microstructures petrofabrics and seismic properties of ultra high pressure eclogites from sulu region china implications for rheology of subducted continental crust and origin of mantle reflections ultra high pressure uhp eclogites from sulu region china represent mafic components of the continental crust which were first subducted to mantle depths greater than 100 km and then exhumed to the earth s surface detailed investigation of microstructures chemical compositions petrofabrics and seismic properties of the uhp eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle we present here results from field optical and tem observations electron back scattered diffraction ebsd measurements and numerical computations of the seismic properties of uhp eclogites collected from fresh surface outcrops at the drill site maobei donghai county jiangsu province of the chinese continental scientific drilling program ccsd two types of eclogites have been distinguished type 1 coarse grained eclogites deformed by recovery accommodated dislocation creep at the peak metamorphic conditions and type 2 fine grained eclogites which are composed of reworked type 1 materials during recrystallizationaccommodated dislocation creep in shear zones which were active during the exhumation of the uhp metamorphic rocks both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the uhp metamorphic conditions plasticity of eclogites under uhp conditions can effectively facilitate channeled flow along the interplate shear zone the

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

modeling dynamic recrystallization of olivine aggregates deformed in simple shear experiments by zhang and karato 1995 have shown that in simple shear dislocation creep of olivine at low strains an asymmetric texture develops with a 100 maximum rotated away from the shear direction against the sense of shear at large strain where recrystallization is pervasive the texture pattern is symmetrical and 100 is parallel to the shear direction the deformation texture can be adequately modeled with a viscoplastic self consistent polycrystal plasticity theory this model can be expanded to include recrystallization treating the process as a balance of boundary migration growth of relatively underformed grains at the expense of highly deformed grains and nucleation strain free nuclei replacing highly deformed grains if nucleation dominates over growth the model predicts a change from the asymmetric to the symmetric texture as recrystallization proceeds and stabilization in the easy slip orientation for the dominant 010 100 slip system this result is in accordance with the experiments and suggests that the most highly deformed orientation components dominate the recrystallization texture the empirical model will be useful to simulate more adequately the development of anisotropy in the mantle where olivine is largely recrystallized

fabric kinematics of the ultrahigh pressure metamorphic rocks from the main borehole of the chinese continental scientific drilling project implications for continental subduction and exhumation abstract the 5158 m deep main borehole of the chinese continental scientific drilling project ccsd mh penetrated granitic gneisses paragneisses eclogites retrograde eclogites amphibolites and ultramafic rocks in the sulu ultrahigh pressure uhp metamorphic terrane eastern china the ccsd mh consists of four petro structural units separated by three se dipping ductile shear zones dfa 835 1280 m dfb 2010 2280 m and dfc 2920 3225 m which are correspondent with the regional shear zones in the northern sulu uhp supracrustal zone using the electron backscatter diffraction ebsd technique we investigated the lattice preferred orientations lpos of omphacite diopside and quartz in core samples from the ccsd mh omphacite from eclogites and diopside from garnet pyroxenites display very strong lpos which are characterized by the maximum concentration of 001 axes parallel to the lineation and 010 poles normal to the foliation quartz in para and granitic mylonites gneisses from the shear zones dfa dfb and dfc developed multiple slip systems 40ar 39ar dating of biotite in para and granitic gneisses from the ccsd mh yields 223 202 ma which constrains the formation ages of the quartz high temperature prism slip systems m and m c the asymmetric lpos of omphacite diopside olivine and quartz with respect to the structural frame reveal three deformation phases in the sulu terrane in the middle triassic the northward subduction of the yangtze plate to depths 100 km produced a top to the south shear sense in lpos of omphacite diopside and olivine and a nearly n s striking foliation and a subhorizontal n s trending lineation in eclogites and ultramafic rocks in the late triassic the uhp rocks were exhumed to the lower crust and quartz developed high temperature slip systems with a top to the nw shear sense which is consistent with the regional se dipping foliation and se plunging lineation in the ductile shear zones in the cretaceous the uhp rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a ne striking antiform structure as a result the activation of quartz low temperature basal slip 0001 is characterized by a top to the se shear sense in the south but a top to the nw shear sense in the north

ultrahigh pressure texture inheritance during retrogression evidence from magnetofabrics in eclogites and ultramafic rocks chinese continental scientific drilling project abstract in order to contribute to a better understanding of exhumation related retrogression processes ultrahigh pressure uhp mafic and ultramafic rocks from the chinese continental scientific drilling ccsd in the maobei eclogite body of the sulu ultrahigh pressure metamorphic belt in eastern china were studied for their magnetofabrics variably retrogressed eclogites and serpentinized ultramafic rocks were retrieved from the depth interval of 100 to 1000 m of the borehole a vein network of irregular shaped veins with a retrograde metamorphic assemblage cuts across the eclogite foliation at both low and high angles sem imaging of the eclogites documents that magnetite associated with retrograde pargasitic amphiboles developed around shape preferred garnet sem imaging of the serpentinized ultramafic rocks documents that magnetite rims grew around shape preferred garnet and that magnetite formed within a mesh texture related to serpentinization syn serpentinization magnetite growth increased bulk susceptibilities but reduced anisotropy maximum susceptibility axes from both eclogites and serpentinized garnet peridotites trend n s i e parallel with the stretching lineation as defined by olivine and omphacite grains this implies that the magnetic fabric mimics the uhp fabric and survived retrogression and that the fabric is inherited from the uhp stage as a consequence retrogression was not associated with substantial ductile deformation and the mafic ultramafic maobei body behaved as a rigid body within a ductile deforming quartzo feldspathic matrix during exhumation internal strain is restricted to brittle fracturing associated fluid circulation and vein formation facilitating retrograde reactions in the mafic ultramafic rocks

