antigorite induced seismic anisotropy and implications for deformation in subduction zones and the tibetan plateau the present study which is a follow up of the journal of geophysical research paper by ji et al 2013a provides a new calibration for both seismic and fabric properties of antigorite serpentinites comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high t serpentinite which is essentially controlled by the antigorite c axis fabric is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock extrapolation of the experimental data with both pressure and temperature suggests that vp anisotropy decreases but shear wave splitting vs and vp vs increase with increasing pressure in either cold or hot subduction zones for a cold steeply subducting slab antigorite is most likely deformed by nearly coaxial flattening or trench parallel movements forming trench parallel seismic anisotropy for a hot shallowly subducting slab however antigorite is most likely deformed by simple shear or transpression trench normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30 the geophysical characteristics of the tibetan plateau such as strong heterogeneity in vp vs and attenuation shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks hydrated zones of subducting lithospheric mantle and the crust mantle boundary if the temperature is below 700 c in the region of interest

olivine antigorite orientation relationships microstructures phase boundary misorientations and the effect of cracks in the seismic properties of serpentinites antigorite bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of subduction zones here we present a detailed study of the microstructures and seismic properties in a sample of antigorite olivine schist previously studied by kern et al 1997 2015 we have measured crystallographic orientations and calculated the seismic properties in three orthogonal thin sections microstructures indicate that deformation is localized in the bands with high antigorite fractions resulting in strong crystallographic preferred orientations cpos with point maxima of poles to 100 parallel to lineation and poles to 001 to the foliation normal olivine cpo suggests deformation under high temperature and low stress with a 100 fiber texture the cpo strength varies with grain size but is strong even in fine grained antigorite and larger grains tend to display higher internal misorientation orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis 100 ol 001 atg being more frequent than 001 ol 010 atg two new orientation relationships between olivine and antigorite have been documented seismic velocities decrease while anisotropy increases with increasing antigorite modal content antigorite grain shape has a weak effect on seismic velocities but it is important on anisotropy comparison between cpo derived seismic velocities using voigt reuss hill averages and geometric mean only showed good agreement in 1 3 of experimental velocities if the crack porosity of 1 63 measured experimentally at 600 mpa was used in the self consistent model with two crack orientations with planes normal to z and y good match with all experimental velocities was achieved the self consistent model implies important crack porosity in the foliation plane at 600 mpa that reduces vp normal to the foliation by 0 3 km s

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

seismic velocities anisotropy and shear wave splitting of antigorite serpentinites and tectonic implications for subduction zones 1 antigorite the high temperature ht form of serpentinite is believed to play a critical role in various geological processes of subduction zones we have measured p and s wave velocities vp and vs anisotropy and shear wave splitting of 17 serpentinite samples containing 90 antigorite at pressures up to 650 mpa the new results combined with data for low temperature lt lizardite and or chrysolite reveal distinct effects of lt and ht serpentinization on the seismic properties of mantle rocks at 600 mpa vp 5 10 and 6 68 km s vs 2 32 and 3 67 km s and vp vs 2 15 and 1 81 for pure lt and ht serpentinites respectively above the crack closure pressure 150 mpa the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 c should be at least twice that of previous estimates based on lt serpentinization the presence of seismic anisotropy high pressure fluids or partial melt is also needed to interpret ht serpentinized mantle with vp 1 81 the intrinsic anisotropy of the serpentinites 3 8 16 9 with an average value of 10 5 for vp and 3 6 18 3 with an average value of 10 4 for vs is caused by dislocation creep induced lattice preferred orientation of antigorite three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics s l and ls tectonites formed by three categories of strain geometry i e coaxial flattening coaxial constriction and simple shear respectively our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

serpentine in active subduction zones abstract serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10 200 km depth range serpentines are a major water carrier and their rheological properties have a strong influence on deformation partitioning and seismicity at depths i review experimental investigations that have been conducted on serpentines with emphasis on the large body of data acquired over the past decade determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology and to link the presence of serpentinites with deformation and fluid circulation the fluid budget can be partially constrained from geophysical data elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography anisotropy suggests the existence of thin serpentinite channels above the plate interface that account for mechanical decoupling inferred from down dip limit of the seismogenic zone and heat flow strain rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high pressure rocks and with the geological record of subduction related deformation circulation of serpentinizing fluids depends on the permeability structure and is imaged by electrical conductivity tomography it could be controlled by fracturing in the undeformed cold nose of the mantle wedge and by plastic deformation along the plate interface fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up to faulting pattern needs to be established especially for water cycling to the transition zone in the core of the slab

