strength of single crystal orthopyroxene under lithospheric conditions creep strength of oriented orthopyroxene single crystals was investigated via shear deformation experiments under lithospheric conditions p pressure 1 3 gpa and t temperature 973 1 373 k for the a orientation shear direction 001 on 100 plane the samples have transformed completely to clinoenstatite and much of the deformation occurred after transformation in contrast for the b orientation shear direction 001 on 010 plane samples remained orthoenstatite and deformation occurred through dislocation motion in orthoenstatite the strength of orthopyroxene with these orientations is smaller than for olivine aggregates under all experimental conditions flow of the b orientation samples is described by a power law and the pre exponential constant the apparent activation energy and the stress exponent are determined to be a 10 9 5 s 1 mpa 4 2 q 114 kj mol and n 4 2 however for the a orientation the results cannot be fit by a single flow law and we obtained the following a 108 9 s 1 mpa 3 0 q 459 kj mol and n 3 0 at high temperatures 1 173 k and a 10 27 4 s 1 mpa 14 3 q 296 kj mol and n 14 3 at low temperatures 1 173 k the stress exponent for the low temperature regime is high suggesting that deformation involves some processes where the activation energy decreases with stress such as the peierls mechanism our study shows that orthopyroxene with these orientations is significantly weaker than olivine under the lithospheric conditions suggesting that orthopyroxene may reduce the strength of the lithosphere although the extent to which orthopyroxene weakens the lithosphere depends on its orientation and connectivity

the stress dependence of olivine creep rate implications for extrapolation of lab data and interpretation of recrystallized grain size abstract based on measured values for the stress exponent n 3 5 combined with the empirically determined relationship between dislocation density and stress 1 37 and an analysis of diffusion kinetics in olivine we conclude that silicon pipe diffusion limits strain rate in the dislocation creep regime furthermore assuming that steady state recrystallized grain size is set by a dynamic balance between strain energy density associated with dislocations and surface energy density associated with grain boundaries the resulting dependence of recrystallized grain size on stress accurately describes experimental observations when the empirical dislocation density versus stress relationship is accounted for d 1 1 37 the improved physical understanding of the stress dependence of creep rate provides justification for incorporation of experimentally derived flow laws into models of geodynamical process and grain size evolution these lab constraints combined with independent analyses of the stress dependence of mantle viscosity based on geophysical data provide bounds on rheological properties such as the yield stress of the lithosphere

switch off of a major enhanced ice flow unit in east antarctica 1 the east antarctic ice sheet eais is the largest reservoir of ice on the planet by an order of magnitude compared with the west antarctic ice sheet wais it is traditionally considered to be relatively stable with only minor adjustments to its configuration over glacial interglacial cycles here we present the results of a radio echo sounding survey from coats land east antarctica which suggests that parts of the eais outlet drainage system may have changed significantly since the last glacial maximum we have identified an enhanced flow unit from buckled internal layering and smooth bed morphology that is no longer active we believe this feature to have shut down at some point since the last glacial maximum 20 ka bp

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

deformation fabrics of the cima di gagnone peridotite massif central alps switzerland evidence of deformation at low temperatures in the presence of water we report a new observation of the olivine b type lattice preferred orientation lpo from the garnet peridotite at cima di gagnone switzerland the olivine b type fabric forms at low temperatures and or high stress in the presence of water and is of particular interest because it may be used to explain the trench parallel shear wave splitting that is often observed at subduction zones in conjunction with the olivine b type fabric we have found strong orthopyroxene lpo that is identical to those formed under water free conditions this suggests that water may not have a significant effect on orthopyroxene fabric from the olivine microstruc ture we determine that a stress of 22 8 mpa was applied during the deformation event that formed the olivine lpo using an olivine flow law and assuming geological strain rates we determine the temperature of deformation to be 800 175c this does not preclude an ultra deep origin for the ultramafic rocks at cima di gagnone but indicates that much of the deformation recorded in the microstructure occurred at modest temperatures

lattice preferred orientation and microstructure of peridotites from odp hole 1274a 15 39 n mid atlantic ridge testing models of mantle upwelling and tectonic exhumation abstract eleven harzburgites and one dunite from ocean drilling program leg 209 hole 1274a preserve high temperature mantle textures electron backscatter diffraction ebsd analysis shows moderately developed crystal lattice preferred orientations lpos in olivine and orthopyroxene m indices 0 1 indicative of crystal plastic deformation at 1250 c these rocks preserve a protogranular texture with a weak olivine foliation a very weak or absent orthopyroxene foliation that may be decoupled from the orthopyroxene lpo and minor interstitial clinopyroxene and spinel olivine grain size distributions along with melt related microstructures in orthopyroxene clinopyroxene and spinel suggest that high temperature deformation textures have been overprinted by pervasive post deformation melt rock interaction paleomagnetic data constrain the olivine 100 axes to be subhorizontal and oriented at low angle 28 6 10 6 to the ridge axis at the onset of serpentinization this orientation is consistent with either complex 3 d mantle upwelling or 2 d mantle upwelling coupled with complex 3 d tectonic emplacement to the seafloor

