不同温压下岩石弹性波速度、衰减及各向异性与组构的关系

结合岩石组构分析 ,阐述了岩石弹性波传播速度和衰减以及它们的各向异性与岩石组构之间的关系。在不同温压条件下对具有很强晶格优选方位的岩石样品的研究表明 ,随着围压的增加 ,波速和Q值均增大 ,但是在相互正交的 3个方向上 (垂直或平行于层理面及线理方向 )增大的速度并不相同 ,这与微裂隙的逐渐闭合密切相关。观测到的波速和Q值的各向异性具有不同的形成机理 ,波速各向异性主要与定向分布的微裂隙和主要矿物的晶格优选方位等构造因素有关 ;高围压下Q值各向异性与速度各向异性正好相反 ,可能是由于定向排列的矿物晶体沿不同方向其边界之间接触程度不同造成的。对岩石组构的研究不仅可以揭示岩体的变形机制、变形的动力学过程及其有关的热力学信息 ,还可以对宏观岩石的各种物理性质 ,尤其是力学特性 ,从微观机理上加以解释。文中特别强调了岩石组构分析对研究岩石物理性质的各向异性具有十分重要的意义。     

Mineralogical and seismic properties of serpentinite of Ait Ahmane fault zone of Bou Azzer ophiolite,central Anti-Atlas of Morocco

This paper presents the first seismic measurements of serpentinite of Bou Azzer ophiolite, central Anti-Atlas of Morocco, including seismic velocities and anisotropy. Two serpentinite samples collected from the Ait Ahmane fault zone were analyzed in order to define the mineralogical and seismic features of the natural serpentinite of the Bou Azzer ophiolite. The mineralogical features were investigated using microscopic observation and Raman spectroscopy, while the seismic features were performed using an Electron Backscatter Diffraction (EBSD) instrument. Microscopy and spectroscopy analyses confirmed that the investigated serpentinite suffers from a variable degree of serpentinization, and the antigorite is the dominant variety of serpentine minerals in the study area. The crystal preferred orientation (CPO) results show that the axis [001] of the antigorite deformation is aligned subnormal to the foliation, while the axis [010] is mostly aligned subparallel to the lineation. The seismic anisotropy results are depending on serpentine amount in the rock samples. The sample with a low serpentine amount (30%) shows lowest P- and S-wave anisotropy (Vp = 7.2% and AVs = 6.55%), while the sample with a high amount of serpentine (85%) presents highest P-wave and S-wave anisotropy (Vp = 8.6% and AVs = 11.06%). Consequently, the results indicate that seismic anisotropy increases when increasing the antigorite amount.     

Directional dependence of P- and S-wave propagation and polarization in foliated rocks from the Kola superdeep well: Evidence from laboratory measurements and calculations based on TOF neutron diffraction

We have measured P- and S-wave velocities on two amphibolite and two gneiss samples from the Kola superdeep borehole as a function of pressure (up to 600 MPa) and temperature (up to 600 °C). The velocity measurements include compressional (Vp) and shear wave velocities (Vs1, Vs2) propagating in three orthogonal directions which were in general not parallel to inherent rock symmetry axes or planes. The measurements are accompanied by 3D-velocities calculations based on lattice preferred orientation (LPO) obtained by TOF (Time Of Flight) neutron diffraction analysis which allows the investigation of bulk volumes up to several cubic centimetres due to the high penetration depth of neutrons. The LPO-based numerical velocity calculations give important information on the different contribution of the various rock-forming minerals to bulk elastic anisotropy and on the relations of seismic anisotropy, shear wave splitting, and shear wave polarization to the structural reference frame (foliation and lineation). Comparison with measured velocities obtained for the three propagation directions that were not in accordance with the structural frame of the rocks (foliation and lineation) demonstrate that for shear waves propagating through anisotropic rocks the vibration directions are as important as the propagation directions. The study demonstrates that proper measurement of shear wave splitting by means of two orthogonal polarized sending and receiving shear wave transducers is only possible when their propagation and polarization directions are parallel and normal to foliation and lineation, respectively.     

