Olivine Slip Systems Research Articles

Recent progress in the study of lattice-preferred orientation of olivine

Recent progress on the study of olivine LPO after (Karato et al., 2008) is reviewed with the emphasis on three issues: (i) LPO formed by the rotation of olivine crystals with anisotropic shape (euhedral crystals) in diffusion creep (Miyazaki et al., 2013), (ii) B-type LPO in the olivine + basaltic melt (Holtzman et al., 2003), and (iii) pressure change in the influence of LPO (Ohuchi and Irifune, 2013). Regarding the role of euhedral crystals, we show that euhedral olivine crystals occur in a mixture of forsterite and diopside (used by (Miyazaki et al., 2013)) but not in a mixture of olivine and enstatite. Consequently, the results by reported by (Miyazaki et al., 2013) are not applicable to the Earth’s upper mantle where olivine co-exists mostly with enstatite. Also we show that the LPO reported by (Miyazaki et al., 2013) is not consistent with the shape of olivine, and the observed LPO is likely due to dislocation glide (A-type fabric) under the conditions near the diffusion-dislocation creep regime boundary.Regarding the LPO of olivine with the presence of melt, (Qi et al., 2018)’s experimental study with the torsion geometry did not reproduce the B-type fabric reported by (Holtzman et al., 2003) indicating that the B-type fabric reported by (Holtzman et al., 2003) was indeed an artifact of the direct shear experiments. The weak LPO found by (Qi et al., 2018) (compared to that by (Zimmerman et al., 1999)) can be explained by the smaller grain size in their experiments. I conclude that a majority of the experimental results on olivine LPO at relatively low pressures (<2 GPa) can be understood based on the basics of deformation mechanism map and LPO caused by various slip systems in olivine. Regarding a claim by (Ohuchi and Irifune, 2013) that the A-type LPO (a-slip) dominates at high water content and c-slip dominates at low water content at pressures higher than ∼7 GPa, a compilation of experimental studies by (Masuti et al., 2019) and the observed LPO of the ultra-deep xenolith do not support their claim. However, experimental studies under these high-pressure conditions are limited and there remain large uncertainties regarding the LPO at high pressures (P>3 GPa).

On the glide of [100] dislocation and the origin of ‘pencil glide’ in Mg2SiO4 olivine

ABSTRACTOlivine with chemical composition (Mg,Fe)2SiO4 is a silicate which is supposed to strongly constrain the flow of the Earth’s upper mantle under thermal convection. Its mechanical properties are thus of primary importance. Slip systems in olivine involve two types of dislocations with [100] and [001] Burgers vectors. In this study, we report atomistic modelling of screw dislocations with [100] Burgers vector and their intrinsic properties. We show that the [100] screw dislocation core exhibits several configurations corresponding to spreading in different planes and different relative stabilities. At low pressure, we identify a clear tendency for core spreading in (010). At higher pressure, relevant for the Earth’s mantle, we show that pressure promotes a change in core configuration with spreading into equivalent {021} planes. Based on the systematic investigation of the minimum energy path between the different configurations, we show that the variability of core structures allows complex glide paths which has been described at the macroscopic level as ‘pencil glide’. Our results suggest that the pencil glide is more efficient at high pressure.

The Influence of Water on the Strength of Olivine Dislocation Slip Systems

AbstractThe nature of lattice‐preferred orientation (LPO) in olivine‐rich rocks strongly influences many important physical properties of Earth's upper mantle. Different LPO types have been observed to develop in deformation experiments on olivine‐rich rocks carried out at different water fugacity conditions. The development of the different LPO types has been attributed to dislocation slip systems in olivine having different sensitivities to water fugacity, but this hypothesis has not been directly tested. To measure the influence of water fugacity on the relative strengths of olivine dislocation slip systems, we carried out a series of deformation experiments on olivine single crystals under either anhydrous or hydrous conditions. The crystals were oriented to activate either the (010)[100], (001)[100], or (100)[001] dislocation slip systems using a direct shear geometry, which allows for isolation of single slip systems, in contrast to the multiple systems activated in experiments carried out in compression. Post‐deformation electron backscatter diffraction analyses reveal orientation gradients consistent with deformation occurring via the motion of dislocations on the activated slip systems. Crystals in all of the investigated orientations exhibit hydrolytic weakening, but crystals oriented to activate the (001)[100] slip system exhibit the largest degree of weakening. These results are consistent with a water‐induced change in LPO in olivine‐rich rocks deforming by dislocation creep. The rheological data obtained from the experiments can be used to improve models of LPO evolution in Earth's mantle, which is critical for imaging the structure of Earth's interior and predicting the movement of Earth's tectonic plates.

