mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

the olivine spinel transformation and the rheology of subducting lithosphere during subduction relatively hot lithospheric slabs are expected to deform in a superplastic flow regime below a depth of 300 km if olivine reacts to fine grained spinel this process reduces the flow strength of such slabs below the phase boundary so that they become aseismic at about this depth in colder slabs temperatures are too low for superplastic deformation and a high flow strength and seismic activity are maintained to a depth of 600 700 km

transformation kinetics of polycrystalline aragonite to calcite new experimental data modelling and implications experimental data are used to model the transformation rate of polycrystalline aragonite grain diameter 80 m to calcite optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280 to 380 c and 0 10 mpa to an expression for a grain boundary nucleated and interface controlled transformation the nucleation rate is 4 5 orders of magnitude faster than for calcite nucleated within aragonite grains and the growing in rate is slower below 300 c than for calcite growing in aragonite single crystals the activation enthalpy for growth in polycrystalline aggregate is 247kj mol compared to 163 kj mol for growth in single crystals permanent deformation of the phases limits the elastic strain energy due to the 7 volume change and reduces the coherency of the calcite aragonite interace theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures and data from 0 10 to 400 mpa are used to evaluate the effect of pressure on the grain boundary nucleation rate because of permanent deformation of the phases the effective strain energy for nucleation is 0 55 kj mol which is less than a quarter of the value for purely elastic deformation these data are used to predict the percent transformation for various p t t paths without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235 c and the kinetic data are also consistent with published p t t paths which include heating during uplift the predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite but strongly dependent on the effective strain energy

glacier volume estimation from ice thickness data applied to the muz taw glacier sawir mountains china in this work ground penetrating radar gpr data global position system gps data landsat remote sensing images and digital elevation model dem srtm were used to study the muz taw glacier in the sawir mountains by a variety of interpolation methods spatial pattern of thickness of the muz taw glacier was obtained the results show that there are obvious differences between the available gpr transverse profiles the lower transverse profile has typical u type topographic feature the maximum glacier thickness is up to 116 4 m glacier thickness distribution in the higher profile is relatively flat with an average about 70 90 m the longitudinal profile forms a ladder like distribution which in the middle of the ice bedrock topography has obvious depression the average longitudinal ice thickness is about 80 89 m the maximum ice thickness is up to 122 61 m in addition the preliminary estimation of the average thickness and total terminus ice volume were approximately 60 5 m and 0 195 km3 respectively

stress distribution due to olivine spinel phase transition in descending plate and deep focus earthquakes the stress field due to olivine spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant clapeyron s slope and for a nonequilibrium transition taking a kinetic effect into consideration the material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures the calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km the magnitude of principal stress being greater than 0 5 gpa however for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces and the central part is compressional in the depth range from 300 to 600 km unlike the case of equilibrium transition the maximum principal stress in the latter case is greater than 2 gpa which is much more than that for the equilibrium transition the principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases comparison of the calculated results to the actually observed seismicity depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

mechanisms of transformation and deformation in mg1 8fe0 2sio4 olivine and wadsleyite under non hydrostatic stress abstract we have studied the effect of non hydrostatic stress on the mechanisms of the olivine wadsleyite ringwoodite phase transformations and deformation mechanisms of olivine and wadsleyite at high pressure experiments were performed at 900 c in the stability field 15 gpa for 0 5 h and in the stability field 16 gpa for 11 h using a multianvil apparatus with san carlos olivine as the starting material a sample assembly designed to produce non hydrostatic stress was used the deformed samples have been characterised using optical and transmission electron microscopy remnant olivine contains high densities of mostly c dislocations and deformation occurs by dislocation glide involving the slip systems 010 001 and 100 001 in wadsleyite dislocations are in a climb configuration which suggests that self diffusion of si and or o is much faster in wadsleyite than in olivine at 900 c wadsleyite also contains 010 stacking faults which are interpreted to be growth defects that anneal out with time during the olivine wadsleyite transformation non hydrostatic stress results in anisotropic reaction textures wadsleyite nucleates preferentially on olivine grain boundaries that are oriented at a high angle to the direction of principal compressive stress and or the growth of wadsleyite occurs preferentially in this direction wadsleyite transforms to ringwoodite by two competing mechanisms i coherent intracrystalline nucleation on a dislocations probably where they intersect 010 stacking faults and ii partially coherent nucleation at wadsleyite grain boundaries non hydrostatic stress enhances transformation rates by increasing the density of dislocations which act as nucleation sites for ringwoodite although the samples were partially reacted under non hydrostatic stress there is no evidence for transformational faulting