Determination of the elastic modulus set of foliated rocks from ultrasonic velocity measurements

Ultrasonic measurements of compressional and shear wave velocities under hydrostatic pressure up to 70 MPa were carried out on cylindrical specimens cored across and along the foliation planes. Our measurements revealed that the foliation of the metamorphic rocks induces a clear velocity anisotropy between two orthogonal directions; faster along the foliation plane and slower across the plane in most rock types. All velocity components monotonically increase with the confining pressure, probably due to the closure of microcracks distributed in rock specimens. We determined the complete set of dynamic moduli of foliated metamorphic rocks with two assumptions; transverse isotropy due to the foliation and ellipsoidal seismic energy propagation from a point source. The calculated elastic moduli referring to different directions could be valuable for the design of various engineering structures in planar textured rock mass.     

北祁连山高压变质带的变形和变质特征及其对俯冲带折返的意义

北祁连造山带是早古生代大洋板块"冷"俯冲的典型地区,形成了一系列高压低温变质的岩石组合.基性榴辉岩是俯冲洋壳变质的产物,根据其结构构造特征可以划分为(1)粒状榴辉岩,峰期变质矿物组合为Grt+Omp+Phn+Rut±Lws,硬柱石呈包裹体和假象分布于石榴石内,变质温压条件T=465～508℃,P=2.30～2.60GPa.(2)片状榴辉岩,岩石具片状构造,其共生矿物组合Omp+Czo/Ep+Phn+Gln构成透入性面理S1和线理L1,以及宏观和显微同斜褶皱F1.其峰期变质温压条件为T=466～510℃,P=1.9～2.2GPa.(3)蓝片岩相退化变质榴辉岩,蓝片岩相变质矿物Gln+Ep+Phn±Ab强烈交代榴辉岩相矿物,并发生强烈D2期变形作用,岩石形成明显的透入性面理S2和线理L2,以及同斜剪切褶皱F2.蓝闪石和多硅白云母线理的统计揭示D2期变形以斜向走滑剪切为主,与D1期变形的榴辉岩的运动学特征大体相似.D2同变质的温压条件为T=422～487℃,P=1.15～1.37GPa.高压变质带晚期的绿片岩相叠加和D3期变形,形成透入性面理S3和线理L3,其运动特征为近于垂直构造线的逆冲剪切.结合榴辉岩变形特征,变质温压条件和同位素年代学资料,我们认为退变质的强弱与变形程度有密切关系.峰期变质后的榴辉岩经过早期绿帘石榴辉岩相到蓝片岩相斜向走滑剪切,和晚期绿片岩相逆冲,在泥盆纪早期快速折返的出露地表.     

Magnetic fabrics in garnet peridotites–pyroxenites and host felsic granulites in the South Bohemian Granulites (Czech Republic): Implications for distinguishing between primary and metamorphism induced fabrics

In the high-grade Moldanubian Zone of the European Variscides, numerous bodies of ultramafic rocks occur embedded in granulite. The anisotropy of magnetic susceptibility and its low-field variation as well as the anisotropy of magnetic remanence were used to investigate magnetic fabrics of some ultramafic bodies and host granulite. In granulite, the magnetic foliation is roughly parallel to the metamorphic foliation and the magnetic lineation is near the mineral alignment lineation. In ultramafite, the magnetic foliation is relatively scattered spatially, but mostly oriented in a different way than that in granulite. The magnetic lineation is also scattered, but still relatively well defined spatially. Again, its orientation is mostly different than that of granulite. The magnetic fabric in ultramafic rocks is therefore different from that in the host granulite even though both rock types underwent at least partially common structural history. The componental movements forming the granulite fabric, mostly during amphibolite facies retrograde metamorphism, were evidently not intensive enough to strongly overprint the magnetic fabric of ultramafite. The ultramafite is therefore strong enough to maintain its pre-metamorphism fabric even at such high temperatures and pressures that are characteristic of high amphibolite facies retrograde metamorphism.     

Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region,southwest Norway,and implications for seismic anisotropy

Chlorite peridotites from Almklovdalen in southwest Norway were studied to understand the deformation processes and seismic anisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electron backscattered diffraction (EBSD)/scanning electron microscopy. A sample with abundant garnet showed [100] axes of olivine aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chlorite showed different olivine LPOs. Two samples showed [001] axes aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned sub-parallel to lineation, and [001] axes aligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormal to foliation with a weak girdle subnormal to lineation. Fourier transform infrared (FTIR) spectroscopy of the specimens revealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines with B-type LPOs contain a large amount (340 ppm H/Si) of water.

The seismic anisotropy of the olivine and chlorite was calculated. Olivine showed Vp anisotropy of up to 3.8% and a maximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1%, and a maximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vp anisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressure and high temperature in the subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slab in a subduction zone where chlorite is stable.     

Olivine [100] normal to foliation: lattice preferred orientation in prograde garnet peridotite formed at high H<Subscript>2</Subscript>O activity,Cima di Gagnone (Central Alps)

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 s^у) 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.     

Elastic properties, fabric and seismic anisotropy of amphibolites and their contribution to the lower crust reflectivity

In this paper the elastic properties of amphibolites from the Serie dei Laghi and the Ivrea zone (Southern Alps, N-Italy) were investigated as a function of their mineralogical composition, microfabric and density.Three orthogonal cores were cut parallel and normal to foliation and lineation; from those, bulk and grain density were measured and the interconnected porosity was calculated. Bulk density varies from 2.75 to 3.07 g/cm³ and calculated porosity ranges from 0.02 to 0.88%.The same cores were also used to measure seismic velocity of ultrasonic waves at room temperature and at increasing confining pressure up to 300 MPa. At high pressure the matrix properties are separated from the crack-induced properties. P-wave velocity varies with respect to the direction of propagation: the slowest direction is always normal to foliation and the fastest parallel to the mineral lineation. The mineral lineation is typically defined by the elongation of amphibole crystals, in which the fastest Vp direction is parallel to the c axis, that is also the elongation axis. The Vp ranges between 6.76 and 7.54 km/s in the direction parallel to lineation and between 6.32 and 7.06 km/s in the direction normal to foliation. This defines a Vp anisotropy of up to 14%, whose shape varies from orthorhombic to axially symmetric (either prolate or flattened). It was observed that both Vp and Vp anisotropy increase with the amount of amphibole and decrease with the amount of plagioclase. Moreover, the c axis distribution of amphiboles is responsible of the Vp anisotropy intensity and shape, in agreement with observations from previous studies. The seismic properties calculated with the approach of Mainprice (1990), using the fabric data, the elastic constants and the modal composition, gave results in good agreement with the measurements.Exposed rocks in the Ivrea and Serie dei Laghi zones show that amphibolites are interlayered with metapelites on a scale from 1 to 100 m. Because of the very large acoustic impedance contrast (20.34 ± 1.75 for amphibolites, 17.16 ± 0.4 for metapelites), they represent a very reflective portion in the middle-lower crust.     

Serpentinization of phlogopite phenocrysts from a micaceous kimberlite

Phenocrysts of phlogopite from a micaceous kimberlite contain finely interlayered serpentine. These phenocrysts occur in the kimberlite groundmass and are altered along the mica layers and are slightly deformed. Lizardite is the predominant serpentine mineral, but chrysotile and mixed structures also occur. The lizardite occurs as lamellae within phlogopite, oriented such that (001) layers of the two minerals are parallel and the [100] direction of lizardite is parallel to the [100] or 110 directions of phlogopite. The serpentinized regions of phlogopite are localized and extensive along the (001) layers. Chrysotile fibers and chrysotile-like curled serpentine occur within regions of disrupted material, and polygonal structures occur in folded lizardite lamellae. Textural relations suggest three events: 1) replacement of phlogopite by lizardite, 2) deformation of the phenocrysts, and 3) partial transformation of the lizardite to chrysotile-like structures. Deformation features include openings along (001), folds, and regions of disrupted or broken material. The folded and broken material consists of lamellar lizardite and phlogopite, indicating that lamellar replacement preceded deformation. Intergrowths of lizardite and curled serpentine are associated with cleavage openings and voids in disrupted material, suggesting that a partial transformation of lizardite to chrysotile occurred within openings created by deformation. Clay minerals also occur within phlogopite as either a minor product of serpentinization or of late-stage alteration.     