dehydration breakdown of antigorite and the formation of b type olivine cpo abstract peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the happo area central japan shows a strong olivine crystallographic preferred orientation ol cpo the lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine cpo in this area did not form as a result of solid state deformation instead the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the cpo formed by topotactic growth of the olivine after antigorite ol cpo is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable the happo ol cpo has a strong a axis concentration perpendicular to the lineation and within the foliation commonly referred to as b type ol cpo seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of b type ol cpo in the mantle wedge of these regions experimental work has shown that b type cpo can form by dislocation creep under hydrous conditions at relatively high stresses there are however several discrepancies between the characteristics of natural and laboratory samples with b type ol cpo 1 the formation conditions stress and temperature of some natural examples with b type cpo fall outside those predicted by experiments 2 in deformation experiments slip in the crystallographic c axis direction is important but has not been observed in natural examples of b type cpo 3 experimental work suggests the presence of h2o and either high shear stress or relatively low temperatures are essential for the formation of b type cpo these conditions are most likely to be achieved close to subduction boundaries but these regions are also associated with serpentinization which prevents strong olivine cpo patterns from forming we show b type ol cpo can form as a result of static topotactic growth of olivine after high temperature breakdown of foliated serpentinite these results resolve the discrepancies between experimental and natural examples of b type cpo and show the need to rethink the formation process of olivine cpo in convergent margins topotactic growth of olivine after antigorite can account for the inferred distribution of b type ol cpo in the mantle wedge more successfully than dislocation creep

equatorial ionospheric electrodynamic perturbations during southern hemisphere stratospheric warming events 1 we use ground based and satellite measurements to examine for the first time the characteristics of equatorial electrodynamic perturbations measured during the 2002 major and 2010 minor southern hemisphere sudden stratospheric warming ssw events our data suggest the occurrence of enhanced quasi 2 day fluctuations during the 2002 early autumnal equinoctial warming they also show a moderately large multi day perturbation pattern resembling those during arctic ssw events during 2002 late equinox as the major ssw was weakening we also compare these data with extensive recent results that showed the fundamentally important role of lunar semidiurnal tidal effects on low latitude electrodynamic perturbations during arctic ssw events

rheological contrast between garnet and clinopyroxene in the mantle wedge an example from higashi akaishi peridotite mass sw japan abstract garnet clinopyroxenites occur within foliated dunite in the higashi akaishi peridotite mass located within the subduction type high pressure low temperature sanbagawa metamorphic belt the garnet clinopyroxenites contain 3 80 garnet and garnet and clinopyroxene are homogeneously distributed garnet crystals contain extensive regular dislocation arrays and dislocation networks suggesting that dislocation creep was the dominant deformation mechanism analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene regardless of modal composition indicating that these minerals deformed with similar degree of plasticity moreover indexes of crystallographic fabric intensity i e j index and m index for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet fourier transform infrared spectroscopy analysis revealed that water content in garnet is 60 ppm whereas that in clinopyroxene is 70 ppm olivine crystal preferred orientations in the higashi akaishi peridotite mass characterized by 0 0 1 0 1 0 are thought to have developed during deformation under wet conditions consequently we argue that the presence of water could act to enhance garnet plasticity during deformation the results reveal contrasting influences of water on the deformation of garnet and diopside under wet conditions compared with dry the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside given the influence of water on the creep strength of garnet garnet within the higashi akaishi mass may have become significantly as weak as clinopyroxene during deformation

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

remote sensing of hydrogen in earth s mantle hydrogen1 in earth s interior is known to play a key role in a number of processes consequently inferring the distribution of hydrogen is a critical step in our study of dynamics and evolution of earth usually hydrogen distribution is inferred from two types of samples at the surface first a magma may contain hydrogen water and under some conditions the water content of the magma can be quenched upon cooling in these cases measurements of the water content of the magma provide us with some constraints on the hydrogen water content of the source region if we know the partitioning of hydrogen between magmas and the source rocks and the degree of melting second hydrogen content of some xenoliths transported by magma can be measured they provide a direct clue as to the hydrogen content in a region from which a xenolith has been carried however these direct petrologic approaches have two major problems firstly the sampling is limited by the distribution of volcanoes and even if there are volcanoes that carry rocks from earth s interior the depth extent that volcanoes sample rocks is limited usually 200 km secondly there is no guarantee that the hydrogen content that one measures on these samples actually represents the hydrogen content in a region where these samples came from for example hydrogen is known to diffuse very easily so hydrogen dissolved in minerals could diffuse out during the transport of a rock or conversely a piece of rock may acquire extra hydrogen during its ascent also hydrogen atoms dissolved in minerals may precipitate in the mineral to form fluid inclusions or micro scale hydrous minerals in summary the direct method to infer the distribution of hydrogen from rock samples has major limitations and an alternative approach i e remote sensing hydrogen content from geophysical

flow behavior and microstructures of hydrous olivine aggregates at upper mantle pressures and temperatures deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation dia apparatus combined with synchrotron in situ x ray observations at pressures of 1 5 9 8 gpa temperatures of 1223 1800 k and strain rates ranging from 0 8 10 5 to 7 5 10 5 s 1 the pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 6 cm3 mol 1 and a stress exponent of 3 2 0 6 at temperatures of 1323 1423 k a comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323 1423 k is much weaker than that for the dislocation creep of water saturated olivine and is similar to that for diffusional creep and dislocation accommodated grain boundary sliding while dislocation microstructures showing the 001 slip or the 001 100 slip system were developed at temperatures of 1633 1800 k a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 7 cm3 mol 1 assuming a stress exponent of 3 5 water fugacity exponent of 1 2 and activation energy of 520 kj mol 1 i e power law dislocation creep of hydrous olivine because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures water weakening of olivine could be effective in the deeper and colder part of earth s upper mantle