crystallographic preferred orientation of olivine in the earth s deep upper mantle abstract simple shear deformation experiments of olivine were conducted at pressures of 11 1 12 7 gpa and temperatures of 1670 1770 k c type olivine fabric developed by the 1 0 0 0 0 1 slip system and similar fabrics were dominantly developed in a wide range of water content coh 49 19 242 ppm h si in olivine because the seismic fast a axes are perpendicular to the shear plane only for c type fabric a positive negative transition of vsh vsv difference in velocity between horizontally and vertically polarized shear waves inevitably occurs when c type fabric is developed by a pressure induced fabric transition the positive negative transition of vsh vsv at 200 km depth reported in global one dimensional models is known to be well explained by the horizontal flow of olivine with a low water content

stratospheric circulation features deduced from sams constituent data abstract stratospheric circulation is investigated by further analyses of three years of stratospheric and mesospheric sounder sams data eddy effects on constituent transport are investigated with two transport formulations the transformed eulerian mean formulation and the effective transport formulation the transformed eulerian mean formulation together with calculated residual mean winds v w is used to delineate regions or times of significant eddy contributions to constituent transport significant regions are found in the stratosphere in all seasons not only in the northern hemisphere nh winter high latitudes where contributions from nonlinear and nonsteady perturbations in sudden and final warming events are expected but also in the midlatitude middle stratosphere in autumn and at winter summer low latitudes near the stratopause questionably large calculated 0 1 m magnitudes near the stratopause during solstice suggest that the use of residual winds calculated here may be i

mineral inclusions in zircons of para and orthogneiss from pre pilot drillhole ccsd pp1 chinese continental scientific drilling project the pre pilot drillhole ccsd pp1 chinese continental scientific drilling project ccsd with depth of 432 m is located in the donghai area in the southwestern sulu terrane the core samples are mainly comprised of paragneiss orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite bearing kyanite quartzite all analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism coesite and coesite bearing ultrahigh pressure uhp mineral assemblages were identified by raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss eclogite and phengite bearing kyanite quartzite samples in the paragneiss samples uhp mineral inclusion assemblages mainly consist of coe omp grt phe coe jd phe ap preserved in the mantles m and rims r of zircons these uhp mineral inclusion assemblages yield temperatures of 814 852 c and pressures of 28 kbar presenting the p t condition of uhp peak metamorphism of these country rocks according to the mineral inclusions and cathodoluminescence images of zircons the orthogneisses can be divided into two types uhp og1 and non uhp og2 in og1 orthogneisses low pressure mineral inclusion assemblage mainly consisting of qtz phe ab ksp ap were identified in zircon cores c while coesite or coesite bearing uhp mineral inclusions were identified in the mantles m and rims r of the same zircons these features suggest that the og1 orthogneisses together with the paragneisses phengite bearing kyanite quartzite and eclogite experienced widespread uhp metamorphism in the sulu terrane however in the zircons of og2 orthogneiss samples no uhp mineral inclusions were found inclusions mainly comprised qtz phe ap and were identified in cores c mantles m and rims r of og2 zircons the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims these features indicate that the og2 orthogneisses in pre pilot drillhole ccsd pp1 did not experience uhp metamorphism therefore we should not rule out the possibility that some orthogneisses in sulu terrane might represent relatively low pressure granitic intrusives emplaced after the uhp event

pressure effect on forsterite dislocation slip systems implications for upper mantle lpo and low viscosity zone abstract in order to better constrained the effect of pressure p on olivine dislocation slip system activities deformation experiments were carried out in a deformation dia apparatus d dia on pure forsterite fo100 single crystals at p 5 7 gpa temperature t 1675 k differential stress and specimen strain rates e were monitored in situ by synchrotron x ray diffraction and radiography respectively two compression directions were tested promoting either 1 0 0 slip or 0 0 1 slip in 0 1 0 crystallographic plane comparison of the obtained high p rheological data with room p data previously reported by darot and gueguen 1981 shows that 1 0 0 slip is strongly inhibited by pressure while 0 0 1 slip is virtually p insensitive this translates in creep power laws into a high activation volume v a 15 3 cm 3 mol for 1 0 0 0 1 0 slip system and v c 0 1 2 cm 3 mol for 0 0 1 0 1 0 slip system using these laws along geotherms at natural condition shows that the 1 0 0 slip 0 0 1 slip transition may occur at 200 km depth in the upper mantle and be responsible for the observed lattice preferred orientation lpo transition a rheological law for polycrystalline forsterite is deduced from the single crystal rheological laws assuming that individual grains are randomly oriented in the aggregate applying the aggregate law within a 2d geodynamic model of upper mantle couette flow suggests that the pressure dependence of olivine dislocation slip activities may partly explain the low viscosity zone lvz observed underneath oceanic plates