Fabric and seismic properties of carrara marble mylonite

Shear deformation in calcite-rich rocks can produce strong lattice preferred orientations (LPO), which result in a high anisotropy of bulk seismic properties because of the high elastic anisotropy of calcite (32% Vp anisotropy). Deformed rocks often show also strong shape preferred orientations (SPO). Theories for averaging the elastic properties have not yet satisfactorily predicted the contribution to the seismic anisotropy caused by the SPO alone.A calcite mylonite from Carrara (Italy) was investigated, which is characterised by a strong SPO and a weak LPO. It was composed of about 80% calcite, then white mica, quartz and hematite. Flattening of mica and of calcite grains defined the mylonitic foliation, and elongation of calcite grains defined the lineation. On average calcite grains have aspect ratios of about 2.5:1.6:1, and grain sizes of about 10 μm. At 400 MPa confining pressure, the measured Vp (km/s) parallel to the lineation (X direction) was highest (6.63), lower in the intermediate Y direction (6.47); the Vp normal to the foliation (Z direction) was lowest (6.30). This yielded a Vp anisotropy of 5%. The LPO, determined by automated electron backscatter diffraction (EBSD), was very weak (texture index 1.1), with intensities between 0.6 and 1.6 m.r.d. in the c-axis pole figure. Extrapolation of the texture index to an infinite number of orientation measurements indicated that the observed variations were mostly random noise in the orientation distributions and that the bulk rock texture was random. The Vp anisotropy of the Voigt, Reuss and Hill averages calculated from this calcite LPO is predicted to be close to zero. Adding 5% of muscovite with (001) perfectly aligned parallel to the foliation, we calculated a total anisotropy of 2.8%. The anisotropy calculated for the special directions X, Y and Z remained at 2.6% only.It was concluded that the measured seismic anisotropy cannot be explained by the LPO of calcite and by 5% of mica alone. It is also attributed to the strong SPO and to further grain boundary effects.     

Minéraux serpentineux extraits de péridotites serpentinisées des Alpes Occidentales

Complete physical and chemical data presented concern serpentine minerals choosen from epizonal to non metamorphic domains. It is tried to correlate well-studied serpentine minerals, generally coming from particular outcrops with the rock-forming ones known only through the microscope.It appears that ribbons filling serpentine veins in a serpentinized dunite, the whole mesh-structure of a non metamorphic serpentine, and bastite lamellae are made of lizardite. Antigorite, only serpentine mineral present in metamorphic domains, shows no significative difference between high-pressure facies (glaucophane schist) and low-pressure facies (green-schist). The data allow a better understanding of the serpentinization process. The geochemically antinomic behaviour of iron and magnesium is emphasized.     

四川石棉矿蛇纹石矿物学研究

四川石棉矿产出四种蛇纹石矿物:纤蛇纹石,Povlen型纤蛇纹石、利蛇纹石和叶蛇纹石。它们的形态、结构、化学成分和红外光谱各具特征,本文对此进行了描述和讨论。纤蛇纹石以纵纤维脉和横纤维脉形式产出,以斜纤蛇纹石为主,含少量正纤和副纤蛇纹石。纵纤维蛇纹石可能由地壳浅层中的大气热水形成。Povlen型纤蛇纹石是蛇纹石族矿物的一个新变种,其形态、结构和化学成分都不同于其他蛇纹石矿物。     

Deformation fabrics of glaucophane schists and implications for seismic anisotropy: the importance of lattice preferred orientation of phengite