an experimental investigation on fluid transfer mechanisms in ultramafic rocks abstract static annealing and crack cook experiments were performed on wet and dry fabricated san carlos olivine aggregates and a core of balsam gap dunite to investigate fluid infiltration mechanisms for low permeability mantle rocks deionized water 30 40 l was added to cold pressed olivine powders before hot pressing to make wet samples samples hot pressed without added water are dry

the preservation potential of microstructures during static grain growth grain growth simulations using the microstructure simulation system elle have been performed in materials with a pre existing grain shape foliation as might be expected the foliation is destroyed by the end of the experiment and grain areas have increased by a factor of seven the area of material swept by the migrating grain boundaries was monitored and it was found that at every stage virtually all of the grains which survived the grain growth process contain one and only one core of unswept material remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation this work suggests that even for samples where no equivalent protolith can be found it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock and perhaps even reconstruct the grain boundary kinematics in addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism as an example of a natural system showing these microstructures we describe a peridotite from almklovdalen norway this peridotite was infiltrated by aqueous fluids at several stages during late caledonian exhumation and retrogressive metamorphism grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite peridotite at the grain scale microstructural mapping of the fluid inclusion rich areas shows that as with the numerical simulations many of the grains retain exactly one core of unswept material examples of other natural systems discussed include dislocation density distributions and trace element zoning

influence of grain size evolution on convective instability 1 grain size one of the most important microstructural properties of materials evolves during creep deformation to minimize the free energy of polycrystalline aggregates we apply a model of grain size evolution to the study of convective instability of cooling boundary layers the grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep the onset of convection is sensitive to grain growth rates and the initial grain size the formation of convective instabilities is enhanced by stresses induced by plate motions therefore small scale convection is more likely to occur beneath fast moving plates in finite amplitude convection grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude such viscosity contrasts are sufficient to influence the dynamics of convection often leading to domains which remain isolated from the well mixed convecting fluid a composite viscosity including diffusion creep which has a lower activation energy than dislocation creep reduces the effective temperature dependence of viscosity

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

effect of temperature on the b to c type olivine fabric transition and implication for flow pattern in subduction zones abstract the effect of temperature on the b to c type fabric transition in olivine was investigated by large strain shear deformation experiments a new series of deformation experiments was conducted at 1273 1373 k 2 0 gpa and strain rates ranging from 3 8 10 4 to 3 2 10 5 s 1 under water saturated conditions and combined with previous results at 1473 k the influence of temperature on deformation fabrics was investigated two types of olivine fabrics were identified one in which the olivine 0 0 1 axis is subparallel to the shear direction and the 0 1 0 plane is parallel to the shear plane b type and another one in which the 0 0 1 axis is parallel to the shear direction and the 1 0 0 plane is parallel to the shear plane c type the b type olivine fabric occurs at higher stress than the c type but the stress magnitude at which the b to c type fabric transition occurs decreases significantly with decreasing temperature the temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study when the experimental results are extrapolated to lower temperatures the fabric transition between b and c type is predicted to occur at 1 mpa and 1000 k this prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate convergent regions the present study indicates that when some amounts of water 200 ppm h si are present the b type fabric will dominate in low temperature regions of the upper mantle whereas the c type or e type fabric will dominate at high temperatures the complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables such as temperature and water content rather than by complexities in mantle wedge flow pattern

theories and applicability of grain size piezometers the role of dynamic recrystallization mechanisms abstract the average grain size d arising from dynamic recrystallization drx is often used as an indicator of flow stress however a theoretical basis for the scaling relation between d and has yet to be well established in this paper theories for the development of recrystallized grain size are reviewed and their applicability is examined special attention is paid to the dependence of the d relation on drx mechanisms steady state drx is classified into discontinuous drx with bulging blg nucleation grain boundary migration gbm and continuous drx with subgrain rotation sgr nucleation gbm the nucleation and growth model derived from derby ashby theory describes the former case whereas that derived from shimizu theory applies to the latter a static energy balance model derived from twiss theory is applicable to subgrain size but not to recrystallized grain size the lower limit of grain size is possibly constrained by a change in deformation mechanism from dislocation creep to diffusion creep because deformation induced grain size reduction ceases in the diffusion creep field scaling relations determined in the laboratory support the shimizu model in the case of sgr gbm the theoretical piezometer calibrated for quartz suggests significant temperature effects under low temperature metamorphic conditions

fabric development in experimentally sheared quartzites abstract a fine grained quartzite was deformed under a combination of simple shear and axial compression to varying amounts of shear strain 1 3 to 2 9 the experiments were conducted at 800 c 1500 mpa and an axial strain rate of 10 6 s in the dislocation creep regime shear zone boundaries were oriented at 45 or 30 to the compressior axis the progressive development of deformation microstructures including planar fabrics and c axis preferred orientations was studied as a function of increasing shear strain elongate original grains define a foliation s which rotates from 45 to 0 to the shear zone boundary with increasing strain producing a sigmoidal foliation in a single shear zone a second foliation invariant in orientation is defined by elongate recrystallized grains which lie at about 30 40 to the shear zone boundaries no c surfaces parallel to the shear zone boundaries were observed however extensional crenulation cleavages c defined by parallel zones of recrystallized grains as well as impurity and melt segregations were observed even at low strain lying at approximately 30 to the shear zone boundaries and these could easily be mistaken for c surfaces c axis preferred orientations of original grains define symmetric type i crossed circle girdles at low strain 2 with the greater concentration of c axes in the direction of shear these features are very similar to those observed in natural ductile shear zones