olivine wadsleyite pyroxene topotaxy evidence for coherent nucleation and diffusion controlled growth at the 410 km discontinuity we have synthesized a hydrous peridotite composition sample at 13 gpa and 1400 c with co existing coarse grains 100 lm of olivine wadsleyite clinoenstatite plus melt in a multi anvil press some of the olivine grains contain fine scale 0 5 2 lm wide lamellae of wadsleyite and clinoenstatite that likely resulted from transformation caused by small temperature fluctuations during the four hour experiment phase compositions were determined by electron probe microanalysis epma and secondary ion mass spectroscopy sims the olivine ranges from fo94 to fo90 in composition and contains about 4000 ppm wt h2o the wadsleyite is fo87 1 in composition and contains about 10 000 ppm wt h2o the clinoenstatite is en93 1 in composition and about 1400 ppm wt h2o transmission electron microscopy of the wadsleyite lamellae and host olivine shows that the two phases share their close packed oxygen planes so that the wadsleyite lamellae are nearly planar and perpendicular to the 1 0 0 of olivine the wadsleyite lamellae thus have their 1 0 1 and 0 2 1 planes parallel to the 1 0 0 plane of olivine additionally larger incoherent grains of wadsleyite in olivine are found dislocation microtexures in the olivine and iron concentration profiles across the lamella interface suggest heterogeneous nucleation and diffusion controlled growth of coherent wadsleyite lamellae on defects in the olivine followed by the nucleation of faster growing incoherent grains on the lamellae the results show that under hydrous conditions the olivine wadsleyite transformation occurs close to equilibrium at conditions of the 410km discontinuity furthermore inheritance of crystallographic preferred orientations and therefore seismic anisotropy across the 410 km discontinuity is unlikely to be significant in addition hydrogen distributions among the various phases indicate that dehydration by melt extraction at 410 km will be inefficient and that h contents greater than about 4000 ppm wt h2o are needed to initiate melting at 410 km 2012 published by elsevier b v

transformation mechanisms of san carlos olivine to mgfe 2sio4 β phase under subduction zone conditions abstract to understand the mechanisms of the high pressure transformations of mg0 9fe0 1 2sio4 under transition zone conditions we have experimentally reacted san carlos olivine fo90 at 1000 c and 13 gpa in a multianvil split sphere apparatus the reaction products and microstructures have been studied in detail by transmission electron microscopy under these disequilibrium conditions san carlos olivine transforms to phase and spinel with mg fe ratios which are not consistent with equilibrium compositions the transformation to both phases occurs by a nucleation and growth mechanism involving nucleation at olivine grain boundaries both phases contain characteristic stacking faults on 010 phase and 110 spinel and some spinel grains show extreme degrees of disorder although there is uncertainty in the exact stability field of the experiment the results are consistent with formation of metastable spinel probably within either the or stability fields metastable spinel progressively transforms back to phase by a shear like mechanism involving a highly disordered spinelloid phase which may be a transitional phase in the transformation based on these results and available kinetic data metastable formation of spinel in a subducting oceanic slab within the or stability fields is possible at temperatures below 1000 c

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

an olivine to beta phase transformation mechanism mg2sio4 fine grained powder samples of mg2sio4 forsterite phase have been partially transformed to the beta phase in a 2000 ton uniaxial split sphere apparatus ussa 2000 at p 15 gpa and t 1000 c in runs of 5 minutes duration this technique allowed us to catch the first steps of the transformation in mg2sio4 a tem study of the run products showed that a structurally disordered phase is an intermediate of the reaction the oxygen sublattice of this intermediate phase is well ordered but a very large density of planar faults on 010 planes destroys the long range order in the cation sublattices a two steps mechanism for the transformation is therefore probable involving first the nucleation of the structurally disordered phase then the ordering of this phase these two steps may be associated with low energy activation barriers and could thus explain the fast kinetics of the transformation

detection and analysis of microseismicity in multi anvil experiments abstract we present a new technique to measure microseismicity at gigapascal pressures this new technique uses existing acoustic emission ae and multi anvil press technologies with minor modifications to make them compatible as a proof of concept we present results from acoustic emission studies on serpentine dehydration to 8 gpa and the olivine wadsleyite transformation at 16 gpa these case studies demonstrate the power of gigapascal acoustic emission monitoring and point to future developments which will further strengthen the technique