Crust-mantle coupling during continental convergence and break-up: Constraints from peridotite xenoliths from the Borborema Province, northeast Brazil

We studied a suite of mantle xenoliths carried by Cenozoic volcanism in the Borborema Province, NE Brazil. These xenoliths sample a subcontinental lithospheric mantle affected by multiple continental convergence and rifting events since the Archean. Equilibrium temperatures indicate a rather hot geotherm, implying a ca. 80 km thick lithosphere. Most xenoliths have coarse-granular and coarse-porphyroclastic microstructures, recording variable degrees of annealing following deformation. The high annealing degree and equilibrated pyroxene shapes in coarse-granular peridotites equilibrated at ~900 °C indicate that the last deformation event that affected these peridotites is several hundreds of Ma old. Coarse-porphyroclastic peridotites equilibrated at 950–1100 °C probably record younger (Cretaceous?) deformation in the deep lithospheric mantle. In addition, a few xenoliths show fine-porphyroclastic microstructures and equilibrium temperatures ≥1200 °C, which imply recent deformation, probably related to the dykes that fed the Cenozoic volcanism. Chemical and microstructural evidence for reactive percolation of melts is widespread. Variation in textural and chemical equilibrium among samples implies multiple melt percolation events well spaced in time (from Neoproterozoic or older to Cenozoic). Crystal preferred orientations of olivine and pyroxenes point to deformation controlled by dislocation creep with dominant activation of the [100](010) and [001]{0kl} slip systems in olivine and pyroxenes, respectively, for all microstructures. Comparison of xenoliths' seismic properties to SKS splitting data in the nearby RCBR station together with the equilibrated microstructures in the low-temperature xenoliths point to coupled crust-mantle deformation in the Neoproterozoic (Brasiliano) continental-scale shear zones, which is still preserved in the shallow lithospheric mantle. This implies limited reworking of the lithospheric mantle in response to extension during the opening of the Equatorial Atlantic in the Cretaceous, which in the present sampling is restricted to the base of the lithosphere.

Resolved shear stress analytical modelling: application to the determination of CRSS in olivine thanks to dislocation electron tomography

ABSTRACTCritical resolved shear stresses (CRSS) are generally obtained from uniaxial mechanical tests on oriented single crystals. However, slip systems are sometimes difficult to isolate with these mechanical tests. The dislocation radii of curvature (observed with transmission electron microscopy) let us estimate the resolved shear stresses but far from high lattice friction conditions, i.e. far from initial deformation conditions. A new analytical model has been developed to evaluate the resolved shear stresses with high lattice friction conditions. This model uses the parallel between beams flexure and dislocation physics in elastic anisotropy conditions, i.e. for most crystalline materials. Moreover with the dislocation transmission electron tomography technique, it is possible to orientate dislocations perfectly in order to obtain their precise geometry. Thanks to this tool, one can also access a sufficient amount of dislocation configurations and so of resolved shear stresses in order to get closer to CRSS. This model has been implemented to estimate the CRSS of the and slip systems in olivine.

Deformation history of Pinatubo peridotite xenoliths: constraints from microstructural observation and determination of olivine slip systems

The deformation history of the Pinatubo peridotite xenoliths was estimated on the basis of the microstructural observations and the determination of olivine slip systems. The latter was performed by using three methods: lattice-preferred orientation (LPO), crystallographic analysis of subgrain boundaries, and direct characterization of dislocations. The Pinatubo peridotites are composed of coarse olivine grains containing numerous fluid inclusions and some fine aggregates of orthopyroxene and amphibole grains, which implies intense fluid–rock interaction. The development of euhedral fine recrystallized olivine grains along the healed cracks within the coarse olivine grains suggests that the strain-free grains were nucleated and grew during static recovery. The LPO patterns and the analyses of subgrain boundaries indicate the activation of a [100]{0kl} slip system that developed under high temperature, low pressure, and dry deformation conditions. Although dislocations showing the [100]{0kl} slip system are dominantly observed, the other slip systems which could be formed by the deformation under moderate–high water content and lower-temperature conditions are also developed. The discrepancy between the results of dislocation characterization and the other two methods might have been caused by fulfilling the von Mises criterion or overprinting dislocation microstructures. Either way, the possible deformation history of the Pinatubo peridotites can be explained by the following scenario. The peridotites plastically moved from the back-arc to the fore-arc adjacent region, where CO2-rich saline fluid was trapped, by the corner flow of a mantle wedge. They were then annealed and metasomatized during entrapment of the upwelling magma.

