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1.
Bridgmanite (Mg,Fe)SiO3 and ferropericlase (Mg,Fe)O are the most abundant phases in the lower mantle and localized regions of the D″ layer just above the core mantle boundary. Seismic anisotropy is observed near subduction zones at the top of the lower mantle and in the D″ region. One source of anisotropy is dislocation glide and associated texture (crystallographic preferred orientation) development. Thus, in order to interpret seismic anisotropy, it is important to understand texture development and slip system activities in bridgmanite and bridgmanite + ferropericlase aggregates. Here we report on in situ texture development in bridgmanite and bridgmanite + ferropericlase aggregates deformed in the diamond anvil cell up to 61 GPa. When bridgmanite is synthesized from enstatite, it exhibits a strong (4.2 m.r.d.) 001 transformation texture due to a structural relationship with the precursor enstatite phase. When bridgmanite + ferropericlase are synthesized from olivine or ringwoodite, bridgmanite exhibits a relatively weak 100 transformation texture (1.2 and 1.6 m.r.d., respectively). This is likely due to minimization of elastic strain energy as a result of Young’s modulus anisotropy. In bridgmanite, 001 deformation textures are observed at pressures <55 GPa. The 001 texture is likely due to slip on (001) planes in the [100], [010] and \(\left\langle {110} \right\rangle\) directions. Stress relaxation by laser annealing to 1500–1600 K does not result in a change in this texture type. However, at pressures >55 GPa a change in texture to a 100 maximum is observed, consistent with slip on the (100) plane. Ferropericlase, when deformed with bridgmanite, does not develop a coherent texture. This is likely due to strain heterogeneity within the softer ferropericlase grains. Thus, it is plausible that ferropericlase is not a significant source of anisotropy in the lower mantle.  相似文献   

2.
This paper reports the results of optical and electron microscopic investigations of mantle olivine samples with H2O contents of tens-hundreds ppm weight. Samples were obtained from the xenoliths and xenocrysts of the Udachnaya pipe. At the scale of optical microscope magnification, a peculiar banded microstructure was observed in thin sections prepared parallel to the olivine (010) plane. It is formed by cross-hatched bands parallel to four crystallographic directions of the olivine structure: [100], [001], [101], and [−101]. At the scale of electron optical magnifications, the banded microstructure is observed as nanometer-sized heterogeneities of various types which are related to olivine deformation: (a) planar defects parallel to (100) and (001) corresponding to the (100)[010] and (001)[100] dislocation glide systems, respectively; they are occasionally transformed into lamellae or decorated by nanoinclusions; and (b) nanometer-sized heterogeneities formed by nanoinclusion arrays not related to planar defects and oriented along the same directions of the olivine structure as the optically visible bands. The deformation structures are decorated by coupled point OH-bearing defects, which were initially present in the olivine. The crystallographically oriented arrays of nanoinclusions of high-pressure hydrous silicates are considered as a result of olivine deprotonization (elimination of OH-bearing defects from the olivine structure) in the zones of previous deformation compression in the crystal. Light refraction effects on the nanoinclusions make these zones optically visible and produce the banded microstructure, which is a consequence of previous deformation.  相似文献   

3.
Automated electron backscattered diffraction (EBSD) was applied using a scanning electron microscope to obtain lattice preferred orientation (LPO) data for olivine in garnet peridotites of the Central Alps. As a reference frame, the LPOs of enstatite were also investigated. In the garnet peridotite at Cima di Gagnone (CDG), a weak foliation carrying a distinct lineation is present. The lineation is characterized by elongated enstatite, olivine and poikiloblastic garnet. Olivine shows a very unusual LPO with [100] normal to foliation and [001] parallel to lineation. Achsenverteilungsanalyse (AVA) maps demonstrate that [001] of olivine grains corresponds quite well to their maximum length axes which are preferentially parallel to the lineation. Numerous planar hydrous defects within (001) planes of olivine are marked by palisades of ilmenite rods and show a preferred orientation normal to lineation. Calculated P-wave velocities for CDG are fastest (8.32 km sу) normal to foliation with a relatively low anisotropy (2.9%). Compared to mantle peridotites with the usual (010)[100] LPO where the fastest Vp direction is towards the lineation, the relationship between flow geometry and seismic anisotropy is significantly different at CDG. Several mechanisms for the formation of the LPO type at CDG are considered, with glide possible on (100)[001] of olivine. On the basis of field data as well as petrographic and petrologic evidence, it has been demonstrated that the CDG garnet peridotite formed by prograde metamorphism from a hydrous protolith at pressures and temperatures of about 3.0 GPa and 750 °C, respectively. The CDG LPO is interpreted to have formed during hydrous subduction zone metamorphism. The same interpretation may hold for the previously investigated olivine LPO at Alpe Arami, which is similar to that at the nearby CDG. The observed anomalous LPO is no proof for ultradeep (>3.0 GPa) conditions.  相似文献   