experimental deformation of a synthetic dunite at high temperature and pressure ii transmission electron microscopy abstract we have performed detailed transmission electron microscope on most of the deformed synthetic dunite specimens prepared in the study by zeuch and green 1984 we have identified three basic types of sub boundaries simple tilt walls in 100 and 001 composed by b 100 and b 001 edge dislocations respectively and twist boundaries in 010 composed of b 100 and b 001 screws we have also observed more complex asymmetric lilt boundaries in 100 and 001 like the 010 twist boundaries these asymmetric tilt walls are common only at the highest temperatures and lowest strain rates subgrain development is extensive at the higher temperatures and lower strain rates and subgrains are composed of the above mentioned three types of sub boundaries edge components in 100 and 001 ire knitted to screw components in 010 as described by kirby and wegner 1978 for naturally deformed olivine in many areas of the samples which we studied subgrain development is not observed but parallel arrays of tilt boundaries of one type or the other are present at higher temperatures and lower strain rates 100 organization durham et al 1977 is common this structure consists of parallel arrays of 100 tilt boundaries with b 100 screws connecting the sub boundaries at lower temperatures we have observed an analogous arrangement of 001 sub boundaries and b 001 screws which we refer to as 001 organization under all experimental conditions dislocations with b 100 and b 001 are present in approximately equal numbers however the two types of dislocations also have distinctly different geometries under all test conditions we suggest that the transition from slip parallel to 001 to slip parallel to 100 with increasing temperature which has been reported in earlier studies may also depend upon water content the substructures which we observe are virtually identical to those seen in many naturally deformed peridolites and we conclude that the mechanisms involved in both natural and laboratory deformation of olivine polycrystals are similar on the other hand the substructures reported here are very different from those observed in experimentally deformed olivine single crystals it seems likely that these substructural differences reflect fundamental differences in the behavior oh single crystals and polycrystals which are in turn reflected in different measured creep strengths

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

strength of mg fe 2sio4 wadsleyite determined by relaxation of transformation stress abstract the growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change the strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth stress relaxation enables growth to continue at a rate controlled primarily by the mechanical properties of the rim we studied this process in high pressure experiments in order to constrain the rheology of wadsleyite one of the high pressure polymorphs of olivine single crystal spheres of san carlos olivine surrounded by a quasi hydrostatic pressure medium of nacl or au were partially transformed to wadsleyite at 1100 c and 16 17 gpa to determine the rate of growth as a function of time microstructural observations indicate that the relict olivine remains largely undeformed while the wadsleyite rims deform by dislocation creep application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 gpa a decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples kubo t ohtani e kato t shinmei t fujino k 1998a experimental investigation of the transformation of san carlos olivine single crystal phys chem miner 26 1 6 kubo t ohtani e shinmei t fujino k 1998b effects of water on the transformation kinetics in san carlos olivine science 281 85 87 modeling these results gives values for the yield strength of wadsleyite between 2 and 5 gpa under various conditions a strength difference of 1 gpa is indicated by experiments at 1030 c conducted under nominally dry 200 ppm h2o and water added 500 ppm h2o conditions supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite the calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions based on an extrapolation of the low temperature plasticity flow law for olivine these results imply a strong increase in the strength of subducting slabs in the mantle transition zone provided that the deformation mechanism is the same as in the experiments furthermore inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high pressure polymorphs in subducting slabs

stability of clinopyroxene at pressure temperature conditions of the transition region abstract the stability of diopside at conditions of 16 19 gpa and 1250 1900 c was determined using a 6 8 type multi anvil apparatus at pressures of 17 18 gpa diopside undergoes a decomposition reaction to an assemblage of casio3 perovskite ca mg sio3 garnet or ilmenite depending on temperature the clapeyron slope for the breakdown reaction download download full size image is 2 mpa k 1 when coupled with a re examination of clinopyroxene stability in other more complex peridotite systems the experimental data are consistent with a recent proposal that the 520 km discontinuity in the transition region is caused by the exsolution of casio3 rich perovskite from garnet rich in majorite component

