deformation in polyphase rocks may occur on any pressure (P),temperature (T)-path where the equilibrium composition of the constituent minerals changes.     

Universal stage measurements and transmission electron microscope observations of fractured plagioclase

At low to moderate temperatures of deformation, fracturing of plagioclase is common. The mechanism of fracturing is generally thought to be either a dislocation assisted process with fractures typically exhibiting some crystallographic regularity or a process of breaking along cleavage planes without the involvement of dislocations. In this study, naturally fractured plagioclase from granodiorites and a gabbro deformed at high strain rates are examined with the transmission electron microscope (TEM) to identify structures at that scale. In addition, fracture orientations are determined with the Universal stage.Some fractures observed in thin section occur parallel to (001) but many are not so simple but are confined to the [112], [1$\text{1}$2], [101], [$\text{1}$01] zones. At the TEM scale, dislocation walls or arrays are common in plagioclase. They also occupy the [101], [$\text{1}$01], [112], [1$\text{1}$2] zones. Microcracks form when dislocations are pinned in these arrays or when a free dislocation interacts with dislocations within a dislocation wall. In this way, large-scale fractures which develop inherit their crystallographic orientation from the dislocation wall.     

Grain boundary dissolution porosity in quartzofeldspathic ultramylonites: Implications for permeability enhancement and weakening of mid-crustal shear zones

Quartzofeldspathic ultramylonites from the Alpine Fault Zone, one of the world's major, active plate boundary-scale fault zones have quartz crystallographic preferred orientations (CPO) and abundant low-angle (<10° misorientation) boundaries, typical microstructures for dislocation creep-dominated deformation. Geometrically necessary dislocation density estimates indicate mean dislocation densities of ∼10⁹ cm⁻². A significant proportion (∼30%) of grain boundaries (>10° misorientation) are decorated by faceted pores, commonly with uniformly-oriented pyramidal shapes. Only grain boundaries with >10° misorientation angles in polymineralic aggregates are decorated by pores. Mean grain boundary pore densities are ∼5 × 10⁸ cm⁻². Grain boundary pores are dissolution pits generated during syn-deformational transient grain boundary permeability, nucleating on dislocation traces at dilatant grain boundary interfaces. They have not been removed by subsequent grain boundary closure or annealing. Pore decoration could have led to grain boundary pinning, triggering a switch in the dominant deformation mechanism to grain boundary sliding, which is supported by evidence of CPO destruction in matrix quartz. Pore-decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion, which would reduce the tensile and shear strength of grain boundaries, and hence, the bulk rock. Grain boundary decoration also significantly decreased the mean distance between pores, potentially facilitating dynamic permeability. Consequently, these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid-crustal 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.     

Fabric and Deformation of Omphacite in Dabie Ultra-high-pressure Ecologites

The rheological characters of omphacites in Dabie ultra-high-pressure eclogite have been studied interms of fabric, dislocation and micro-structures. 1. The eclogite has undergone high-temperature deformation, thus forming omphacite lattice preferred orientation. In addition to creep dislocation, the omphacite ductile deformation may have other mechanisms, such as diffusion creep and grain boundary migration. 2. The main-phase deformation of eclogite is coaxial, but asymmetry strain also exists due to strain partitioning in the Dabie erogenic belt. 3.The twin measured by the universal stage is (100), indicating that omphacite high-T deformation was superimposed by low-T deformation. 4. Subgrain structure is common in omphacite, but the deformation features of the omphacites in the Shuanghe area and Bixiling area are different, the latter being dominated by dynamic recrystallization. 5. The Flinn plots show that the strain of omphacite belongs to the constriction ellipsoid and stretching strain, which is