ABSTRACT

Strong seismic anisotropy is observed in many subduction zones. This effect is attributed partly to subducting oceanic crust that is transformed into blueschist facies rocks. Because blueschist facies constituents such as glaucophane, epidote, and phengite show strong anisotropic elasticity, seismic anisotropy in subducting oceanic crust can be attributed to the lattice preferred orientation (LPO) of these minerals. We studied the deformation fabrics and seismic properties of phengite-rich epidote–glaucophane schists from the Franciscan Complex of Ring Mountain, California. The samples are composed mainly of glaucophane, epidote, and phengite. Some samples contain abundant phengite, the maximum being 40%. The LPOs of glaucophane showed that the [001] axes are aligned subparallel to lineation, and both (110) poles and [100] axes are aligned subnormal to foliation. The epidote [001] axes are aligned subnormal to foliation, with both (110) and (010) poles aligned subparallel to lineation. The LPOs of phengite are characterized by the maxima of [001] axes subnormal to foliation, and both (110) and (010) poles and [100] axes are aligned in a girdle subparallel to foliation. The phengite showed substantially strong seismic anisotropy (AV_P = 42%, max.AV_S = 37%). The glaucophane schist with abundant phengite showed significantly stronger seismic anisotropy (AV_P = 30%, max.AV_S = 23%) than the epidote–glaucophane schist (AV_P = 13%, max.AV_S = 9%). When the subduction angle of phengite-rich glaucophane schist is considered, the polarization direction of the fast S-waves for vertically propagating S-waves changed to a nearly trench-parallel direction for the subduction angle of 45?60°, and the S-wave anisotropy became stronger for vertically propagating S-waves with increasing subduction angles. Our data showed that phengite-rich blueschist facies rock can therefore contribute to the strong trench-parallel seismic anisotropy occurring at the subducting oceanic crust and at the slab–mantle interface in many subduction zones.     

Transformation of magnetite to hematite and its influence on the dissolution of iron oxide minerals

Deformed rocks of the Itabira Iron Formation (itabirites) in Brazil show microstructural evidence of pressure solution of quartz and iron oxides; it appears that magnetite was dissolved and hematite precipitated. The dissolution of magnetite seems to be related to its transformation to hematite by oxidation of Fe²⁺ to Fe³⁺. The transformation of magnetite to hematite occurs along {111} planes, and results in the development of hematite domains along {111} that are parallel to the foliation. The difference in volume created by the transformation of magnetite to hematite and the shear stress acting on the interphase boundaries allow fluids to migrate along these planes. The dissolution of magnetite involves the hydrolyzation of the Fe²⁺—O bonds at interphase boundaries of high normal stress. The high fugacity of oxygen in the fluid phase promotes the reaction of Fe²⁺ (in solution) with oxygen. Fe²⁺ ions oxidize to Fe³⁺ and precipitate as hematite platelets with their longest axes oriented parallel to the direction of maximum stretching. The transformation of magnetite to hematite during deformation plays an important role in the fabric evolution of the iron formation rocks. The transformation along {111} creates planes of weakness that facilitate fracturing. The fracturing plus the dissolution result in a reduction of magnetite grain size, and the oriented precipitation results in layers of hematite platelets. These processes produce a new fabric characterized by a penetrative foliation and lineation.     

A petrographie study of the process of serpentinisation in ophiolites and the ocean crust

The process of serpentinisation is illustrated by 17 samples showing different degrees of serpentinisation chosen from a large number of peridotites and serpentinites from the ocean floor and from ophiolite complexes. Observations of textural relationships were made by optical and scanning electron microscope. Mineral identification was confirmed by X-ray diffraction. Of the serpentine polymorphs, lizardite forms during early stages of serpentinisation and displays a characteristically platy morphology. Olivine and pyroxene are replaced by lizardite until no olivine remains. At this stage chrysotile begins to crystallise as fine fibres characteristically filling veins and actually replacing lizardite throughout the rock. Antigorite is confined to sheared surfaces and is rare in the ocean floor forming the latest polymorph. Both ocean floor and ophiolite samples show well developed mesh textures in hand specimen, while a much smaller web network of serpentine occurs on some ocean floor samples. Serpentines from ophiolites show two morphological types of lizardite which may have formed at different temperatures.     