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

hydrolytic weakening of olivine at mantle pressure evidence of 100 010 slip system softening from single crystal deformation experiments abstract to date most of the experiments performed to quantify the hydrolytic weakening of olivine single crystals have been performed at low confining pressure and low water fugacities in order to determine if the degree of hydrolytic weakening changes at higher water fugacities higher water contents we have performed a series of high pressure p and high temperature t deformation experiments in hydrous condition on san carlos olivine fo92 and forsterite fo100 single crystals in a deformation dia apparatus d dia deformation was carried out in axial compression along the 1 1 0 c crystallographic reference frame direction in order to activate 1 0 0 0 1 0 dislocation slip system alone at p ranging from 4 to 7 gpa and t 1200 c water was supplied during the experiments through dehydration of a talc sleeve talc enstatite coesite h 2 o around the sample while the differential stress and strain rate of the specimen were calculated from in situ x ray diffraction and time resolved imaging respectively run products were investigated using fourier transform infrared ftir spectroscopy and optical electron microscopy sem and tem structurally bound hydroxyl content c oh derived from absorbance in ftir spectra based on the paterson 1982 relation were typically between 290 and 720 ppm h si in the run products and the majority of these hydroxyls were observed as distinct peaks in the range of wavenumber from 3620 to 3500 cm 1 tem reveals that the 1 0 0 0 1 0 slip system was effectively activated and responsible for crystal deformation and that dislocation glide was significantly assisted by recovering processes such as dislocation climb and dipole annihilation combining the experimental data from this study and mackwell et al 1985 we estimate the activation volume v 110 c for san carlos olivine in order to produce a reasonable water fugacity exponent r in the flow law and obtain v 110 c 12 1 cm 3 mol for r 0 and v 110 c 17 3 cm 3 mol for r 1 2 comparisons of the strengths of the wet single crystals to the strengths of dry single crystals calculated from the rheological laws bai et al 1991 darot and gueguen 1981 raterron et al 2009 indicate that the wet single crystal cylinders were between 1 3 and 1 6 times weaker than their dry counterpart at same conditions which is consistent with previous studies of 1 0 0 0 1 0 olivine rheology performed at low confining pressure 300 mpa

the effect of dynamic recrystallization on olivine fabric and seismic anisotropy insight from a ductile shear zone oman ophiolite abstract subhorizontal mantle structures subparallel to the moho are rotated into nw se subvertical orientations across a shear zone in a sinistral sense of shear within the northern fizh mantle section of the oman ophiolite dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture mean olivine grain size stabilized at 0 7 mm within the shear zone center this may reflect the steady state grain size of dynamically recrystallized olivine as determined by the deviatoric stress which in this case was as low as 10 mpa crystal preferred orientation cpo patterns of olivine are consistently 100 fiber or partial fiber texture indicating that olivine slip systems did not change during shearing dynamic recrystallization causes a weakening of olivine fabric intensity toward the shear zone center but this weakening is counterbalanced by cpo strengthening due to dislocation glide this process resulted in an abrupt decrease in seismic anisotropy at the center of the shear zone in contrast to a gradual decrease in olivine fabric intensity and mean grain size the measured seismic anisotropy patterns did not change in ways that would be significantly measurable by seismological observations despite the development of the shear zone dispersion of both p and s waves in the shear zone may be of little effect with respect to the overall seismic anisotropy this is not only because the shear zone occurs substantially in a narrow region but also because the seismic anisotropy is weaker in the shear zone than the high t structure region it suggests that a record of simple systematic seismic anisotropy observed in the upper mantle may indicate a simplified mantle flow structure as localized structures may be obscured in the region of the observation

accurate differential stress measurement using the molten salt cell and solid salt assemblies in the griggs apparatus with applications to strength piezometers and rheology abstract calibrations have been developed for the molten salt cell and solid salt assemblies used in tri axial griggs type piston cylinder deformation apparatus griggs apparatus that enable accurate differential stress determinations in both assemblies at high pressures and high resolution differential stress measurements in the molten salt cell elastic response of the apparatus was characterized by loading alumina and wc rods and flow strengths were measured for high purity nickel and molybdenum and a ti zr mo alloy in replicate experiments in a griggs apparatus and a heard tri axial gas apparatus gas apparatus at identical pressures temperatures and strain rates the elastic response of the griggs apparatus is nonlinear at low total loads 1 760 mpa on a 5 mm diameter sample but becomes linear at loads greater than 15 kn plastic deformation experiments on water insensitive metals indicate that differential stresses measured in the molten salt cell msc and solid salt ssa assemblies in the griggs apparatus are greater than those measured in the gas apparatus at all conditions but vary systematically and reproducibly a simple linear calibration gas apparatus 0 73 griggsmsc 10 mpa where differential stress 1 3 can be applied to the differential stresses obtained using the msc in the griggs apparatus to obtain sample flow strength a similar linear calibration gas apparatus 0 73 griggsssa 48 mpa 30 mpa can be applied to the differential stresses obtained using the griggs apparatus with a solid salt assembly to obtain accurate sample flow strength these calibrations have been tested by applying them to mechanical data for carbonates deformed in the griggs apparatus and gas apparatus the calibrations are also applied to data from previous studies performed in the griggs apparatus on clinopyroxenites and quartzites and compared to results from similar experiments using the gas apparatus when the experimental starting materials and conditions are the same the recalculated flow strengths measured in the griggs apparatus experiments agree with those measured in the gas apparatus however discrepancies in differential stress are still observed in cases where starting materials differ or water available to silicate samples varies presumably through hydrogen diffusion through sample jackets and hydrogen fugacities of the surrounding confining media