4.
The microstructure and texture in cordierites of a moldanubian gneiss from the Bohemian Massif has been analysed by transmission electron microscopy (TEM) and universal stage in order to get information on the deformation mechanisms and textural development of this rock-forming mineral. Deformation may have taken place at temperatures between about 500° C and 630° C and pressures smaller than about 3 kb. The elongated cordierite xenoblasts show a typical dislocation creep microstructure consisting of subgrain boundaries and free dislocations. The dislocations have [001], [010] and 1/2<110> Burgers vectors. [001] dislocations often have pure screw and edge character the latter type being climb-dissociated on (001). Among the dislocations reactions are common. The main subgrain boundaries observed are (010)[001], {110}[001] and (001)[010] tilt boundaries. Burgers vectors and dislocation line directions reveal (100)[001], (010)[001], (100)[010], {110} 1/2<110> and (001)1/2<110> as activated slip systems. The crystallographic preferred orientation (here referred to as texture) consists of a [001] maximum in the foliation parallel to the mineral lineation. [100] and [010] maxima are perpendicular to it within and normal to the foliation, respectively, with a girdle tendency normal to the lineation. The texture may be explained by simple shear deformation on the {hkO}[001] slip systems with preference of (010)[001].  相似文献   

5.
Lattice-preferred orientations (LPO) of olivine, diopside, enstatite and garnet from the Zhimafang garnet peridotite body in the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China) were measured using the electron backscatter diffraction (EBSD) technique. The peridotite was captured from a mantle wedge immediately adjacent the subducted Yangtze slab and then experienced the UHP metamorphism at 750–950 °C and 4–7 GPa. The olivine LPO is characterized by the [001] axis close to the stretching lineation and the (100) plane subparallel to the foliation, indicating the prevailing of (100) [001] slip. Enstatite LPO displays the dominance of (100) [001] slip. Diopside developed complex LPO patterns that are difficult to explain using a single slip system of (100) [001]. Garnet is almost randomly oriented due to its low volume fractions, cubic symmetry and the presence of numerous slip systems. Calculated seismic properties of the peridotite yield a maximum P-wave velocity normal to the foliation and a minimum along the foliation, with anisotropy up to 8% in strongly sheared samples. The S-wave velocity pattern is complex but the fast polarization plane generally normal to the foliation. The inferred shear sense from the olivine LPO is top-to-SE, in contrary to exhumation-induced top-to-NW thrusting recorded in the quartz LPO, implying that the olivine LPO formed at early UHP metamorphic conditions. The olivine crystals have relatively low water contents (141–475 H/106 Si), indicating a fluid-deficient environment for the LPO formation. The present study suggests that a combination of low temperature and UHP plays a much more important role than the water content to promote the activation of (100) [001] slip in olivine.  相似文献   

6.
A San Carlos olivine polycrystal has been deformed under uppermost mantle conditions, by compression at 900 °C, at a strain rate of 1.1 × 10?5 s?1, under a confining pressure of 300 MPa, using the Paterson press. Transmission electron tomography of dislocations has been performed by scanning transmission electron microscopy, by conventional transmission electron microscopy using the weak-beam dark-field technique, associated with precession or not, in order to determine the glide planes of [001] screw dislocations. This recent technique is the most suitable one since most [001] dislocations exhibit straight screw segments due to the high lattice friction on this character at low temperature. We find that [001] dislocations glide in (100), (010) and {110} as already reported, but also more unexpectedly in {120} and {130}. We show that at 900 °C, [001] {110} glide is dominant in polycrystals. We have, however, noted and characterized numerous cross-slip events in the specimen.  相似文献   