the rheology of faults triggered by the olivine spinel transformation in mg2geo4 and its implications for the mechanism of deep focus earthquakes the relevance of the anticrack faulting mechanism to the origin of deep focus earthquakes depends critically on the physical processes operating during fault generation and movement we show here that the resistance to sliding on fault zones produced by this mechanism in mg2geo4 depends only weakly on confining pressure but strongly on sliding rate both of these characteristics are inconsistent with friction the microstructures of the fault zones and the crystal plastic rheology of the constituent phases indicate that sliding must occur primarily by grain boundary sliding previously we reported that anticrack faulting is accompanied by elastic radiation of energy acoustic emissions and that it can operate during the transformation in mg fe 2sio4 at the pressures and temperatures at which deep earthquakes occur the present results and the recent demonstration that metastable olivine is present in the subducting slab beneath japan provide additional support for the anticrack theory of deep focus earthquakes

transmission electron microscope observation of α β and γ mg fe 2sio4 in shocked meteorites planar defects and polymorphic transitions abstract high pressure polymorphs of olivine mg fe 2sio4 have been observed by transmission electron microscopy in the shocked tenham and catherwood meteorites planar defects are characterized in and polymorphs and their possible role in polymorphic transitions is assessed and discussed in relation with the published literature the stacking fault 1 2 1 01 010 observed in the phase can produce a shear transformation to the phase if it occurs on every other 010 plane conversely the stacking fault 1 2 1 1 2 110 observed in the phase can be transformed to the phase if it is repeated on every other 110 plane and to an intermediate yet unobserved e phase if it is repeated on every 110 plane shear transformation between olivine and spinel could involve the e phase as an intermediate stage

shear promoted phase transitions in fe2sio4 and mg2sio4 and the mechanism of deep earthquakes phase transformations in end member olivines have been investigated in the temperature range comparable to the interior of subducting slabs this work constitutes the experimental evidence that the kinetics of transformation of silicate olivine phase to modified spinel and spinel phases is enhanced by shear deformation natural fayalite fe2sio4 subjected to a pressure gradient from 0 to 25 gpa at 380 c in the diamond anvil cell dac developed a ring of phase where the pressure was in the stability field of the phase and shear stress was large enough to promote the transition the sample inside the ring despite being at higher pressure remained dominantly as phase the outermost lower pressure region of the sample also remained as phase in the mg2sio4 system the transition from to was observed at 575 c in runs in which pressure covered the stability fields of phase phase and mixed oxides these results show that the characteristic transformation temperature tch can be lowered as much as 200 c by shear deformation on the basis of these observations we propose a nonhydrostatic kinetic boundary for the and transitions in mantle olivine for temperatures below this boundary the transformations are kinetically inhibited while above it the transformations can be promoted by shear deformation therefore olivine carried to a depth of several hundred kilometers in a subducting slab can remain as metastable phase until shear deformation causes its transformation we suggest that this region of shear promoted transformation in the cold interior of the subduction zone is responsible for the generation of deep earthquakes

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

stress determination in multianvil deformation experiments based on dislocation curvatures measurements application to wadsleyite and ringwoodite 1 the curvature of dislocation lines can serve as a probe for the resolved shear stress in multianvil deformation experiments this method is very well adapted to deformation experiments in the multianvil apparatus where the microstructure and in particular the dislocation density can be largely determined by early deformation stages on the contrary individual dislocation movements can adjust more rapidly to stress level changes and can thus record the last stress level borne by the specimen before it is quenched the method has been applied to dense specimens of wadsleyite and ringwoodite deformed at 15 to 22 gpa and 1000 to 1500 1600 c the resolved shear stress is very similar in wadsleyite 400 mpa and ringwoodite 350 mpa deformed at 1000 c showing that these minerals exhibit very similar rheological properties under these conditions

transformation induced faulting requires an exothermic reaction and explains the cessation of earthquakes at the base of the mantle transition zone abstract in many subduction zones earthquakes occur continuously to the bottom of the mantle transition zone and stop abruptly at the endothermic phase change marking the top of the lower mantle suggesting cause and effect new experiments on polycrystals undergoing the ilmenite perovskite transformation show that transformation induced faulting occurs only in the exothermic direction of the reaction the close structural and thermodynamic analogies between the transformation studied here in cdtio 3 and those expected in the silicates of the earth s mantle argue strongly that these results are relevant to transformations occurring in subducting lithosphere moreover consideration of the thermodynamics of this asymmetry show that this result should be a general phenomenon if so it follows that faulting by this mechanism cannot occur in the lower mantle providing a natural explanation for the termination of earthquakes just before 700 km depth if a wedge containing metastable olivine exists at depth in subduction zones and is necessary for deep earthquakes our results remove the constraint that this wedge must taper to zero where the deepest earthquakes occur the characteristics of the large deep earthquakes of 1994 and their aftershocks show that deep earthquakes can propagate beyond the zone of previous hypocenters and trigger aftershocks there providing important new information about the mechanism of deep earthquakes and pointing to the need for additional experiments

