internally consistent thermodynamic data set for dense hydrous magnesium silicates up to 35 gpa 1600 c implications for water circulation in the earth s deep mantle abstract fluid undersaturated high pressure experiments were carried out in the system mgo sio 2 h 2 o in a multi anvil apparatus from 15 5 to 21 5 gpa and from 1000 to 1200 c in order to constrain fluid absent solid solid reactions from the experimentally determined fluid absent phase equilibria thermodynamic parameters for high pressure hydrous phases were retrieved by a linear programming method together with dry mantle minerals from the existing data set and high pressure hydrous phases from our previous study an internally consistent set of thermochemical and thermophysical parameters for dense hydrous magnesium silicates dhmss was evaluated up to 35 gpa and 1600 c high pressure hydrous phases involved in the data set are phase a phase e clinohumite phase d superhydrous phase b hydrous wadsleyite and hydrous ringwoodite in addition by calculating water bearing reactions we evaluated water activities in the fluid phase coexisting with mantle minerals the water activity decreases with increasing pressure or temperature suggesting that the amount of silicate component dissolved in the fluid phase increases with pressure or temperature the calculated petrogenetic grid consisting of 89 univariant reactions can be used for the discussion of water circulations in the mantle in a subducting slab peridotite water in phase a as the post serpentine phase is transferred to other dhmss via solid solid reactions down to the bottom of the transition zone at around 660 km depth in the stagnant slab heated from the surrounding mantle a free fluid will be released by the dehydration of superhydrous phase b or phase d the released fluid will be trapped by wadsleyite or ringwoodite in the surrounding mantle in a cold subducting slab without stagnation at 660 km depth dhmss will survive into the lower mantle in an upwelling plume through the water bearing transition zone a free fluid will be produced when the plume passes 410 km depth finally water will be released to the earth s surface by magmatism the fluid generated in the deep mantle dissolves significant amounts of a silicate component therefore at 410 and 660 km depths some chemical differentiation will be processed

mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

stability limits of hydrous minerals in sediment and mid ocean ridge basalt compositions implications for water transport in subduction zones phase relationships in hydrated natural sediment and mid ocean ridge basalt morb compositions were investigated experimentally from 6 to 15 gpa and 700 c to 1400 c in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle phengite and hydrous aluminum silicates topaz oh and phase egg are stable in the sediment composition at depths greater than 150 km and constitute h2o reservoirs similarly lawsonite is an h2o reservoir in morb composition at depths greater than 150 km according to mass balance calculations based on experimental run products the weight proportions of phengite and h2o at 6 gpa in the sediment are about 50 and 2 respectively at pressures from 8 to 12 gpa topaz oh al2sio4 oh 2 appears as the hydrous mineral and the sediment retains about 0 5 0 7 wt h2o at higher pressures phase egg alsio3 oh appears as the hydrous phase with a large stability field the h2o content in the sediment is about 0 5 wt at pressures greater than 12 gpa in natural morb compositions lawsonite is the hydrous phase at pressures lower than 10 gpa about 1 wt h2o in the morb composition is retained by lawsonite at 6 gpa and 800 c above 6 gpa a lawsonite out reaction is controlled by compositional changes of garnet and clinopyroxene if the oceanic crust descends with an overlying sediment layer water can be carried down into the transition zone by the sediment the amount of transported water is controlled by the chemical compositions of the subducting sediments water in the morb layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle causing the generation of a peridotite region including hydrous phases chondrodite clinohumite phase a and phase e if this hydrous peridotite is dragged down by the descending oceanic crust water can be transported into the transition zone by the subduction process the amount of transported water is controlled by the thermal state of the slab and the chemical composition of morb

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

high temperature stability limit of phase egg alsio3 oh the stability relations of phase egg alsio3 oh have been investigated at pressures from 7 to 20 gpa and temperatures from 900 to 1700 c in a multi anvil apparatus at the lower pressures phase egg breaks down according to the univariant reaction phase egg stishovite topaz oh which extends from 1100 c at 11 gpa to 1400 c at 13 gpa where it terminates at an invariant point involving corundum at pressures above the invariant point the stability of phase egg is limited by the breakdown reaction phase egg stishovite corundum fluid which extends from the invariant point to 1700 c at 20 gpa stishovite crystallized in the al2o3 sio2 h2o system contains al2o3 and the amount of al2o3 increases with increasing temperature it is inferred that the al2o3 content is controlled by the charge balanced substitution of si4 by al3 and h aluminum bearing stishovite coexisting with an h2o rich fluid may contain a certain amount of water therefore phase egg and stishovite in a subducting slab could transport some h2o into the deep earth

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

melting experiments and thermodynamic analyses on silicate h2o systems up to 12 gpa abstract melting experiments on pyrope mg 3 al 2 si 3 o 12 h 2 o system were conducted in an ma8 type high pressure apparatus up to 12 gpa and approximately 2000 k pyrope melts congruently with its solidus temperature increasing from 1430 k at 4 2 gpa to 1510 k at 12 gpa these temperatures are lower than those of the anhydrous condition by 640 k and 930 k respectively a thermodynamic analysis was carried out on the hydrous melting of two major mantle silicates namely pyrope and forsterite mg 2 sio 4 on the basis of the ideal mixing model proposed by silver and stolper 1985 j geol 93 161 178 the experimental phase diagrams were reproduced successfully within experimental uncertainty by adjusting only three thermodynamic parameters the equilibrium constant for the chemical reaction between silicate and h 2 o in the melt the equilibrium constant for the dissolution of h 2 o gas into the melt at 0 5 gpa and the molar volume of h 2 o in the melt at atmospheric pressure furthermore the thermodynamic analysis suggests that pressure enhances the silicate h 2 o reaction in the melt

decomposition of phase d in the lower mantle and the fate of dense hydrous silicates in subducting slabs abstracthigh pressure high temperature quench type experiments were carried out on serpentine to pressures of 53 gpa andtemperatures between 800 1800oc x ray analyses show that recovered phase assemblages varied considerably for thedifferent high pressure and high temperature studies the dense hydrous magnesium silicate phases decompose sequentiallyas the pressure increases and appear to serve as reservoirs for h 2 o in the mantle eventually releasing it at the highestpressure superhydrous phase b is stable up to pressure equivalent to the boundary between the transition zone and lowermantle phase d decomposes at pressures of 44 gpa equivalent to about 1250 km depth this may de ne the lower depthlimit for the presence of dense hydrous magnesium silicates beyond this pressure only nominally anhydrous phases havebeen recovered in high pressure experiments this may indicate a lack of stoichiometric hydrogen bearing silicate phasesat higher pressure 1998 elsevier science b v all rights reserved keywords x ray diffraction analysis lower mantle serpentine high pressure