pressure and temperature dependence of p and s wave velocities seismic anisotropy and density of sheared rocks from the sierra alpujata massif ronda peridotites southern spain seismic wave velocities and densities and their pressure and temperature dependence of rock samples recovered from surface exposures at the sierra alpujata were determined in order to better understand the petrological controls on velocity and density the rock samples studied are representative of the major lithologies of the ronda ultramafic body and its metamorphic aureole the suite of ultramafic rocks includes dunite harzburgite and lherzolite exhibiting different degrees of deformation and serpentinization the rocks representing the metamorphic aureole comprise coarse grained migmatites to ultramylonitic migmatites gneisses and eclogites the experimental data include measurements of elastic wave velocities vp vs and densities at confining pressures up to 600 mpa and temperatures up 600 c at 600 mpa the determination of the pressure and temperature derivatives of velocities and densities and the determination of velocity anisotropy average p and s wave velocities at 600 mpa for rocks of the ultramafic body vary from 6 47 to 8 22 km s 1 and 3 27 to 4 75 km s 1 respectively mainly due to different degrees of serpentinization maximum vp and vs anisotropy is about 8 0 and 9 5 respectively and is primarily related to the preferred orientation of olivine in rocks of the metamorphic aureole average p velocities are in the range 5 61 7 23 km s 1 and s velocities vary from 3 48 to 4 04 km s 1 p and s velocity anisotropy is highest about 6 4 and 4 6 in sheared rocks mylonites and is mainly due to the preferred orientation of biotite and sillimanite the geophysical significance of the experimental data is briefly discussed

petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites abstract ji s salisbury m h and hanmer s 1993 petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites tectonophysics 222 195 226 in order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones we have undertaken experimental measurements of p wave velocities up to 600 mpa for 20 granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada the rocks whose composition ranges from ultramafic to felsic have experienced extremely large ductile strain it is found that the amphibolitic mylonites exhibit significant vp anisotropy 6 13 at 600 mpa whereas the pyroxene bearing granulite facies mylonites and the quartz or feldspathic tonalitic granitic and diatexitic mylonites are quasi isotropic in order to constrain interpretation of the measured vp properties microstructural analyses have been systematically performed in the samples studied it is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low the amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong lpo 001 lineation and 100 foliation the anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic vp x vp y vp z where x is parallel to stretching lineation y is normal to the lineation in the foliation plane and z is normal to the foliation plane but the anisotropy pattern in the mica rich mylonites is transversely isotropic vp x vp y vp z if sufficiently large volumes of the crust display such patterns this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques the study demonstrates that while seismic reflectivity is strongly lithology controlled fabric induced anisotropy can both enhance and decrease seismic reflectivity hence the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones but may indicate changes in composition or metamorphic grade

first results on the lpo derived seismic properties of iron ores from the quadrilatero ferrifero region southeastern brazil abstract determinations of the lpo derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy hematite magnetite and quartz and deformed in different conditions all the samples are seismically quasi isotropic or weakly anisotropic which reflect i an absent or weak crystallographic preferred orientation cpo in some samples ii the high modal content of magnetite iii the relatively weak anisotropy of elastic stiffness of hematite single crystal such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite a plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite magnetite aggregates and in itabirites with strong cpo of hematites in these high strained aggregates some relationships between the crystallographic axes of hematite and the propagation velocities can be observed the magnitudes of p and s wave velocities derived of hematite cpo are lower than the values experimentally determined in iron ores such a difference probably reflects other microstructural variable which were not taken into account in the present contribution e g shape preferred orientation

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

natural examples of olivine lattice preferred orientation patterns with a flow normal a axis maximum tectonic plate motion is thought to cause solid state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals the mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction this has been explained by the existence of an olivine preferred orientation with an a axis maximum parallel to the induced mantle flow direction in subduction zones however the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion which has usually been ascribed to localized complex mantle flow patterns recent experimental work suggests an alternative explanation under conditions of high water activity a b type olivine preferred orientation may form with the a axis maximum perpendicular to the flow direction natural examples of such b type preferred orientation are however almost entirely unknown here we document widespread b type olivine preferred orientation patterns from a subduction type metamorphic belt in southwest japan and show that these patterns developed in the presence of water our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

reply to the comments of s karato on petrofabrics and seismic properties of garnet peridotites from the uhp sulu terrane china by xu et al tectonophysics 421 2006 111 127 abstract we thank karato for his interest in our recent paper in tectonophysics xu z q wang q ji s c chen j zeng l s yang j s chen f y liang f h wenk h r 2006 petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab tectonophysics 421 111 127 however karato is incorrect to take water or water related species herein referred to generically as water as an almost unique cause for any olivine fabric transition in nature no matter what geological conditions are in this reply we will first give a succinct introduction to the context of the ongoing debate and then provide some additional arguments in favour of our conclusion that the olivine c type fabric of the zhimafang garnet bearing peridotites was formed under the conditions of high pressure low temperature and low water content during the subduction of cold and dry supracrustal rocks of the yangtze plate the map of olivine slip systems as a function of differential stress and water content drawn by karato and coworkers cannot be reliably extrapolated to earth s interior until relevant scaling laws have been established the role of high water content on the fabric transitions is more likely to modify recrystallization kinetics than to switch the dominant slip systems

olivine 100 normal to foliation lattice preferred orientation in prograde garnet peridotite formed at high h2o 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 1 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