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

pressure solution in rocks focused ion beam transmission electron microscopy study on orthogneiss from south armorican shear zone france in order to characterize the m to nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks we have performed a detailed focused ion beam transmission electron microscopy study in ultramylonite samples from south armorican shear zone france that focused on grain boundary scale we have studied phase boundaries between quartz k feldspar and white mica in both 2d and 3d and compare our evidences with theoretical dissolution precipitation models in the current literature the dissolution re precipitation processes lead to the development of different features at different phase boundaries in both quartz white mica and quartz k feldspar phase boundaries voids were ubiquitously observed these voids have different shapes and the development of some of them is crystallographically controlled in addition part of these voids might be filled with vermiculite amorphous leached layers with kaolinite composition were observed at the boundaries of k feldspar quartz and k feldspar white mica the development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface together with the presence of fluids that either leaches or directly dissolve the mineral phases in addition the local dislocation density in quartz may play an important role during pressure solution we suggest that the nanoscale observations of the quartz white mica phase boundaries show direct evidence for operation of island and channel model as described in wassmann and stockhert tectonophysics 608 1 29 2013 while k feldspar quartz phase boundaries represents amorphous layers formed via interface coupled dissolution reprecipitation as described by hellmann et al chem geol 294 295 203 216 2012

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

small scale sublithospheric convection reconciles geochemistry and geochronology of superplume volcanism in the western and south pacific abstract cretaceous volcanism in the west pacific seamount province wpsp and tertiary volcanism along the cook australs in the south pacific are associated with the same broad thermochemical anomaly in the asthenosphere perhaps related to the pacific superplume abundant volcanism has usually been attributed to secondary plumelets rising from the roof of the superplume the cook australs display distinct geochemical trends that appear to geographically project backward in time to corresponding trends in the wpsp however the implied close proximity of source regions i e 1000 km with very different geochemical fingerprints and their longevity over geological time 100 myrs appear to be at odds with the secondary plumelet hypothesis a mechanism with a typical timescale of 30 myrs moreover ages sampled along the individual volcano chains of the cook australs and of the wpsp violate the predictions of the plumelet hypothesis in terms of linear age distance relationships our numerical models indicate that small scale sublithospheric convection ssc as likely triggered by the thermochemical anomaly of the superplume instead reconciles complex age distance relationships because related volcanism occurs above elongated melting anomalies parallel to plate motion hot lines furthermore ssc melting of a mantle source that consists of pyroxenite veins and enriched peridotite blobs in a matrix of depleted peridotite creates systematic geochemical trends over seafloor age during volcanism these trends arise from variations in the amount of pyroxenite derived lavas relative to peridotite derived lavas along a hot line therefore stretching between the geochemical end members himu and emi these predicted trends are consistent with observed trends in radiogenic isotopic composition from the wakes marshalls gilberts i e the individual volcano groups of the wpsp and the cook australs for increasing mantle temperatures volcanism is further predicted to occur at greater seafloor ages and with a more emi like signature a result that can explain many of the observed systematics thus ssc explains many of the geochemical observations with long term temporal variations in mantle temperature instead of persistent intermediate scale 1000 km compositional heterogeneity

stability and dynamics of serpentinite layer in subduction zone abstract the hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths in the absence of data newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions hilairet n et al 2007 high pressure creep of serpentine interseismic deformation and initiation of subduction science 318 5858 1910 1913 using a simple 1d model of a serpentinized channel and as opposed to previous models in this power law rheology we examine the influence of channel thickness temperature and subduction angle on serpentine flow driven by density contrast serpentinization degree with the surroundings at temperatures of 200 500 c relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2 3 km for channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 pa s the velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed hp rocks upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation at shallower levels the thickness allowed for a channel may be up to 8 10 km if the rheology has a higher dependence on stress we therefore propose that the exhumation of hp oceanic units in serpentinite channels is organized in two levels the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow the thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies the deepest serpentinite channel may thus be difficult to detect by seismic methods but it will have a strong influence on the mechanical coupling between the slab and mantle wedge