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

hydrous phases and water transport in the subducting slab arc volcanoes are typically located 90 180 km above the surface of downgoing slabs as shown by wadati benioff deep seismic foci gill 1981 tatsumi 1989 the intimate relationship between the dip angles of the subducting slab and the locations of volcanic arcs indicates that subduction zone magmatism is triggered by material input from the subducting slab tatsumi and eggins 1995 the slab derived components are thought to be aqueous fluids or h2o rich partial melts of subducted oceanic crust therefore knowledge of the stability of hydrous phases and the chemical and physical properties of aqueous fluids in downgoing slabs is essential to understand the material transport in subduction zones in this section i will review the stability of hydrous phases in downgoing peridotite basalt and sediment systems and the chemical and the wetting properties of aqueous fluids recent experimental studies indicate that 3 4 gpa equivalent to 90 120 km depth is a key pressure where 1 the chemical compositions of silicate components dissolved in aqueous fluids equilibrated with mantle minerals approach the composition of mantle peridotite itself stalder et al 2001 mibe et al 2002 kawamoto et al 2004 2 the dihedral angle between olivine and aqueous fluids starts becoming smaller than 60 watson et al 1990 mibe et al 1998 1999 and 3 the immisciblity gap between peridotitic melts and aqueous fluids disappears and consequently hydrous minerals liberate supercritical aqueous fluids mibe et al 2004a 2006 the similarity between these pressures and the depths of downgoing slab underneath volcanic fronts where the maximum numbers of volcanoes are formed 124 38 km gill 1981 or 112 19 km tatsumi 1986 suggests that subduction zone magmatism can be triggered by the input of supercritical fluids from the downgoing peridotite and basalt many hydrous crystalline phases

two dimensional electrical section beneath the eastern margin of japan sea 1 four ocean bottom electromagnetometers provided seafloor geoelectromagnetic data for more than nine months in order to reveal mantle dynamics beneath the eastern margin of japan sea where several large and hazardous earthquakes have repeatedly occurred so far high quality magnetotelluric mt data from the seafloor array together with those of four additional land sites in northeast japan were analyzed and carefully corrected for topographic bathymetric effects to extract two dimensional 2d mt responses at each site here we show how the 2d electrical section derived by inverting the corrected 2d mt responses depicts fluid injection deep into the wedge and back arc mantle by the cold and fast subducting pacific plate a high conductivity anomaly found beneath the sado ridge at depths ranging from 150 to 200 km may be direct field evidence that supports water transportation from the earth s surface to the wedge and back arc mantle in the form of serpentinite

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

the role of volatiles in the transition zone phase relations in the system mgo sio2 with low volatile contents of h2o f2 and cl2 were experimentally investigated at 100 234 kbar pressures and 850 2050 c temperatures using a split sphere anvil apparatus ussa 2000 superphase b mg10si3o14 oh f 4 was observed at 160 234 kbar fluorous phase b mg12si4o19f2 at 178 kbar and hydrous phase f mg4si7o16 oh 4 also at 178 kbar at lower pressures the volatiles were present in phase a mg7si2o8 oh f 6 phase c mg 7 x si 2 x o 9 x oh f 4 and phase e mg 2 x si 1 x o 4 x y oh f 2y with the exception of the fluorous phase b all these phases can coexist with stishovite at pressures above 130 kbar in a purely hydrous system the dehydration temperatures of the assemblage superphase b stishovite increase from 1200 c at 160 kbar to 1400 c at 234 kbar dehydration produces beta phase mg2sio4 possibly containing up to 7 wt of structural h2o about 3 wt of h2o in beta phase expands the stability of the assemblage beta phase stishovite at the expense of ilmenite by 30 kbar in more complex compositions the volatile solidus is below 1350 c thus at the temperatures which are much lower than the expected mantle temperatures at the relevant depths consequently most of the mantle is in a partially molten state the volatile bearing phases can only be present in the colder subducted slabs but not in the rest of the transition zone 400 670 km hydrous beta phase and stishovite are the most likely constituents of the slabs incongruent melting of the hydrous beta phase coexisting with stishovite producing garnet and melt could be the cause of deep focus earthquakes evidence for a mineralogically and chemically stratified upper mantle is reviewed and its structure is discussed in the context of the present results

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

wet plume atop of the flattening slab insight into intraplate volcanism in east asia abstract geophysical observations imply the intraplate volcanism in east asia is related to dehydration of slab stagnating in the transition zone to better understand the dynamics of such process a thermochemical mantle convection model is constructed to simulate numerically the thermal evolution of slab and the transportation of water in the process of slab downgoing flattening and stagnation equation of water transfer is included and water effects on density and viscosity are considered model results indicate the warming of slab by surrounding mantle is rather slow water could be successfully dragged into the transition zone if the reference viscosity of the hydrous layer with initial water of 2 wt is higher than 1017 pa s and that of mantle is 1021 pa s wet plumes could then originate in the flat lying part of the slab relatively far from the trench generally the viscosity of the hydrous layer governs the initiation of wet plume whereas the viscosity of the overlying mantle wedge controls the activity of the ascending wet plumes they are more active in the weaker wedge the complex fluid flow superposed by corner flow and free thermal convection influences greatly the water transport pattern in the upper mantle modeling results together with previous modeling infer three stages of water circulation in the big mantle wedge 1 water is brought into the mantle transition zone by downward subducting slab under some specific thermo rheological conditions otherwise water is released at shallow depth near wedge tip 2 wet plume generates from surface of the flattening slab warmed by surrounding mantle and 3 water spreads over the big mantle wedge wet plume from the flattening pacific plate arrives at the lithospheric base and induces melting which can explain the intraplate cenozoic volcanoes in east asia

global scale water circulation in the earth s mantle implications for the mantle water budget in the early earth abstract we investigate the influence of the mantle water content in the early earth on that in the present mantle using numerical convection simulations that include three processes for redistribution of water dehydration partitioning of water into partially molten mantle and regassing assuming an infinite water reservoir at the surface these models suggest that the water content of the present mantle is insensitive to that of the early earth the initial water stored during planetary formation is regulated up to 1 2 oms om ocean mass 1 4 10 21 kg which is reasonable for early earth however the mantle water content is sensitive to the rheological dependence on the water content and can range from 1 2 to 3 oms at the present day to explain the evolution of mantle water content we computed water fluxes due to subducting plates regassing degassing and dehydration for weakly water dependent viscosity the net water flux is almost balanced with those three fluxes but for strongly water dependent viscosity the regassing dominates the water cycle system because the surface plate activity is more vigorous the increased convection is due to enhanced lubrication of the plates caused by a weak hydrous crust for strongly water dependent viscosity the degassing history is insensitive to the initial water content of the early earth as well as rheological strength the degassing flux from earth s surface is calculated to be approximately o 10 13 kg yr consistent with a coupled model of climate evolution and mantle thermal evolution