ultrahigh pressure metamorphism in the forbidden zone the xugou garnet peridotite sulu terrane eastern china the xugou garnet peridotite body of the southern sulu ultrahigh pressure uhp terrane is enclosed in felsic gneiss bounded by faults and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite the peridotite is composed of variable amounts of olivine fo91 enstatite en92 93 garnet alm20 23prp53 58knr6 9grs12 18 diopside and rare chromite the ultramafic protolith has a depleted residual mantle composition indicated by a high mg number very low cao al2o3 and total ree contents compared to primary mantle and other sulu peridotites most garnet prp44 58 clinopyroxenites are foliated except for rare kyanite bearing eclogitic bands most eclogites contain a simple assemblage of garnet alm29 34prp32 50grs15 39 omphacite jd24 36 minor rutile clinopyroxenite and eclogite exhibit lree depleted and lree enriched patterns respectively but both have flat hree patterns normalized la sm and yb contents indicate that both eclogite and garnet clinopyroxenite formed by high pressure crystal accumulation variable trapped melt from melts resulting from two stage partial melting of a mantle source recrystallized textures and p t estimates of 780 870 c 5 7 gpa and a metamorphic age of 231 11 ma indicate that both mafic and ultramafic protoliths experienced triassic uhp metamorphism in the p t forbidden zone with an extremely low thermal gradient 5 c km 1 and multistage retrograde recrystallization during exhumation develop of prehnite veins in clinopyroxenite eclogite felsic blocks and country rock gneiss and replacements of eclogitic minerals by prehnite albite white mica and k feldspar indicate low temperature metasomatism

continental mantle seismic anisotropy a new look at the twin sisters massif abstract recent interpretations of upper continental mantle seismic anisotropy observations have often relied on fabric measurements and calculated anisotropies of upper mantle xenoliths seismic ray paths of p and s waves which provide information on azimuthal compressional wave anisotropy and shear wave splitting are tens to hundreds of kilometers whereas xenoliths are usually only a few centimeters in diameter to place better constraints on field based anisotropy observations and to evaluate anisotropy information provided by xenoliths it is important to examine anisotropy in large ultramafic massifs which have originated in the upper mantle one such massif is the twin sisters range located in the western portion of the north cascades of washington state usa the twin sisters massif a slab of unaltered dunite is 16 km in length 6 km in width and 3 km thick exposed along its south and west sides are mafic granulite facies rocks which likely represent lower continental crustal fragments the ultramafic rocks are porphyroclastic in texture consisting of strained flattened porphyroclasts of olivine and enstatite and strain free olivine mosaics olivine fabrics are typical of those formed at high temperatures and low strain rates petrofabrics and calculated anisotropies of individual samples vary throughout the massif however overall anisotropy of the body is significant with maximum p and s waves anisotropies of 5 4 and 3 9 respectively the maximum delay time for split shear waves traveling through a 100 km thick slab is 0 8 s and two directions of shear wave singularity are observed the directions of maximum shear wave splitting and shear wave singularities do not coincide with the directions of maximum and minimum compressional wave velocity in general individual hand samples show significantly higher anisotropy than the overall anisotropy of the massif it is concluded that simple averages of xenolith anisotropies are unreliable for use in the interpretation of field anisotropy observations

evidence of high water content in the deep upper mantle inferred from deformation microstructures deep upper mantle rocks from the norwegian caledonides show evidence for large strain deformation in both olivine and garnet under varying water contents using microstructural observations including lattice preferred orientation of olivine and subgrain boundaries of majoritic garnet we infer the following deformation history at depths exceeding 150 km large strain deformation occurred at low stress 10 mpa and modest temperature 1300 k involving high water content 1000 h 10 6 si in olivine this was followed by low strain deformation at lower water content 200 1000 h 10 6 si and modest stress 40 mpa in the shallower parts these observations show that the deep upper mantle in this region had a considerably higher water content than the upper mantle near mid ocean ridges

experimental deformation of olivine single crystals at mantle pressures and temperatures abstract deformation experiments were carried out in a deformation dia high pressure apparatus d dia on oriented san carlos olivine single crystals at pressure p ranging from 3 5 to 8 5 gpa temperature t from 1373 to 1673 k and in poor water condition oxygen fugacity fo2 was maintained within the olivine stability field and contact with enstatite powder ensured an orthopyroxene activity aopx 1 two compression directions were tested promoting either 1 0 0 slip alone or 0 0 1 slip alone in 0 1 0 crystallographic plane here called respectively a slip and c slip constant applied stress and specimen strain rates e were monitored in situ using time resolved x ray synchrotron diffraction and radiography respectively transmission electron microscopy tem investigation of run products revealed that dislocation creep was responsible for sample deformation comparison of the obtained high p deformation data with the data obtained at room p by bai et al bai q mackwell s l kohlstedt d l 1991 high temperature creep of olivine single crystals 1 mechanical results for buffered samples journal of geophysical research 96 2441 2463 on identical materials deformed at comparable t fo2 aopx conditions allowed quantifying the p effect on a slip and c slip rheological laws a slip transition with increasing pressure from dominant a slip to dominant c slip is documented a slip appears sensitive to pressure which translates into the high activation volume v a 12 4 c m 3 mol in the corresponding rheological law while pressure has little effect on c slip with v c 3 4 c m 3 mol these results may explain the discrepancy between olivine low p and high p deformation data which has been debated in the literature for more than a decade