deformation mechanisms of antigorite serpentinite at subduction zone conditions determined from experimentally and naturally deformed rocks abstract we performed deformation dia experiments on antigorite serpentinite at pressures of 1 3 5 gpa and temperatures of between 400 and 650 c bracketing the stability of antigorite under subduction zone conditions for each set of pressure temperature p t conditions we conducted two runs at strain rates of 5 10 5 and 1 10 4 s 1 we complemented our study with a sample deformed in a griggs type apparatus at 1 gpa and 400 c chernak and hirth 2010 and with natural samples from cuba and the alps deformed under blueschist eclogitic conditions optical and transmission electron microscopies were used for microstructural characterization and determination of deformation mechanisms our observations on experimentally deformed antigorite prior to breakdown show that deformation is dominated by cataclastic flow with observable but minor contribution of plastic deformation microkinking and 001 gliding mainly expressed by stacking disorder mainly in contrast in naturally deformed samples plastic deformation structures are dominant stacking disorder kinking pressure solution with minor but also perceptible contribution of brittle deformation when dehydration occurs in experiments plasticity increases and is coupled to local embrittlement that we attribute to antigorite dehydration in dehydrating samples collected in the alps embrittlement is also observed suggesting that dehydration may contribute to intermediate depth seismicity our results thus show that semibrittle deformation operates within and above the stability field of antigorite however the plastic deformation recorded by naturally deformed samples was likely acquired at low strain rates we also document that the corrugated structure of antigorite controls the strain accommodation mechanisms under subduction conditions with preferred inter and intra grain cracking along 001 and gliding along both a and b we also show that antigorite rheology in subduction zones is partly controlled by the presence of fluids which can percolate within the exhumation channel via deformation induced interconnected porosity

p and s wave anisotropy and shear wave splitting at pressure and temperature in possible mantle rocks and their relation to the rock fabric the seismic properties and their directional dependence have been measured in three potentially mantle derived rocks peridotite ivrea zone italy lherzolitic xenolith massif central france serpentinite the experimental data include the measurement of elastic wave velocities and densities at confining pressures up to 600 mpa and temperatures up to 600 c 600 mpa and the determination of velocity anisotropy and shear wave splitting the petrophysical data can be related to mineral composition and to microstructural characteristics of the rocks macroscopic anisotropy of p and s wave velocities is strongly controlled in the peridodites by the olivine and in the serpentinite by the serpentine antigorite lattice preferred orientation lpo respectively shear wave splitting was found to be an important characteristic of the anisotropic rocks the study shows that for the interpretation of shear wave splitting the direction of wave propagation in relation to the lpos of the major minerals and to the symmetry of their lattice fabric is of great importance in the ivrea peridotite exhibiting rhombic symmetry there is practically no shear wave splitting to be observed for s waves propagating parallel to the olivine 100 axes and the maximum shear wave splitting is not coupled with the direction of highest p wave velocity in the transversely isotropic massif central xenolith shear waves do not split when propagating parallel to the olivine 010 maximum coinciding with the direction of the radial symmetry axis of the olivine fabric the experimental data are principally in accordance with velocity calculations based on the measured olivine lpo and the olivine single crystal stiffness coefficients implications for the interpretation of seismic data are discussed

sequential deformation from serpentinite mylonite to metasomatic rocks along the sashu fault sw japan the deformation of serpentinites along the sashu fault saganoseki peninsula sw japan involved both mylonitization and brecciation the brecciation was accompanied by metasomatism and the metasomatized rocks record additional ductile deformation serpentinite mylonite is composed mainly of antigorite with minor magnetite and cr spinel foliation and lineation in this rock type are defined by the shape preferred orientation of antigorite and the alignment of fine grained magnetite and carbonate minerals the antigorite also shows a lattice preferred orientation lpo as measured with a universal stage with c axes oriented near perpendicular to the foliation and b axes oriented parallel to the lineation brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation consequently they contain composite planar fabrics and drag folds the breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200 300 c at depths of 3 8 11 8 km metasomatic reactions particularly the formation of talc contributed to weakening of this serpentinite bearing fault zone