the stability of antigorite in the systems mgo sio2 h2o msh and mgo al2o3 sio2 h2o mash the effects of al3 substitution on high pressure stability the high pressure stability of antigorite in the systems msh and mash was investigated using two structurally and chemically well constrained natural samples careful sample selection and characterization ensured that the only significant difference between the samples was al content one sample being essentially al free and the other containing 3 06 2 wt al 2 o 3 in the system msh the reaction antigorite forsterite clinoenstatite water was bracketed under water saturated conditions between 630 and 650 c at 1 6 gpa between 620 and 660 c at 2 5 gpa between 620 and 660 c at 3 9 gpa and between 4 5 and 5 0 gpa at 520 c in the system mash the reaction antigorite forsterite clinoenstatite chlorite water was bracketed under water saturated conditions between 660 and 700 c at 2 0 gpa between 660 and 680 c at 2 9 gpa and between 5 0 and 5 5 gpa at 600 c at pressures above 5 8 gpa intersection of this reaction with the reaction chlorite clinoenstatite pyrope forsterite water leads to an additional reaction whereby the al component of the antigorite is transferred to pyrope upon antigorite breakdown the addition of a few weight percent al 2 o 3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets this effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure in addition antigorite breakdown in the system mash transfers a significant volume of water to chlorite bearing assemblages thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system msh

phase transitions of serpentine in the lower mantle abstract phase transformations in natural serpentine and chlorite have been studied at 25 50 gpa and 900 1280 c by in situ x ray diffraction measurements using the multianvil apparatus with sintered diamond anvils antigorite was found to transform to a phase assemblage including phase h at pressures above about 35 40 gpa corresponding to those in the upper part of the lower mantle the zero pressure bulk modulus k 0 of phase h was determined to be 160 gpa assuming its pressure derivative k 4 the thermal stability of phase h is significantly enhanced by a solid solution with a alooh component suggesting that aluminous phase h in cold slabs would deliver a certain amount of water into the deepest part of the lower mantle

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

stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle abstract we evaluated the water storage capacity in the mantle based on the phase relations of silicate systems containing water we conducted high pressure and temperature synthesis experiments on some silicate systems such as the superhydrous phase b phase c and phase g phase d and f compositions in the mgo sio2 h2o system and the cmas pyrolite 2 wt h2o system up to the uppermost part of the lower mantle superhydrous phase b is stable at temperatures below 1200 c at 18 gpa and below 1300 c at 20 gpa the following reaction sequences were observed with increasing temperature from 1200 to 1600 c in the pressure range from 18 to 20 gpa superhydrous phase b phase b liquid wadsleyite periclase liquid anhydrous phase b periclase liquid phase g is stable at temperatures below 1000 c at 18 gpa below 1100 c at 20 22 gpa and below 1200 c at 25 gpa wadsleyite stishovite liquid are stable above 1000 c at 18 gpa ringwoodite or phase e stishovite liquid above 1100 c at 22 gpa ilmenite stishovite liquid above 1100 c at 23 5 gpa and perovskite stishovite liquid above 1200 c at 25 gpa superhydrous phase b is stable at 1200 c at 18 5 and 25 gpa in cmas pyrolite 2 wt h2o composition whereas it decomposes at around 1400 c at 25 gpa in the composition there may be a layered structure in the mantle in its water storage potential solubility of water per unit mass i e the upper and lower mantles have relatively small water storage potentials whereas the transition zone has a larger potential the water content in the transition zone might be large because the primordial water trapped in the lower mantle and the recycled water transported by the slab subduction might have been released upwards and stored in the transition zone during geological time

the high pressure stability of mg sursassite in a model hydrous peridotite a possible mechanism for the deep subduction of significant volumes of h2o the stability of the high pressure phase mg sursassite previously mgmgal pumpellyite in ultramafic compositions has been determined in experiments in the system mgo al2o3 sio2 h2o mash the breakdown of mg sursassite forsterite enstatite to pyrope vapour with increasing temperature was bracketed at 6 0 and 7 0 gpa below 6 0 gpa mg sursassite forsterite vapour reacts to chlorite enstatite this reaction provides a mechanism for transfer of water from chlorite to mg sursassite bearing assemblages at pressures of 7 0 gpa and above the assemblage mg sursassite phase a enstatite was found phase relations involving mg sursassite and phase a are considered for bulk compositions with a low water content the vapour absent reaction mg sursassite forsterite pyrope phase a enstatite determines the upper pressure stability of mg sursassite and provides a mechanism for the complete transfer of water from mg sursassite to phase a bearing assemblages mg sursassite plays an important role in peridotite compositions in the subducting slab because at temperatures below 700 c it can transfer water from hydrous phases such as antigorite and chlorite to high pressure stable phases such as phase a

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

melting relations of hydrous pyrolite in cao mgo al2o3 sio2 h2o system at the transition zone pressures phase relations and melt compositions in cao mgo al2o3 sio2 pyrolite under hydrous 2 of h2o and anhydrous conditions have been determined at 13 20 gpa and 1600 2220 c liquidus and solidus temperatures for the hydrous system are about 50 100 c and 180 240 c lower than those for the dry system respectively majorite is a liquidus phase of the hydrous pyrolite from 13 to 20 gpa olivine is a liquidus phase at 13 gpa and both periclase and majorite are the liquidus phases at 20 gpa in the dry pyrolite we observed expansion of the stability field of anhydrous phase b in hydrous experiments compositions of partial melts at 13 20 gpa are generally similar in dry and hydrous systems but hydrous melts contain more sio2 at 13 17 gpa the melts formed by low degree of melting have al2o3 depleted and cao rich compositions trends of hydrous melt compositions are generally consistent with those of aluminum depleted komatiite magmas

