is the transition zone an empty water reservoir inferences from numerical model of mantle dynamics abstract water is probably present everywhere in the earth s mantle today with abundances ranging between scales of percent to the parts per million ppm mantle total water content is estimated to be between 10 to several times that of the present day hydrosphere numerous studies have been devoted to the determination of water solubility in mantle material d r bell g r rossmann science 255 1992 1391 1397 j ingrin h skogby eur j mineral 12 2000 543 570 they all show strong solubility variations from one mineral phase to another principally water partitioning has made the transition zone a probable trap for water from the earth s mantle n bolfan casanova et al earth planet sci lett 182 2000 209 221 d l kohlstedt et al contrib mineral petrol 123 1996 345 357 nevertheless water distribution within the mantle is still debated we have studied the role of mantle dynamics in water distribution by modeling water transport and mantle convection in a two dimensional 2 d cartesian geometry the model takes into account water partitioning between the mantle s transition zone and the upper mantle of 10 1 and between the lower mantle and the transition zone of 1 100 i e respectively between olivine and spinel and spinel and post spinel we have modeled the mantle temperature field using depth dependent viscosity and plate like surface conditions water injection at the trench has also been simulated our numerical experiments suggest that diffusivity of water has to be very high at least two orders of magnitude higher than the one experimentally determined d r bell g r rossmann science 255 1992 1391 1397 j ingrin h skogby eur j mineral 12 2000 543 570 to significantly influence water distribution in earth s mantle in fact the diffusion process is not efficient enough to balance the mixing due to mantle dynamics and to force water into the transition zone we show that the distribution of water should be quite homogeneous throughout the mantle if advection and diffusion are the only processes involved in water transport in the mantle this homogeneity implies that water below the transition zone could be in excess according to the lower mantle rocks solubility this addresses the question of stability of free water in the lower mantle and its mobility by percolation process which could be a very efficient transport process previously unconsidered in this field of research

lithospheric structure of the chaco and parana basins of south america from surface wave inversion surface wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the chaco and parana basins of central south america the upper mantle velocity structure beneath the parana basin is cratonic in character whereas that beneath the chaco basin is tectonic or asthenospheric in character the surface wave analysis used broadband recordings from a subset of a 14 station array deployed in a roughly east west sawtooth arrangement along 20 s latitude with a total e w aperture of 1 400 km results from receiver function analysis as well as direct p wave regional travel time data were used in the inversions to help constrain moho depths and crust and upper mantle velocities s wave structure for the intracratonic parana basin was determined using interstation phase and group velocities for rayleigh waves fundamental and first higher mode and love waves fundamental mode only based on seven events with paths which traverse the eastern parana basin and one event with a path across the western parana basin the average moho depth in the eastern parana basin is 42 km the high velocity upper mantle lid has a maximum s wave velocity of 4 7 km s with no resolvable low velocity zone to at least 200 km depth this cratonic velocity structure indicates the presence of a lithospheric root beneath the parana basin despite emplacement of the parana plume the limited data from the western parana basin are consistent with a homogeneous upper mantle structure throughout the parana basin waveform inversion of fundamental mode and first higher mode rayleigh waves from a single subandean event was used to obtain estimates for pure path dispersion along propagation paths through the chaco basin and the western half of the parana basin the data were partitioned to isolate the partial path contribution of the phase and group velocities for the chaco basin the phase and group velocities from this somewhat sparse data set were inverted to obtain a velocity depth model for the chaco basin the distinguishing features of the chaco model consist of a rather shallow moho depth 32 km and low asthenospheric upper mantle s wave velocities about 4 2 km s with velocity increasing only slightly to about 4 3km s at 150 km depth

on the relative influence of heat and water transport on planetary dynamics the dynamics of a planet and its evolution are controlled to a large extent by its viscosity in this study we demonstrate that the dependence of mantle viscosity on temperature and water concentration introduces strong dynamic feedbacks we derive a dimensionless parameter to quantitatively evaluate the relative strength of those feedbacks and show that water and heat transport are equally important in controlling present day dynamics for the earth a simple parameterized evolution model illustrates the strong feedbacks and behavior of the system and agrees well with our analytic results the analysis identifies characteristic times for changes of viscosity temperature and water concentration and demonstrates for time scales greater than a few hundred million years that the system should either be degassing while warming or regassing while cooling this yields a characteristic evolution in which after an initial period of rapid adjustment the mantle warms while degassing and subsequently cools rapidly while regassing as the planet continues to cool the entire surface ocean may eventually return to the mantle our results suggest that a simple relationship may exist between the rate of change of water concentration and the rate of change of temperature in the mantle this connection is extended by deriving an explicit equation for the urey ratio that depends on both heat and water transport