p wave velocities of polymineralic rocks comparison of theory and experiment and test of elastic mixture rules abstract we compare the p wave velocities vp of 696 dry samples measured at pressures up to 0 6 1 0 gpa with values calculated from the volume fraction and room pressure elastic constants of each constituent mineral using 16 averaging methods the exceptional large number of samples covers almost all common lithologic types of igneous and metamorphic rocks the calculated vp data are in good agreement with laboratory values measured at about 300 mpa even though elastic constants of only 22 common minerals are used in the computation the mean vp of a polymineralic rock is exclusively controlled by the volume fractions of its constituent minerals while grain shape and crystallographic preferred orientations anisotropy and other perturbations have minimum effects the mean vp can be fairly well predicted as long as a relevant mixture rule is used and the volume fraction is accurately determined for each mineral however none of the mixture rules can simultaneously produce the best fit to measured p wave velocities for all the lithologic types one method may work well for one lithology but poor for other lithology even for a given lithology an averaging method may yield good agreement at moderate pressure but poor agreement at high pressure applications of an inappropriate mixture rule will potentially cause the misinterpretation of the crust and mantle vp data in terms of mineralogical compositions and structures

geochemistry of high grade eclogites and metarodingites from the central alps analyses for major minor and trace element contents of metamorphosed variably rodingitized mafic rocks demonstrate substantial removal of na and as much as three fold gains in ca as a consequence of rodingitization modest declines in si and fe can be explained in terms of dilution effects losses in k and ba do not correlate with ca and may have been caused by an alteration process not related to the rodingitization the ca metasomatism was not accompanied by a gain in sr the relative contents of ti zr hf y co sc and heavy ree show no readily detectable changes despite the rodingitization other alteration and subsequent metamorphisms namely eclogite facies t 800 c p 20kbar followed by amphibolite facies sillimanite zone protoliths were tholeiitic basalt or diabase and gabbro with trace element contents indicative of a spreading center origin trace element and ree patterns indicate low pressure fractionation of this magma with plagioclase stable this petrogenesis is consistent with prior conclusions on the shallow crustal origin of the protolith of the eclogite metarodingite garnet lherzolite suite in the cima lunga adula nappe central alps based on their bulk chemical composition the mafic rocks in this suite could be the equivalent of mesozoic ophiolitic rocks in the more external parts of the alps

fabric development in a middle devonian intraoceanic subduction regime the careon ophiolite northwest spain abstract a middle devonian suprasubduction zone ophiolite the careon unit northwest spain displays amphibolite facies ductile deformation fabrics related to the onset of the rheic ocean closure two different fabrics an early high t foliation and a subsequent lower t foliation each of which characterized by distinct deformation mechanisms have been identified in two distinct crustal scale shear zones of the same ophiolitic thrust sheet combined quantitative texture analysis by electron backscattered diffraction and time of flight neutron diffraction were carried out on the shear zones and correlated with micro and macrostructural data the results indicate that the regional lineation and shear zone kinematics east west top to the east represent fabrics developed essentially during the intraoceanic subduction of the rheic ocean and their orientation may be considered a reference vector for convergence models in this part of the variscan belt

fluid generation and evolution during exhumation of deeply subducted uhp continental crust petrogenesis of composite granite quartz veins in the sulu belt china

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

pressure sensitivity of olivine slip systems and seismic anisotropy of earth s upper mantle the mineral olivine dominates the composition of the earth s upper mantle and hence controls its mechanical behaviour and seismic anisotropy experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks have led to the conclusion that olivine at upper mantle conditions deforms essentially by dislocation creep with dominant 100 slip the resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth the rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle but new high pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle but with a different slip direction here we show that this high pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy consistent with seismological observations below 250 km depth these results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

a novel ebsd based finite element wave propagation model for investigating seismic anisotropy application to finero peridotite ivrea verbano zone northern italy a novel electron backscatter diffraction ebsd based finite element fe wave propagation simulation is presented and applied to investigate seismic anisotropy of peridotite samples the fe model simulates the dynamic propagation of seismic waves along any chosen direction through representative 2d ebsd sections the numerical model allows separation of the effects of crystallographic preferred orientation cpo and shape preferred orientation spo the obtained seismic velocities with respect to specimen orientation are compared with voigt reuss hill estimates and with laboratory measurements the results of these three independent methods testify that cpo is the dominant factor controlling seismic anisotropy fracture fillings and minor minerals like hornblende only influence the seismic anisotropy if their volume proportion is sufficiently large up to 23 the spo influence is minor compared to the other factors the presented fe model is discussed with regard to its potential in simulating seismic wave propagation using ebsd data representing natural rock petrofabrics