7.
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.  相似文献   

8.
Transmission electron microscopy (TEM) has been used to investigate deformation microstructures of synthetic stishovite specimens deformed at 14 GPa, 1,300°C. Geometrical characteristics of numerous dislocations have been characterized by dislocation contrast and stereographic analyses in order to identify the easy slip systems of stishovite. TEM data allowed us to characterize the following slip systems: 〈100〉{001}, 〈100〉{010}, 〈100〉{021}, [001]{100}, [001]{110}, [001]{210} and Observation of sub-grain boundaries and scalloped edge dislocations suggest that climb has been activated in the specimens.  相似文献   

9.
崔军文 《地球学报》1987,9(3):89-101
随着深部构造研究的不断深入,岩石流变作用日益受到人们重视,普遍认为塑性流变是地壳深部构造形成的主要机制。超基性岩的流变作用目前已成为研究上地幔流变、岩石圈板块动力学和热对流,甚至震源机制的重要内容。 本文拟通过对构成雅鲁藏布江蛇绿岩套底部的藏南超基性岩的研究,重点划分塑性流变的结构类型,探讨不同结构类型中橄榄石的组构特征及实际存在的滑动系,用不同方法计算超基性岩形成和侵位过程中的流动应力值,并认为由于橄榄石塑性流变而获得的晶格方位排列是造成地震波速度不连续性的主因,岩石的塑性流动有可能是深部能量释放和诱发  相似文献   

10.
To understand the deformation mechanism and seismic anisotropy in the uppermost mantle beneath Spitsbergen, Svalbard, in the Arctic, the deformation microstructures of olivine in the peridotite of Spitsbergen were studied. Seismic anisotropy in the upper mantle can be explained mainly by the lattice-preferred orientation (LPO) of olivine. The LPOs of the olivine in the peridotites were determined using electron backscattered diffraction patterns. Eight specimens out of 10 showed that the [100] axis of the olivine was aligned subparallel to the lineation and that the (010) plane was subparallel to the foliation, showing a type A LPO. In the other two specimens the [100] axis of olivine was aligned subparallel to the lineation and both the [010] and [001] axes were distributed in a girdle nearly perpendicular to the lineation, showing a type D LPO. The dislocation density of the olivine in the samples showing a type D LPO was higher than that in the samples showing a type A LPO. The result of an Fourier transformation infrared study showed that both the types A and D samples were dry. These observations were in good agreement with a previous experimental study ( Tectonophysics , 421 , 2006, 1 ): samples showing a type D LPO for olivine were observed at a high stress condition and samples showing both types A and D LPO were deformed under dry condition. Observations of both strong LPOs and dislocations of olivine indicate that the peridotites studied were deformed by dislocation creep. The seismic anisotropy calculated from the LPOs of the olivine could be used to explain the seismic anisotropy of P - and S -waves in the lithospheric mantle beneath Spitsbergen, Svalbard.  相似文献   

11.
Cordierite — (Mg,Fe)2Al4Si5O18 — occurs as porphyroclasts within metapelitic and metavolcanic rocks from the Kemiö-Orijärvi belt, SW Finland. After crystallisation the cordierites have been deformed at temperatures between 550–825° C and pressures of 3–5 kbar. Optical microscopy reveals the following deformation-induced microstructures: a bimodal size distribution between host, 0.3 to 4.0 mm, and recrystallised (new) grains, 0.1 to 0.5 mm; the intracrystalline defect-structures of host grains yield undulatory extinction, subgrains and some twinning. Recrystallised grains are optically strain free. Grain and subgrain boundaries are generally straight and parallel to crystallographic low-index planes. Orientation distribution diagrams for host and recrystallised grains yield similar fabric diagrams, i.e. [010] perpendicular to foliation -S-, [001] and [100] parallel to S and [001] parallel to lineation -L-. The fabric diagrams indicate that [001] (010) is the dominant slip system. Transmission electron microscopy reveals straight free dislocations, glide and climb loops, minor {130} and {110} microtwins, isolated nodal points and dislocation walls. Contrast analyses yield Burgers vector b = [001] being dominant and b = [100] subordinate. Climb loops consist of 〈c〉-dislocations that are dissociated in (001) planes, glide loops are defined by [100] [010] and [001] (100). The cordierite microstructures have been interpreted to be generated by dislocation creep. The dominant recrystallisation mechanism is thought to be subgrain rotation subsequently followed by minor grain or twin-band boundary migration.  相似文献   

