in situ x ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions implications for density of subducted oceanic plate 1 phase relationships in klb 1 peridotite normal type mid oceanic ridge basalt nmorb and k rich basalt compositions were investigated experimentally from 28 to 143 gpa and 2050 to 3000 k a laser heated diamond anvil cell with the synchrotron x ray diffraction was developed to perform in situ observations of samples at simultaneous high pressures and temperatures we confirmed that the peridotite composition crystallized into an assemblage of mg perovskite ca perovskite magnesiowustite at pressures below 97 gpa and that nmorb and k rich basalt compositions both crystallized as mg perovskite ca perovskite stishovite an aluminous phase with a cafe2o4 type structure at pressures below 68 gpa the orthorhombic mg perovskite is stable in natural rock compositions at pressures less than 97 gpa corresponding to about 2200 km depth phase transition of silica phase from stishovite to cacl2 type phase was observed at 78 gpa in nmorb composition moreover we observed that nmorb crystallized in to an assemblage of cairo3 type mg fe sio3 ca perovskite pbo2 type silica an aluminous phase with a calcium cati2o4 type structure at 143 gpa corresponding to the core mantle boundary depth we measured the unit cell volumes of minerals in the nmorb composition at high p t and thereby estimated the density of subducted morb to a depth of 1800 km in the lower mantle in this depth regime morb is denser than model mantle densities e g prem and ak135 the estimated density of morb may contribute to an understanding of dynamics of the subducted slab in the deep lower mantle

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

on the separation of crustal component from subducted oceanic lithosphere near the 660 km discontinuity the recent mineral physics observations suggest that the crustal and mantle components of subducted oceanic lithosphere have distinct densities and viscosities the deformation of such chemically stratified oceanic lithosphere as it collides with the 660 km discontinuity is examined using a simple analytical model it is shown that the separation of the crustal component can occur through a small scale folding 10 20 km if the viscosity of the crustal garnetite layer is significantly higher than the surrounding mantle 1021 pa s but is less than 1023 pa s leading to a garnet enriched transition zone behind subduction zones

mixing of heterogeneities in the mantle effect of viscosity differences the effect of viscosity variations associated with large scale chemical heterogeneities on mixing in the mantle is studied for a model two dimensional problem low viscosity regions are rapidly deformed develop long tendrils and are entrained towards regions of surface divergence very viscous regions are only slowly stretched thus geochemical reservoirs can persist relatively undisturbed for long periods of time if they are o 10 100 times more viscous than the surrounding mantle high viscosity ratio blobs have a tendency to aggregate leading to the formation of large scale heterogeneities from smaller ones

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

seismic evidence for accumulated oceanic crust above the 660 km discontinuity beneath southern africa 1 high pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle thus subducted oceanic crust may accumulate at the base of the upper mantle direct observational evidence for this hypothesis however remains elusive we present an analysis of a negative polarity shear wave converted from compressional wave at a seismic discontinuity near 570 600 km depth beneath southern africa the negative polarity of the converted phase indicates a 2 2 0 2 s velocity decrease with depth at the seismic discontinuity this velocity reduction is associated however with a low velocity contrast at the 660 km discontinuity the exsolution of ca perovskite in former oceanic crust at depth greater than 600 km and the associated small volume fraction of ringwoodite are plausible explanations for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660 km discontinuity

intracrystalline nucleation during the post garnet transformation under large overpressure conditions in deep subducting slabs 1 the mechanism of the post garnet transformation in natural single crystalline pyropic garnet has been examined under large overpressure conditions of 31 43 gpa using the multianvil apparatus with sintered diamond anvils intracrystalline nucleation was found to be dominant above 35 38 gpa while only grain boundary nucleation is responsible at lower pressures in addition natural pyropic garnet transformed to a single phase of perovskite without decomposing to multiple phases above 38 gpa both intracrystalline nucleation and polymorphic growth of the post garnet transformation under large overpressure conditions would strongly enhance the transformation kinetics consequently the post garnet transformation may pwroceed rapidly in the subducting slab at depths near 950 km which would cause substantial rheological weakening of the subducting slab the metastable post garnet transformation may also cause the seismic discontinuities at a depth of 900 1080 km depth which is indeed observed beneath some subduction zones