p wave velocities anisotropy and hysteresis in ultrahigh pressure metamorphic rocks as a function of confining pressure 1 the chinese continental drilling project ccsd has drilled to a depth of 5100 m at maobei n34 40 e118 67 donghai county jiangsu province in the eastern segment of the dabie sulu ultrahigh pressure uhp metamorphic terrane the borehole which penetrated through all of the high velocity layers and seismic reflectors observed within the uppermost crust on seismic refraction and reflection profiles reveals the main lithologies to be coesite bearing felsic gneisses metabasic rocks i e amphibolite retrogressed and non retrogressed eclogites and ultramafic rocks i e garnet peridotite and serpentinite p wave velocities anisotropy and hysteresis of 31 typical ccsd core samples and 35 representative surface samples collected from the sulu uhp belt were measured at hydrostatic confining pressures up to 800 mpa the velocity pressure curves can be well described by a four parameter exponential equation derived from theory v p v0 dp b0 exp kp where v0 is the projected velocity at zero pressure if pores cracks were absent d is the intrinsic pressure derivative of velocity in the linear elastic regime b0 is the initial velocity drop caused by the presence of pores cracks at zero pressure and k is the decay constant of the velocity drop in the nonlinear poro elastic regime the seismic hysteresis is caused by irreversible changes in grain contact increases in microcrack aspect ratios and reduction of void space during the pressurization depressurization cycle the statistical properties of p wave velocities in the uhp rocks provide an important set of basic information for the interpretation of field seismic data from the root zones of continental convergent orogenic belts and modern and ancient subduction zones

petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab abstract 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

olivine flow mechanisms at 8 gpa abstract the mechanisms responsible for high temperature olivine deformation are investigated at a pressure of 8 gpa and temperatures up to 1780 k san carlos olivine specimens of different average grain sizes 0 5 and 5 m were deformed simultaneously between hard alumina pistons during relaxation experiments these experiments are carried out in a multi anvil high pressure apparatus coupled with synchrotron x ray radiation the different grain size specimens experienced identical p t stress condition at any given time a new method for measuring strains and strain rates 10 6 s 1 of specimens at high pressure is documented this method uses time resolved in situ x ray imaging and an image analysis computation the microstructures of run products recovered after being quenched at different temperatures were characterized by transmission electron microscopy tem we find that high temperature olivine flow is grain size insensitive at 8 gpa which suggests that dislocation creep dominates olivine deformation at high pressure this result is confirmed by the tem investigation of our deformed specimens in which we find evidences of the activation of olivine dislocation slip systems specimen microstructures are consistent with dynamic recrystallization as an assisting process in olivine deformation during the high pressure experiments extrapolation of our results to the low stress level and large grain size expected in the mantle suggests that dislocation creep assisted by dynamic recrystallization may also dominate natural olivine deformation in the upper mantle

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

into the forbidden zone very low temperatures at high pressures have long been believed to constitute a forbidden zone not realized in earth however recent studies of ultrahigh pressure uhp terranes indicate that these rocks have experienced such forbidden zone conditions liou et al discuss the implications of these observations for plate tectonics

rate of dissolution of carbonate sediments by microboring organisms davies reef australia abstract skeletal carbonate sediments on the lagoon floor of davies reef great barrier reef australia are subject to intense attack by microboring organisms principally algae the rate at which this microboring process dissolves carbonate was measured experimentally fresh unbored molluscan sand samples placed in the shallow lagoon 5 m water depth experienced a weight loss of about 0 9 g 30 g sample or 3 in one year due to dissolution of carbonate by microborers revealed by direct measurement of weight change and point count analysis this figure may be converted to about 350 g caco3 dissolved m2 lagoon floor year which is equivalent to between 18 and 30 of the sediment influx rate to this lagoon averaged over the past 9 000 years consideration of the exp rimental design suggests that the experiment underestimated the true rate of microboring in lagoon floor sediments we conclude that the extent of microboring in carbonate sands may provide information on the rate of sediment deposition and that dissolution of carbonate sediments by microborers is a significant factor in whole reef caco3 budgets

crystal preferred orientations of garnet comparison between numerical simulations and electron back scattered diffraction ebsd measurements in naturally deformed eclogites observations of dislocations sub grains and elongated crystal shapes support plastic deformation of garnet in laboratory experiments and naturally deformed eclogites to evaluate the crystal preferred orientations cpo of garnet formed in axial shortening pure shear and simple shear we performed numerical simulations of cpo development during plastic flow using the visco plastic self consistent model as input for the models we use the slip systems determined by transmission electron microscopy using experimentally deformed specimens although in garnet 66 slip systems are available slip on the k111l 110 system provides over 86 of the total strain in the simulations characteristic cpo distributions are produced for the three deformation paths with the cpo being strongest for axial shortening and weakest for simple shear compared with low symmetry minerals the pole figure densities of garnet which has cubic symmetry are weak k100l axes tend to align with the shortening direction in all three deformation modes the simulations are compared with cpo of naturally deformed garnet from nine eclogite samples from the alps norway and mali which contain 20 40 garnet all samples show weak garnet cpo only two samples have a cpo pattern similar to the simulations for simple shear no samples are similar to simulations for axial strain or pure shear the presence of other weaker minerals such as omphacite and quartz with volume fractions higher than garnet probably prevented garnet from becoming highly strained and developing characteristic cpos in these eclogites higher volume fractions of garnet and higher temperature conditions may however allow the development of garnet cpo in the mantle transition zone particularly within subducted oceanic material morb q 2004 published by elsevier ltd

