Weak Crystallographic Preferred Orientation Research Articles

Episodic deformation and reactions in mylonitic high-grade metamorphic granulites from Dronning Maud Land, Antarctica

Microfabrics of mylonitic orthopyroxene-bearing quartzofeldspatic granulites from central Dronning Maud Land, Antarctica, have been investigated to evaluate the deformation behaviour at lower crustal conditions. The microfabric is characterized by a fine-grained matrix of dispersed feldspar, quartz and orthopyroxene, which contains monophase elongate quartz and feldspar aggregates, so-called ribbons, and elongate monocrystalline orthopyroxene ribbon grains. The observed crystallographic preferred orientation (CPO) of orthopyroxene ribbon grains that contain clinopyroxene exsolution lamellae in (100) and kink bands in equidimensional porphyroclasts indicate (100)[001] dislocation glide. Fine-grained matrix orthopyroxene formed by deformation of porphyroclasts and growth in strain shadows during diffusional creep, indicating a separate stage of deformation and reaction. Although dislocation creep of coarse ribbon quartz is indicated by sutured grain boundaries at high angle to the ribbon boundary and few subgrains, the weak CPO is rather unspecific. Fine-grained matrix quartz shows no indication of dislocation creep. We suggest that epitactic growth during quartz-producing mineral reactions played a major role for ribbon formation during diffusional creep. Our study demonstrates the importance of episodic deformation and metamorphic reactions in lower continental crust with transient high stresses allowing for dislocation glide of orthopyroxene enclosed in a fine-grained polyphase matrix undergoing diffusional creep on long term.

Understanding the emplacement of Martian volcanic rocks using petrofabrics of the nakhlite meteorites

In order to validate calculated ages of the Martian crust we require precise radiometric dates from igneous rocks where their provenance on the Martian surface is known. Martian meteorites have been dated precisely and quantitatively, but the launch sites are currently unknown. Inferring the formation environment of a correlated suite of Martian meteorites can constrain the nature and complexity of the volcanic system they formed from. The nakhlite meteorites are such a suite of augite-rich rocks that sample the basaltic crust of Mars, and as such can provide unique insights into its volcanic processes. Using electron backscatter diffraction we have determined the shape-preferred and crystallographic-preferred orientation petrofabrics of four nakhlites (Governador Valadares, Lafayette, Miller Range 03346 and Nakhla) in order to understand the conditions under which their parent rocks formed. In all samples, there is a clear link between the shape-preferred orientation (SPO) and crystallographic-preferred orientation (CPO) of augite phenocrysts. This relationship reveals the three-dimensional shape of the augite crystals using CPO as a proxy for 3D SPO, and also enables a quantitative 3-dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite <c> axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their <c> axis foliation that is interpreted to have developed in a combined pure shear/simple shear flow regime, indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow. By contrast, the foliation dominated CPO petrofabrics of Lafayette and Miller Range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow. These CPO petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies, or the flattening area of spreading lava flows. The CPO foliation of Lafayette's is substantially weaker than Miller Range 03346, probably due to its higher phenocryst density causing grain-grain interactions that hindered fabric development. The CPO petrofabrics identified can also be used to determine the approximate plane of the Martian surface and the line of magma flow to within ∼20°. Our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on Mars.

Grain-size reduction of feldspar and flow of deformed granites within the Gaoligong shear zone, southwestern Yunnan, China