seismic wave attenuation energy dissipation in viscoelastic crystalline solids seismic imaging as a tool to understand the structure and perhaps the dynamics of the planet at depth involves the spatially resolved combined study of wave velocities and velocity dispersion wave birefringence and wave attenuation mechanical absorption when integrated with insights from petrology plus or minus input from magneto tellurics specific hypotheses concerning structure at depth can be formulated cf karato 1993 karato and karki 2001 the recent advances in tomography have allowed significant improvements in spatial resolution which also have allowed ever more exacting hypotheses of structure to be articulated nevertheless as is nicely illustrated by the relatively recent seismic analyses of an accreting plate margin the east pacific rise e g toomey et al 1998 webb and forsyth 1998 the structures inferred at depth can vary significantly clearly the interpretation of seismic data is limited specifically by a lack of understanding of the physical processes by which low frequency wave absorption occurs particularly in the cases a where melt is present and or b where the material is being actively plastically deformed this chapter emphasizes the mineral physics materials science of mechanical absorption specifically in dense materials at elevated temperature the interest then is in employing the physical study of mechanical absorption to understand natural phenomena such as those mentioned above beyond questions of the basic mineral physics of absorption the guiding interest the community of scholars pursuing the ideas is in isolating actual absorption mechanisms operative in the geological setting thereby allowing for a greater discrimination in the interpretation of seismic data at the outset i note that this contribution does not seek to be a comprehensive review of the ideas and work involved in the physics of mechanical absorption there are marvelous reviews in the literature that are both authoritative and current i refer to these frequently in contemplating my own

detecting seismic anisotropy above the 410 km discontinuity using reflection coefficients of underside reflections abstract we investigate the effect of various types of deformation mechanisms on the reflection coefficients of p and s waves underside reflections off the 410 km discontinuity to develop a diagnostic tool to detect the style of deformation at boundary layers we calculate the reflection coefficient for p and sh underside reflections using velocity perturbations resulting from aligned minerals above the 410 km discontinuity for different deformation scenarios the results show that in the case of an anisotropic olivine layer above an isotropic wadsleyite layer the p wave reflection coefficient amplitudes are only slightly influenced by the joint effect of angle of incidence and the strength of imposed deformation without any polarity reversal and for all deformation styles for the sh wave underside reflections the incidence angle for which a polarity reversal occurs changes with distance for all scenarios and in addition changes with azimuth for shear deformation scenarios these differences in amplitude and polarity patterns of reflection coefficients of different deformation geometries especially for s wave at shorter distances potentially provide a possibility to distinguish between different styles of deformation at a boundary layer we also show a first test using currently available elastic constants of anisotropic wadsleyite beneath anisotropic olivine

deformation of olivine under mantle conditions an in situ high pressure high temperature study using monochromatic synchrotron radiation polycrystalline samples of san carlos olivine were deformed at high pressure 2 8 7 8 gpa high temperature 1153 to 1670 k and strain rates between 7 10 sup 6 and 3 10 sup 5 s sup 1 using the d dia apparatus stress and strain were measured in situ using monochromatic x rays diffraction and imaging respectively based on the evolution of lattice strains with total bulk strain and texture development we identified three deformation regimes one at confining pressures below 3 4 gpa one above 4 gpa both below 1600 k and one involving growth of diffracting domains associated with mechanical softening above approx 1600 k the softening is interpreted as enhanced grain boundary migration and recovery below 1600 k elasto plastic self consistent analysis suggests that below 3 4 gpa deformation in olivine occurs with large contribution from the so called a slip system 100 010 above approx 4 gpa the contribution of the a slip decreases relative to that of the c slip 001 010 this conclusion is further supported by texture refinements thus for polycrystalline olivine the evolution in slip systems found by previous studies may be progressive starting from as low as 3 4 gpa and up to 8 gpa during such a gradual change activation volumes measured on polycrystalline olivine cannot be linked to a particular slip systemmore straightforwardly the quest for the activation volume of olivine at high pressure should cease at the expense of detailed work on the flow mechanisms implied such evolution in slip systems should also affect the interpretation of seismic anisotropy data in terms of upper mantle flow between 120 and 300 km depth less

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

origin of deformed halite hopper crystals pseudomorphic anhydrite cubes and polyhalite in alpine evaporites austria germany the alpine haselgebirge formation represents an upper permian to lower triassic evaporitic rift succession of the northern calcareous alps eastern alps although the rocksalt body deposits are highly tectonised consisting mainly of protocataclasites and mylonites of halite and mudrock the early diagenetic history can be established from non tectonised mudrock bodies cm sized euhedral halite hopper crystals formed as displacive cubes within mud just during shallow burial the crystals were deformed by subsequent compaction later migrating fluids led to the replacement of halite by anhydrite retaining the shapes of deformed halite cubes polyhalite formed from subsequent enhanced fluid migration mudrock provided water by dewatering while potassium and magnesium were dissolved from primary salt minerals when these fluids interacted with sulphates polyhalite precipitated 40ar 39ar analyses date the polyhalite from within the retaining shapes of deformed halite hopper shaped cubes from two localities to ca 235 232 ma middle triassic at this time ca 20 25 ma after sedimentation polyhalite crystallised at shallow levels

