Dislocation-accommodated Grain Research Articles

Patched removal of the mantle lithosphere under orogens: A systematic numerical study

Delamination or convective thinning could cause large-scale and complete removal of the mantle lithosphere under orogens. However, geological and geophysical observations suggest that patched removal of the mantle lithosphere has occurred in some orogens, such as the northeastern Tibetan Plateau, the central Tianshan, and the central Andes. Dislocation-creep-induced strain localization cannot promote effective removal of the mantle lithosphere to the Moho on a small-scale. Recent rheological studies propose that dislocation-accommodated grain boundary sliding (DisGBS) may dominate upper mantle deformation. DisGBS could make the lower lithospheric mantle rheologically weaker than dry olivine. With 2-D high-resolution thermo-mechanical modeling, we systematically investigated the conditions for the initiation of small-scale lithospheric thinning under orogens and explored the minimum range of removal of the mantle lithosphere. The numerical results indicate that classic convective drip cannot effectively thin the mantle lithosphere to the Moho on a small-scale. In contrast, small-scale thinning can be induced by lithospheric heterogeneity with DisGBS and plasticity. The rheological heterogeneity can be verified by magmatism and metasomatism under the central Andes and orogens between terranes under the northeastern Tibetan Plateau or in Tianshan.

Grain-size-sensitive creep of olivine induced by oxidation of olivine in the Earth's deep upper mantle: Implications for weakening of the subduction interface

Creep behavior of olivine has been studied at pressures of 9–16 GPa and temperatures of 960–1450 K, which is equivalent to the conditions in/around the slab subducted into the deep upper mantle, using a combination of synchrotron radiation and a deformation-DIA apparatus. At pressures above 12 GPa, the creep strength of olivine was primarily controlled by Peierls creep, and nucleation of wadsleyite caused softening by a contribution of grain-size-sensitive creep to deformation. Similarly, oxidation of deforming olivine caused a significant softening due to another grain-size-sensitive creep presumed to be dislocation-accommodated grain boundary sliding (dislGBS) of olivine with a low activation enthalpy (~260 kJ/mol) at pressures of 9–12 GPa. Extrapolation of the obtained flow law of dislGBS to mantle conditions predicts a formation of a weak layer along the slab-asthenosphere interface in the deep upper mantle. The viscosity contrast between the weak layer and the surrounding asthenosphere is estimated to be up to 3 orders of magnitude, resulting in a partial slab-asthenosphere decoupling in the deeper part of the subduction zones. The weak layer would allow for the occurrence of trench-parallel flow above/beneath the slab.

Crystallographic preferred orientations of plagioclase via grain boundary sliding in a lower-crustal anorthositic ultramylonite

To clarify the rheological properties and deformational mechanisms that operate within lower-crustal shear zones, we analyzed deformation microstructures and fabrics in plagioclase grains from high-strain regions of intrafolial drag folds of an anorthositic ultramylonite from Langoya, Vesteralen, northern Norway. These grains have developed weak crystallographic preferred orientations (CPOs). Other than deformation twinning, they do not show any indications of intracrystalline plasticity or dynamic recrystallization. In some domains of the drag fold, plagioclase CPOs are more distinct, but the majority of inferred slip systems are not consistent with the main slip systems in plagioclase. These suggest that neither dislocation creep nor dislocation-accommodated grain boundary sliding (GBS) produced the plagioclase CPOs but rather grain rotation via GBS along specific grain boundaries parallel or subparallel to crystallographic planes of (010), (100) or (110), which led to an alignment of the easy-slip grain boundaries in the flow direction that produced the observed CPO patterns. The anisotropy of dissolution and growth rates may have been responsible for the development of the specific grain boundaries.