Mineral deformation and rock flow mechanism in the lithosphere are related to the rheological behavior and weakening mechanism of the continent. Natural deformation behaviors of feldspars are not well understood due to the complexity of their mineral compositions, crystal structures, as well as changing deformation conditions. The refined microstructure, fabric and composition of major minerals in the deformed granitic rocks within the Gaoligong shear zone (GLGSZ), southwestern Yunnan, China, were studied. With increasing mylonitization, two fabric types of end-members have been distinguished (type-I banded granitic mylonite and type-II banded ultramylonite). The two types of deformed granitic rocks have the same mineral assemblage, but different mineral modes. The type-I banded granitic mylonite has a greater proportion of K-feldspar (mostly present as porphyroclasts)>plagioclase>quartz±biotite, however, the type-II banded ultramylonite has a greater proportion of fine-grained plagioclase>K-feldspar>quartz±biotite. The crystallographic preferred orientation (CPO) patterns of quartz combined with two-feldspar geothermometer, confirm that the quartz grains in the type-I and type-II granitic rock have undergone high-temperature dislocation creep deformation. The K-feldspar grains in the matrix of type-II banded ultramylonite show a dominant (100) [010] slip system with dislocation creep recrystallization, while the fine-grained plagioclase grains present a weak CPO pattern with superplastic flow. The K-feldspar porphyroclasts show grain-size reduction associated with mineral composition and fabric transformation. The myrmekite formation with the fine-grained neocrystallization of plagioclase and quartz significally replaced the K-feldspar porphyroclasts. Finally, the fine-grained neocrystallization plagioclases were formed further into the high strain localized ultramylonites with superplastic flow.

Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha, Aravalli–Delhi Fold Belt, NW India

In the present study we investigate the microstructural development in mullite, quartz and garnet in an anatectic migmatite hosted within a Grenvillian-age shear zone in the Aravalli–Delhi Fold Belt. The migmatite exhibits three main deformation structures and fabrics (S1, S2, S3). Elongated garnet porphyroblasts are aligned parallel to the metatexite S2 layers and contain crenulation hinges defined by biotite–sillimanite–mullite–quartz (with S1 axial planar foliation). Microstructural evidence and phase equilibrium relations establish the garnet as a peritectic phase of incongruent melting by breakdown of biotite, sillimanite ± mullite and quartz at peak P–T of ~ 8 kbar, 730 °C along a tight-loop, clockwise P–T path. Monazite dating establishes that the partial melting occurred between ~ 1000 and 870 Ma. The absence of subgrains and systematic crystal lattice distortions in these garnets despite their elongation suggests growth pseudomorphing pre-existing 3-D networks of S1 biotite aggregates rather than high-temperature crystal plastic deformation which is noted in the S1 quartz grains that exhibit strong crystallographic preferred orientation (CPO), undulatory extinction and subgrains. Mode-I fractures in these garnet porphyroblasts induced by high melt pressure during late stage of partial melt crystallization are filled by retrograde biotite–sillimanite. Weak CPO and non-systematic crystal lattice distortions in the coarse quartz grains within the S2 leucosome domains indicate these crystallized during melt solidification without later crystal plastic deformation overprint. In the later stages of deformation (D3), strain was mostly accommodated in the mullite–biotite–sillimanite-rich restite domains forming S3 which warps around garnet and leucosome domains; consequently, fine-grained S3 quartz does not exhibit strong CPOs.

Microstructural and rheological evolution of calcite mylonites during shear zone thinning: Constraints from the Mount Irene shear zone, Fiordland, New Zealand

Layers of calc-mylonite in the Mount Irene shear zone, Fiordland, New Zealand, show substantial variations in thickness due to deflection of the shear zone boundaries around wall rock asperities. In relatively thick parts (c. 2.6 m) of the shear zone, calcite porphyroclasts are internally strained, contain abundant subgrain boundaries and have a strong shape preferred orientation (SPO) and crystallographic preferred orientation (CPO), suggesting that deformation occurred mainly by dislocation creep involving subgrain-rotation recrystallization. In relatively thin parts (c. 1.5 m) of the shear zone, aggregates of fine-grained recrystallized calcite surrounding flattened porphyroclasts have a weak SPO and CPO, and contain polygonal calcite grains with low degrees of internal misorientation. The recrystallized aggregates also contain microstructures (e.g. grain quadruple junctions, randomized misorientation axes) similar to those reported for neighbor-switching processes during grain-boundary sliding. Comparison of subgrain sizes in the porphyroclasts to published grain-size differential-stress relationships indicates that stresses and strain rates were substantially higher in relatively thin parts of the shear zone. The primary microstructural response to higher stresses and strain rates was an increase in the amount of recrystallization to produce aggregates that deformed by grain-boundary sliding. However, even after the development of interconnected networks of recrystallized grains, dislocation creep by subgrain-rotation recrystallization continued to occur within porphyroclasts. This behavior suggests that the bulk rheology of shear zones undergoing thinning and thickening can be controlled by concomitant grain-size insensitive and grain-size sensitive mechanisms. Overall, our observations show that shear zone thickness variations at constant P-T can result in highly variable stresses and strain rates, which in turn modifies microstructure, deformation mechanism and shear zone rheology.