strong heterogeneity caused by deep mantle layering 1 numerical simulations are used to characterize the volumetric heterogeneity associated with proposed chemical layering in the deep mantle a globally continuous undulating layer generates enormous amounts of volumetric heterogeneity which is caused by lateral variations in the layer boundary depth and by strong hot upwellings arising from the top of the layer in the middle lower mantle region in contrast the heterogeneity in the deepest mantle is very weak partial cancellation of the temperature and compositional contributions to seismic velocity could mask the layer itself but the strong signatures of the thermochemical boundary layer and induced upwellings remain even when fields are filtered to approximate the resolution of global seismic tomographic models this distinctive heterogeneity is not observed in global seismic tomographic models dense material that is restricted to discontinuous piles generates heterogeneity that is more consistent although not entirely with seismic observations with strong heterogeneity at the core mantle boundary radial correlation functions display a characteristic narrowing in the vicinity of any chemical layer but are very sensitive to the applied seismic tomographic filtering

seismic and mineralogical structures of the lower mantle from probabilistic tomography 1 one of the most powerful approaches for understanding the 3 d thermo chemical structure of the lower mantle is to link tomographic models with mineral physics data this is not straightforward because of strong trade offs between thermal and chemical structures and their influence on seismic structures they can be reduced by mapping simultaneously perturbations of wave speeds and density anomalies and by the quantitative assessment of the accuracy and uniqueness of seismic and mineralogical data here we present new tomographic maps of low order even degree seismic structures which are an improvement on earlier models they satisfy constraints from body wave surface wave and normal mode data simultaneously thereby enhancing the spatial resolution furthermore the seismic structure at a given location is represented by a probability density function pdf which takes into account the uncertainty and non uniqueness of the solution due to modeling and data restrictions following a robust statistical procedure we fit heterogeneity of wave speeds and density from hypothetical thermo chemical models to those of our tomographic maps we thereby constrain lateral variations of temperature as well as iron silica and post perovskite concentration in terms of pdfs our work shows that large scale chemical variations are likely everywhere in the lower mantle in most of the d region post perovskite is most abundant in the circum pacific belt but near the core its lateral variation is more complex furthermore post perovskite concentration trades off with the amplitudes of temperature and silicate variations but not with their lateral distribution this might be the reason why temperature and silicate variations appear not constrained by our data in the lowermost few hundred km of the mantle

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

influence of initial cmb temperature and other parameters on the thermal evolution of earth s core resulting from thermochemical spherical mantle convection 1 here we present a wide ranging parameter study of the effects of initial core mantle boundary cmb temperature concentration of radioactive potassium in the core and density difference between harzburgite and mid ocean ridge basalt morb in a coupled spherical model of thermochemical mantle convection and parameterized core heat balance the initial cmb temperature is expected to be much higher than the solidus temperature of silicates at the base of the mantle the results indicate that as with previous purely thermal convection models the final state of the system is only weakly dependent on initial cmb temperature unless the cmb becomes blanketed by a global layer of dense material fully 3 d spherical cases have a very similar core evolution to cases in a 2 d spherical annulus giving confidence in the applicability of 2 d spherical annulus geometry for modeling earth s evolution obtaining a successful thermal evolution in the sense of obtaining the correct present day inner core size and maintaining a geodynamo over geological time is helped by the accumulation of piles of dense material at the cmb subducted morb in the present calculations and a concentration of radiogenic k in the core in the range 400 800 ppm the present day cmb heat flow is predicted to be around 9 tw while this is lower than estimates based on calculating temperature gradients in regions where the postperovskite transition is seismically imaged these tend to be areas of higher than average heat flux and thus likely overestimate the global heat flow

a preliminary numerical study on water circulation in convecting mantle with magmatism and tectonic plates abstract we present a preliminary self consistent numerical model of solid state convection in a wet mantle with magmatism and tectonic plates to understand how magmatism and tectonic plates affect circulation of water injected into deep mantle the solidus temperature of mantle materials depends on water content but their water saturation limit is assumed to be constant and water content dependence of viscosity is neglected when the mantle is mildly heated by heat producing elements plate tectonics steadily occurs and subducted basaltic crusts accumulate on the core mantle boundary water injected into deep mantle is mostly transported to basaltic accumulation areas which serve as a major storage of injected water the characteristic time for the water circulation is 2 6 gyr water significantly enhances magmatism when the mantle is more strongly heated and the internal heating rate exceeds a threshold however basaltic accumulations become convectively unstable cause vigorous magmatism and split the lithosphere into smaller fragments water transported into deep mantle by subducting lithospheric fragments distributes more uniformly and the water storage capacity of the mantle becomes much smaller than that under the mild internal heating below the threshold the characteristic time for water circulation is 1 2 gyr the characteristic time of water circulation in the mantle is comparable to the age of the earth under mild internal heating and the water circulation depends on the regime of magmatism and mantle convection which in turn depends on the internal heating rate a full understanding of dynamics of magmatism and mantle convection is crucial for understanding water circulation in the mantle and the effect of water on mantle evolution