prediction intervals for climate reconstructions with autocorrelated noise an analysis of ordinary least squares and measurement error methods the estimation of past climate variations is a statistical prediction problem where climate proxy data are calibrated against instrumental observations although noise is always present in both instrumental and proxy data motivating the use of so called errors in variables or measurement error methods such methods have not yet been widely accepted by palaeoclimatologists we define a univariate measurement error model that allows for white noise in instrumental and red noise in proxy data and derive new formulae to construct prediction intervals for past climate values the new method can be applied to either unsmoothed data or to data smoothed after calibration using synthetic simulated data we demonstrate that the new formulae perform well for noise levels and calibration period lengths typical of many palaeoclimate series in particular tree rings and other annually resolved data with an example using a recently published 500 year long temperature reconstruction we demonstrate that conclusions about the statistical significance of the difference between the present and past climates may be incorrect if the noise is not adequately modeled

plastic deformation and development of antigorite crystal preferred orientation in high pressure serpentinites we have inferred the deformation mechanisms of antigorite by high resolution ebsd mapping of samples from cerro del almirez ultramafic massif betic cordillera se spain textural relations and phase diagram calculations constrain the foliation development conditions to the subduction prograde path at up to 600 630 c and 1 6 1 9 gpa deformation was followed by static annealing of antigorite at ca 680 c the crystal preferred orientation cpo of antigorite is characterised by a strong alignment of 001 poles normal to the foliation plane and a weaker but clear parallelism between 100 axes and the macroscopic lineation defined by the elongation of magnetite aggregates analysis of misorientations across subgrains shows predominance of 010 rotation axis consistent with activation of the 100 001 slip system however tilt subgrain boundaries subparallel to 100 probably formed by edge dislocations of this system are subsidiary most subgrain boundaries are subparallel to 001 planes they are interpreted as 001 twins wherein continuing viscoplastic deformation resulted in a slight increase of the misorientation between the twins modelling of the evolution of the antigorite cpo using a lower bound approach and considering different deformation regimes and sets of basal and non basal slip systems has shown that intensities of 100 and 010 maxima reflect the relative strength of the antigorite 100 001 and 010 001 systems activation of other basal or non basal slip systems does not change significantly the cpo patterns but results in less concentrated cpo 3d transpression models better reproduce the cpo of natural antigorite serpentinites we propose that the widespread occurrence of strong cpo in high pressure antigorite serpentinite is consistent with deformation by dislocation creep with dominant glide on hk0 001 together with the activation of twinning implying that a power law rheology would better account for the mechanical behaviour of antigorite serpentinite deep in the subduction channel and mantle wedge

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

elastic anisotropy modeling of kimmeridge shale 1 anisotropy of elastic properties in clay rich sedimentary rocks has been of long standing interest these rocks are cap rocks of oil and gas reservoirs as well as seals for carbon sequestration elasticity of shales has been approached by direct velocity measurements and by models based on microstructures here we are revisiting the classical kimmeridge shale studied by hornby 1998 by first quantifying microstructural features such as phase volume fractions grain shapes and grain orientations and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties it is shown that by application of a self consistent algorithm based on eshelby s 1957 model of inclusions in a homogeneous medium it is possible to explain most experimental elastic constants though some discrepancies remain which may be due to the interpretation of experimental data using a differential effective medium approach an almost perfect agreement with experimental stiffness coefficients can be obtained though the physical basis of this method may be questionable the influence of single crystal elastic properties grain shapes preferred orientation and volume and shapes of pores on elastic properties of shale is explored