Grain‐ to multiple‐grain‐scale deformation processes during diffusion creep of forsterite + diopside aggregate: 1. Direct observations

AbstractWe uniaxially deformed fine‐grained (~ 1 μm) forsterite + diopside (5 and 20 vol %) aggregates in the diffusion creep regime. Prior to deformation, line markers were milled on a lateral surface of a cylindrical sample to detect single‐ to multiple‐grain‐scale deformation. We performed deformation experiments and observations of the marker‐etched surface after sample cooling multiple times on the same specimens. The strain measured at the scale of several tens of grains from macroscopic shortening of the markers parallel to the compression axis is consistent with the total strain of the sample. However, microscopically, the markers are intensely segmented and rotated at the grain scale increasing with the sample strain. Meanwhile, essentially, no deformation is observed within the grains in most of the samples. The surface microstructures, including the deformation of the markers, reveal the serial operations of grain boundary migration, grain boundary sliding, rigid body grain rotation, and grain neighbor switching, which correspond well to processes expected in diffusion‐controlled superplasticity. This sequence is commonly observed in both samples consisting of forsterite grains of tabular and equiaxed grain shapes, which have been shown to develop notable crystallographic preferred orientation (CPO) and random (or weak) CPO, respectively, during diffusion creep. Intragranular regions of relatively larger forsterite grains in the specimens deformed at stresses near the transition between deformation mechanisms from diffusion creep to dislocation creep reveal marker deformation and formation of surface creases and subgrain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well.

Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province, Norway)

A dry mafic dyke from a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway) experienced syn-kinematic melt-rock interaction. Viscous shearing occurred at T≈800°C, P≈0.75–0.95GPa and was coeval with infiltration of felsic melt from adjacent migmatitic metapelites. The dyke has a mylonitic microstructure where porphyroclasts of orthopyroxene, clinopyroxene and plagioclase are wrapped by a fine-grained (4–7μm) polyphase mixture of clinopyroxene+orthopyroxene+plagioclase+quartz+ilmenite±K-feldspar±apatite. Microstructural observations and electron backscatter diffraction analysis indicate that the porphyroclasts deformed by a combination of dislocation glide and fracturing, with only a limited record of dislocation creep, recovery and dynamic recrystallization. We identified diffusion creep as the dominant deformation mechanism in the mixture based on the small grain size, phase mixing and weak crystallographic preferred orientation of all phases (interpreted as the result of oriented grain growth during viscous flow). The polyphase mixture did not form by dynamic recrystallization or by mechanical fragmentation of the porphyroclasts, but rather by melt-rock interaction. Thermodynamic models indicate that the syn-kinematic mineral assemblage results from the chemical interaction between a pristine mafic dyke and ca. 10vol.% of felsic melt infiltrating from the adjacent partially molten metapelites. Extrapolation of laboratory-derived flow laws to natural conditions indicates that the formation of interconnected layers of fine-grained reaction products deforming by diffusion creep induces a dramatic weakening in the mafic granulites, with strain rates increasing up to 2–3 orders of magnitude. The reaction weakening effect is more efficient than the weakening associated with melt-assisted diffusion creep in the presence of up to 10vol.% of infiltrated melt without formation of fine-grained reaction products. Thus, we conclude that weakening by dramatic grain-size reduction through nucleation of fine-grained material that subsequently deforms by diffusion creep is a major effect of melt infiltration during lower crustal shearing.