12.
Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} 〈100〉, {100} 〈001〉, and {010} 〈100〉 in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes.  相似文献   

13.
We have used a first-principle approach based on the calculation of generalised stacking faults (GSF) to study the influence of pressure on the mechanical properties of forsterite. Six cases corresponding to [100] glide over (010), (021) and (001), and [001] glide over (100), (010) and (110) have been considered. The relaxed energy barriers associated with plastic shear have been calculated by constraining the Si atoms to move perpendicular to the fault plane and allowing Mg and O atoms to move in every direction. These conditions, which preserve dilations as a relaxation process, introduce Si–O tetrahedral tilting as an additional relaxation mechanism. Relaxed GSF show little plastic anisotropy of [100] glide over different planes and confirms that [001] glide is intrinsically easier than [100] glide. The GSF are affected by the application of a 10 GPa confining pressure with a different response for each slip system that cannot be explained by sole elastic effect. In particular, [100](010) is found to harden significantly under pressure compared to [001](010). Our results give the first theoretical framework to understand the pressure-induced change of dominant slip systems observed by Couvy et al. (in Eur J Mineral 16(6):877–889, 2004) and P. Raterron et al. (in GRL, submitted). It appears necessary to account for the influence of pressure on the mechanical properties of silicates in the context of the deep Earth.  相似文献   

14.
We report results from axisymmetric deformation experiments carried out on forsterite aggregates in the deformation-DIA apparatus, at upper mantle pressures and temperatures (3.1–8.1 GPa, 1373–1673 K). We quantified the resulting lattice preferred orientations (LPO) and compare experimental observations with results from micromechanical modeling (viscoplastic second-order self-consistent model—SO). Up to 6 GPa (~185-km depth in the Earth), we observe a marked LPO consistent with a dominant slip in the (010) plane with one observation of a dominant [100] direction, suggesting that [100](010) slip system was strongly activated. At higher pressures (deeper depth), the LPO becomes less marked and more complex with no evidence of a dominant slip system, which we attribute to the activation of several concurrent slip systems. These results are consistent with the pressure-induced transition in the dominant slip system previously reported for olivine and forsterite. They are also consistent with the decrease in the seismic anisotropy amplitude observed in the Earth’s mantle at depth greater than ~200 km.  相似文献   

15.
Previous experiments by Raleigh et al. (1971) have shown that at strain rates of 10−2.sec−1 to 10−7.sec−1 only slip occurs in dry enstatite at temperatures above 1300°C and 1000°C, respectively.The present experiments have been conducted on polycrystalline enstatite under wet conditions in this regime where enstatite only slips, polygonizes and recrystallizes. Slip occurs throughout the whole regime on the system (100)[001] and at strains greater than 40% the system (010)[001] is observed. Polygonization and intragranular recrystallization begin at about 1300°C and 10−4.sec−1 and the orientation of these neoblasts is host-controlled. At lower strain rates intergranular neoblasts develop and their fabric is one of [100] maximum parallel with σ1 and [010] and [001] girdles in the σ2 = σ3 plane, similar to those in natural enstatite tectonites.Dislocation substructures of experimentally deformed enstatite have been examined by transmission electron microscopy. The samples were deformed within the field in which slip polygonization and recrystallization are the dominant deformation mechanisms. Samples within this regime have microstructures that are characterized by stacking faults and partial dislocations. Under the conditions of steady-state flow in olivine, these microstructures inhibit the operation of recovery mechanisms in enstatite.Other samples deformed within the polygonization and recrystallization field have microstructures that confirm the optical observations of intragranular and intergranular growth of neoblasts. It is suggested that the former result from strain-induced tilt of subrains, whereas the latter may result from bulge nucleation into adjacent subgrains.Mechanical data from constant strain-rate experiments at steady state, stress relaxation and temperature-differential creep tests are best fit to a power-law creep equation with the stress exponent, n~3 and the apparent activation energy for creep, Q~65 kcal/mole. Extrapolation of this equation to a representative natural geologic strain rate of 10−4. sec−1, over the temperature interval 1000–2000°C, gives an effective viscosity range of 1020–1018 poise and stresses in the range of 7-0.1 bar, respectively. Comparison with corrected wet-olivine mechanical data (Carter, 1976) over the same environment indicates that olivine is consistently the weaker of the two minerals and will recrystallize whilst enstatite will only slip and kink, thus accounting for the different habits of olivine and enstatite in ultramafic tectonites.  相似文献   

