Relationship among titanium,rare earth elements,U–Pb ages and deformation microstructures in zircon: Implications for Ti-in-zircon thermometry

A zircon grain in an orthopyroxene–garnet–phlogopite–zircon–rutile-bearing xenolith from Udachnaya, Siberia, preserves a pattern of crystallographic misorientation and subgrain microstructure associated with crystal–plastic deformation. The zircon grain records significant variations in titanium (Ti) from 2.6 to 30 ppm that corresponds to a difference in calculated Ti-in-zircon temperatures of over several hundred degrees Celsius. The highest Ti concentration is measured at subgrain centres (30 ppm), and Ti is variably depleted at low-angle boundaries (down to 2.6 ppm). Variations in cathodoluminescence coincide with the deformation microstructure and indicate localised, differential enrichment of rare earth elements (REE) at low-angle boundaries. Variable enrichment of U and Th and systematic increase of Th/U from 1.61 to 3.52 occurs at low-angle boundaries. Individual SHRIMP-derived U–Pb ages from more deformed zones (mean age of 1799 ± 40, n = 22) are systematically younger than subgrain cores (mean age of 1851 ± 65 Ma, n = 7), and indicate that open system behaviour of Ti–Th–U occurred shortly after zircon growth, prior to the accumulation of significant radiogenic Pb. Modelling of trace-element diffusion distances for geologically reasonable thermal histories indicates that the observed variations are ~ 5 orders of magnitude greater than can be accounted for by volume diffusion. The data are best explained by enhanced diffusion of U, Th and Ti along deformation-related fast-diffusion pathways, such as dislocations and low-angle (< 5°) boundaries. These results indicate chemical exchange between zircon and the surrounding matrix and show that Ti-in-zircon thermometry and U–Pb geochronology from deformed zircon may not yield information relating to the conditions and timing of primary crystallisation.     

Trace-element heterogeneity in rutile linked to dislocation structures: Implications for Zr-in-rutile geothermometry

The trace-element composition of rutile is commonly used to constrain P–T–t conditions for a wide range of metamorphic systems. However, recent studies have demonstrated the redistribution of trace elements in rutile via high-diffusivity pathways and dislocation-impurity associations related to the formation and evolution of microstructures. Here, we investigate trace-element migration in low-angle boundaries formed by dislocation creep in rutile within an omphacite vein of the Lago di Cignana unit (Western Alps, Italy). Zr-in-rutile thermometry and inclusions of quartz in rutile and of coesite in omphacite constrain the conditions of rutile deformation to around the prograde boundary from high pressure to ultra-high pressure (~2.7 GPa) at temperatures of 500–565°C. Crystal-plastic deformation of a large rutile grain results in low-angle boundaries that generate a total misorientation of ~25°. Dislocations constituting one of these low-angle boundaries are enriched in common and uncommon trace elements, including Fe and Ca, providing evidence for the diffusion and trapping of trace elements along the dislocation cores. The role of dislocation microstructures as fast-diffusion pathways must be evaluated when applying high-resolution analytical procedures as compositional disturbances might lead to erroneous interpretations for Ca and Fe. In contrast, our results indicate a trapping mechanism for Zr.     

Quantitative characterization of plastic deformation of zircon and geological implications

The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolite grade has been quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic orientations that accommodate 20° of misorientation in the zircon crystal. These variations are manifested by discrete low-angle (<4°) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation change is documented by crystallographic pole figures which show systematic small circle distributions, and disorientation axes associated with 0.5–4° disorientation angles, which lie parallel to rational low index crystallographic axes. In the most distorted part of the grain (area A), this is the [100] crystal direction. A quaternion analysis of orientation correlations confirms the [100] rotation axis inferred by stereographic inspection, and reveals subtle orientation variations related to the local boundary structure. Microstructural characteristics and orientation data are consistent with the low-angle boundaries having a tilt boundary geometry with dislocation line [100]. This tilt boundary is most likely to have formed by accumulation of edge dislocations associated with a 〈001〉{100} slip system. Analysis of the energy associated with these dislocations suggest they are energetically more favorable than TEM verified 〈010〉{100} slip. Analysis of minor boundaries in area A indicates deformation by either (001) edge, or [100](100) and [001](100) screw dislocations. In other parts of the grain, cross slip on (111), and (112) planes seems likely. These data provide the first detailed microstructural analysis of naturally deformed zircon and indicate ductile crystal-plastic deformation of zircon by the formation and migration of dislocations into low-angle boundaries. Minimum estimates of dislocation density in the low-angle boundaries are of the order of ∼3.10¹⁰ cm⁻². This value is sufficiently high to have a marked effect on the geochemical behavior of zircon, via enhanced bulk diffusion and increased dissolution rates. Therefore, crystal plasticity in zircon may have significant implications for the interpretation of radiometric ages, isotopic discordance and trace element mobility during high-grade metamorphism and melting of the crust.     