Strain localization in ultramylonitic marbles by simultaneous activation of dislocation motion and grain boundary sliding (Syros, Greece)

Abstract. Extreme strain localization occurred in the centre of the cross-cutting element of a flanking structure in almost pure calcite marbles from Syros, Greece. At the maximum displacement of 120 cm along the cross-cutting element, evidence of grain size sensitive deformation mechanisms can be found in the ultramylonitic marbles, which are characterized by (1) an extremely small grain size ( ∼ 3 µm), (2) grain boundary triple junctions with nearly 120° angles, (3) a weak crystallographic preferred orientation with very low texture index (J = 1.4), (4) a random misorientation angle distribution curve and (5) the presence of small cavities. Using transmission electron microscopy, a deformation sequence is observed comprising recrystallization dominantly by bulging, resulting in the development of the fine-grained ultramylonite followed by the development of a high dislocation density ( ∼ 1013 m−2) with ongoing deformation of the fine-grained ultramylonite. The arrangement of dislocations in the extremely fine-grain-sized calcite differs from microstructures created by classical dislocation creep mediated by combined glide and thermally activated climb. Instead, it exhibits extensive glide and dislocation networks characteristic of recovery accommodated by cross-slip and network-assisted dislocation movement without formation of idealized subgrain walls. The enabling of grain boundary sliding to dislocation activity is deemed central to initiating and sustaining strain softening and is argued to be an important strain localization process in calcite rocks, even at a high strain rate ( ∼ 10−9 s−1) and low temperature (300 °C).

Influence of stress and strain on dolomite rim growth: a comparative study

Triaxial compression and torsion experiments were performed to investigate the influence of non-isostatic stress and strain on dolomite reaction rim growth using orientated natural calcite and magnesite single crystals at a temperature of 750 °C, 400 MPa confining pressure, stresses between 7 and 38 MPa, and test durations up to 171 h. Reaction products were composed of a polycrystalline magnesio-calcite layer, palisade-shaped dolomite, and granular dolomite grains. In all experiments, inelastic deformation was partitioned into calcite and reaction products, while magnesite remained undeformed. Calcite deformed by twinning and dislocation creep, where the activation of additional glide systems at high stress allowed high strain. Depending on grain size, magnesio-calcite deformed by diffusion creep and/or grain boundary sliding, twinning, and dislocation creep. Dolomite deformed mainly by diffusion creep, assisted by enhanced dislocation activity allowing Ca enrichment in the granular rim. A weak crystallographic preferred orientation of the reaction products was observed. In triaxial compression, dolomite rim growth was diffusion-controlled and showed no influence of axial stresses up to 38 MPa on the reaction kinetics. At high strain (>0.1), the magnesio-calcite layer is wider suggesting faster growth kinetics. This may be related to additional diffusion pathways provided by enhanced dislocation activity. At very high strain (>0.3–0.6), twisted samples showed a gradual decrease in layer thickness of dolomite and magnesio-calcite with increasing strain (-rate).

Sillimanite deformation mechanisms within a Grt-Sil-Bt gneiss: effect of pre-deformation grain orientations and characteristics on mechanism, slip-system activation and rheology

AbstractSillimanite-bearing metapelites are common in the middle to lower crust and play an important role in the rheology of these crustal levels. We report deformation microstructures in a metasedimentary gneiss (Greenland) produced during simple shear at c. 700 °C and c. 4.5 kbar. Electron backscatter diffraction analysis shows activation of the (001)[010] and (001)[100] slip systems in large grains. Additionally, (100)[001] and (010)[001] are activated in needles with their c-axes parallel to lineation. Medium-grained sillimanite aggregates mixed with biotite show a weak crystallographic preferred orientation (CPO) suggesting activation of (001)[010] and (001)[100] together with grain boundary sliding (GBS). GBS is the dominant deformation mechanism in the outer parts of fine-grained clusters of sillimanite stemming from andalusite replacement. Associated features are phase mixing, clockwise dispersion of c-axis orientations and change of misorientation axes from original c-axis rotation to rotation close to the kinematic Y axis. The latter features which result in strain localization around these clusters allow their ‘preservation’. Rheologically, the medium-grained aggregates together with zones around clusters take up most of the deformation. Our study shows that deformation mechanism and slip-system activation, and CPOs are strongly dependent on the orientation relationship between stress axes and pre-deformation grain orientations and characteristics.