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

metastability of enstatite in deep subducting lithosphere olivine and mg fe sio3 pyroxene the most abundant minerals in the earth s upper mantle are believed to transform to high pressure phases at 400 km depth1 5 the possible metastable persistence of olivine to greater depths in some subduction zones with consequences for the origin of deep focus earthquakes and the dynamics of subduction has been discussed extensively6 14 but the role of other mantle minerals has not been considered we report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite mgsio3 at upper mantle conditions we find that whereas forsterite mg olivine mg2sio4 transforms rapidly to phase at 1 200 c and 16 gpa the transformation of enstatite to phase plus stishovite on the same timescale requires much higher temperatures at lower temperatures enstatite transforms directly to the ilmenite structure but only at pressures greater than 20 gpa enstatite should therefore persist metastably to greater depths than olivine in subduction zones transforming directly to the ilmenite structure the enstatite ilmenite transformation is accompanied by a large decrease in volume which should increase the stresses in subducting slabs change the buoyancy forces that drive subduction and might also contribute to the origin of deep focus earthquakes

grain growth kinetics in wadsleyite effects of chemical environment abstract grain growth kinetics in wadsleyite was investigated using a multianvil high pressure apparatus fine grained wadsleyite aggregates were synthesized by isostatic hot pressing and were subsequently annealed under high pressure and temperature in a controlled chemical environment wadsleyite samples show normal grain growth characterized by a log normal grain size distribution following the relation l n l 0 n k t where n is a constant l the grain size at time t l0 the grain size at time t 0 and k is a rate constant that depends on temperature t and chemical environments f o 2 oxygen fugacity in pa coh water content in h 106si as k a d f o 2 r d exp h d r t a w f o 2 r w c oh q exp h w r t with a d 10 4 9 6 1 8 0 7 4 m n s 1 p a r d rd 0 12 0 11 0 20 0 14 h d 410 230 500 270 kj mol a w 10 18 2 1 4 24 0 1 7 m n s 1 p a r w rw 0 14 0 05 0 22 0 06 q 1 7 0 3 2 2 0 3 and h w 120 60 160 70 kj mol with assumed value of n 2 3 values in parentheses denote parameters for n 3 both water and oxygen fugacities significantly enhance grain growth kinetics the large value of the parameter describing the water fugacity dependence q 1 5 2 5 cannot be explained solely by a simple model in which grain growth is controlled by diffusion of atoms defects across the grain boundaries the interaction of grain boundaries with charged defects or the density of hydrated ledges may be important factors that control the grain growth kinetics of wadsleyite when compared at similar thermo chemical conditions grain growth of wadsleyite is found to be more sluggish than grain growth of olivine the present results show that a small wadsleyite grain size 100 000 h 106si is present a small grain size

the influence of oh in coesite on the kinetics of the coesite quartz phase transition metastable coesite is an important pressure indicator for ultrahigh pressure rocks however in many cases coesite does not survive exhumation but reacts back to quartz although it was shown experimentally that incorporation of h in coesite increases with increasing pressure most coesite relics found in nature are essentially dry i e oh concentrations are below detection limit 1 wt ppm h2o thus does the incorporation of h promote the back reaction of coesite to quartz during exhumation the influence of intrinsic oh on the kinetics of the coesite quartz phase transition was determined using synthetic dry coesite with approximate to10 wt ppm h2o and wet coesite with approximate to105 wt ppm h2o tem analysis of the quenched samples proved the presence and absence of water in the wet and dry samples respectively the kinetics of the coesite quartz transition was investigated in situ using the multi anvil apparatus max 80 at the hamburger synchrotron radiation laboratory hasylab the transition rates were measured by observing changes in selected diffraction line intensities as a function of time the transformation and growth rates were derived using cahn s model of nucleation and growth at grain boundaries under the same experimental conditions the transformation rate of the wet coesite is more than ten times higher than that of the dry coesite this difference may explain why oh bearing natural coesite is rare this study reveals the importance of structurally bound oh for the kinetics of phase transitions of nominally anhydrous minerals