韧性剪切带中片麻岩和超高压榴辉岩变形特征及其与地震波速各向异性的关系：来自中国大陆科学钻探（CCSD）680～1200米岩心的证据

中国大陆科学钻探(CCSD)680-1200米区段发育了多个韧性剪切带,带中主要岩石类型包括片麻岩和超高压榴辉岩。片麻岩中的变形石英、面理化榴辉岩中的拉长石榴石和绿辉石的应变轴比都表现为X>Y>Z,Flinn系数分别为0.11-0.27、0.22-0.23和0.23-0.24。随着糜棱岩化作用的增强,变形石英的C轴组构由Z轴极密逐渐向Y轴极密和叶理面上的大圆环带转变。在常温常压下测试了样品的波速,计算出片麻岩Vp和Vs的各向异性分别为30.17％-60.97％和11.52％-35.79％,榴辉岩Vp和Vs的各向异性分别为0.17％-11.19％和2.41％-6.70％。影响各向异性的主要因素有岩石的结构构造、矿物的晶格优选方位(LPO)、形态优选方位(SPO)和定向微裂隙。随着糜棱岩化作用的增强,岩石的P波各向异性逐 渐升高。变形岩石中的黑云母、石英、绿辉石的LPO和SPO是地震波各向异性的主要控制因素。饱水后的片麻岩样品的P波各向异性明显低于干燥片麻岩样品。在东海钻井中的强反射带主要是由于不同岩层之间的波阻抗差异而造成的,榴辉岩/强退变榴辉岩和黑云斜长片麻岩之间的接触界面会产生较强的地震深反射。此外,与LPO相关的地震波各向异性会增强地震波的反射,所以韧性剪切带中的糜棱岩化片麻岩可能是地震反射的良好载体。韧性剪切带中岩石弹性波速度的强各向     

Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy)

Central Italy is an active tectonic area that has been recently studied by several regional mantle, Pn and SKS, studies which revealed the presence of a strong regional anisotropy. In this paper, we present the first petrophysical results on the only mantle xenoliths from Central Italy, which place new constraints on the upper mantle structures of this region. The Torre Alfina mantle xenoliths are very small in size, from few millimetres to about 1.5 cm. They are mainly dunites and harzburgites, with subordinate lherzolites and wehrlites. Since olivine and spinel are always present, they should have crystallised in the spinel-bearing lherzolite field. Their mineralogical composition is ol+spl±opx±cpx. Both olivines and pyroxenes are present as porphyroclasts and as neoblasts. The xenoliths show different degrees of recrystallization. Geothermobarometry on these xenoliths give a temperature range of 1040±40 °C and a pressure estimate of about 1.5 GPa, corresponding to 50 to 60 km depth. Previous seismic studies have estimated the Moho to be at 20 to 25 km in this region, hence the xenoliths come from a hot mantle, probably asthenospheric, below a lithosphere of about 25 to 40 km in thickness below the Moho. We measure the crystallographic preferred orientation (CPO) of olivines and pyroxenes using a SEM and the Electron Back Scattered Diffraction (EBSD) technique. The CPO shows all three axes of olivine are tightly clustered: [100] axis is typically more tightly clustered than [010] and [001] is the most widely distributed axis. The fabric strength expressed by the integral J index, varies from 4.5 to 25.9, and decreases with the degree of recrystallization. We use CPO data to calculate anisotropic seismic properties of the xenoliths. They are very homogenous and probably statistically representative of the mantle below the Torre Alfina area. Vp ranges from 8.4 to 9.1 km/s, Vs₁ from 4.8 to 5.0 km/s. The seismic anisotropy is more variable; AVp ranges from 9.8% to 19.3% and AVs from 7.3% to 13.4%. The majority of the xenoliths display an orthorhombic seismic symmetry, but xenoliths with a transverse isotropic behaviour have also been observed.