Experimental deformation and recrystallization of olivine – processes and timescales of damage healing during postseismic relaxation at mantle depths

Abstract. Experiments comprising sequences of deformation (at 300 or 600 °C) and annealing at varying temperature (700 to 1100 °C), time (up to 144 h) and stress (up to 1.5 GPa) were carried out in a Griggs-type apparatus on natural olivine-rich peridotite samples to simulate deformation and recrystallization processes in deep shear zones that reach mantle depth as continuations of seismically active faults. The resulting olivine microfabrics were analysed by polarization and electron microscopy (SEM/EBSD, TEM). Core-and-mantle-like microstructures are the predominant result of our experiments simulating rapid stress relaxation (without or with minor creep) after a high-stress deformation event: porphyroclasts (&gt; 100 μm) are surrounded by new grains comprising fragments and recrystallized grains with a wide range in size (2 to 40 μm). Areas with small grains (&amp;amp;leq; 10 μm) trace former high-strain zones generated during initial high-stress deformation even after annealing at a temperature of 1100 °C for 70 h. A weak crystallographic preferred orientation (CPO) of new olivine grains is related to the orientation of the original host crystals but appears unrelated to the strain field. Based on these findings, we propose that olivine microstructures in natural shear-zone peridotites with a large range in grain size, localized fine-grained zones, and a weak CPO not related to the strain field are diagnostic for a sequence of high-stress deformation followed by recrystallization at low stresses, as to be expected in areas of seismic activity. We extended the classic Avrami-kinetics equation by accounting for time-dependent growth kinetics and constrained the involved parameters relying on our results and previous studies devoted to the kinetics of defect processes in olivine. Extrapolation to natural conditions suggests that the observed characteristic microstructure may develop within as little as tens of years and less than ten thousands of years. These recrystallization microstructures have a great diagnostic potential for past seismic activity because they are expected to be stable over geological timescales, since driving forces for further modification are not sufficient to erase the characteristic heterogeneities.

The influence of hydrous phases on the microstructure and seismic properties of a hydrated mantle rock

To better understand the microstructural evolution of a “serpentinized” mantle rock and the influence of various hydrous phases on the seismic properties of the mantle wedge, we have conducted the detailed microstructural analyses of a sample of tremolite–chlorite–antigorite schist collected from the Moses Rock dike (central part of the Colorado Plateau). We performed differential effective media (DEM) modelling to study the effect of three hydrous phases forming two-phase aggregates with olivine, considering the crystallographic preferred orientation (CPO) of each phase and the shape ratio of the hydrous phases. We have demonstrated that in a partially serpentinized peridotite, the olivine CPO characteristic of [100](010) dislocation glide is still preserved, and the high-temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist. The transformation of olivine into antigorite occurs predominantly (~75%) by the relationship (100)ol || (001)atg with [001]ol || [010]atg, with the (010)ol || (001)atg and [001]ol || [010]atg relationship observed in areas of weak antigorite CPO. Chlorite results from the phase transformation of olivine in a relatively static environment, as shown by the correlation between the olivine–chlorite CPOs with (100)ol || (100)ch, (010)ol || (001)ch and (001)ol || (010)ch. The fluid percolation that caused the localized metasomatism and partial hydration of the mantle occurred possibly along trans-lithospheric shear zones. The presence of chlorite induces the most important drop on the P-wave velocities and may help to explain some local low velocities in the fore-arc mantle wedges, but is unlikely to be of global importance due to its very high Vp/Vs ratio~1.9. On the other hand, antigorite is the only phase that causes important modification on the propagation directions of P and S-waves, and the only phase to explain the polarization of the fastest shear waves parallel to the subduction trench.

Transition from fracturing to viscous flow in granulite facies perthitic feldspar (Lofoten, Norway)