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

water solubility in mg perovskites and water storage capacity in the lower mantle the water storage capacity of the major constituent of the lower mantle mg perovskite is a matter of debate here we report water solubility of mg perovskites with different compositions observed in peridotite and morb systems ir spectra of pure mgsio3 perovskite show bands at 3397 3423 3448 and 3482 cm 1 and suggest about 100 ppm h2o the h2o content in al mg perovskite 4 7 wt al2o3 mg 100 is 1000 1500 ppm major band at 3448 cm 1 whereas al fe mg perovskite in morb al2o3 13 17 wt mg 58 61 contains 40 110 ppm h2o major band at 3397 cm 1 the h2o content in al fe mg perovskite observed in peridotite al2o3 5 6 wt mg 88 90 is 1400 1800 ppm major band at 3397 cm 1 al fe mg perovskite from the morb system has a high fe3 content fe3 fe 0 6 determined by electron energy loss spectroscopy measurements water can enter into the perovskite structure with oxygen vacancies originating from the substitution of si by al and fe3 oxygen vacancy incorporation is favored for aluminous perovskite synthesized from the mgo rich peridotite system the substitution of si4 mg2 2 al fe 3 prevails however in the al fe mg perovskite from the morb system mgo poor al and fe rich explaining its restricted water solubility the maximum amount of water stored in the lower mantle is estimated to be 3 42 1021 kg which is 2 5 times the present ocean mass comparison of the phase relations in hydrous pyrolite and hydrous morb indicates that pyrolite is more important as water container and water carrier in the mantle pyrolite contains 1 dense hydrous magnesium silicates existing under conditions of subducting slabs and 2 hydrous wadsleyite hydrous ringwoodite and water bearing perovskite under the normal mantle and hotter conditions distribution of water to the morb is restricted at the conditions of the transition zone and lower mantle

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

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

an assessment of the seawater effect by geochemical and isotopic data on the brackish karst groundwater from the karaburun peninsula izmir turkey the study area is located in the center of the karaburun peninsula turkey where several karstic springs discharge near the coast 0 2 m above the sea level the most important discharge points are the ildiri and gerence springs groundwater obtained from ildiri springs and several drill wells provide water to the region during the summer periods the electrical conductivity values are between 1900 and 3900 s cm springs are slightly salty and are not used directly as drinking water salinity was observed in the spring waters under natural flow conditions the salinity values of the drillings and springs were measured in dry and wet seasons spring waters have conductivity values between 3000 and 4000 s cm preliminary surveys reveal that seawater contribution to ildiri springs is 6 5 maximum gerence spring waters discharge to the surface from a region very close to the sea the dominant cation is na and the dominant anion is cl conductivity values were between 7000 and 13 000 s cm gerence spring waters discharge to the sea with seawater contributions ratios between 8 5 and 20 3 depending on the seasonal changes while some spring waters discharge 0 5 1 0 m above sea level the rest of them discharge directly into the sea salinization of gerence spring water is higher than the other springs

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

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

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

a temperature profile of the mantle transition zone the temperature distribution across the transition zone has been determined on the high pressure phase equilibria in the system mg2sio4 fe2sio4 recently determined by katsura and ito 1988 and ito and takahashi 1988 the temperature at the base of the transition zone 655 km depth is estimated to be about 1600 c assuming that the spinel dissociation is responsible for the sharp seismic discontinuity implied by study of reflected waves this estimation would be little modified for the chemically stratified mantle models since the lower mantle is characterized by the stability of perovskite the temperatures in the region from 350 km to 655 km depth are calculated for a peridotitic mantle model assuming an adiabatic temperature gradient the and transformations in mg fe 2sio4 make the temperature profile steep in the depth regions of 390 415 km and 500 530 km respectively the temperature would increase from about 1400 c to 1600 c over the depths interval 350 between 655 km

observations of the mid mantle discontinuity beneath indonesia from s to p converted waveforms 1 s to p conversions were employed to derive a coherent discontinuity structure beneath indonesia analysis of data recorded by three regional arrays from nine deep earthquakes not only confirmed the results of previous studies on the existence of the mid mantle discontinuity beneath the java arc but also revealed its presence north to kalimantan island s to p waves converted at the discontinuity were observed on the stacked diagrams with a negative slowness relative to the p wave and a conversion depth ranging from 1080 km in the west to 930 km in the east