dislocation recovery in olivine under deep upper mantle conditions implications for creep and diffusion the kinetics of dislocation recovery in olivine have been studied up to 10 gpa and 1500 c using a multianvil apparatus dislocations in the 010 100 slip system were generated by high temperature deformation at room pressure and then the specimens were statically annealed at high pressures and temperatures the rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion a small activation volume v 6 1 cm3 mol was obtained which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport limiting species oxygen or silicon combined with the previous observation of a small activation volume for mg fe diffusion we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle we have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters such as stress exponents grain size exponents and activation volume for dislocation creep the results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle

kinking in mg2geo4 olivine an ebsd study we have used electron backscatter diffraction ebsd to characterize the internal deformation and orientation of olivine grains within an experimentally deformed mg2geo4 olivine aggregate we observe that a significant fraction of the grains contain kink bands our observations of kink band geometry combined with the work of earlier authors indicate that 001 slip is the dominant slip system operating in mg2geo4 olivine at 1000 1473 k at 0 6 1 3 gpa since mg2geo4 is a high pressure analog for natural compositions of olivine this is consistent with the observations made by others that the dominant slip system for olivine in the mantle switches from 100 to 001 as pressure increases we also observed that kink bands formed in all measured olivine grains with 001 axes oriented within 35 of the compression direction we thus conclude that in a randomly oriented polycrystal undergoing uniaxial deformation at low to moderate temperatures 18 of all grains will deform by kinking and that kink band formation is an important accommodation mechanism for olivine at these conditions

anisotropic growth in the olivine spinel transformation of mg2geo4 under nonhydrostatic stress abstract the olivine spinel phase transformation in mg 2 geo 4 does not occur by a martensitic mechanism the evidence from samples transformed in a griggs type solid medium deformation apparatus are 1 1 lack of microstructural features in the olivine phase which can be specifically associated with a martensitic mechanism 2 2 the orientation relationship between the two phases that is predicted by the martensitic mechanism does not occur nor is there any apparent consistency of relative orientations 3 3 application of a differential stress to the transforming sample resulted in an anisotropic growth rate for the spinel phase indicating that growth was externally controlled rather than crystallographically controlled anisotropic growth of the spinel phase results in elongation of the residual olivine phase grains in the plane normal to the direction of maximum principal compressive stress a velocity ratio of 1 7 0 7 5 4 has been determined for the growth rate of the spinel from measurements on residual olivine grains the interphase grain boundary in samples transformed under stress has cusp shaped fingers of spinel with a blunt end separated by thin spikes of olivine samples transformed isostatically do not exhibit this feature providing further confirmation of anisotropic growth of the spinel the preferred growth of the spinel is consistent with a theory of phase transformation under nonhydrostatic stress the predicted spinel finger shape based on this theory is generally consistent with observed shapes except for the blunt end the discrepancy may be due to surface energy which has not been considered here or to local deviations of the applied macroscopic stress

the nature and significance of stacking faults in wadsleyite natural β mg fe 2sio4 from the peace river meteorite abstract the phase spinelloid polymorph of mg fe 2 sio 4 makes up a major part of the transition of the earth s mantle naturally occurring mg fe 2 sio 4 wadsleyite from the peace river meteorite was found to carry a variety of stacking faults the nature of which have been studied using high resolution transmission electron microscopy the faults lie on 010 and are generally of a complex nature best described in terms of various stacking sequences of the component spinelloid units the stacking faults locally transform the cation distribution so that in the plane of the fault the structure is that of spinel the development of such stacking faults is consequently a significant feature of the martensitic transformation of spinel to phase the possible occurrence of transformation enhanced plasticity associated with this inversion is discussed and the probable deformation mechanisms of mg fe 2 sio 4 are outlined