Recrystallization of perthites in granulite facies (T = 700–730 °C, P = 0.65–0.8 GPa) shear zones in mangerite-charnockite rocks from Lofoten (Norway) is localized along intracrystalline bands parallel to fractures. Fracturing preferentially occurred along the cleavage planes (010) and (001). EBSD analysis of perthite porphyroclasts indicates a very low degree of internal misorientation (within 5°) and the lack of recovery features. Recrystallized grains show coarsening with increasing width of the bands, and chemical changes with respect to the host grains. Crystallographic orientation of the new grains does not show a host-control relation to the parent perthite grains. In summary, the microstructure and CPO data consistently indicate intragranular recrystallization by nucleation and growth from fractured grains. Perthite porphyroclasts are surrounded by a matrix of recrystallized plagioclase + K-feldspar ± amphibole ± biotite. There is extensive evidence of syndeformational nucleation of new phases and of phase boundary migration in the matrix, with plagioclase grains forming bulges and protrusions towards K-feldspar. The spatial distribution of K-feldspar and plagioclase in the recrystallized matrix is characterized by the predominance of phase boundaries over grain boundaries. All these observations are consistent with diffusion creep as the dominant deformation mechanism in the matrix, associated with grain boundary sliding. Accordingly, recrystallized plagioclase and K-feldspar show a very weak crystallographic preferred orientation, which is interpreted in terms of oriented growth during diffusion creep. Fracturing of perthites promoted extensive grain size reduction, recrystallization, fluid infiltration, and operation of grain-size sensitive creep, resulting in strain localization.

Gabbro layering induced by simple shear in the Oman ophiolite Moho transition zone

We investigate the origin of modal layering in gabbro lenses of the Moho transition zone in the Oman ophiolite with a microstructural study. Gabbro lenses exhibit a shape preferred orientation of plagioclase crystals, that are euhedral and devoid of any intracrystalline deformation. This texture and field kinematic indicators show a strong simple shear deformation in magmatic conditions. The parallelism of the lineation in gabbros and the plastic lineation of the host dunite indicate that their development is contemporaneous, and that the magmatic features are passively coupled to the solid-plastic flow of the host mantle. We also found undeformed-isotropic and weakly deformed-roughly layered gabbros. The host rock is plagioclase and clinopyroxene impregnated dunite; it contains euhedral plagioclase suggesting that some grains crystallized in suspension. There is no clear boundary between the gabbro and the host rock; on the contrary, the limit is diffuse, with increasing plagioclase and clinopyroxene content from the dunite to the gabbro over tens of meters. This suggests that the gabbro corresponds to melt impregnated dunite. We defined 4 gabbro types, from unlayered (type 1) to well layered (type 4) with progressively more continuous and distinct layers. We characterized deformation with crystal shape and crystallographic preferred orientation (CPO) measurements. Unlayered samples have a random fabric; type 2 have a very weak shape fabric and planar CPO, defined by olivine and plagioclase (010) planes; type 3 have a weak fabric, with a lineation defined by the [001] olivine axis and the [100] plagioclase axis, which reflects the shape fabric; type 4 have a strong shape fabric and CPO, with a lineation defined by the [100] olivine and plagioclase axis. We interpret the progression from isotropic to clear S–L fabrics as a result of increasing deformation imposed by the flowing host mantle. The type 2 gabbros must result from compaction; with increasing simple shear, the lineation is developed in type 3 samples; finally, with lower melt content, suspension flow switches to plastic deformation in the olivine-rich layers, leading to plastic olivine fabrics in type 4 samples. We deduce that layering originates from simple shear, which other studies show induces strong segregations in suspended particles, and plastically deformed rocks. To build the isotropic stage, melt must accumulate faster than the deformation, suggesting that melt is extracted by dikes.

Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, Étang de Lers, Pyrenees, France

In naturally deformed upper mantle rocks a strong olivine crystallographic preferred orientation (CPO) occurs in rocks with grain sizes larger than about 15 μm. Finer grained peridotites tend to have weak to random olivine CPO. The different types of olivine CPO are usually interpreted in relation to the dominant deformation mechanisms: with a strong CPO indicating dislocation creep and a random CPO indicating grain size sensitive (GSS) mechanisms involving grain boundary sliding and diffusion creep. Here we report the occurrence of a weak but systematic olivine CPO in ultra-fine-grained (0.5–10 μm) ultramylonites from the Étang de Lers lherzolite, Pyrenees, France. The microstructures, with elongated grains and mixed distribution of phases in the ultramylonites indicate deformation by dominant GSS creep. In theory, an olivine CPO may develop in rocks deformed by GSS creep during dislocation accommodated boundary sliding or phase boundary controlled diffusion creep in pyroxene-rich rocks. Alternatively, the CPO may be a relict texture preserved because the elongated grain shapes limit rotation during grain boundary sliding. Our observations confirm theoretical and experimental studies, which predict that systematic CPO and an associated seismic anisotropy can occur in naturally deformed upper mantle rocks during deformation by grain boundary sliding and diffusion creep.