Plastic Deformation and Recrystallization of Garnet: A Mechanism to Facilitate Diffusion Creep

Elongate and deformed garnets from Glenelg, NW Scotland, occurwithin a thin shear zone transecting an eclogite body that hasundergone partial retrogression to amphibolite facies at circa700°C. Optical microscopy, back-scattered electron imaging,electron probe microanalysis and electron back-scatter diffractionreveal garnet sub-structures that are developed as a functionof strain. Subgrains with low-angle misorientation boundariesoccur at low strain and garnet orientations are dispersed, aroundrational crystallographic axes, across these boundaries. Towardshigh-strain areas, boundary misorientations increase and thereis a loss of crystallographic control on misorientations, whichtend towards random. In high-strain areas, a polygonal garnetmicrostructure is developed. The garnet orientations are randomlydispersed around the original single-crystal orientation. Somegarnet grains are elongate and Ca-rich garnet occurs on thefaces of elongate grains oriented normal to the foliation. Commonly,the garnet grains are admixed with matrix minerals, and, wherein contact with other phases, garnet is well faceted. We suggestthat individual garnet porphyroclasts record an evolution fromlow-strain conditions, where dislocation creep and recoveryaccommodated deformation, through increasing strain, where dynamicrecrystallization occurred by subgrain rotation, to higheststrains, where recrystallized grains were able to deform bydiffusion creep assisted grain boundary sliding with associatedrotations. KEY WORDS: diffusion creep; EBSD; garnet; plastic deformation; recrystallization     

Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD) mapping reveals a c.18° variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (<4°) boundaries. These microstructural data are consistent with crystal-plastic deformation of zircon associated with the formation and migration of dislocations. A heterogeneous pattern of dark cathodoluminescence, with the darkest domains coinciding with low-angle boundaries, mimics the deformation microstructure identified by EBSD. Geochemical data collected using the Sensitive High Resolution Ion MicroProbe (SHRIMP) shows a positive correlation between concentrations of the elements U, Th and Pb (ranging from 20–60 ppm, 30–110 ppm, and 14–36 ppm, respectively) and Th/U ratio (1.13 – 1.8) with the deformation microstructure. The highest measured concentrations and Th/U coincide with low-angle boundaries. This enrichment is interpreted to reflect enhanced bulk diffusion of U and Th due to the formation and migration of high-diffusivity dislocations. ²⁰⁷Pb/²⁰⁶Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 ± 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent ²⁰⁷Pb/²⁰⁶Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data.     

Deformation-related microstructures in magmatic zircon and implications for diffusion

An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common <100> misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of <001>{100} and <100>{010} slip systems. The restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates.     

Microstructural evolution and trace element mobility in Witwatersrand pyrite

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of <010>{100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple <010>{100} deformation. These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized <001>{120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing.     

Cordierite microstructure and texture in a moldanubian gneiss

The microstructure and texture in cordierites of a moldanubian gneiss from the Bohemian Massif has been analysed by transmission electron microscopy (TEM) and universal stage in order to get information on the deformation mechanisms and textural development of this rock-forming mineral. Deformation may have taken place at temperatures between about 500° C and 630° C and pressures smaller than about 3 kb. The elongated cordierite xenoblasts show a typical dislocation creep microstructure consisting of subgrain boundaries and free dislocations. The dislocations have [001], [010] and 1/2<110> Burgers vectors. [001] dislocations often have pure screw and edge character the latter type being climb-dissociated on (001). Among the dislocations reactions are common. The main subgrain boundaries observed are (010)[001], {110}[001] and (001)[010] tilt boundaries. Burgers vectors and dislocation line directions reveal (100)[001], (010)[001], (100)[010], {110} 1/2<110> and (001)1/2<110> as activated slip systems. The crystallographic preferred orientation (here referred to as texture) consists of a [001] maximum in the foliation parallel to the mineral lineation. [100] and [010] maxima are perpendicular to it within and normal to the foliation, respectively, with a girdle tendency normal to the lineation. The texture may be explained by simple shear deformation on the {hkO}[001] slip systems with preference of (010)[001].     

Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy

Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} 〈100〉, {100} 〈001〉, and {010} 〈100〉 in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes.     