deformation of antigorite serpentinite at high temperature and pressure abstract deformation experiments were conducted in a griggs type apparatus to constrain the rheology of antigorite serpentinite at high temperatures and pressures samples deformed at temperatures of 400 625 c and pressures of 0 85 1 5 gpa experienced semi brittle deformation resulting in localization on faults at 5 to 25 strain samples deformed at 300 c exhibited transitional behavior the stresses measured for our samples are in good agreement with recent experiments conducted in the deformation dia apparatus however our results indicate that steady state ductile flow is not achieved in contrast with interpretations based on the deformation dia experiments extrapolation of our data to geologic conditions suggests that serpentinites may be weak enough during localized frictional sliding to account for slab wedge decoupling inferred from geologic observations experiments conducted above the thermal stability of antigorite at 700 c and 1 5 gpa show evidence for distributed deformation this observation does not support the hypothesis that intermediate depth earthquakes result directly from dehydration embrittlement in addition samples exhibit velocity strengthening from 300 to 700 c a frictional behavior that inhibits earthquake nucleation

a new method for calculating seismic velocities in rocks containing strongly dimensionally anisotropic mineral grains and its application to antigorite bearing serpentinite mylonites abstract seismic velocity is one of the most important sources of information about the earth s interior for its proper interpretation we must have a thorough understanding of the dependence of seismic velocity on microstructural elements including the modal composition the crystal preferred orientation cpo the grain shape the spatial distribution of mineral phases etc the conventional voigt reuss and hill averaging schemes take into account only the modal composition and the cpo the information about the earth s interior is thus poorly constrained for a better interpretation it is critical to have a calculation method which accounts for the grain shape and the spatial distribution of mineral phases etc we propose a calculation method which accounts for the grain shape of strongly dimensionally anisotropic minerals like micas and serpentines our method can be applied to a distributed geometrical orientation of mineral grains comparison was made between calculated and measured velocities in three antigorite serpentinite mylonites judging from the root mean square relative error our method provides velocities closer to measured values than the voigt reuss and hill averaging schemes the input of the grain shape considerably improves the prediction of seismic properties however large discrepancies 0 1 km s between measured and calculated velocities can be seen in some directions the discrepancies might come from microstructural elements which were not considered in the calculation layer structures and cracks

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

seismic anisotropy of serpentinite from val malenco italy 2015 american geophysical union all rights reserved serpentinites deformed in mantle subduction zones are thought to contribute significantly to seismic anisotropy of the upper mantle and have therefore been of great interest with studies on deformation preferred orientation and elastic properties here we present a combined study of a classical sample from val malenco italy investigating the microstructure and texture with state of the art synchrotron x ray and neutron diffraction methods and measuring ultrasonic velocities both with a multi anvil apparatus and a novel instrument to measure 3 d velocities on spheres both results from diffraction methods and velocity measurements are compared discussing advantages and disadvantages from spherical velocities elastic tensor properties can be derived by inversion also from quantitative texture measurements elastic properties can be modeled by self consistent averaging good agreement between the velocity and microstructural models is observed

shearing instabilities accompanying high pressure phase transformations and the mechanics of deep earthquakes deep earthquakes have been a paradox since their discovery in the 1920s the combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers yet earthquakes occur continually in subduction zones to 700 km the expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self organized failure analogous to but different from brittle failure the only high pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material this fluid spontaneously forms mode i microcracks or microanticracks that self organize via the elastic strain fields at their tips leading to shear failure growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher pressure polymorphs in either case fault propagation could be enhanced by shear heating just as is sometimes the case with frictional sliding in the crust extensive seismological interrogation of the region of the tonga subduction zone in the southwest pacific ocean provides evidence suggesting significant metastable olivine with implication for its presence in other regions of deep seismicity if metastable olivine is confirmed either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast

high pressure elasticity of serpentine and seismic properties of the hydrated mantle wedge 1 a subset of the single crystal elastic moduli of natural antigorite has been measured using brillouin scattering at high pressure up to 9 gpa aggregate properties and axial compressibilities are in good agreement with equation of state results from x ray diffraction stiffness along the c axis increases becoming close to those within the silicate layer near 7 gpa a slight discontinuity in the evolution of the elastic moduli near 7 gpa is associated with a phase transition raman spectroscopy shows that the transition does not occur in subducting slabs except in the core of the coldest slabs where serpentine may be subducted to depths of about 200 km varying temperature and pressure has limited effects on the interpretation of seismic velocities principally because of the limited depth range of serpentine stability mostly above 100 km depth and also due to the compensating effects of pressure and temperature that maintain velocity variations well within the uncertainties and statistical variability of seismological studies serpentinites are excellent candidates for explaining low velocities in the hydrated mantle wedge inversion of the moho and thin anisotropic low velocity layers at the plate interface serpentinite layers provide an alternative explanation to fluid saturated oceanic crust for explaining these phenomena and account for a transition to an aseismic decoupled plate interface