Characterization of microstructures and interpretation of flow mechanisms in naturally deformed, fine-grained anhydrite by means of EBSD analysis

AbstractAnhydrite-rich layers within foreland fold and thrust belts are frequently observed to be the weakest horizon of the sequence. Characterizing the microstructure of anhydrite is therefore important for interpreting the larger-scale deformation history of these rocks. Two microstructures from naturally deformed, fine-grained (&lt;15 µm mean grain size) anhydrite samples from the Triassic Evaporites of the Umbria–Marche Apennines, Italy were analysed using electron backscatter diffraction (EBSD). Microstructural observations, misorientation analysis and crystallographic preferred orientation (CPO) determination were carried out on these samples. Both samples have a CPO characterized by alignment of 〈001〉 and distribution of 〈100〉 and 〈010〉 on a great circle normal to this. This anhydrite 〈001〉 ‘fibre texture’ has not been described before. Microstructure A is characterized by a moderate to weak CPO and a weak shape preferred orientation at 55° to 70° from the trace of the 〈001〉 maximum. Low-angle boundaries are revealed by misorientation analysis. A change in grain size from c. 10 to c. 7 µm corresponds to reduction in strength of CPO and reduction in the number of low-angle grain boundaries. Microstructure B is characterized by a very strong CPO. The orientation of the CPO changes between different microstructural domains. The 〈001〉 maximum is always perpendicular to the trace of a strong grain elongation and high-angle grain boundaries have misorientations close to 〈001〉, suggesting that the CPO is geometrically controlled: anhydrite grains are platy with 〈001〉 short axes. The origin of the CPO is therefore unclear but it need not relate to dislocation creep deformation. Whether or not CPO relates to dislocation creep, both samples have a high number of lower-angle grain boundaries and internal grain distortions with 〈010〉 and 〈001〉 misorientation axes. These are indicative of dislocation activity and the data are best explained by slip on either (100)[010] (dominant) and (001)[100] or a combination of these. Neither of these slip systems has been recognized before. Both microstructures are interpreted to have undergone dynamic recrystallization, and the weakening of the CPO with decreasing grain size in microstructure A is suggested to be indicative of a grain-boundary sliding mechanism becoming active. Comparison with experimental data shows that creep mechanisms involving dislocations at the observed grain sizes require the differential stress magnitudes driving deformation to be greater than c. 100 MPa.

Development of Microstructure and Texture of Hematite Ores Deformed to Large Strain in Torsion: Can Texture Identify the Prevailing Strength and Creep Mechanisms During Deformation?

AbstractSamples of fine‐grained (approximately 9 µm) and coarse‐grained (approximately 45 µm) hematite ores with almost random crystallographic preferred orientation (CPO) were deformed in high pressure, high temperature (400 MPa, 850–1 000 °C) torsion experiments up to shear strains of 4.7. Samples with large initial grain size, preferably deformed by dislocation creep attended by dynamic recrystallization, showed grain size reduction and a weak CPO (J ∼ 1.4 at 950 °C). In contrast, fine‐grained ores, deformed mainly by grain boundary sliding accompanied by dislocation activity with slip on the dominant basal glide system, showed grain growth and a strong CPO (J ∼ 2.9). At high strain both ores attained similar strength and grain size, but different CPOs. The experiments demonstrate that the texture intensity of highly deformed rocks strongly depends on initial microstructure and may not reflect unambiguously the prevailing deformation mechanism and strength as often assumed in field studies.