elasticity of superhydrous phase b mg10si3o14 oh 4 abstract we have used first principles simulation based on density functional theory to calculate the equation of state and elasticity of superhydrous phase b mg 10 si 3 o 14 oh 4 the pressure volume results for superhydrous phase b is well represented by a third order birch murnaghan formulation with k 0 161 8 0 2 gpa and k 0 4 4 0 01 the calculated full elastic tensor at 0 gpa is in good agreement with brillouin scattering results with the compressional elastic constants c 11 329 5 gpa c 22 294 9 gpa c 33 306 8 gpa the shear elastic constants c 44 99 8 gpa c 55 98 gpa and c 66 99 gpa the off diagonal elastic constants c 12 82 5 gpa c 13 84 6 gpa and c 23 98 7 gpa at the depths corresponding to the mantle transition zone the aggregate sound wave velocities for superhydrous phase b is slower compared to dry ringwoodite which is the dominant mineral phase however hydrous ringwoodite bulk sound velocities are comparable to that of superhydrous phase b majoritic garnet the second most abundant mineral in the transition zone has bulk sound wave velocities slower than superhydrous phase b an assemblage consisting of hydrous ringwoodite superhydrous phase b and majorite garnet could account for the low velocities observed in certain subduction zone settings at depths corresponding to the base of the transition zone and upper mantle superhydrous phase b exhibits moderate single crystal elastic anisotropy with av p 3 and av s 5 at the base of the transition zone single crystal elastic anisotropy of other dense hydrous magnesium silicate phases phase such as hydrous phase d is significantly larger at these conditions and might play a major role in explaining the observed mid mantle seismic anisotropy

experimental and theoretical studies of the stabilities of talc antigorite and phase a at high pressures with applications to subduction processes we have experimentally determined the equilibrium talc enstatite quartz coesite h2o to 40 kbar in the system mgosio2h2o msh using both synthetic and nearly pure mg end member natural talc and other synthetic starting materials for the other solid phases at 40 kbar the equilibrium dehydration boundary lies 150 c higher than that calculated using data from the existing internally consistent thermochemical data bases the reason for this discrepancy lies in the erroneous compressibility data of talc in the data bases we have retrieved the compressibility of talc from the experimental phase equilibrium data and have also calculated sereral other equilibria in the msh system involving talc antigorite and the dense hydrous magnesium silicate dhms commonly referred to as phase a comparison of these equilibria with selected thermal profiles at the leading edge of young and old subducting oceanic slabs along with the dehydration condition of basaltic amphibole and solidus of mantle peridotite provides an explanation for the observed heights of the volcanic fronts above subducting oceanic lithosphere further it is found that in cold oceanic slabs 50 ma with subduction velocity of 10 cm y antigorite will transform to the dhms phase a through a vapor conserved reaction at a depth of 200 km phase a will then serve as a carrier of water into the deeper mantle

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

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

is phase a relevant to the earth s mantle abstract dense hydrous magnesium silicates dhms such as phase a mg 7 si 2 o 8 oh 6 are potential hosts for water in the earth s mantle previous work has delineated the stability field of phase a pha coexisting with clinoenstatite cen and forsterite fo in the system mgo sio 2 h 2 o msh in this study the effects of the presence of magnesite mc diopside di and pyrope py on the stability of coexisting pha cen fo have been determined in msh the breakdown reaction pha cen fo h 2 o was bracketed between 720 and 740 c at 7 gpa between 800 and 820 c at 8 gpa and between 860 and 880 c at 9 gpa at 10 gpa pha cen transforms to a clinohumite ch bearing assemblage between 920 and 940 c which in turn dehydrates between 940 and 960 c to fo v at higher pressures pha becomes unstable relative to phase e another dhms kanzaki 1991 addition of carbonate to msh changes the breakdown reaction to pha cen mc fo v where v is an h 2 o co 2 fluid at 8 gpa this reaction occurs between 800 and 850 c indistinguishable from the position of the co 2 free reaction in msh in the presence of diopside in the cao mgo sio 2 h 2 o system the reaction pha en ss fo di ss h 2 o occurs between 700 and 730 c at 8 gpa where en ss and di ss are ca poor and ca rich pyroxene solid solutions respectively in the pyrope containing assemblage in the mgo al 2 o 3 sio 2 h 2 o system the pha bearing assemblage reacts to form ch between 740 and 760 c at 8 gpa the ch bearing assemblage dehydrates to fo py v between 800 and 830 c at 8 gpa in the system mgo feo sio 2 h 2 o with molar mg mg fe 0 90 the reaction pha cen ss ch v occurs at ss ch fo v reaction occurs between 850 and 900 c in no case did the presence of additional components such as cao feo al 2 o 3 or co 2 or the additional phases diopside magnesite or pyrope stabilize the hydrous phases pha or ch to significantly higher temperatures therefore pha will not be a stable phase in lherzolite assemblages under normal geothermal gradients in the mantle in the interior of subducting slabs temperatures may be sufficiently low that assemblages containing pha may be stable to pressures greater than 10 gpa allowing pha to transport h 2 o to the deeper mantle it is more likely that the higher pressure phase e will be stabilized instead

effects of h2o on the phase behaviour of the forsterite enstatite system at high pressures and temperatures and implications for the earth abstract in addition to many of the naturally occurring hydrous minerals known in the system mgo sio2 h2o various dense hydrous magnesium silicates 10 a phase phases a b c and d have been synthesized by high pressure and high temperature experiments in the past two decades in conjunction with these high pressure hydrous phases the effects of h2o on the observed phase transformations in the forsterite mg2sio4 enstatite mgsio3 system at high pressures and high temperatures are investigated on the basis of both experimental data and theory for forsterite the modified spinel phase may exist up to 7 1 wt h2o along with enstatite phase a up to 9 9 wt h2o along with stishovite phase a and up to only 3 2 wt h2o along with stishovite phase b the spinel phase of mg2sio4 is no longer stable when more than 3 2 wt h2o is present although both stishovite and the ilmenite type mgsio3 do not occur in dry mg2sio4 at high pressures a small amount of h2o would yield assemblages containing stishovite and the ilmenite phase in mg2sio4 at high pressures the post spinel phase assemblage perovskite periclase may be stable along with phase d up to 11 wt h2o in the case of enstatite except for the assemblage modified spinel stishovite which exists along with phase a up to 7 1 wt h2o and for the perovskite type mgsio3 along with periclase and phase d which exists up to 15 wt h 2 o all the high pressure phase transformations observed in dry mgsio3 may not occur at high pressures if 2 3 wt h2o is present based on experimental observations the various high pressure hydrous phases are stable from as low as 40 kbar to more than 280 kbar at various temperatures thus there is no lack of storage phases for h2o in the deeper interior of the earth s mantle according to the impact induced dehydration model advanced recently by shock wave experiments it is estimated that the earth s interior contains probably at least more than five times its present water contents in the near surface geochemical reservoirs hydrosphere crust so long as both forsterite olivine and h2o coexist in the upper mantle even in trace amounts there must exist a water line which is defined by the reaction 5mg 2 sio 4 3h 2 o 3mgsio 3 mg 7 si 2 o 14 h 6 forsterite water enstatite phase a in the upper mantle the equilibrium boundary of the water line has been in fact determined in two different phase assemblages by experiments but was not noticed if the earth s surface was covered by large scale melts during and shortly after its accretion h2o would be stored in phases a b below the water line and would liberate as water above the water line during freezing today s oceans were most likely derived from water released in the mantle when the palaeogeotherm dropped below the solidus of mantle materials stripping the free water above the water line from depths near 350 km thus the oceans were formed rapidly and early in the earth s history and slow degassing is still going on today the low velocity zone observed in today s mantle is interpreted as a region containing free water between the water line and the stability field of amphiboles in the suboceanic mantle and between the water line and the stability field of phlogopite in the subcontinental mantle in other words the low velocity zone is the free water zone and the water line is the base of the low velocity zone in the earth s mantle