kinetics of the post garnet transformation implications for density and rheology of subducting slabs abstract mechanisms and kinetics of the post garnet transformation in natural pyrope were examined at 28 1 31 3 gpa and 1320 2150 k by time resolved x ray diffraction measurements using synchrotron radiation combined with microstructural observations of the recovered samples the water content of samples recovered was estimated to be approximately 400 wt ppm h 2 o from infrared spectroscopic measurements the transformation occurs by the grain boundary reaction and reaction rims are formed along the grain boundaries of parental garnet grains decomposed post garnet assemblages consisting of mg and ca perovskites aluminous phase and stishovite show very fine grained symplectic texture analyses of kinetic data obtained have revealed that growth rates in the post garnet transformation are strongly time dependent extrapolations of the present kinetic data suggest that the post garnet transformation cannot complete at less than ca 1730 k even under geological timescales because of the slow growth kinetics due to the presence of the metastable garnet subducting slabs become buoyant in the lower mantle at less than ca 1730 k for the basaltic crust and at ca 1400 1600 k for the underlying peridotite layer rheological weakening of the basaltic crust is expected in the lower mantle through grain size reduction by the metastable post garnet transformation occurring under large overpressure conditions the kinetics of the post garnet transformation significantly affect both the density and rheology of subducting slabs in the lower mantle and possibly has important roles in the mixing and survival of chemically differentiated slab materials

role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization for plate tectonics to operate on a terrestrial planet the surface layer the lithosphere must have a modest strength earth 200 mpa but a standard strength profile based on olivine far exceeds this threshold value consequently it is essential to identify mechanisms that reduce the strength of the lithosphere on earth here we report results of high strain laboratory deformation experiments on a representative olivine orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window the reduction in strength is associated with the formation of small orthopyroxene and olivine grains our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones fine grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated a model is developed to apply these results to geological conditions such a model combined with our experimental observations suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range leading to long term weakening associated with strain localization in the lithosphere

effects of water on dynamically recrystallized grain size of olivine abstract recrystallized grain size versus stress relationship for olivine has been determined under water poor and water rich conditions we found that when the water content in olivine exceeds 800 ppm h si the size of recrystallized grains is about two to three times larger than under water poor conditions compared at the same stress level microstructural observations of deformed samples show that the ratio of recrystallized grain size to that of subgrains increases significantly with water content and that the volume fraction of grains containing subgrain boundaries is smaller under water rich conditions than under water poor conditions these observations suggest that the enhancement of grain boundary mobility by water is more significant than that of dislocation recovery leading to a larger size of recrystallized grains under water rich conditions

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

dislocation creep of fine grained olivine 1 deformation experiments conducted in a gas medium apparatus at temperatures from 1200 to 1350 c with a fine grained solution gelation derived fe bearing olivine show a stress dependence of the strain rate at stresses above 150 mpa which is much stronger than previously reported for polycrystalline samples the data can be fit by a power law with n with n 7 8 or equally well by a peierls creep law with exponential stress dependence due to the observed strong stress dependence the samples deform at significantly higher strain rates at a given stress than single crystals or coarse grained polycrystals with n 3 5 tem observations indicate the presence of dislocations with at least two different burgers vectors with free dislocations predominantly of screw character subgrain walls are present but are only weakly developed and have small misorientation angles both the rheology and dislocation structures are consistent with creep rate limited by dislocation glide or cross slip for aggregates with grain sizes smaller than or approaching the recrystallized grain size deformation mechanism maps extrapolated to lithospheric temperatures using the melt free diffusion creep rheology of faul and jackson 2007 the dislocation creep rheology of hirth and kohlstedt 2003 and the results described here indicate that deformation conditions of ultramylonitic shear zones fall near the triple point of peierls dislocation and diffusion creep

stress dependence of microstructures in experimentally deformed calcite abstract optical measurements of microstructural features in experimentally deformed carrara marble help define their dependence on stress these features include dynamically recrystallized grain size dr subgrain size sg minimum bulge size l and the maximum scale length for surface energy driven grain boundary migration l taken together with previously published data dr defines a paleopiezometer over the range 15 291 mpa and temperature over the range 500 1000 c with a stress exponent of 1 09 ci 1 27 to 0 95 showing no detectable dependence on temperature sg and dr measured in the same samples are closely similar in size suggesting that the new grains did not grow significantly after nucleation l and l measured on each sample define a relationship to stress with an exponent of approximately 1 6 which helps define the boundary between a region of dominant strain energy driven grain boundary migration at high stress from a region of dominant surface energy driven grain boundary migration at low stress

high resolution subducting slab structure beneath northern honshu japan revealed by double difference tomography the high resolution seismic velocity structure of the subducting slab beneath northern honshu japan has been obtained by double difference tomography capitalizing on the existence of two planes of seismicity the upper plane lies in the region with average to high vp vs ratios 1 72 1 85 which may correspond to the transformations of metabasalt and metagabbro to blueschist the lower plane is associated with very low vp vs ratios 1 6 1 7 in sharp contrast with high vp vs ratios 1 8 1 85 in the region between the two planes these features may be explained by forsterite enstatite h2o formation from serpentine dehydration in the lower plane and partial hydration of the region between the two planes our results provide direct constraints for petrologic modeling of the subducting slab