Rheology of Synthetic Omphacite Aggregates at High Pressure and High Temperature

Eclogite rheology, an essential control on the mechanics of subduction zones, is probably governed by the mechanical properties of the volumetrically dominant constituent, omphacite. Triaxial compression tests were performed in a solid-medium apparatus on synthetic aggregates with five compositions along the solid-solution series jadeite^diopside. The bulk water content of the synthetic material is of the order of 10 � 2 H/Si.The deformation experiments were performed at a confining pressure of 2·5 GPa and temperatures between 900 and 11008C (i.e. conditions in the stability field of jadeite). Strain rates ranged from 10 � 6 to 10 � 4 s � 1 . For a given composition, strength decreases with increasing temperature and decreasing strain rate. The relation between strength and composition changes with increasing temperature. Whereas jadeite is the weaker endmember at all explored conditions, intermediate compositions are stronger than either endmember at 9008C and 10008C. Samples with calcic compositions deformed at differential stresses above 500 MPa have abundant mechanical (100)[001] twins. In contrast, almost all deformation steps on jadeite aggregates yield strengths below 500 MPa and the resulting microstructures are dominated by well-developed subgrain boundaries and locally recrystallized grains. A weak crystallographic preferred orientation is developed in the deformed samples. Combining mechanical data and observed microstructures we hypothesize that the rate-controlling deformation mechanism differs for the two compositional endmembers. Deformation of diopside aggregates is controlled by dislocation glide, whereas the kinetics of dislocation climb controls the deformation of jadeite aggregates at the explored conditions. Reasonable parameters are found when fitting corresponding, micromechanically motivated flow laws to our data.The variation in strength with composition obeys scaling based on the dependence of melting temperature and shear modulus on composition. Extrapolation of the constrained flow laws to geological strain rates indicates considerable strength for omphacite aggregates at temperatures up to 10008C. Continuous deformation of eclogites by dislocation creep therefore appears unlikely in subduction zones. We suspect that exhumed eclogites exhibiting microstructures indicative of crystal plastic deformation experienced high-stress episodes related to seismic events.

First results on the LPO-derived seismic properties of iron ores from the Quadrilátero Ferrífero region, southeastern Brazil

Determinations of the LPO-derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy (hematite, magnetite and quartz) and deformed in different conditions. All the samples are seismically quasi-isotropic or weakly anisotropic, which reflect (i) an absent or weak crystallographic preferred orientation (CPO) in some samples; (ii) the high modal content of magnetite, (iii) the relatively weak anisotropy of elastic stiffness of hematite single crystal. Such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite. A plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite ± magnetite aggregates and in itabirites with strong CPO of hematites. In these high strained aggregates, some relationships between the crystallographic axes of hematite and the propagation velocities can be observed. The magnitudes of P and S-wave velocities derived of hematite CPO are lower than the values experimentally determined in iron ores. Such a difference probably reflects other microstructural variable which were not taken into account in the present contribution (e.g. shape preferred orientation).

Deformation and recrystallization mechanisms in mylonitic shear zones in naturally deformed extrusive Eocene–Oligocene rocksalt from Eyvanekey plateau and Garmsar hills (central Iran)

Microstructural processes in mylonitic shear zones from extrusions of Eocene–Oligocene rocksalts from Garmsar hills and Eyvanekey plateau (central Iran) are described and analyzed. The halite samples were studied by transmitted light microscopy of gamma-irradiated thin sections, subgrain size palaeopiezometry of polished and chemically etched samples and texture measurements by electron backscatter diffraction (EBSD). The studied shear zones are thin (<5 cm), subhorizontal and bedding parallel. The protomylonite comprises 2–6 mm sized grains, occasionally rich in primary fluid inclusions indicative of their primary non-recrystallized state. Abundant, well-developed subgrains suggest that the protomylonite was deformed mainly by dislocation processes. Elongated subgrains at grain edges point to recrystallization by fluid-assisted grain boundary migration. Recrystallized, strain-free grains are common. The material in the mylonitic zones is fine grained (∼0.6 mm) with strong shape-preferred orientation. Microstructures such as oriented fibrous overgrowths and growth banding (observed in gamma-irradiated sections) suggest that the principal deformation mechanism was solution-precipitation creep (non-conservative grain boundary migration and grain boundary sliding accommodated by solution-precipitation). Crystal fabrics measured by EBSD show a weak crystallographic preferred orientation that is consistent with solution-precipitation accommodated grain boundary sliding. Using published flow laws, the strain rate in the fine-grained mylonites is about 10 −10 l/s, which is in a good agreement with earlier in situ measurements on glacier salt flow rate.