deformation of olivine at subduction zone conditions determined from in situ measurements with synchrotron radiation abstract we report measurements of the deformation stress for san carlos olivine at pressures of 3 5 gpa temperatures of 25 1150 c and strain rates of 10 7 10 5 s 1 we determine a deformation stress of approximately 2 5 gpa that is relatively temperature and strain rate independent in the temperature range of 400 900 c the deformation experiments have been carried out on a deformation dia d dia apparatus sam85 at x17b2 nsls powder samples are used in these experiments enstatite mgsio 3 3 5 total quality of sample is used as the buffer to control the activity of silica ni foil is used in some experiments to buffer the oxygen fugacity water content is confirmed by ir spectra of the recovered samples samples are compressed at room temperature and are then annealed at 1200 c for at least 2 h before deformation the total plastic and elastic strains macroscopic are derived from the direct measurements of the images taken by x ray radiograph technique the differential stresses are derived from the diffraction determined elastic strains in the regime of 25 400 c there is a small decrease of stress at steady state as temperature increases in the regime of 400 c to the transition temperature the differential stress at steady state 2 5 gpa is relatively insensitive to the changes of temperature and strain rate however it drastically decreases to about 1 gpa and becomes temperature dependent above the transition temperature and thereafter the transition temperature is near 900 c above the transition temperature the flow agrees with power law creep measurements of previous investigations the anisotropy of differential stress in individual planes indicates that the deformation of olivine at low temperature is dominated by 0 0 1 1 0 0 accounting to a slower strain rate in the natural system the transition temperature for the olivine in the slab is most likely in the range of 570 660 c

high pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy the effects of pressure on crystallographic preferred orientation cpo of olivine aggregates were investigated through simple shear deformation experiments at pressures between 2 1 and 7 6 gpa and temperatures of 1493 1673 k under dry conditions using a deformation dia apparatus and the variations in seismic anisotropy were evaluated under the earth s upper mantle conditions we found that the monotonic decrease in seismic anisotropy with depth is caused by the pressure dependency of the seismic properties of a type developed by the 010 100 100 slip system olivine fabric while the rapid decrease is caused by the fabric transition from a type to b c type by the hk0 001 001 slip systems at 7 6 gpa and 1673 k moreover an alternative transition from a type fabric to b type like fabric by the 010 001 001 slip system occurs at 7 6 gpa and lower temperature these two temperature dependent fabric transitions occurring at 7 6 gpa result in low seismic anisotropy with vsh vsv the ratio of horizontally and vertically polarized shear waves 1 at low temperatures i e old continental mantle conditions and vsh vsv 1 at high temperatures i e oceanic mantle conditions at greater depths consistent with seismological observations thus the variations of cpo with pressure and temperature in olivine under dry conditions can explain the seismic anisotropy signatures observed in the upper mantle without invoking other mechanisms

development of a type olivine fabric in water rich deep upper mantle abstract 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 2130 ppm h si in olivine while b type fabric by the 010 001 slip system was observed under moderately wet conditions 750 2130 ppm h si developments of c type by the 100 001 slip system fabric was limited to water poor conditions v sh v sv 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 v sh v sv 200 km depth a strong anomaly of v sh v sv

comments on petrofabrics and seismic properties of garnet peridotites from the uhp sulu terrane china by xu et al tectonophysics 421 2006 111 127

a review of water contents and ductile deformation mechanisms of olivine implications for the lithosphere asthenosphere boundary of continents abstract water plays an important role in the ductile deformation and evolution of the upper mantle water contents of natural olivine from 240 samples reveal a wide variation of 0 170 ppm h2o suggesting heterogeneous water distribution in the continental upper mantle the average water contents 17 13 ppm h2o in kimberlite nodules provide the best estimation of water concentrations in olivine in the lithosphere beneath cratons the very low water contents 7 9 ppm h2o of olivine from basalt xenoliths are caused by significant hydrogen loss during transport while the high values 44 34 ppm h2o in olivine megacrysts from kimberlites reflect restricted fluid rich conditions in the upper mantle to compare deformation in different tectonic environments the western superior province canada the dabie mountains and the north jiangsu basin china are selected to represent an archean craton an orogenic belt and a rift basin respectively using recent flow laws of olivine deformation maps of dry and wet olivine are constructed under p t conditions of the three tectonic units and in a continental subduction zone characterized by p 6 28 gpa and t 900 c for dry olivine diffusion creep is the dominant mechanism in all the cases which is contrary to the widely observed crystal preferred orientation of olivine in peridotites and seismic anisotropy observations for wet olivine only a small amount of water 50 h 106 si can remarkably decrease the stress of dislocation creep and increase contribution of dislocation creep to the deformation of olivine the strain rate profiles of olivine indicate a transition from dislocation creep to diffusion creep at a depth of 220 km which can be related with the lehmann discontinuity characterized by a rapid decrease in seismic anisotropy however the pressure induced fabric transition from 100 slip to 001 slip may be responsible for the lehmann discontinuity in subduction zones therefore rheology of the continental upper mantle is controlled by power law creep of wet olivine and diffusion creep is the dominant deformation mechanism in the deep upper mantle especially for fine grained peridotites the mechanical lithosphere asthenosphere boundary lab can be defined by the characteristic pressure derivative of effective viscosity the sharp lab beneath the dabie mountains and the sulu terrane favors the lithosphere asthenosphere decoupling while the diffuse lab beneath the western superior province will protect the continental root from convective erosion and mantle metasomatism the long term preservation of the continental roots can be attributed to a large viscosity contrast temperature contrast at a depth of 150 km