Flow laws and fabric transitions in wet quartzite

We investigated quartzite flow laws based on data from 21 different experimental studies on samples deformed with added water at temperatures from 700 to 1200°C. Two clear trends emerge: one with a power-law stress exponent of n=4±0.3 and the other, at higher stresses and lower temperatures, with a stress exponent of n=2.7±0.3. The change in stress exponent indicates a transition in the rate-limiting process of dislocation creep. The deformation conditions where the change in n occurs also correlate with conditions where a distinctive change in the c-axis fabric is observed for samples deformed in general shear. At low stresses, quartz fabrics are defined by a Y-max (where c-axes align parallel to the Y direction of the strain ellipse), suggesting easy slip on the prism <a> system. At higher stresses quartz fabrics indicate easy slip on the basal <a> system. The correlation between the stress exponents and c-axis fabrics suggests that the flow law transition results from a concomitant switch in the dominant slip system and the slip system that rate-limits deformation. Alternatively, recent studies on both experimental and natural samples provide evidence for dislocation-accommodated grain boundary sliding (disGBS) in quartz aggregates; microstructures and mechanical data acquired on samples deformed at conditions near the transition between the n=4 and n=2.7 flows laws are indicative of this process, providing evidence for a rheological model similar to that proposed for ice and olivine. Extrapolation of these newly constrained flow laws delineates estimated deformation conditions where the Y-max and basal <a> fabrics are preserved in the crust. These results provide further support for the application of quartzite flow laws to investigate the rheology of the continental crust and motivate more analyses on the grain size dependence of creep in quartz aggregates.

Laboratory investigation of mechanisms for phase mixing in olivine + ferropericlase aggregates.

To investigate the role of grain boundary pinning and the mechanisms by which phase mixing occurs during deformation of polymineralic rocks, we conducted high-strain torsion experiments on samples consisting of olivine plus 30 vol% ferropericlase. Experiments were performed in a gas-medium deformation apparatus at 1524 K and 300 MPa. Samples were deformed to outer radius shear strains of up to γ(R) = 14.1. The value of the stress exponent and the small grain sizes of our samples indicate that our two-phase material deformed by dislocation-accommodated grain boundary sliding. In samples deformed to 1 < γ < 7, elongated clusters of ferropericlase grains form thin layers in the olivine matrix, and small grains of ferropericlase appear at olivine grain boundaries and three- and four-grain junctions. By γ ≈ 14, a well-distributed mixture of small ferropericlase grains among the olivine grains developed. Microstructures exhibit similarities to both mechanical and chemical models proposed to describe the processes leading to phase mixing. Our results provide evidence for grain size reduction during phase mixing that results in a grain size significantly smaller than the value predicted by the single-phase recrystallization piezometer for olivine. Thus, phase mixing provides a mechanism for the persistent weakening of rocks that is important for developing and maintaining shear zones necessary for plate tectonics.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

A cavitation-seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi-brittle field (Vivero fault, NW Spain)

Bands of multi-phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions. Indeed, they are one of the most typical manifestations of strain localization in high-strained polymineralic rocks in the lithosphere, especially in the semi-brittle field. Yet many questions remain on their origin, and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove, particularly for the inter-grain mixing of phases. Here we present a case study of ultramylonite bands developed in a coarse-grained granite deformed at conditions near the base of the seismogenic zone, i.e. showing coseismic deformation microstructures overprinted by thermally activated ones. We found that in the mylonite to ultramylonite transition, two deformation mechanisms coexist promoting inter-grain phase mixing through Kfs precipitation: (1) dislocation-accommodated grain boundary sliding in quartz and (2) diffusion-accommodated grain boundary sliding in feldspar. The model proposed to explain grain mixing is based on a cavitation-seal mechanism, which is strongly dependent on grain boundary sliding for the opening of transient micro-cavities and on the diffusivity of potassium feldspar for their progressive sealing.

Flow behavior and microstructures of hydrous olivine aggregates at upper mantle pressures and temperatures

Deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation-DIA apparatus combined with synchrotron in situ X-ray observations at pressures of 1.5–9.8 GPa, temperatures of 1223–1800 K, and strain rates ranging from 0.8 × 10−5 to 7.5 × 10−5 s−1. The pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 ± 6 cm3 mol−1 and a stress exponent of 3.2 ± 0.6 at temperatures of 1323–1423 K. A comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323–1423 K is much weaker than that for the dislocation creep of water-saturated olivine and is similar to that for diffusional creep and dislocation-accommodated grain boundary sliding, while dislocation microstructures showing the [001] slip or the [001](100) slip system were developed. At temperatures of 1633–1800 K, a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 ± 7 cm3 mol−1 assuming a stress exponent of 3.5, water fugacity exponent of 1.2, and activation energy of 520 kJ mol−1 (i.e., power-law dislocation creep of hydrous olivine). Because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures, water weakening of olivine could be effective in the deeper and colder part of Earth’s upper mantle.