Direct shear of olivine single crystals

Knowledge of the strengths of the individual dislocation slip systems in olivine is fundamental to understanding the flow behavior and the development of lattice-preferred orientation in olivine-rich rocks. The most direct measurements of the strengths of individual slip systems are from triaxial compression experiments on olivine single crystals. However, such experiments only allow for determination of flow laws for two of the four dominant slip systems in olivine. In order to measure the strengths of the (001)[100] and (100)[001] slip systems independently, we performed deformation experiments on single crystals of San Carlos olivine in a direct shear geometry. Experiments were carried out at temperatures of 1000 ° to 1300 °C, a confining pressure of 300 MPa, shear stresses of 60 to 334 MPa, and resultant shear strain rates of 7.4 × 10−6 to 2.1 × 10−3 s−1. At high-temperature (≥1200 °C) and low-stress (≤200 MPa) conditions, the strain rate of crystals oriented for direct shear on either the (001)[100] or the (100)[001] slip system follows a power law relationship with stress, whereas at lower temperatures and higher stresses, strain rate depends exponentially on stress. The flow laws derived from the mechanical data in this study are consistent with a transition from the operation of a climb-controlled dislocation mechanism during power-law creep to the operation of a glide-controlled dislocation mechanism during exponential creep. In the climb-controlled regime, crystals oriented for shear on the (001)[100] slip system are weaker than crystals orientated for shear on the (100)[001] slip system. In contrast, in the glide-controlled regime the opposite is observed. Extrapolation of flow laws determined for crystals sheared in orientations favorable for slip on these two slip systems to upper mantle conditions reveals that the (001)[100] slip system is weaker at temperatures and stresses that are typical of the asthenospheric mantle, whereas the (100)[001] slip system is weaker at conditions typical of the lithospheric mantle. These observations demonstrate that the relative strength of the dislocation slip systems in olivine and, thus, the development of lattice-preferred orientation and anisotropic viscosity in olivine-rich rocks are strongly dependent upon temperature and stress.

Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model

AbstractThe significant viscous anisotropy that results from crystallographic alignment (texture) of olivine grains in deformed upper mantle rocks strongly influences a large variety of geodynamic processes. Our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution. Unfortunately, existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths, making them less general than desired for many large‐scale geodynamic scenarios. Here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories. For the mechanical behavior, we explore two extreme scenarios, one in which each grain experiences the same stress tensor (Sachs model) and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate (pseudo‐Taylor model). For the textural evolution, we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation. Only recently has enough laboratory data on the deformation of olivine become available to calibrate these models. We use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models. These inversions demonstrate that the calibrated pseudo‐Taylor model best reproduces the mechanical observations. Additionally, the pseudo‐Taylor textural evolution model can reasonably reproduce the observed texture strength, shape, and orientation after large and complex deformations. A quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution. In addition, we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large‐strain torsion tests. This set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large‐scale numerical simulations.

Strength of orthoenstatite single crystals at mantle pressure and temperature and comparison with olivine

Oriented single crystals of orthopyroxenes (OPx) were deformed in axisymmetric compression in the D-DIA at pressure and temperature in excess of 3 GPa and 1040 °C. Two crystal orientations were tested with the compression axis parallel to either [101]c crystallographic direction, to investigate [001](100) dislocation slip-system strength, or [011]c direction to investigate [001](010) slip-system strength. These slip systems are the most active in orthopyroxenes. Applied differential stresses and specimen strain rates were measured in situ by synchrotron X-ray diffraction and radiography. We used these data and comparison with previously reported low-pressure flow laws for protoenstatite and orthoenstatite to determine the power law parameters for the deformation of orthoenstatite crystals, which characterize OPx dislocation slip-system strengths. Applying these laws at reasonable mantle stresses along oceanic and continental geotherms indicates that OPx [001](100) slip system is weaker than OPx [001](010) slip system to ∼260 km depth where the strengths converge. It also indicates that both OPx slip systems are significantly stronger than olivine slip systems throughout the upper mantle, except in the upper most mantle, in the lithosphere, were OPx [001](100) slip system may be as weak or even weaker than olivine [100](010) easy slip system.