deformation mechanisms and rheology of serpentines in experiments and in nature microstructures in serpentine samples recovered from deformation experiments performed at high pressure 1 8 gpa high temperature 150 500 degrees c and laboratory strain rates 4 10 4 10 6 s 1 were studied using transmission electron microscopy on thin sections prepared by focused ion beam lizardite crystals deform by easy glide along the basal planes associated with microkink this mechanism is associated with a plastic strength of similar to 100mpa that defines the upper bound of lizardite strength in natural conditions antigorite crystals deform essentially by conjugated slip along 101 and 10 1 over bar planes observed in sections close to 010 this conjugate system results in an apparent global slip system akin to 100 001 in all samples delamination and comminution produce fine grained interconnected regions at grain boundaries because intracrystalline deformation mechanisms are insufficient to satisfy the von mises criterion the deformation laws of lizardite plastic flow and antigorite strain rate dependent differ because of their differing intracrystalline deformation mechanisms subgrain boundary geometry in natural samples and 100 001 crystal preferred orientations indicate the activation of slip systems similar to those observed in experimentally deformed samples suggesting that the strain rate dependent rheology of antigorite derived from experiments applies to subduction zone conditions delamination of antigorite crystals of a few tens to hundreds of nanometers is not observed in natural samples from subduction zones suggesting plastic deformation of serpentinites at natural low strain rates and stresses is likely accompanied by recrystallization through dissolution precipitation processes that act as a low temperature equivalent of dynamic recrystallization through diffusion at high temperature

spatial variations in antigorite fabric across a serpentinite subduction channel insights from the ohmachi seamount izu bonin frontal arc abstract we conducted a microstructural study of samples from a natural serpentinite shear zone in the ohmachi seamount izu bonin frontal arc the serpentinite samples consist mainly of columnar antigorite grains that show marked variations in texture from two approximately orthogonal sets of grains interpenetrating to aligned schistose forms because the two types of grains have similar compositions these textural differences are interpreted to reflect the existence of a strain gradient toward a plate interface in a subduction zone the crystal preferred orientation cpo of antigorite with interpenetrating texture is almost randomly oriented whereas in the case of schistose texture the cpo shows a typical 010 001 pattern we also found that with increasing intensity of schistosity the polarization plane of v s 1 for antigorite grains becomes aligned parallel to the flow plane consistent with a plane oriented normal to the maximum concentration of slow antigorite c axes this configuration results in seismic anisotropy that is approximately five times higher than that for olivine grains these findings indicate that if a serpentinite layer on the plate interface attains large bulk shear strains 2 the resultant alignment of antigorite grains within the layer strongly influences the orientation and magnitude of seismic anisotropy in the mantle wedge depending on the dip angle of the subducting slab

the influence of hydrous phases on the microstructure and seismic properties of a hydrated mantle rock abstract to better understand the microstructural evolution of a serpentinized mantle rock and the influence of various hydrous phases on the seismic properties of the mantle wedge we have conducted the detailed microstructural analyses of a sample of tremolite chlorite antigorite schist collected from the moses rock dike central part of the colorado plateau we performed differential effective media dem modelling to study the effect of three hydrous phases forming two phase aggregates with olivine considering the crystallographic preferred orientation cpo of each phase and the shape ratio of the hydrous phases we have demonstrated that in a partially serpentinized peridotite the olivine cpo characteristic of 100 010 dislocation glide is still preserved and the high temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist the transformation of olivine into antigorite occurs predominantly 75 by the relationship 100 ol 001 atg with 001 ol 010 atg with the 010 ol 001 atg and 001 ol 010 atg relationship observed in areas of weak antigorite cpo chlorite results from the phase transformation of olivine in a relatively static environment as shown by the correlation between the olivine chlorite cpos with 100 ol 100 ch 010 ol 001 ch and 001 ol 010 ch the fluid percolation that caused the localized metasomatism and partial hydration of the mantle occurred possibly along trans lithospheric shear zones the presence of chlorite induces the most important drop on the p wave velocities and may help to explain some local low velocities in the fore arc mantle wedges but is unlikely to be of global importance due to its very high vp vs ratio 1 9 on the other hand antigorite is the only phase that causes important modification on the propagation directions of p and s waves and the only phase to explain the polarization of the fastest shear waves parallel to the subduction trench