phase relations of a serpentine composition between 5 and 14 gpa significance of clinohumite and phase e as water carriers into the transition zone run products from high pressure experiments at 800 1 200 c and 5 14 gpa corresponding to depths of 150 to 420 km on a serpentine bulk composition close to mg3si2o5 oh 4 were analysed by optical microscopy micro raman spectroscopy and electron microprobe all charges exhibit strong chemical zoning fluid melt and hydrous solids were mostly concentrated at the top bottom and along the wall of the capsules the central part of the charge was devoid of h2o both fluid and hydrous magnesian phases exhibit a mg si ratio higher than forsterite in contrast the centre of the capsule was enriched in sio2 the observed zoning can neither be explained by gravitational settling nor by a thermal gradient alone most likely the fluid was separated from the solids by surface forces and thereby established the chemical gradient by preferentially dissolving mgo if strong chemical zoning is taken into account the occurrence of more phases than allowed by the phase rule can be explained by separating the bulk into several domains of different bulk compositions results indicate that small amounts of f increase the stability field of clinohumite mg9si4o16 oh f 2 compared to oh clinohumite in pure msh previously reported clinohumite coexists with enstatite up to 975 c at 5 gpa and up to 1 100 c at 12 gpa at 14 gpa close to the mg2sio4 transition phase e becomes the most important water carrier the new results indicate that clinohumite could be an important mantle mineral for transporting water into the earth s transition zone due to its high thermal stability compared to other important water carriers such as serpentine and phase a

stability of hydrous phases in the transition zone and the upper most part of the lower mantle phase relation for mg2sio4 20 4 wt h2o composition was studied in the pressure range from 11 gpa to 22 5 gpa the sequence of the phase transitions in the temperature range from 800 c to 1000 c with increasing pressure was as follows forsterite water phase a clinoenstatite water phase e water superhydrous phase b phase f water brucite phase f water coexistence of brucite and phase f was observed at pressures above 20 gpa these phases are stable at least up to 22 5 gpa and around 1000 c and are the candidates for the hydrous phase in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle

melting and seismic structure beneath the northeast japan arc transportation of h2o associated with subduction of the pacific plate beneath the northeast japan arc is modeled to predict distribution of aqueous solution and melt and the consequent p wave velocity structure the observed velocity structure coincides well with the model for equilibrium transport of h2o most of h2o subducted is brought down to 150 200 km depth along the slab then the aqueous fluid generated ascends to initiate melting which explains the low velocity regions 6 reduction observed beneath the backarc rather than beneath the volcanic front if equilibrium transport of h2o occurs initiation of melting beneath the backarc with deep subduction of h2o is likely to be the case also for other subduction zones with slabs older than several tens of m y cold enough to stabilize serpentine to a great depth

chlorite stability in mantle peridotite the reaction clinochlore enstatite forsterite pyrope h2o chlorite crystallising in hydrated mantle peridotite is a potential source of water for subduction zone volcanism the reaction clinochlore enstatite forsterite pyrope h2o is the important reaction for defining the stability of chlorite in these rocks it has been investigated in phase equilibrium experiments in piston cylinder and multi anvil apparatus between 2 and 5 gpa the reaction has a steep negative p t slope at low pressures with brackets at 860 880 c at 2 5 gpa 840 860 c at 3 0 gpa and 820 840 c at 3 5 gpa the reaction flattens out with increasing pressure to 5 0 gpa at 650 c chlorite compositions close to the reaction were determined from unit cell parameters measured using high resolution synchrotron powder diffraction these showed a decrease in al content with increasing pressure along the reaction consistent with calculations using a previously published thermodynamic data set the experimental results are consistent with the results of earlier studies on the reaction defining the maximum thermal stability of clinochlore clinochlore forsterite pyrope spinel h2o they have been used in the derivation of revised thermodynamic data for clinochlore in the latest version of the previously published data set the new results show that chlorite in subducting slab or overlying mantle wedge could dehydrate below subduction zone volcanoes and be a major source of the water required for melting however the dehydration reaction is not sufficiently pressure dependent to be responsible for the narrow range in depth from the volcanoes to the slab and chlorite will not be the only hydrous mineral involved

along strike variation in subducting plate seismicity and mantle wedge attenuation related to fluid release beneath the north island new zealand abstract we analyze seismicity in the subducted slab of the hikurangi subduction zone along a 600 km length beneath the north island new zealand the volcanic character changes along strike with extremely productive rhyolitic volcanism in the central taupo volcanic zone moderate andesitic volcanism in the northern and south central zones and subduction without volcanism in the southernmost hikurangi zone we have relocated slab earthquakes with 3 d velocity models the relocated seismicity shows more detail of the varied distribution and abundance of slab seismicity which below 50 km depth may be related to embrittlement from high fluid pressure the depth of the buoyant subducted hikurangi plateau ranges from 40 to 140 km depth at depths where the subducting slab interacts with the mantle wedge along arc variation in slab seismicity is a fundamental characteristic with patches of abundant seismicity separated by low seismicity zones the largest most numerous patch at 150 220 km depth underlies a pronounced low qp zone in the mantle wedge which is associated with the rhyolite dominant taupo caldera extensive melt in the region of low qp requires high h 2 o flux from the underlying slab the abundant taupo seismicity suggests a correlation between melt production and regions of earthquake fracture permeability following embrittlement which promote migration of dehydration fluid the hydration history of the incoming slab may be a key factor in producing variations in dehydration and intraslab fluid migration broader extensive outer rise yielding and hydration may have occurred near the approach of the re entrant hikurangi plateau forming the slab section that currently has high seismicity slab seismicity deepens from 240 to 330 km along arc as the subduction rate increases from 40 mm yr the southwestern slab seismicity is bounded by an unusually narrow zone with 110 km depth extent this is inferred to be a dehydration front related to heating at a slab edge that is located 70 km further southwest