origin of allanite in gneiss and granite in the dabie orogenic belt central east china abstract allanite is a common accessory mineral phase representing an important carrier of rare earth elements th u sr and other trace elements in most continental rocks as th and u can be incorporated into the allanite lattice the mineral is a good geochronological tool for constraining geological events moreover the trace element features eu th u ratio and common lead content of allanite are indicators of the forming conditions allanite and coexisting epidote group minerals are abundant in ultrahigh pressure uhp metamorphic rocks from the dabie sulu orogen in central east china however if these minerals formed in the neoproterozoic as magmatic phases or in the triassic as metamorphic phases is a matter of long standing controversy we report major and trace element analyses of whole rocks allanite and coexisting epidote group minerals together with u th pb isotopic compositions of allanite in uhp gneiss from the dabie sulu orogen and allanite in the adjacent jingshan granite the granite is emplaced along the southeastern margin of the north china craton and considered a product of partial melting of the subducted dabie sulu gneiss trace elements low th u and la sm high eu and high sr and high common lead concentrations indicate a metamorphic origin of allanite epidote in the uhp gneiss on the other hand coarse grained allanite from the jingshan granite shows a corrosion core and a magmatic rim with common 208 pb up to 70 in the core and less than 30 in the rim the allanite cores are of peritectic and the rims of magmatic origin with ages of 160 ma consistent with the granite crystallization age in combination with previous studies we conclude that the allanite of the jingshan granite has form from the subducted and remolten dabie sulu gneiss allanite records triassic uhp metamorphic ages as well as jurassic peritectic magmatic ages as a part of the evolution of the dabie sulu orogen

pressure induced slip system transition in forsterite single crystal rheological properties at mantle pressure and temperature deformation experiments were carried out in a deformation dia high pressure apparatus d dia on oriented mg2sio4 olivine fo100 single crystals at pressure p ranging from 2 1 to 7 5 gpa in the temperature t range 1373 1677 k and in dry conditions these experiments were designed to investigate the effect of pressure on olivine dislocation slip system activities responsible for the lattice preferred orientations observed in the upper mantle two compression directions were tested promoting either 100 slip alone or 001 slip alone in 010 crystallographic plane constant applied stress sigma and specimen strain rates formula were monitored in situ using time resolved x ray synchrotron diffraction and radiography respectively transmission electron microscopy tem investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation a slip transition with increasing pressure from a dominant 100 slip to a dominant 001 slip is documented extrapolation of the obtained rheological laws to upper mantle p t and sigma conditions suggests that 001 slip activity becomes comparable to 100 slip activity in the deep upper mantle while 001 slip is mostly dominant in subduction zones these results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle and for the puzzling seismic anisotropy anomaliesmore commonly observed in subduction zones less

exhumation tectonics of the ultrahigh pressure metamorphic rocks in the qinling orogen in east china new petrological structural radiometric insights from the shandong peninsula 1 in eastern china the sulu area is recognized as the eastern extension of the qinling dabie belt which is famous for its ultrahigh pressure uhp metamorphism although numerous petrologic and geochemical works are available structural data are still rare this paper provides the first extensive study of bulk geometry and kinematic analysis of the shandong peninsula the study area is divided into three tectonic areas by cretaceous faults namely a southern uhp belt or sulu area a northern migmatite area and an eastern eclogite and migmatite area or weihai area conversely to the deeply entrenched idea that the later area belongs to the north china belt and the two others to the south china block scb we argue that all three areas are parts of the scb structural petrologic 40ar 39ar and u pb data comply with this new interpretation in the north shandong area mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the sulu and weihai eclogites which also experienced a granulite facies overprint before migmatization the circa 210 200 ma age of the main ductile deformation is related to an extensional event during the triassic or indosinian orogeny this date corresponds to the temperature climax but the time of the pressure peak i e the real age of the uhp metamorphism is discussed

hydroxyl rich topaz in high pressure and ultrahigh pressure kyanite quartzites with retrograde woodhouseite from the sulu terrane eastern china hydroxyl rich topaz was recognized in thick kyanite quartzites from both high pressure hp and ultrahigh pressure uhp belts of the sulu terrane china these quartzites contain variable proportions of quartz kyanite and topaz with minor phengite pyrite and rutile some topaz grains from the uhp belt contain abundant inclusions of oriented kyanite whereas those from the hp belt are partially replaced by woodhouseite caal 3 po 4 so 4 oh 6 most topaz crystals contain 9 5 to 13 5 wt f 0 92 1 30 atoms per formula unit apfu indicating 35 55 substitution of f by oh such naturally occurring hydroxyl rich topaz has not been previously reported some topaz grains from the hp belt show distinct zoning 1 decreasing f content from narrow cores 13 3 16 5 wt to thick rims 9 5 wt or 2 oscillatory zoning 9 44 12 77 wt unit cell parameters of topaz show a positive linear correlation between the oh content and a and b as well as volume based on our petrologic data the experimentally determined curve of ky h 2 o topaz oh at very high pressures and calculated oh oh f isopleths of topaz at low pressures the topaz with x oh 0 35 from the uhp belt may have formed at p t conditions within the coesite stability field the p t condition of hp topaz is less well constrained its high x oh 0 40 0 55 may have been caused by lower metamorphic temperatures and higher initial x oh in comparison with uhp topaz hydroxyl rich topaz together with other hydrous minerals in uhp rocks may be important carriers of h 2 o to mantle depths of 100 200 km during continental subduction