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

phase relations of peridotites under h2o saturated conditions and ability of subducting plates for transportation of h2o phase relations of peridotites under h2o saturated conditions up to 28 gpa and 2600 c have been clarified based on the high pressure experimental results in the mgo feo sio2 h2o mgo sio2 al2o3 h2o cao mgo al2o3 sio2 h2o and natural systems based on the phase assemblages deduced and the chemical compositions of the phases the maximum h2o contents for two bulk rock compositions lherzolite and harzburgite have been calculated using mass balance then the potential ability of subducting plates for transportation of h2o is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a 15 130 ma the subduction velocity u0 2 25 18 cm year and angle 30 and 60 and the mantle potential temperature tp 1300 and 1350 c the results confirm the importance of choke point a cusp around 6 2 gpa and 550 c at which the stability field of major hydrous phases is minimized in terms of temperature although the choke point is exceeded by the mg sursassite bearing assemblage that has the maximum h2o content of 0 4 0 7 wt around 5 7 gpa and 550 700 c if the geotherm of the coldest part across the subducting plate passes below the choke point maximum 4 6 wt h2o can subduct whereas h2o less than 0 4 0 7 wt can subduct above the choke point the critical conditions are clarified as follows if 30 and tp 1300 c a 15 ma with u0 9 cm year or a 30 ma with u0 2 25 cm year is the critical condition if 60 and tp 1300 c or 30 and tp 1350 c the critical conditions slightly shift to an older age or a greater velocity e g a 30 ma with u0 3 5 cm year the slab thermal parameter au0sin is useful to predict roughly the critical condition however within the younger plate the spatial extent of a cold region with major hydrous phases is thinner hence less h2o is subducted even with the same slab thermal parameter this indicates that the age of the plate and the subduction velocity act differently on the h2o transportation ability of subducting plates

the effects of deep water cycling on planetary thermal evolution 1 we use a parameterized convection model to investigate the effects of deep water cycling on the thermal evolution of an earth like planet the model incorporates two water reservoirs a surface and an interior mantle reservoir exchange between the two is calculated using a mantle convection parameterization that allows for temperature and water dependent mantle viscosity together with internally self consistent degassing and regassing parameterizations the balance between degassing and regassing depends on the average spreading rate of tectonic plates the amount of water partitioned into melt the thickness of a mantle melt zone and of a hydrated layer at the top of subducting plates degassing scales with melt zone thickness such that an early period of extensive melting would create a drier and more viscous mantle shifting the solidus line in a direction that would reduce the melt zone thickness and the rate of mantle heat loss coupling a hydrated zone thickness dependent regassing factor to the model to mimic water delivery to the mantle via a serpentinized layer allows for the potential of a reversing point where the overall water flow direction switches from degassing to regassing as the mantle cools the water effect on viscosity creates a negative feedback that tends to regulate the final amount of water in the mantle so it is not strongly dependent on the initial amount of planetary water the final amount of water in the surface reservoir is then determined by this feedback effect together with the initial water budget of the entire planet this implies that if the initial water budget of a planet can be estimated from planetary formation models then the volume of surface water can be used to estimate the volume of water in the mantle of an earth like planet applying this methodology to the earth leads to predictions for water concentration in the earth s mantle that are in line with geochemical and petrological constraints

water circulation and global mantle dynamics insight from numerical modeling we investigate water circulation and its dynamical effects on global scale mantle dynamics in numerical thermochemical mantle convection simulations both dehydration hydration processes and dehydration melting are included we also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed due to effective slab dehydration at shallow depths water still affects significantly the global dynamics by weakening the near surface oceanic crust and lithosphere enhancing the activity of surface plate motion compared to dry mantle case as a result including hydrous minerals the more viscous mantle is expected with several orders of magnitude compared to the dry mantle the average water content in the whole mantle is regulated by the dehydration hydration process the large scale thermochemical anomalies as is observed in the deep mantle is found when a large density contrast between basaltic material and ambient mantle is assumed 4 5 comparable to mineral physics measurements through this study the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