Kinetic processes for plastic deformation of olivine in the Poyi ultramafic intrusion, NW China: Insights from the textural analysis of a ~ 1700 m fully cored succession

The ubiquitous presence of undulose extinction and subgrain boundaries in olivine crystals is commonly perceived as originating in the mantle, however these plastic deformation features are also well developed in the Poyi ultramafic intrusion, NW China. In this case, olivine was deformed through kinetic processes in a crustal magma chamber, rather than by deformation processes in the upper mantle. Moreover, accumulation and textural coarsening were critical to the characteristics of crystal size distributions (CSDs) of olivines in the Poyi intrusion. The axial deformational compaction of crystal mush was revealed by virtue of other quantitative textural analyses (e.g., spatial distribution patter, alignment factor and aspect ratio). Additionally, based on the contrast of density between crystal matrix and interstitial melt, adequate stress was generated by the km-scale crystal framework in Poyi body (~2–11MPa) which triggered the distortion of grain-lattice in olivine. The deformation mechanisms of olivine primarily are dislocation creep and dislocation-accommodated grain boundary sliding (DisGBS), while diffusion creep is subsidiary. This study has revealed various kinetic processes in a magmatic system by first demonstrating the genetic relationship between mineral deformation and axial compaction of crystal mush while highlighting the uncertainty of employing the deformation features of olivine in peridotite xenoliths as an indicator for a mantle origin. In contrast to the olivine populations of xenocrysts that underwent fragmentation during ascent, the deformed primitive olivines in compaction exhibit a distinct shortage of small grains, which is conducive to delimiting these two types of deformed grains.

Microstructural evidence for the transition from dislocation creep to dislocation-accommodated grain boundary sliding in naturally deformed plagioclase

We use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction (EBSD) data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite. The sample is from lower oceanic crust exposed near the Southwest Indian Ridge, and it has a high ratio of recrystallized matrix grains to porphyroclasts. Microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization; recrystallized grain (average diameter ∼8 μm) microstructures indicate that subsequent grain boundary sliding (GBS) was active in the continued deformation of the recrystallized matrix. The recrystallized matrix shows four-grain junctions, randomized misorientation axes, and a shift towards higher angles for neighbor-pair misorientations, all indicative of GBS. The matrix grains also exhibit a shape preferred orientation, a weak lattice preferred orientation consistent with slip on multiple slip systems, and intragrain microstructures indicative of dislocation movement. The combination of these microstructures suggest deformation by dislocation-accommodated GBS (DisGBS). Strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive GBS, and sustained by the maintenance of a small grain size during superplasticity.

Recrystallisation, phase mixing and strain localisation in peridotite during rapid extrusion of sub-arc mantle lithosphere

The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200–700 MPa and strain rate ∼10−15 s−1, probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730–770 °C, stress ∼52–700 MPa and strain rate ∼10−15 s−1. The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69–137 MPa and strain rate ∼10−15 s−1, and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a “creep cavitation” process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate, with strain distributed across the entire section. This widespread deformation caused rapid exhumation of the peridotite from the lithospheric mantle into the overlying arc crust. The massif therefore records multiple overprinting phases of deformation under mantle and crustal conditions associated with the rapid exhumation of a large orogenic peridotite.

Dislocation-accommodated grain boundary sliding: the major deformation mechanism of olivine in the Earth's upper mantle

Dislocation-accommodated grain boundary sliding: the major deformation mechanism of olivine in the Earth's upper mantle

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle.

Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

The origin and geophysical implications of a weak C-type olivine fabric in the Xugou ultrahigh pressure garnet peridotite

We report here a comprehensive investigation of mineral chemistry, water content and deformation features (including the crystallographic preferred orientation and dislocation substructures of olivine) of the Xugou garnet peridotite from the southern Sulu ultrahigh pressure (UHP) terrane. Our results suggest: (1) the Xugou garnet peridotite was equilibrated and deformed at high pressure (4.8–6.6 GPa) and relatively low temperature (810–930 °C) conditions. (2) In contrast to the pronounced shape-preferred orientations of olivine, pyroxene and garnet, electron backscattered diffraction (EBSD) measurements show a weak C-type fabric in olivine, a strong (100)[001] fabric in clinopyroxene and orthopyroxene and a nearly random fabric in garnet, respectively. The deformation mechanism map suggests a dominant deformation mechanism of dislocation-accommodated grain boundary sliding (DisGBS) in olivine. (3) The trivial amount of water in olivine (15±10 ppm), orthopyroxene (21±17 ppm), clinopyroxene (64±48 ppm) and garnet (dry) correspond to a water-poor deformation condition. We proposed that the C-type olivine fabric was generated by a dominant DisGBS with some contribution from diffusion creep at elevated pressures. Calculated seismic properties of the peridotite show the fastest P-wave velocity and the fast polarization direction of S-waves are oriented normal to the foliation plane and perpendicular to the lineation in the foliation plane, indicating that the C-type olivine fabric could also be a candidate to explain the observed strike-parallel faster S-wave polarization near trenches and in collisional orogenic belts.

Low oxygen fugacity dependency for the deformation of partially molten lherzolite

We present the results of an experimental investigation of the influence of oxygen fugacity on the deformation of a partially molten spinel lherzolite using a 0.1MPa gas-media creep rig under temperatures of 1160–1190°C, stresses of 4–74MPa and well-controlled oxygen fugacities of 10−7 to 10−11MPa. The partially molten spinel lherzolite was deformed by a dislocation-mediated creep process such as dislocation creep or dislocation-accommodated grain boundary sliding (DGBS) (n=3.5±0.3) as well as in the diffusion creep regime (n=1.2±0.2). The average oxygen fugacity exponent of lherzolite is 0.04±0.02, which is significantly smaller than the values measured for olivine single crystals (0.10–0.36) and dunite rocks (0.20) deformed by dislocation creep. We attribute the low oxygen fugacity exponent of partially molten lherzolite samples either to the operation of grain boundary sliding, if DGBS dominates the deformation, or to the presence of pyroxenes whose deformation has a weak or no dependency on oxygen fugacity, if dislocation creep dominates the deformation. In the latter case, the oxygen fugacity exponent decreases rapidly with decreasing volume fraction of olivine. The low oxygen fugacity exponent for our partially molten lherzolite samples implies that dislocation creep or DGBS of mantle peridotite will most likely be insensitive to variations of oxygen fugacity in the upper mantle.

Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation

[1] We performed triaxial compressive creep experiments on aggregates of San Carlos olivine to develop a flow law and to examine microstructural development in the dislocation-accommodated grain boundary sliding regime (GBS). Each experiment included load and temperature steps to determine both the stress exponent and the activation energy. Grain boundary maps, created with electron backscatter diffraction data, were used to quantify grain size distributions for each sample. Inversion of the resulting data produced the following flow law for GBS: GBS = 104.8 ± 0.8 (σ2.9 ± 0.3/d0.7 ± 0.1) exp[(−445 ± 20 kJ mol−1)/RT], with σ, d, and GBS in units of MPa, μm, and s−1, respectively. Although relatively weak, crystallographic-preferred orientations (CPOs) have [010] maxima parallel to the compression direction along with [100] and [001] girdles perpendicular to the compression direction. CPOs and subgrain boundary misorientation axes suggest that the (010)[100] slip system contributes significantly to deformation. We propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations. Extrapolation of our flow laws to mantle conditions suggests that GBS is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow, and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep.