hydration adjacent to a deeply subducting slab the roles of nominally anhydrous minerals and migrating fluids subduction related transport of water into the mantle has significant dynamical and geochemical implications dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle potentially hydrating nominally anhydrous minerals nam and impacting the viscosity and density structure of the mantle over a wide area we present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle a sharp decrease in water storage capacity across the lower boundary of the transition zone may produce secondary dehydration of hydrated nam leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle water stabilized in nam and as a free fluid can initiate upwelling within and above the transition zone a less abrupt change in water storage capacity across the base of the transition zone leads to high nam water contents in a channel adjacent to the slab where viscosity is reduced however seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity

long term stability of plate like behavior caused by hydrous mantle convection and water absorption in the deep mantle we investigate the cycling of water regassing dehydration and degassing in mantle convection simulations as a function of the strength of the oceanic lithosphere and its influence on the evolution of mantle water content we also consider pseudo plastic yielding with a friction coefficient for simulating brittle behavior of the plates and the water weakening effect of mantle materials this model can generate long term plate like behavior as a consequence of the water weakening effect of mantle minerals this finding indicates that water cycling plays an essential role in generating tectonic plates in vigorous plate motion the mantle water content rapidly increases by up to approximately 4 5 ocean masses which we define as the burst effect a burst is related to the mantle temperature and water solubility in the mantle transition zone when the mantle is efficiently cooled down the mantle transition zone can store water transported by the subducted slabs that can pass through the choke point of water solubility the onset of the burst effect is strongly dependent on the friction coefficient the burst effect of the mantle water content could have significantly influenced the evolution of the surface water if the burst started early in which case the earth s surface cannot preserve the surface water over the age of the earth

stability of hydrous magnesium silicates in the mantle transition zone phase relations in the mg oh 2 sio2 system have been studied up to 18 gpa a new hydrous phase phase e mg2 3si1 3h2 4o6 was encountered at 13 17 gpa and 1000 c phase f mg1 2si1 8h2 4o6 another new hydrous phase was observed at 15 5 gpa and 800 c the stability field of phase a was also briefly studied these new hydrous phases might be stabilized in the mantle transition zone especially in subducting slabs at those depths

intraplate volcanism originating from upwelling hydrous mantle transition zone most magmatism occurring on earth is conventionally attributed to passive mantle upwelling at mid ocean ridges to slab devolatilization at subduction zones or to mantle plumes however the widespread cenozoic intraplate volcanism in northeast china1 3 and the young petit spot volcanoes4 7 offshore of the japan trench cannot readily be associated with any of these mechanisms in addition the mantle beneath these types of volcanism is characterized by zones of anomalously low seismic velocity above and below the transition zone8 12 a mantle level located at depths between 410 and 660 kilometres a comprehensive interpretation of these phenomena is lacking here we show that most or possibly all of the intraplate and petit spot volcanism and low velocity zones around the japanese subduction zone can be explained by the cenozoic interaction of the subducting pacific slab with a hydrous mantle transition zone numerical modelling indicates that 0 2 to 0 3 weight per cent of water dissolved in mantle minerals that are driven out from the transition zone in response to subduction and retreat of a tectonic plate is sufficient to reproduce the observations this suggests that a critical amount of water may have accumulated in the transition zone around this subduction zone as well as in others of the tethyan tectonic belt13 that are characterized by intraplate or petit spot volcanism and low velocity zones in the underlying mantle the widespread intraplate volcanism in northeast china and the unusual petit spot volcanoes offshore japan could have resulted from the interaction of the subducting pacific slab with a hydrous mantle transition zone

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

petrogenetic grid in the system mgo sio2 h2o up to 30 gpa 1600 c applications to hydrous peridotite subducting into the earth s deep interior 1 a semiquantitative petrogenetic grid in the system mgo sio2 h2o up to 30 gpa and 1600 c was constructed using schreinemakers analysis on previous experimental data the grid includes stability relations of hydrous wadsleyite hydrous ringwoodite and dense hydrous magnesium silicates dhmss phase a phase d phase e and superhydrous phase b a sequence of chemical reactions among these hydrous phases was clarified in the mantle transition zone 410 660 km depth hydrous wadsleyite and hydrous ringwoodite are stable even at a standard mantle temperature of 1600 c whereas in the other depths nominal hydrous phases including dhmss are stable below 1400 c newly found phase transitions in hydrous wadsleyite and hydrous ringwoodite are multiphase and multireaction because their compositions are on neither mgo sio2 nor mg2sio4 h2o tie lines different from single transition in the dry mg2sio4 system the grid indicates that water in the subducting slab peridotite would be transported to the bottom of the upper mantle by several dhmss and hydrous polymorphs of olivine via solid solid reactions after antigorite decomposition finally dhmss would dehydrate to liberate free water at the upper mantle lower mantle boundary layer or deeper level we also examined a relation between the predicted depth distributions of dehydration reactions in slab peridotite along several possible pressure temperature paths and the mode of seismic frequency in each subduction zone the result suggests a possible origin of intermediate to deep seismicity by the dehydration of hydrous phases in the subducting slab

shear wave anisotropy in textured phase d and constraints on deep water recycling in subduction zones abstract regions of low seismic velocity and high shear anisotropies in cold subducted slabs have often been related to anisotropic fabrics in hydrous phases mainly induced by slab deformation the interpretation of these seismic anomalies in terms of hydration thus relies on a better knowledge of the elasticity and plastic deformation mechanisms of candidate hydrous phases here we investigate the development of lattice preferred orientations lpo in phase d mgsi 2 h 2 o 6 10 18 wt h 2 o the ultimate water carrier in hydrous subducted peridotite the samples were deformed non hydrostatically up to 48 gpa in a diamond anvil cell and the texture and strength were obtained from analysis of the x ray diffraction patterns collected in radial diffraction geometry we find that at low strains the layered structure of phase d displays strong 0001 texture where the stacking fault axis c axis preferentially align parallel to the compression axis a subsidiary 1 0 1 0 texture develops at higher strains plasticity simulations in polycrystalline aggregates using a viscoplastic self consistent model suggest that these lpo patterns are consistent with shape preferred orientation mechanism during the first compaction steps and with dominant easy glide on basal planes and harder first order pyramidal slip respectively upon further compression we find that phase d displays the lowest strength and the highest anisotropy among phases in hydrous peridotite in the uppermost lower mantle and might thus control the shear wave anisotropy generated in subducted slabs below the transition zone we further evaluate the effect of textured phase d on the seismic velocity structure and shear wave anisotropy of deformed hydrous peridotite and compare the results to seismic observations in tonga subduction we show that 16 vol of phase d in hydrous subducted peridotite is required to explain the negative velocity anomalies of 3 the extent of shear wave splitting 0 9 0 3 and the shear wave ray polarization geometry v sh v sv observed in a detached fragment of the tonga slab laying sub horizontally below the transition zone seismic observations may thus place constraints on the degree of hydration of the tonga slab beyond the transition zone leading to the requirement that a minimum of 1 2 wt h 2 o is retained in the slab by hydrous peridotite below 670 km depth