Natural olivine crystal-fabrics in the western Pacific convergence region: A new method to identify fabric type

Crystallographic preferred orientations (CPOs) of olivine within natural peridotites are commonly depicted by pole figures for the [100], [010], and [001] axes, and they can be categorized into five well-known fabric types: A, B, C, D, and E. These fabric types can be related to olivine slip systems: A with (010)[100], B with (010)[001], C with (001)[001], D with {0kl}[100], and E with (001)[100]. In addition, an AG type is commonly found in nature, but its origin is controversial, and could involve several contributing factors such as complex slip systems, non-coaxial strain types, or the effects of melt during plastic flow. In this paper we present all of our olivine fabric database published previously as well as new data mostly from ocean floor, mainly for the convergent margin of the western Pacific region, and we introduce a new index named Fabric-Index Angle (FIA), which is related to the P-wave property of a single olivine crystal. The FIA can be used as an alternative to classifying the CPOs into the six fabric types, and it allows a set of CPOs to be expressed as a single angle in a range between −90° and 180°. The six olivine fabric types have unique values of FIA: 63° for A type, −28° for B type, 158° for C type, 90° for D type, 106° for E type, and 0° for AG type. We divided our olivine database into five tectonic groups: ophiolites, ridge peridotites, trench peridotites, peridotite xenoliths, and peridotites enclosed in high-pressure metamorphic rocks. Our results show that although our database is not yet large enough (except for trench peridotites) to define the characteristics of the five tectonic groups, the natural olivine fabrics vary in their range of FIA: 0° to 150° for the ophiolites, 40° to 80° for the ridge peridotites, −40° to 100° for the trench peridotites, 0° to 100° for the peridotite xenoliths, and −40° to 10° for the peridotites enclosed in high-pressure metamorphic rocks. The trench peridotites show a statistically unimodal distribution of FIA consisting of the high peak equivalent of the A type, but with some FIAs close to the AG and D types. The variations in the olivine fabrics in the trench peridotites could result from variations in deformation within the supra-subduction uppermost mantle, possibly related to evolution of the mantle since the subduction initiation of the Pacific plate.

Protracted fabric evolution in olivine: Implications for the relationship among strain, crystallographic fabric, and seismic anisotropy

Crystallographic fabrics in olivine-rich rocks provide critical information on conditions and mechanisms of deformation as well as seismic properties of Earthʼs upper mantle. Previous interpretations of fabrics produced in laboratory experiments were complicated by uncertainty as to whether the steady-state fabric was attained. To examine the systematics of the evolution of olivine crystallographic fabrics at high strain, we conducted torsion experiments on olivine aggregates to shear strains of up to ∼20. Our results demonstrate that a steady-state fabric is not reached until a shear strain >10, a much higher value than previously thought necessary. Fabrics characterized by girdles of [010] and [001] axes or by clusters of [010] and [001] axes are both observed. Until now, these fabrics were associated with either two different deformation mechanisms or two different sets of deformation conditions. Here we establish that both fabrics are, in fact, part of the same evolutionary process. An eigenvalue analysis allows the fabric shape to be quantitatively correlated with the magnitude of shear strain. Misorientation analysis suggests that the observed fabric evolution results from the competition of the two easiest slip systems in olivine, (010)[100] and (001)[100]. Our results open up the possibility of using olivine crystallographic fabrics or seismic anisotropy to quantitatively evaluate strain histories in both field studies and geophysical investigations of upper-mantle rocks.