influence of the zone of weakness on dip angle and shear heating of subducted slabs we study the importance of the zones of weakness and the pattern of downgoing flow in steady state models of subducting lithosphere which interacts mechanically and thermally with the ambient mantle the non linear system of governing equations consists of i the momentum equation in stream function formulation and ii the steady state heat transfer equation including conduction and advection of heat and dissipation a finite element method has been applied to this system we consider the viscosity to be a non linear function of both the temperature and the stream function in steady state two dimensional 2d flow the stream function isolines follow material trajectories they are used to follow the top of the subducting slab which because of its possible increase in water content is assumed to have a lower viscosity the zone of weakness has been thus obtained in the self consistent fashion since the stream function as well as the temperature are the output from our modeling and no a priori assumptions about the shape of the bending lithosphere are taken into account it was shown that several orders decrease of viscosity in the zone of weakness is required to obtain the dip angle of about 45 if the decrease of viscosity is not sufficient enough the subducted slab either sinks almost vertically or does not exhibit a plate like behavior we have also demonstrated that shear heating can unrealistically increase at the zone of weakness for fast subductions if decrease of viscosity is underestimated 2004 elsevier b v all rights reserved

deformation fabrics of olivine in val malenco peridotite found in italy and implications for the seismic anisotropy in the upper mantle abstract an exhumed huge body of ultramafic rocks is preserved in val malenco italy in this study petrofabrics and deformation microstructures of olivine in the val malenco peridotite were examined and new observations of lattice preferred orientation lpo of olivine such as type b and type e lpo in the peridotite are reported three types depending on the grain size of olivine fabrics were observed type a fabric exhibiting the 010 100 slip system in the large relic grain area and type b and type e fabrics exhibiting the 010 001 and 001 100 slip systems respectively in the widely distributed recrystallized grain area the slips occurred mainly on 010 001 and the 001 100 slip system has only rarely been reported to be the dominant slip system of olivine found in natural rocks further seismic velocities and seismic anisotropies were calculated for the three types of fabrics found the seismic anisotropies in the upper mantle can be attributed to the type b type e or type a lpos of olivine moreover the anomaly in seismic low velocity in the mantle wedge can be attributed to the serpentinization as well as the mixed fabrics of the type a type b and type e

strong variations in seismic anisotropy across the hikurangi subduction zone north island new zealand abstract shear wave splitting measurements from sks skks s and scs phases reveal the anisotropic structure of north island new zealand waveforms were recorded on temporary and permanent broadband seismographs strong anisotropy varies rapidly across the hikurangi subduction zone these changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow stations in the fore arc region eastern central north island record fast polarisations subparallel to the trench and moderate delay times of around 2 5 s consistent with earlier studies of north island this is interpreted as caused by trench parallel mantle flow beneath the subducted slab with a possible contribution of trench parallel fossil anisotropy within the subducted slab delay times increase towards the extending central volcanic region cvr with values up to 4 5 s one of the largest delay times measured worldwide strong s wave anisotropy 7 5 through 80 km in the mantle wedge in addition to the sub slab anisotropy can explain the observed high delay times one mechanism to create such high anisotropy is melt segregation of partially molten rock a frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements to the west delay times decrease systematically measurements in the far back arc region of western central north island show no apparent splitting this suggests that mantle wedge dynamics terminate beneath the western border of the cvr the apparent isotropy beneath the far back arc may be related to local small scale mantle convection or vertical mantle flow possibly caused by the detachment of lithosphere

new developments in deformation studies high strain deformation plastic deformation of rocks in the earth is most strikingly illustrated in exposures of shear zones very large strains shear strains of 10 or more may be developed in these zones they appear to be ubiquitous within the continental and oceanic crust and may provide the dominant locus for deformation in these regions and potentially also in the lithospheric upper mantle it is therefore of the greatest interest to obtain information about the deformation properties of rock at large strains in the past various approaches have been made to achieving in the laboratory the types of deformation seen in natural rocks in some cases it has been possible to achieve plastic deformation in single crystals and fine grained aggregates of minerals at atmospheric pressure at sufficiently high temperatures and a number of such experiments have been carried out mackwell et al 1990 dimanov et al 1998 however it is generally necessary to conduct experiments under conditions of high confining pressure as well as high temperature the earliest experiments of this nature were those of adams and nicholson 1901 and von karman 1911 followed by the pioneering work of bridgman 1949 and griggs 1936 and many subsequent papers these studies were carried out using fluid pressure media although otto mugge carried out some deformation experiments on single crystals of minerals in solid confining media already in the late 19th to early 20th century the extensive use of solid media for high pressure experiments was not introduced until the development of the griggs solid medium apparatus in the 1960 s e g griggs et al 1966 griggs 1967 since the 1960s extensive studies have been carried out in both gas medium and solid medium apparatus the apparatus and techniques have been reviewed in several publications griggs et al 1960 paterson

microstructure evolution of aqueous fluid bearing wehrlites implications for the fluid distribution in polymineralic rocks 1 the microstructure of aqueous fluid bearing wehrlites was experimentally studied in order to investigate the fluid distribution in polymineralic rocks wehrlites of various forsterite diopside ratios were synthesized with 1 0 1 5 wt water at 1200 c and 1 2 gpa for 1 5 668 hours conspicuous grain growth drag and coagulation of pore fluids were observed in the time series experiments the pore fluids were classified into two types those surrounded only by a single phase g type and those surrounded by both forsterite and diopside i type in the short time runs the relative volume fraction of i type pores in all the pores fi agrees well with the value calculated from an ideal pore distribution model in which all the grains are randomly distributed the fi of wehrlites with various forsterite diopside ratios increased with average grain size and reached a steady state value at 80 90 in about 160 163 hours the increase of fi is explained by interphase boundary trapping g type pore fluids move faster than i type and the number of g type becoming i type by being trapped into the interphase boundary per unit time is larger than that of i type pores transforming to g type the experimental results suggest that large fractions of pore fluids might be distributed at the grain junctions surrounded by more than two mineral phases in natural rocks as a result of grain growth the connectivity of the fluid network in polymineralic rocks may be strongly influenced by modal composition of the rocks as well as by the dihedral angles among the fluid and two mineral phases