16.
A set of sanidine single crystals were previously deformed at 700° C in a Griggs triaxial press with different crystallographic orientations of the core so as to induce dislocation glide of different slip systems respectively. Deformed crystals have been studied by transmission electron microscopy (TEM) and the activated slip systems have been characterized for two orientations. (010)[001] and (001)1/2[ \(\overline 1 \) 10] systems expected for one orientation (main stress nearly parallel to [012]) are observed, whereas the (001)[100] system expected for the other orientation (main stress nearly parallel to [101]) is never observed. In the latter specimen the deformation is rather difficult and occurs through unexpected systems characterized as (110)1/2[1 \(\overline 1 \) 2] and (1 \(\overline 1 \) 1)1/2[110]. In all the samples studied the deformation is heterogeneous, exhibiting dislocation configurations related to temperature variations.  相似文献   

17.
Dislocations in intermediate plagioclase feldspars, which were deformed under granulite facies conditions, have been analysed. The study reveals extensive ductile deformation by intracrystalline slip and by twinning. Six out of the seven possible Burgers vectors were identified: \(b = \left[ {001} \right],\tfrac{1}{2}\left[ {110} \right],\tfrac{1}{2}\left[ {1\bar 10} \right],\left[ {101} \right],\tfrac{1}{2}\left[ {112} \right]and\tfrac{1}{2}\left[ {1\bar 12} \right]\) . Most, perhaps all, dislocations are dissociated by up to 200 Å. The microstructure is dominated by [001] screw dislocations, most of which appear to be dissociated in (010). The dominant slip system appears to be (010) [001]. Large grain-to-grain variations in the density of free dislocations indicate that the plastic strain in individual grains depended upon the Schmid factor for (010) [001]. The microstructure suggests that the rate-controlling step for high-temperature creep of plagioclase is cross-slip of extended [001] screw dislocations. The rheological contrast between feldspar and quartz is partly due to a difference in stacking fault energy.  相似文献   

18.
平南幔源包体中橄榄石的显微构造研究及其意义   总被引：1，自引：0，他引：1  
平南玄武岩中的尖晶石二辉橄榄岩包体的平衡温度为930~980℃, 平衡深度为59~74km, 包体中橄榄石的扭折带滑移系多为(010) [100], 但也有(001) [100]的滑移系类型; 斜方辉石的滑移系为(100) [001], 它们均为高温低应变速率下的滑移系, 说明该区的上地幔主要是在高温低应变速率条件下经历了塑性变形作用.橄榄石位错组态多样, 有自由位错、位错壁、位错弓弯、缠结、{110}滑移带, 反映了上地幔的塑性变形特征.根据位错壁的大小估算, 上地幔差异流动应力为24.5~42.1MPa, 流动速率为2.93×10-17~8.36×10-16s-1, 有效粘度为1.72×1023~2.80×1024 Pa·s, 特征与中国东部新生代上地幔较为一致, 反映同处于拉张环境.   相似文献   

19.
Microhardness experiments have been performed on faces of olivine single crystals oriented at 45° to the [100] and [001] axis. Experiments have been performed between 20°C and 900°C in order to follow the evolution of the slip systems and the evolution of the nature of dislocations with temperature. Slip systems were identified using interferential contrast, decoration and transmission electron microscopy (TEM). Although the two possible Burgers vectors [001] and [100] are acted upon symmetrically, the only activated slip systems are (100) [001] and {110} [001]. The latter system is less developed and appears only at 600°C and above. Dislocations are mainly [001] screws, which are always very straight. Microcracks are observed together with dislocations. Above 600°C there is a clear relationship between both as a narrow plastic zone is developed around the cracks.  相似文献   

20.
Dislocation structures in naturally deformed olivine from garnet peridotite xenoliths from South African kimberlites have been studied by electron microscopy. The substructure consists mainly of straight subboundaries of dislocations with Burgers vectors [001]. Most of the dislocations have both edge and screw components, and the slip planes are mainly (100). The dislocation density between the subboundaries is low.The slip planes in olivine are discussed in relation to the olivine structure. The observed dislocation structures seem to indicate that the large difference in strain rate between natural and experimental deformation will produce a difference in the slip mechanisms.The nature of the deformation lamellae visible in optical microscope is discussed.  相似文献   

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