We consider four geodynamic models for the source region of the xenoliths (extension, shear, upwelling, slab tilted), defined by different orientations of the structural reference frame, and we calculated for each model the variation of the seismic properties with temperature, pressure and volume fraction of orthopyroxene. After comparing this variation of calculated seismic parameters with seismic observations from the region, we form the hypothesis that the xenoliths come from either an extensional tectonic zone (lineation X and foliation plane XY horizontal) or transcurrent shear zone (lineation X horizontal and foliation plane XY vertical) and that the mantle beneath Torre Alfina is composed by 70% olivine and 30% orthopyroxene forming an anisotropic layer of about 160 or 110 km in thickness, respectively.     

Geochemistry, petrofabrics and seismic properties of eclogites from the Chinese Continental Scientific Drilling boreholes in the Sulu UHP terrane, eastern China

We present an integrated study of geochemistry, petrofabrics and seismic properties of strongly sheared eclogites from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. First, geochemical data characterize diverse protoliths of the studied eclogites. The positive Eu- and Sr-anomalies, negative Nb anomaly and flat portion of heavy rare earth elements in coarse-grained rutile eclogites (samples B270 and B295) suggest a cumulate origin in the continental crust, whereas the negative Nb anomaly and enrichment of light rare earth elements in retrograde eclogites (samples B504, B15 and B19) imply an origin of continental basalts or island arc basalts. Second, P-wave velocities (V_p) of three typical eclogite samples were measured under confining pressures up to 500 MPa and temperatures to 700 °C. At 500 MPa and room temperature, the mean V_p reaches 8.50-8.53 km/s in samples B270 and B295 but drops to 7.86 km/s in sample B504, and the P-wave anisotropy changes from 1.7-2.7% to 5.5%, respectively. The pressure and temperature derivatives of V_p are larger in the retrograde eclogite than in fresh ones. Third, the electron backscatter diffraction (EBSD) measurements of the eclogites reveal random crystal preferred orientation (CPO) of garnet and pronounced CPO of omphacite, which is characterized by a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles perpendicular to the foliation. The asymmetric CPO of omphacite in sample B270 recorded a top-to-the-south shear event during subduction of the Yangtze plate. The calculated fastest V_p is generally sub-parallel to the lineation, but a different deformation environment during exhumation could form second-order variations in omphacite CPO and affect the V_p distribution in eclogites (e.g., the fastest V_p is at ~ 35° from the foliation in sample B295). Comparison between measured and calculated seismic properties indicates that the CPO of omphacite controls the seismic anisotropy of eclogites at high pressure, and compositional layering and retrograde minerals will increase the anisotropy. Calculated P-wave velocities agree well with velocities measured at 500 MPa and room temperature for fresh eclogites, but much higher than those of retrograde eclogite. As a case study, the laboratory-derived V_p-P and V_p-T relationships were used to estimate P-wave velocities of eclogites and peridotites beneath the Western Superior Province, Canada. The results indicate that besides the fabric-induced anisotropy, the direction dependence of pressure and temperature derivatives of V_p can significantly increase seismic anisotropy of eclogites with depth, which results in eclogites being an important candidate for the seismic anisotropy in the upper mantle. Due to their very high density and velocity, garnet-rich eclogites within peridotite could be detected in seismic reflections in subduction zones.     

Seismic anisotropy of the crystalline crust: what does it tell us?

The study of the directional dependence of seismic velocities (seismic anisotropy) promises more refined insight into mineral composition and physical properties of the crystalline crust than conventional deep seismic refraction or reflection profiles providing average values of P-and S-wave velocities. The alignment of specific minerals by ductile rock deformation, for instance, causes specific types of seismic anisotropy which can be identified by appropriate field measurements.
Vice versa , the determination of anisotropy can help to discriminate between different rock candidates in the deep crust. Seismic field measurements at the Continental Deep Drilling Site (KTB, S Germany) are shown as an example that anisotropy has to be considered in crustal studies. At the KTB, the dependence of seismic velocity on the direction of wave propagation in situ was found to be compatible with the texture, composition and fracture density of drilled crustal rocks.