diapir versus along channel ascent of crustal material during plate convergence constrained by the thermal structure of subduction zones abstract subduction channel processes are crucial for understanding the material and energy exchange between the earth s crust and mantle crustal rocks can be subducted to mantle depths interact with the mantle wedge and then exhume to the crustal depth again which is generally considered as the mechanism for the formation of ultrahigh pressure metamorphic rocks in nature in addition the crustal rocks generally undergo dehydration and melting at subarc depths giving rise to fluids that metasomatize and weaken the overlying mantle wedge there are generally two ways for the material ascent from subarc depths one is along subduction channels the other is through the mantle wedge by diapir in order to study the conditions and dynamics of these contrasting material ascent modes systematic petrological thermo mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates ages of the subducting oceanic plate as well as the plate convergence rates the model results suggest that the thermal structures of subduction zones control the thermal condition and fluid melt activity at the slab mantle interface in subcontinental subduction channels which further strongly affect the material transportation and ascent mode the thick overriding continental plate and the low angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge where the along channel exhumation occurs exclusively to result in the exhumation of hp uhp metamorphic rocks in contrast the thin overriding lithosphere and the steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels which lead to significant hydration and metasomatism melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge derived melts by diapir through the mantle wedge this may correspond to the origination of continental arc volcanism from mafic to ultramafic metasomatites in the bottom of the mantle wedge in addition the plate convergence rate can also affect the material ascent mode e g diapiric extrusion versus along channel exhumation by changing the amount of supracrustal rocks carried into the subduction channels which further regulate the fluid melt activity and thermo rheological properties

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

equilibrium experiments in the system mgo sio2 h2o msh stability fields of clinohumite oh mg9si4o16 oh 2 chondrodite oh mg5si2o8 oh 2 and phase a mg7si2o8 oh 6 the water pressure and temperature stability fields of clinohumite oh chondrodite oh and phase a were determined in reversed equilibrium experiments up to 100 kbar within the system mgo sio2 h2o their pt fields differ from results from former synthesis experiments bracketing experiments on the reaction phase a low p clinoenstatite forsterite water resulted in a slightly steeper dp dt slope compared to earlier experiments for this equilibrium clinohumite oh and chondrodite oh both have large stability fields which extend over pressure ranges of more than 80 kbar however they are hardly relevant as hydrous minerals within the subducted oceanic lithosphere both are too mg rich for a typical mantle bulk composition in addition the dehydration of subducted oceanic lithosphere due to forsterite water forming reactions will occur before the two humite group phases even become stable restricted to the cool region of cold subducting slabs phase a however might be formed via the reactions phase a low p high p clinoenstatite forsterite water or antigorite brucite phase a water before dehydration of the oceanic lithosphere occurs

effect of water on melting phase relations and melt composition in the system mg2sio4mgsio3h2o up to 15 gpa melting experiments in the system mg2sio4 mgsio3 h2o up to pressures of 15 gpa show that the presence of h2o drastically changes melting temperature liquidus phases and the coexisting melt composition the system mg2sio4 h2o shows incongruent melting characterized by a high mgsi ratio of the eutectic composition exceeding 2 0 above 12 gpa whereas it exhibits congruent melting up to at least 7 7 gpa in the mgsio3 h2o system incongruent melting is observed up to 3 gpa which switches to congruent melting above 5 5 gpa and persists to at least 12 gpa the variation of the eutectic composition in the system mg2sio4 mgsio3 h2o is consistent with the variations of the melting temperatures in the systems mg2sio4 h2o and mgsio3 h2o the melting temperature solidus in the system mg2sio4 h2o decreases from 1800 c at atmospheric pressure to less than 1200 c at 10 gpa while in the system mgsio3 h2o it is reduced to a minimum value of 1250 c at approximately 5 5 gpa and then increases with pressure the composition of the initial melt in this system drastically changes at around 5 5 gpa the mgsi ratio being about 3 at 15 gpa variations of the ternary phase diagrams mgo sio2 h2o with pressure are compiled and compared with previous work on the relevant systems a modified spinel like hydrous phase is synthesized at 15 5 gpa the maximum amount of h2o accommodated in this phase is estimated to be 3 3 wt from observed chemical composition and crystal chemical considerations implying that a significant amount of h2o can be accommodated in the mantle transition zone the overall effect of h2o on ultrabasic magma genesis in the mantle is discussed a possibility is presented for the formation of komatiite magma involving the effect of h2o in the mantle of the early earth mg rich komatiite magma was possibly generated at a depth of only 200 km at temperatures much lower than that having been assumed previously

orthoenstatite clinoenstatite phase transformation in mgsio3 at high pressure and high temperature determined by in situ x ray diffraction implications for nature of the x discontinuity 1 in the upper mantle the x discontinuity at the depths of 240 to 340 km is observed in many localities but its origin is under debate the phase transformation of mg fe sio3 from orthopyroxene to high pressure clinopyroxene has been proposed to cause this discontinuity in this study in situ x ray diffraction experiments were performed and the orthoenstatite high pressure clinoenstatite c2 c phase boundary in the mgsio3 system was determined all the experiments were carried out using a kawai type multianvil apparatus installed at bl04b1 in spring 8 hyogo japan two types of experiments were carried out 1 phase observation experiments at varying pressures and temperatures and 2 quench experiments at fixed pressure and temperature the phase boundary determined precisely by integrating the results of both sets of experiments was found to be p gpa 0 0035t c 3 7 using a recent mgo pressure scale the orthoenstatite high pressure clinoenstatite phase boundary at 1400 c determined here is 0 7 1 0 gpa lower than previous reports based on quench experiments the phase transformation of orthopyroxene is expected to occur at 260 km depth along a typical mantle geotherm and the shallower part of the observed x discontinuity can be explained by this phase transformation