Mantle deformation during rifting: Constraints from quantitative microstructural analysis of olivine from the East African Rift (Marsabit, Kenya)

Lherzolitic and harzburgitic peridotite xenoliths from the Marsabit volcanic field of the East-African Rift, Kenya, display a range of porphyroclastic to ultramylonitic textures that record intense deformation and strain localisation during decompression, cooling and exhumation of subcontinental mantle lithosphere during the early stages of continental rifting. Quantitative microstructural analysis by electron backscatter diffraction of both bulk fabrics and intragrain low-angle boundaries have been applied to these xenoliths to establish how deformation has evolved during this exhumation history. Bulk rock fabric analysis indicates the operation of (001)[100] (E-type) as the dominant slip system in the constituent olivines, with only one porphyroclastic xenolith recording the development of the classical (010)[100] (A-type) fabric. A weak E-type fabric is also present within the fine-grained matrix of the mylonitic and ultramylonitic xenoliths. Low-angle boundaries, preserved within individual olivine grains within the different xenoliths indicate the evolution of intra-grain slip systems from (010)[100] (A-slip) and (001)[100] (E-slip) in porphyroclastic peridotites, to dominant (001)[100] (E-slip) in the protomylonite and mylonite and dominant (100)[001] (C-slip) in the ultramylonite. Bulk fabric and intragrain low-angle boundary analyses are therefore concordant and indicate a systematic evolution of olivine slip systems from A-type to E-type to C-type during strain localisation associated with cooling during mantle exhumation. Both approaches confirm the dominant activity of the (001)[100] E-type slip system during rifting. However, the ultramylonites also provide the first evidence of [001] slip in such an environment. The activation of these two slip systems in olivine might be a useful indicator of an extensional rift margin setting in other peridotites.

Development of A-type olivine fabric in water-rich deep upper mantle

Water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation (i.e., fabric) in mantle rocks. The A-type olivine fabric is the most commonly observed olivine fabric in natural peridotites. Development of A-type olivine fabric (developed by the (010)[100] slip system) is known to be limited to the water-poor conditions of the shallow upper mantle (<200km depth). We have performed simple-shear deformation experiments of olivine at 7.2–11.1GPa and 1400–1770 K. Here we show that A-type olivine fabric was developed under water-rich conditions (>2130ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) was observed under moderately wet conditions (750–2130ppm H/Si). Developments of C-type (by the (100)[001] slip system) fabric was limited to water-poor conditions (<220ppm H/Si). We found that monotonic decrease in the seismic anisotropy VSH/VSV (the ratio of horizontally and vertically polarized shear waves) with depth in the global one-dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water-poor mantle. Only A-type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of VSH/VSV<1 in the deep upper mantle (>200km depth). A strong anomaly of VSH/VSV<1 observed in the deep upper mantle beneath the East Pacific Rise is well explained by the distribution of water-rich regions (in which A-type olivine fabric is dominantly developed) in the deep upper mantle and upwelling mantle flows.

Activities of olivine slip systems in the upper mantle

We investigated the effect of pressure (P) on olivine [100](001) and [001](100) dislocation slip systems by carrying out deformation experiments in the Deformation-DIA apparatus (D-DIA) on single crystals of Mg2SiO4 forsterite (Fo100) and San Carlos (SC) olivine (Fo89), at P ranging from 5.7 to 9.7GPa, temperature T=1473 and 1673K, differential stress σ in the range 140–1500MPa, and in water-poor conditions. Specimens were deformed in axisymmetry compression along the so-called [101]c crystallographic direction, which promotes the dual slip of [100] dislocations in (001) plane and [001] dislocations in (100) plane. Constant σ and specimen strain rates (ε˙) were monitored in situ by synchrotron X-ray diffraction and radiography, respectively. Comparison of the obtained high-P rheological data with room-P data, previously reported by Darot and Gueguen (1981) for Fo100 and Bai et al. (1991) for SC olivine, allowed quantifying the activation volume V* in classical creep power laws. We obtain V*=9.1±1.6 cm3/mol for Fo100. For SC olivine, we obtain V*=10.7±5.0cm3/mol taking into account the oxygen-fugacity uncertainty during the high-P runs. These results, combined with previous reports, provide complete sets of parameters for quantifying the activities of olivine dislocation slip systems. Extrapolation of the rheological laws obtained for SC olivine crystals to conditions representative of natural deformations show that [100](010) slip largely dominates deformation in the shallow upper mantle. At depths greater than ∼65km along a 20-Ma oceanic geotherm or ∼155km along a continental geotherm, the dual activity of [100](001) and [001](100) slips becomes comparable to that of [100](010) slip. At depths greater than ∼240km, [001](010) slip becomes dominant over all other investigated slip systems. Such changes in olivine dislocation-slips relative activity provide a straightforward explanation for the seismic anisotropy contrast and attenuation with depth observed in the Earth’s upper mantle.