modeling shear zones in geological and planetary sciences solid and fluid thermal mechanical approaches abstract shear zones are the most ubiquitous features observed in planetary surfaces they appear as a jagged network of faults at the observable brittle surface of planets and in geological exposures of deeper rocks they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of mylonitized reduced grain sizes behind the overall size of the entire shear network rarely exceeds kilometer scale at depth although mylonitic shear zones are only visible to the observer when uplifted and exposed at the surface they govern the mechanical behavior of the strongest part of the lithosphere below 10 15 km depth mylonitic shear zones dissect plates thus allowing plate tectonics to develop on the earth we review the basic multiscale physics underlying mylonitic ductile shear zone nucleation growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization we also discuss a framework for analytic and numerical modeling including the effects of thermal mechanical couplings thermal elasticity the influence of water and void volatile feedback the physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium positive feedback can amplify strength heterogeneities by cascading through different scales we define basic intrinsic length scales of strength heterogeneity such as those associated with plasticity grain size fluid inclusion and thermal diffusion length scale for an understanding ductile shear zones we need to consider the energetics of deformation shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates for the long time scale deformational energy stored inside the shear zone through plastic dilation or crystallographic and shape preferred orientation consumes only a maximum of 10 of energy dissipated in the shear zones but creates structural anisotropy shear zones become long living features with a long term memory a special role is attributed to the presence of water in nominally anhydrous minerals we show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale we have shown that fully coupled finite element calculations using mechanical data from the laboratory can reproduce the basic mode of deformation of an entire mylonitic shear zone the next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure without losing the large scale constraint numerical methods capable of fulfilling the goal are emerging these are adaptive wavelet techniques and hybrid particle finite element codes which can be run over a computational grid across the net

improving flow laws and piezometers for quartz and feldspar aggregates we have modified the molten salt cell msc in order to measure the flow stress of quartz and feldspar ag gregates deforming by dislocation creep the flow stress of cararra marble measured in the msc is within 5 mpa of that measured in a gas apparatus quartz and feldspar aggregates deformed at 900o 1100oc and confining pressures of 1000 1500 mpa in copper and calcite solid confining pressure media are about twice as strong as those deformed in the msc whereas quartz aggregates deformed in talc and solid nac1 have about the same strength as that measured in the msc these results suggest that the stress exponent in the flow law is larger and the dynamically recrystalized grain size is more sensitive to stress than previously reported acking rings stainless steel

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

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

evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation 1 the influence of second phases e g pyroxenes on olivine grain size was studied by quantitative microfabric analyses of samples of the hilti massif mantle shear zone semail ophiolite oman the microstructures range from porphyroclastic tectonites to ultramylonites from outside to the center of the shear zone starting at conditions of ridge related flow they formed under continuous cooling leading to progressive strain localization the dependence of the average olivine grain size on the second phase content can be split into a second phase controlled and a dynamic recrystallization controlled field in the former the olivine grain size is related to the ratio between the second phase grain size and volume fraction zener parameter in the latter dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size in both fields the average olivine and second phase grain size decreases with decreasing temperature combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites 1100 c and mylonites 800 c formed under the predominance of dislocation creep since olivine rich layers are intercalated with layer parallel polymineralic bands in the mylonites nearly equiviscous conditions can be assumed in the ultramylonites diffusion creep represents the major deformation mechanism in the polymineralic layers it is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine grained polymineralic domains at low temperatures 700 c underlining the role of the second phases on strain localization in cooling mantle rocks

new developments in deformation experiments at high pressure although the importance of rheological properties in controlling the dynamics and evolution of the whole mantle of earth is well recognized experimental studies of rheological properties and deformation induced microstructures have mostly been limited to low pressure conditions this is mainly a result of technical limitations in conducting quantitative rheological experiments under high pressure conditions a combination of factors is changing this situation increased resolution of composition and configuration of earth s interior has created a greater demand for well resolved laboratory measurement of the effects of pressure on the behavior of materials higher strength materials have become readily available for containing high pressure research devices and new analytical capabilities in particular very bright synchrotron x ray sources are now readily available to high pressure researchers one of the biggest issues in global geodynamics is the style of mantle convection and the nature of chemical differentiation associated with convectional mass transport although evidence for deep mantle circulation has recently been found through seismic tomography e g van der hilst et al 1997 complications in convection style have also been noted they include 1 significant modifications of flow geometry across the mantle transition zone as seen from high resolution tomographic studies fukao et al 1992 masters et al 2000 van der hilst et al 1991 and 2 complicated patterns of flow in the deep lower mantle 1500 2500 km perhaps caused by chemical heterogeneity kellogg et al 1999 van der hilst and karason 1999 these studies indicate that while large scale circulation involving the whole mantle no doubt occurs significant deviations from simple flow geometry are also present two mineral properties have strong influence on convection 1 density and 2 viscosity rheology contrasts in the past the effects of density contrast have been emphasized honda et al 1993 kellogg et al 1999 tackley et al 1993 and the influence of