The initiation of strain localisation in plagioclase-rich rocks: Insights from detailed microstructural analyses

In order to shed light on the cause for onset of strain localisation in plagioclase-rich rocks we have performed detailed microstructural analyses on a sheared anorthosite–leucogabbro using optical microscopy, electron backscatter diffraction (EBSD) and chemical analyses. The analysed sample is from an Archaean unit, SW Greenland, deformed at lower to mid crustal conditions ( T = 675–700 °C and moderate pressure). The initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation. Recrystallised plagioclase grains (average size 80 μm) are dominantly found in (i) clusters, (ii) lenses and (iii) continuous bands subparallel to shear zone boundaries. Recrystallised grains in clusters and lenses display inherited crystallographic orientations. Their bulk crystallographic preferred orientation (CPO) is random; however, crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems. Recrystallised grains in continuous bands show a CPO with a single dominant active slip system, (001)<110>, aligned with the structural (XYZ) framework. For these parent and daughter grains, misorientation axes are random and the dominant slip system is different. Grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast. We infer that the cause for strain localisation is recrystallisation and development of a CPO in continuous recrystallised bands. Microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation-accommodated grain boundary sliding (DisGBS) in continuous bands. This inferred shift in dominant deformation mechanism would lower the strength of the shear zone.

Effect of iron content on the creep behavior of olivine: 1. Anhydrous conditions

High-temperature, high-pressure triaxial compressive creep experiments were carried out on polycrystalline samples of olivine, (Fe x Mg 1 − x ) 2SiO 4, with x ≡ Fe/(Fe + Mg) = 1.0, 0.75, 0.5, and 0.3 to investigate the influence of Fe content on rheological properties of olivine under anhydrous conditions. Samples with grain sizes of < 50 μm were deformed at differential stresses of 50 to 300 MPa primarily in the dislocation-accommodated grain boundary sliding creep regime for temperatures between 1273 and 1473 K with a confining pressure of 300 MPa. Our results, combined with published results on aggregates of Fa 10, demonstrate that samples of Fa 30, Fa 50, Fa 75, and Fa 100 creep ~ 15, ~ 75, ~ 365, and ~ 1480 times faster than samples of Fa 10, respectively, under the same conditions. Our experimental results can be described by the flow law ε ˙ = ε ˙ gbs 0 ( σ μ ) n gbs ( b d ) 2 X Fa p exp ( - Q gbs o + α X Fa R T ) with ε ˙ gbs 0 = ( 6.2 ± 5.2 ) × 10 5 s - 1 , n gbs = 3.9 , p = 1.5 ± 0.1, Q gbs 0 = 440 ± 10 kJ/mol, and α = − 45 ± 4 kJ/mol. Based on measured dependence of strain rate on iron concentration, we conclude that, at the same thermodynamic conditions, the Martian mantle, which is more rich in Fe than Earth's mantle, will be a factor of ~ 10 less viscous than Earth's mantle.

Dynamic recrystallization and strain softening of olivine aggregates in the laboratory and the lithosphere

AbstractThe effects of dynamic recrystallization on the deformation mechanisms and rheology of olivine aggregates in the laboratory and the lithosphere are reviewed in this paper. The low-strain rheology of olivine is well documented; however, deformation in the lithosphere often involves large strains. Large strain experiments show that recrystallization can result in both hardening and softening during deformation. Moderate strain softening in experimental shear and torsion can be explained by the operation of dislocation-accommodated grain boundary sliding in bands of fine recrystallized grains.Data on the temperature dependence of recrystallized grain size are needed to extrapolate the effects of dynamic recrystallization to the lithosphere. Theories of dynamic recrystallization suggest that grain size is strongly stress dependent and moderately temperature dependent. A re-analysis of experimental grain size data indicates that the recrystallized grain size is temperature independent for olivine aggregates with low water content (&lt;300 ppm H/Si).Rheological regime maps have been constructed for the lithospheric mantle. The maps suggest that grain size sensitive power law creep, involving both grain boundary sliding and dislocation creep, will produce strong strain softening, greater than found so far in experimental studies, in dry and wet lithosphere shear zones.