Water content of the mantle xenoliths from Kimberley and implications for explaining textural variations in cratonic roots

AbstractWater contents estimates based on infrared spectroscopy and deformation microstructures are used to constrain the origin of textural variations observed in mantle xenoliths from Kimberley, South Africa. Infrared spectra indicate water contents of 10–370 ppm (H2O by weight) in olivine, 20–370 ppm in orthopyroxene, up to 340 ppm in garnet and 30–550 ppm in clinopyroxene, although no systematic differences are detected among texturally distinct xenoliths (i.e. granular and sheared peridotites). In contrast, active slip systems in olivine, as inferred from tilt boundaries, are different between the granular and sheared peridotites; the former deformed via the [100](010) slip system, which is commonly active under hot, dry conditions, whereas the latter deformed via the [001](100) slip system, which is commonly activated in water‐rich environments. The discrepancy in water contents estimates based on infrared spectroscopy and deformation microstructures may reflect different stages of the evolution of water content: the present water contents, as analysed by infrared spectroscopy, may have been modified by the most recent events (e.g. eruption by kimberlite magma), whereas the water contents inferred from tilt boundaries may have resulted from the various deformation processes in the deep cratonic mantle. Accordingly, we suggest that the observed heterogeneity in water content within cratonic roots facilitates local deformation, and gives rise to the range of textures seen in cratonic mantle xenoliths. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Deformation and Fluid-Rock Interaction in the Supra-subduction Mantle: Microstructures and Water Contents in Peridotite Xenoliths from the Avacha Volcano, Kamchatka

The mantle above a subducting slab is the site of complex interactions between deformation, partial melting, fluid migration and magma transport. To constrain these interactions and their effects on olivine deformation, we analyze microstructures, crystal preferred orientations, and water contents of peridotite xenoliths entrained by andesites of the Avacha volcano, southern Kamchatka arc. These xenoliths are refractory spinel harzburgites that have coarse-grained microstructures with widely spaced subgrain boundaries and sinuous grain boundaries in olivine, consistent with deformation by dislocation creep under low deviatoric stress (< 13 MPa) and with a significant contribution from diffusional processes. Analysis of crystal preferred orientations (CPO) indicates dominant activation of high-temperature, low-stress {0kl}[100] slip systems in olivine and of(100)[001] in orthopyroxene. In most samples, coarse opx crystals, elongated parallel to the lineation, enclose small olivine grains in crystallographic continuity with neighbouring crystals, indicating crystallization of orthopyroxene at the expense of olivine as a result of reactive percolation of Si-rich fluids coeval with the high-temperature deformation. Secondary crystallization of interstitial orthopyroxene led locally to development of opx-rich lenses parallel to the foliation, characterized by a decrease in olivine grain size and dispersion of the olivine CPO without changing the dominant slip systems. Half of the samples also show acicular orthopyroxene aggregates with which is associated a fine-grained matrix composed of rounded strain-free olivine, orthopyroxene, spinel, and rare amphibole crystals. This matrix occurs pervasively along grain boundaries or forms millimeter-scale irregular lenses and anastomosing veinlets that crosscut the coarse crystals and their ductile deformation structures. Both acicular orthopyroxene and the fine-grained matrix are interpreted as resulting from reactive transport of H2O-rich fluids under static conditions, probably in the lithospheric mantle. Infrared analyses show that olivine contains 1-8 center dot 6 ppm by weight of water. These low water contents are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at low pressure and dehydration during exhumation of the xenoliths. Water contents in orthopyroxene are highly variable (25-506 ppm H2O), probably recording spatially heterogeneous interaction with fluids or melts and compositional disequilibrium in the studied samples. Change in the dominant percolation mechanism from porous flow to fracturing suggests cooling, consistent with the low temperatures estimated from pyroxene thermometry (< 800-900 degrees C). The Avacha xenoliths therefore record pervasive deformation of a region of the mantle under asthenospheric conditions, followed by its accretion to the base of the lithosphere, probably as a result of cooling of the mantle wedge. Percolation of Si-rich fluids or hydrous melts is recorded at all stages; this probably enhanced diffusion and lowered deviatoric stresses during ductile deformation, but did not change the dominant slip direction in olivine from [100] to [001].