high temperature creep of olivine single crystals 2 dislocation structures dislocation structures and densities in experimentally deformed single crystals of san carlos olivine were examined using the oxidation decoration technique the high temperature deformation experiments were conducted at various temperatures t differential stresses and oxygen fugacities o2 the samples were buffered against either orthopyroxene opx or magnesiowustite mw solid state powders and compressed along one of the three 45 orientations in samples that were deformed and subsequently quenched under load seven distinct dislocation structures were observed a for samples compressed parallel to 110 c at 16 50 mpa and o2 10 6atm two different dislocation arrangements were identified for opx buffered samples at 1400 t 1475 c the dislocation structure was composed of curved screw segments that had usually cross slipped and straight edge segments of comparable length at 1300 t 1350 c for opx buffered samples and at 1350 t 1500 c for mw buffered samples the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations b for both opx buffered and mw buffered samples compressed parallel to 101 c at 1340 t 1400 c and 30 60 mpa the dislocation configurations are characterized by zigzag near edge dislocations at o2 10 5atm and by straight cross slipped screw dislocations plus pinned edge dislocations at o2 10 9atm c for samples compressed parallel to 011 c at t 1400 c and 70 150mpa the primary dislocations change from long straight edge dislocations for o2 10 4atm and opx buffer to a combination of straight edge dislocations and polygon shaped half loops for o2 10 4atm and mw buffer to gently curved near edge dislocations for o2 10 9atm for both solid state buffers abundant long straight screw dislocations were present in each case for the 011 c samples all of the above variations in dislocation structure are paralleled by changes in the measured power law equation that describes the behavior of creep mechanisms of olivine thus the different dislocation structures are associated with different rate controlling creep mechanisms for all of the dislocation structures the dislocation density increases with differential stress according to the relation 1 4 dislocation structures in samples that were deformed and then statically annealed have also been studied for both 110 c and 011 c samples deformed over a wide range of experimental conditions the dislocation structures on the slip planes generally consist of short curved edge or mixed dislocation segments in 101 1 samples deformed at low o2 50 mpa and 1400 c the dislocation structure was much less altered by annealing than the dislocation structures in 110 c and 011 c samples the density of dislocations decreased much less for these 101 c samples than for the 110 c and 011 c samples due to climb of the edge segments the straight screw segments found in quenched samples became curved and many low angle 100 tilt boundaries connected by straight 100 screw dislocations developed during the annealing process

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

analytical electron microscopy of a synthetic peridotite experimentally deformed in the β olivine stability field a synthetic harzburgite prepared by isostatic hot pressing of finely ground natural olivine admixed with 3 enstatite was deformed at high temperature and high pressure such that the olivine phase boundary was traversed both residual and new phases their grain sizes their deformation induced lattice defects and their compositions were then characterized by analytical electron microscopy owing to large heterogeneities in temperature pressure and deformation conditions contrasting microstructures were observed some areas consist essentially of untransformed olivine while the polymorph is the main constituent of other areas crystals of the polymorph are generally free of dislocations except at the grain boundaries of the larger grains where slip bands were nucleated probably induced by stress concentrations the activated glide systems are 021 100 the orthopyroxene grains have been transformed to very fine grained clinopyroxene and have reacted with chromite to form garnet with a majoritic component compared to olivine crystals of the phase contain significantly higher concentrations of fe and trivalent cations all crystals are free of inclusions confirming the conclusion from previous infrared spectroscopic analysis that the high oh concentration in this specimen is dissolved in the phase coupled with these previously measured oh concentrations our new measurements suggest that incorporation of hydrogen into the phase and substitution of trivalent cations into the octahedral sites may be coupled such increased complexity of mineral chemistry points to a need for detailed investigation of the spectrum of compositions stable in the structure and coexisting phases in order to better estimate the modal mineralogy of the transition zone

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

evolution of grain sizes and orientations during phase transitions in hydrous mg2sio4 transformation microstructures in mantle minerals such as mg fe 2sio4 are critical for predicting the rheological properties of rocks and the interpretation of seismic observations we present in situ multi grain x ray diffraction experiments on hydrous mg2sio4 at the p t conditions relevant for deep cold subducting slabs up to 40 gpa and 850 c at a low experimental strain rate 4 10 6s 1 we monitor the orientations of hundreds of grains and grain size variations during the series of forsterite wadsleyite ringwoodite phase transformations microtextural results indicate that the and an intermediate phase grow incoherently relatively to the host phase consistent with a nucleation and growth model the and phase exhibit orientation relationships which are in agreement with previous ex situ observations the and intermediate show texturing due to moderate differential stress in the sample both the and transformation induce significant reductions of the mean sample grain size of up to 90 that starts prior to the appearance of the daughter phase apart from the in the newly formed and phases the nucleation rate is faster than the growth rate inhibiting the formation of large grains these results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models