the legacy of crystal plastic deformation in olivine high diffusivity pathways during serpentinization crystal plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence of asthenospheric mantle flow although crystal plastic deformation and serpentinization are spatio temporally decoupled we identified compositional readjustments expressed on the micrometric level as a striped fe enriched bar x text fe 0 24 0 02 zones 0 12 0 02 bulk or fe depleted bar x text fe 0 10 0 01 zones 0 13 0 01 bulk zoning in partly serpentinized olivine grains from two upper mantle sections in norway focused ion beam sample preparation combined with transmission electron microscopy tem and aberration corrected scanning tem enabling atomic level resolved electron energy loss spectroscopic line profiling reveals that every zone is immediately associated with a subgrain boundary we infer that the zonings are a result of the environmental fe2 mg 1 exchange potential during antigorite serpentinization of olivine and the drive toward element exchange equilibrium this is facilitated by enhanced solid state diffusion along subgrain boundaries in a system which otherwise re equilibrates via dissolution reprecipitation fe enrichment or depletion is controlled by the silica activity imposed on the system by the local olivine orthopyroxene mass ratio temperature and the effect of magnetite stability the fe mg exchange coefficients k text d text atg text ol between both types of zoning and antigorite display coalescence toward exchange equilibrium with both types of zoning mn is enriched and ni depleted compared with the unaffected bulk composition nanometer sized heterogeneously distributed antigorite precipitates along olivine subgrain boundaries suggest that water was able to ingress along them crystallographic orientation relationships gained via electron backscatter diffraction between olivine grain domains and different serpentine vein generations support the hypothesis that serpentinization was initiated along olivine subgrain boundaries

trench parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine the strong trench parallel anisotropy observed in several subduction systems is difficult to explain in terms of olivine anisotropy even if the entire mantle wedge were to act as an anisotropic source using high pressure deformation experiments ikuo katayama and colleagues show that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in such subduction systems although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine the strong trench parallel anisotropy observed in several subduction systems is difficult to explain in terms of olivine anisotropy using high pressure deformation experiments it is now shown that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in such subduction systems seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the earth s interior as it primarily reflects the deformation induced preferred orientation of anisotropic crystals1 2 although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine3 the strong trench parallel anisotropy delay time of one to two seconds observed in several subduction systems4 5 is difficult to explain in terms of olivine anisotropy even if the entire mantle wedge were to act as an anisotropic source here we show that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in subduction systems high pressure deformation experiments reveal that the serpentine c axis tends to rotate to an orientation normal to the shear plane during deformation consequently seismic velocity propagating normal to the shear plane plate interface is much slower than that in other directions the seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine6 and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench parallel seismic anisotropy in the case of a steeply subducting slab this hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge as inferred from anomalously low seismic wave velocities7

trench parallel anisotropy and large delay times elasticity and anisotropy of antigorite at high pressures 1 using theoretical methods we calculate the full elastic constant tensor and equation of state of antigorite up to 10 gpa a pressure range that encompasses its experimentally observed stability field at ambient conditions the elastic constant tensor reveals significantly large acoustic anisotropy 38 in vp 35 in vs compared to the dominant mantle phase olivine the shear anisotropy is enhanced upon compression upon compression the full elastic constant tensor reveals anomaly at 2 69 gm cm 3 at 2 69 gm cm 3 we find that the pressure volume results for antigorite are well represented by a third order birch murnaghan formulation with k0 52 gpa k0 8 5 and v0 3037 a3 0 2 48 gm cm 3 our results on elasticity and anisotropy at conditions relevant to the mantle wedge indicates that a 10 20 km layer of antigorite might account for the observed shear polarization anisotropy with fast polarization parallel to the trench and associated large delay times of 1 2 sec observed in some subduction zone settings