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

in situ x ray diffraction study of post spinel transformation in a peridotite mantle implication for the 660 km discontinuity abstract we present the phase relations in anhydrous cao mgo feo al2o3 sio2 pyrolite to examine the influence of compositional difference between pyrolite and mg2sio4 on the post spinel phase transformation it is shown that in the pyrolite system the transformation occurs at about 0 5 gpa lower pressure relative to mg2sio4 we have carried out several in situ x ray diffraction experiments on ringwoodite to mg perovskite ferropericlase and backward transformations and found that post spinel transformation boundary can be expressed as p gpa 0 0005 t k 23 54 using the gold equation of state by tsuchiya t tsuchiya first principles prediction of the p v t equation of state of gold and the 660 km discontinuity in earth s mantle j geophys res 108 2003 doi 10 1029 2003jb002446 the interval for coexisting ringwoodite and mg perovskite was found to be 0 1 0 5 gpa the discrepancy between our data and the depth of the seismic discontinuity global average 654 km is about 20 km at 1850 k based on the results of in situ measurements we confirmed that the difference in chemical composition between pyrolite and mg2sio4 cannot modify the clapeyron slope of the post spinel transformation using experimental data and assuming the average mantle temperature 1850 k at 660 km we can account for only a half of variations in the depth of the 660 km discontinuity in subduction zones and at hot spots an additional explanation for the observed seismological variations at the 660 km discontinuity is required and may reflect influence of other minor components or volatiles

transport of water into the lower mantle role of stishovite when subjected to lower mantle pressures and temperatures natural anhydrous basalt containing 0 2 wt h sub 2 o forms a phase assemblage in which sio sub 2 stishovite is a significant carrier of hydrogen up to 500 ppm h sub 2 o by weight as hydroxide whereas the coexisting mg fe al ca sio sub 3 perovskite appears to be not upper bound of 50 ppm wt h sub 2 o contrary to the devolatilization characteristically observed at lower pressures we find that the abundance of h sub 2 o in residual stishovite increases from approx 100 to approx 400 ppm by weight upon partially melting the high pressure mineral assemblage at 28 60 gpa we infer that the trace concentration of al within residual stishovite increases upon partial melting thereby increasing the coupled abundance of h in this crystalline phase the anhydrous component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the earth with subducted stishovite potentially returning approx 10 sup 2 times the amount of water present in today s atmosphere

bulk attenuation in the earth s mantle due to phase transitions abstract the calculated bulk attenuation due to phase transition from mineral physics data is reported here with relaxation time less than 1 s the calculated value for a pyrolite mantle is consistent with the inverted bulk attenuation of the upper mantle from seismic observations the two important mechanisms of phase transitions diffusion controlled and kinetics controlled have different relaxation time as indicated by the models here the diffusion controlled is more likely to contribute to the observed seismic bulk attenuation than the kinetic controlled process based on the available diffusivity and kinetics data the correlation between the bulk attenuation and relaxation time emphasizes the importance of a number of parameters in the mineral physics database such as fe mg diffusivity and kinetics in olivine wadsleyite ringwoodite perovskite mg ca al si diffusivity and kinetics in pyroxene garnet ca perovskite some of which are still unknown

depth variation of the mid mantle seismic discontinuity short period array seismograms of deep events that occurred in the indonesia japan and izu bonin arcs are stacked and beam formed to identify the near source s p converted waves that result from the mantle transition discontinuities most of the resulting images reveal the existence of a mid mantle seismic discontinuity 920 km discontinuity in these regions of the 15 events analyzed three that occurred at the western end of the indonesia arc show clear s p arrivals observable even in individual seismograms the mid mantle discontinuity is characterized by large depth variation 900 1080 km and velocity contrast variation in different subduction zones especially the depth variation of the mid mantle discontinuity beneath the indonesia arc where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end appears to be well correlated with the location of the high velocity anomalies in recent tomographic models however the mid mantle discontinuity cannot be simply coincided with the bottom of the high velocity anomalies because a velocity increase at the discontinuity is observed from the waveform analysis