Intragranular dynamic recrystallization in naturally deformed calcite marble: diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization

Intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains. Intragranular recrystallized grains within large, twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study. Lattice distortions, low angle boundaries and some high angle boundaries (>15°) in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb-accommodated dislocation creep involving subgrain rotation recrystallization. Changes in the ratio of host and twin domain, as the deformation zone is entered, show that twin boundary migration also occurred. Recrystallized grains have similar sizes (10–60 μm) to subgrains, suggesting that they formed by subgrain rotation. However, within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15° and misorientation axes are randomly oriented. Moreover recrystallized grain orientations average around the porphyroclast orientation. We suggest that the recrystallized grains, once formed, are able to deform partly by diffusion accommodated grain boundary sliding, which is consistent with predictions made from lab flow laws.     

Ultrahigh temperature deformation microstructures in felsic granulites of the Napier Complex, Antarctica

Detailed electron microscope and microstructural analysis of two ultrahigh temperature felsic granulites from Tonagh Island, Napier Complex, Antarctica show deformation microstructures produced at  1000 °C at 8–10 kbar. High temperature orthopyroxene (Al 7 wt.% and  11 wt.%), exhibits crystallographic preferred orientation (CPO) and frequent subgrain boundaries which point to dislocation creep as the dominating deformation mechanism within opx. Two different main slip systems are observed: in opx bands with exclusively opx grains containing subgrain boundaries with traces parallel to [010] and a strong coupling of low angle misorientations (2.5°–5°) with rotation axes parallel to [010] the dominating slip system is (100)[001]. Isolated opx grains and grain clusters of 2–5 grains embedded in a qtz–fsp matrix show an additional slip system of (010)[001]. The latter slip system is harder to activate. We suggest that differences in the activation of these slip systems is a result of higher differential stresses imposed onto the isolated opx grains and grain clusters. In contrast to opx, large qtz grains (up to 200 μm) show random crystallographic orientation. This together with their elongate and cuspate shape and the lack of systematic in the rotation axes associated with the subgrain boundaries is consistent with diffusion creep as the primary deformation mechanism in quartz.Our first time detailed microstructural observations of ultrahigh temperature and medium to high pressure granulites and their interpretation in terms of active deformation mechanisms give some insight into the type of rheology that can be expect at lower crustal conditions. If qtz is the mineral phase governing the rock rheology, Newtonian flow behaviour is expected and only low differential stress can be supported. However, if the stress supporting mineral phase is opx, the flow law resulting from dislocation creep will govern the rheology of the rock unit; hence, an exponential relationship between stress and strain rate is to be expected.     

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

The microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ～8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.     

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.     

Fluid-assisted fracturing,cataclasis, and resulting plastic flow in mylonites from the Moresby Seamount detachment,Woodlark Basin

The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust.     

Dislocations associated with optical features in naturally-deformed olivine

Transmission electron microscopy has been used for the direct observation of dislocations in naturally-deformed olivine. The dislocations are arranged in arrays forming low-angle sub-boundaries which have been identified with features observed in the optical microscope. Comparison of this dislocation substructure with that observed in olivine, and in metals, experimentally deformed under various conditions, suggests that the deformation in nature has occurred by creep. Possible mechanisms of creep, involving the cooperative glide and climb of dislocations, are discussed.     

Dislocation deformation mechanisms in peridotite xenoliths in kimberlites

The dislocation substructures in olivine from coarse-grained peridotite xenoliths in kimberlites from the Lesotho region have been determined. The [100] dislocations may be located in simple (100) tilt boundaries while the density of free or individual [100] dislocations is 10⁶/cm² or less. The [001] dislocations form (010) twist arrays or more complex (100) subboundaries with the [100] dislocations; the density of free [001] dislocations increases to 8 × 10⁸/cm² in those grains in which tangles are observed. The simple (100) subboundaries are considered to result from a high temperature, slow strain-rate deformation (creep-like process) while the more complex subboundaries composed of [100] and [001] dislocations, as well as the high density of [001] dislocations, indicate faster strain rates and/or lower deformation temperatures than the creep deformation. These two broad phases of deformation have been interpreted as an early stage of mantle-type flow followed by deformation during or subsequent to the emplacement of the kimberlite.     

Garnet crystal plasticity in the continental crust,new example from south Madagascar

Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ～300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (10¹⁰ m^?3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.     

Some garnet microstructures: an illustration of the potential of orientation maps and misorientation analysis in microstructural studies

The microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction. In both cases there is a very strong crystallographic preferred orientation, with measurements clustered round a single dominant orientation. Each garnet grain is divided into domains with similar orientations, limited by boundaries with misorientations of 2° or more. In both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains. One sample is a deformed garnet that shows considerable distortion within the domains. Lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes. The domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery. The second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes. These boundaries probably formed by coalescence of originally separate garnets. We suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy. The data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material.     

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.