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

basement control on past ice sheet dynamics in the amundsen sea embayment west antarctica abstract the development of landscapes and morphologies follows initially the tectonic displacement structures of the basement and sediments such fault zones or lineaments are often exploited by surface erosional processes and play therefore an important role in reconstructing past ice sheet dynamics observations of bathymetric features of the continental shelf of the amundsen sea embayment and identification of tectonic lineaments from geophysical mapping indicate that the erosional processes of paleo ice stream flows across the continental shelf followed primarily such lineaments inherited from the tectonic history since the cretaceous break up between new zealand and west antarctica three major ice flow trends correspond to different tectonic phases in east west northwest southeast and north south directions east west oriented basement trends correlate with coastline trends and overlay tectonic lineaments caused by former rift activities directional trends with northwest southeast orientation are observed for the glacial troughs of the western embayment outer shelf the western pine island bay coastal zones and the inner pine island glacial trough and are associated with a distributed southern plate boundary zone of the former bellingshausen plate the north south trend of the main pine island glacial trough and the north northeast trend of the abbot ice shelf trough on the outer shelf follow the predicted lineation trend of an eastern branch of the west antarctic rift system extending from the thwaites drainage basin northward into pine island bay an understanding of this context helps better constrain the geometries and sea bed substrate conditions for regional paleo ice sheet models

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

stagnant slab wet plumes and cenozoic volcanism in east asia abstract data from seismology and mineral physics indicate the existence of low velocity zones in the deep part of the upper mantle wedge and the mantle transition zone under east asia zhao and ohtani 2009 zhao et al 2009 it suggests a link between the deep subduction and stagnation and deep dehydration of the pacific slab below this area and the cenozoic volcanism observed at the surface here we present a numerical model describing the dynamics of a hydrated layer covering a slab that is stagnating in the earth tm s mantle transition zone specifically our model includes the effects of water on density and viscosity of the mantle minerals we conclude that the presence of water is likely to induce rayleigh taylor type instabilities wet plumes that are able to transport water up to the base of the surficial lithosphere in a relatively short time scale a few million years under initial conditions representative of the subducting pacific slab under east asia melting is induced where wet plumes reach the base of the lithosphere according to our results and analysis of the known composition of cenozoic basalts from southwest japan iwamori 1991 iwamori 1992 wet plumes and their melting induced by water related to the stagnant slab provides an appealing hypothesis to explain cenozoic volcanism in east asia

elastic properties of hydrous ringwoodite at high pressure conditions 1 the sound velocities and single crystal elastic moduli of hydrous mg2sio4 ringwoodite containing 2 3 wt of h2o have been measured by brillouin spectroscopy at high pressures to 23 4 gpa spanning the pressure range in earth s transition zone the resulting pressure derivatives of the adiabatic bulk modulus k s and shear modulus are 4 4 1 and 1 7 1 respectively compared with results for anhydrous ringwoodite the pressure derivatives of the elastic moduli are consistent with an increase due to hydration of as much as 7 for the k s and 30 for depending on the data sets used for comparison however the gradients of velocity as a function of pressure for hydrous ringwoodite are significantly less than the corresponding gradients in the earth s transition zone we conclude that transition zone seismic velocity gradients are not due to wet ringwoodite as previously speculated

stability of hydrous silicate at high pressures and water transport to the deep lower mantle hydrous magnesium rich silicates play an important role in transporting water into the deep mantle when oceanic plates subduct as slabs but were thought to dissociate at pressures of 44 gpa in situ x ray measurements in conjunction with a multi anvil apparatus show that hydrous phases of magnesium rich silicate are stable under lower mantle conditions up to 50 gpa and may transport water to deeper layers of the mantle the presence of water strongly influences the structure composition and dynamics of the earth s deep mantle1 2 3 4 hydrous magnesium rich silicates play an important role in transporting water into the deep mantle when oceanic plates subduct as slabs the highest pressure form of such hydrous silicates phase d was reported to dissociate into an assemblage of nominally anhydrous phases plus water at pressures of about 44 gpa equivalent to 1 250 km depth5 in this way free water would be released in the middle region of the lower mantle which suggests that no water can reach the deeper regions of the lower mantle here we use in situ x ray measurements in conjunction with a multi anvil apparatus using sintered diamond anvils to show that hydrous phases are stable under lower mantle conditions up to 50 gpa we found that phase d transforms to an assemblage with another hydrous silicate mgsih2o4 named phase h at pressures above about 48 gpa our results suggest that phase h is the dominant hydrous silicate in descending slabs and may be found at depths far deeper than 1 250 km we conclude that a significant amount of water is retained in the descending slabs and may be delivered to the deepest part of the lower mantle