Determination of slip system in olivine based on crystallographic preferred orientation and subgrain-rotation axis: examples from Ichinomegata peridotite xenoliths, Oga peninsula, Akita prefecture

In this study of peridotite xenoliths from Ichinomegata volcano, Oga peninsula, NE Japan, we used electron backscattered diffraction to assess the validity of two methods of identifying the olivine slip system that operated during deformation. The xenoliths are harzburgite in composition and contain a distinct foliation and lineation defined by aligned spinel and pyroxene grains. Using the first method, the dominant slip system in olivine was estimated based on the crystallographic preferred orientation (CPO) of olivine with respect to the foliation and lineation. The second method was based on an analysis of subgrain rotation: given that intracrystalline subgrain boundaries were observed within several olivine grains, we measured the misorientation angles across these boundaries to define sets of slip planes, slip directions, and rotation axes. The olivine slip systems estimated using the two methods are essentially identical. The CPO pattern provides an important constraint on the dominant slip system that operated during deformation, whereas the analysis of subgrain rotation yields the slip system responsible for the development of individual subgrain boundaries composed of edge dislocations. The two methods are therefore complementary.

Upper mantle seismic anisotropy resulting from pressure-induced slip transition in olivine

Seismic anisotropy in the oceanic lithosphere results from flow-induced crystallographic preferred orientation of dry olivine during lithosphere creation. Recent experiments, however, showed that high water activity changes the flow mechanisms of olivine and hence the crystallographic preferred orientation, better explaining the seismic anisotropy in the mantle wedge above subduction zones. Whether changes in the crystallographic preferred orientation of olivine are unique to the effects of water has become controversial and is critical to resolve. Here we report low-stress, high-strain experiments on typical dry mantle rock at high pressures and temperatures, showing that at ∼3 GPa, pressure induces the same profound transition in olivine crystallographic preferred orientation that is produced by high water activity at lower pressure. One important consequence for global tectonics is that alignment of fast seismic waves parallel to trenches beneath subducting slabs probably reflects entrainment of sub-lithospheric mantle in the direction of subduction, rather than trench-parallel flow as interpreted at present. From the large variety of crystallographic preferred orientations in olivine in both experiments and natural rocks, we infer that in addition to the pressure-induced changes in olivine slip systems implied here, there are probably further changes in slip systems at higher pressure and temperature. Seismic anisotropy in Earth’s oceanic lithosphere and in the mantle wedge above subduction zones is associated with crystallographic preferred orientations of olivine. Experiments at high pressure and temperature suggest that a pressure of ∼3 GPa can induce the same changes in the crystal structure of olivine as high water activity at lower pressures.

Evolution of olivine lattice preferred orientation during simple shear in the mantle

Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M-indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models.

かんらん岩の構造敏感性と弾性的異方性

Peridotites derived from the uppermost mantle consist dominantly of olivine and subsequently of pyroxene, spinel, garnet, and plagioclase. Crystal-plastic flow of mantle rocks results in various types of structure within peridotite being developed to varying degrees, depending upon the structure sensitivity of the different mineral phases. Plastic deformation leads to the simultaneous development of shape-preferred orientations and crystal-preferred orientations. A shape-preferred orientation is the expression of the average orientation of flattening (foliation) and elongation (lineation) directions, as defined by the orientations of individual grains. A crystal-preferred orientation (CPO) is the expression of crystallographic orientations of grains within the rock, as developed via dislocation creep and recrystallization. During intense homogeneous plastic deformation of a peridotite composed of minerals with a dominant slip system, the preferred orientation of the slip plane and slip direction tends to coincide with the plane of plastic flow and the flow direction, respectively. Recently, a new olivine CPO classification (A, B, C, D, and E types) has been proposed by Karato and co-workers to illustrate the roles of stress and water content as controlling factors of olivine slip systems. An additional CPO type (AG) has also been proposed in recognition of its common occurrence in nature. Given that olivine and the other constituent minerals in peridotites contain intrinsic elastic anisotropies, the development of CPO within peridotite during plastic deformation gives rise to seismic anisotropy in the upper mantle. Thus, the anisotropic properties of mantle rocks derived from the upper 100 km of the mantle, such as Ichinomegata peridotite xenoliths from the northeast Japan arc, have been calculated and applied with the aim of understanding the seismic anisotropy of the Earth's mantle.