predicted elastic properties of the hydrous d phase at mantle pressures implications for the anisotropy of subducted slabs near 670 km discontinuity and in the lower mantle the dense hydrous magnesium silicate called the d phase is the most likely candidate to recycle hydrogen into the lower mantle in subduction zones as seismology represents the preferred method to detect this mineral in subduction zones the single crystal elastic tensor has been calculated using first principle methods a triple cell p3 m1 was used to account for an ordered ideal composition of the d phase to lower mantle pressures the elastic tensor of the mineral at ambient conditions is predicted to be c11 387 7 c33 287 7 c44 100 4 c12 108 0 c13 51 1 c14 14 6 in gpa with bulk modulus at zero pressure ko 163 gpa and a statistical error of about 4 0 gpa for all constants the pressure derivatives of the elastic constants have been determined to pressure of 80 gpa our tensor correctly predicts the published experimental measurements of linear compressibility along the a and c axes the logarithmic pressure derivative of the c a axial ratio and bulk modulus at zero pressure it contradicts the experimentally measured elastic tensor reported by liu l g okamoto k yang y j chen c c lin c c 2004 elasticity of single crystal phase d a dense hydrous magnesium silicate by brillouin spectroscopy solid state commun 132 517 520 which fails to reproduce the experimental compressibility data using the elastic constants and density predicted to 85 gpa the seismic anisotropy of p and s waves has been calculated as a function of pressure the single crystal has high p wave velocities and shear wave splitting in the basal plane below 20 gpa at high pressures the maximum p wave velocities have threefold symmetry normal to a axes in the basal plane whereas the maximum shear wave splitting is parallel to a axes with twofold symmetry the d phase has high vp and vs anisotropy at ambient pressure 17 6 for vp and 19 9 for vs the vp anisotropy decreases with increasing pressure to about 30 gpa while it remains constant at 8 from 30 to 85 gpa the vs anisotropy decreases to 17 at 20 gpa and then increases to 22 at 85 gpa the isotropic velocity ratios of s to p rs p bulk sound to s r s and density to s r s are calculated as a function of pressure the rs p shows very little variation with pressure with low value of 0 6 whereas both r s and r s increase with pressure to peak values of 2 8 and 2 0 respectively at about 60 gpa the presence of the d phase in subducted horizontal stagnant plates in the circum pacific could contribute to a number of seismic observations such as the observed velocity heterogeneity at depths between 500 and 1000 km and the fast s waves travelling horizontally with horizontal polarization in this depth range further the lower density of the d phase compared with

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

water flow to the mantle transition zone inferred from a receiver function image of the pacific slab variations in seismic velocity near the subducting slab provide constraints on the thermal structure and the distribution of hydrous minerals and water in a subduction zone we investigated the seismological structure in the upper mantle beneath japan using both radial and transverse receiver functions rfs radial rf can image horizontal layers of velocity contrast while transverse rf can image dipping layers the combination allows us to simultaneously image both horizontal and dipping velocity discontinuities we investigated the records of 45 teleseismic events observed with tiltmeters at 678 hi net stations the frequency band we used is from 0 02 to 0 16 hz the resultant rf image shows an elevated 410 km discontinuity and more importantly the top surface of the pacific slab down to below the 410 km discontinuity with forward modeling we determined that the mantle wedge is about 8 slower in shear wave speed than the subducting slab at depths deeper than 200 km the seismic velocity contrast is presumably caused by a sequence of hydrous minerals at the base of the mantle wedge which receives water released by dehydration reactions in the oceanic crust we employed numerical simulations to determine the distribution of water in and around the subducting slab the result suggests that hydrous minerals are continuously stable above the subducting slab in relatively cool conditions and carry water to the mantle transition zone

sapphirine bearing granulites from the sipiwesk lake area of the late archean pikwitonei granulite terrain manitoba canada sapphirine occurs in the orthopyroxene cordierite and feldspar sillimanite granulites in the sipiwesk lake area of the pikwitonei granulite terrain manitoba 97 40 w 55 05 n the orthopyroxene cordierite granulites have extremely high al2o3 24 5 wt and mgo 24 6 wt contents and contain sapphirine up to 69 2 wt al2o3 aluminous orthopyroxene up to 8 93 wt al2o3 cordierite spinel phlogopite and corundum sapphirine forms coronas mantling spinel and corundum corona sapphirine is zoned and its composition varies through the substitution mg fe mn si 2 al as a function of the phases with which it is in contact textural and chemical relationships of sapphirine with coexisting phases indicate that spinel cordierite reacted to form orthopyroxene sapphirine under conditions of increasing pressure moreover decreasing core to rim variation of al2o3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation on the basis of experimentally determined p t stability of the assemblage enstatite sapphirine cordierite and the al content of hypothetical fe2 free orthopyroxene associated with sapphirine and cordierite metamorphic temperatures and pressures are estimated to be 860 890 c and 3 0 11 2 kbar

deformation and mantle flow beneath the sangihe subduction zone from seismic anisotropy abstract subduction of oceanic lithosphere is the most direct feedback between the earth s surface and deep interior however the detail of its interaction with the broader convecting mantle is still unclear mantle flow around subduction zones can be constrained using seismic anisotropy but despite many such studies a simple global picture is lacking the sangihe subduction zone where the molucca sea microplate is subducting westward beneath the eurasian plate is part of the tectonically complex sulawesi philippine region and an ideal natural laboratory to study complex subduction processes we investigate the anisotropic structure of the sangihe subduction zone with shear wave splitting measurements of local s and sks phases at two stations mni in sulawesi dav in the philippines as well as downgoing s phases at five stations at teleseismic distances combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase the broad depth distribution of local events 60 630 km allows us to observe a change in splitting behaviour at 380 km depth above fast directions are trench parallel and delay times t are 0 34 0 53 s with no increase with depth we suggest this anisotropy is caused by aligned cracks possibly melt filled beneath the volcanic arc and fossil anisotropy in the overriding plate below 380 km is predominantly trench normal and t are slightly higher 0 53 0 65 s as no correlation is observed with inferred distance travelled inside the slab we attribute this anisotropy to shear layers atop the slab which are coherent from 200 to 400 km depth and perhaps extend into the transition zone sks and source side measurements show larger t 1 53 and 1 33 s respectively and trench parallel since these phases predominantly sample sub slab mantle we consider along strike lateral flow associated with the double sided subduction of the molucca sea microplate to be the most likely explanation we thus infer three dominant regions of anisotropy at the sangihe subduction zone one within the overriding lithosphere one along the slab wedge interface and one below the subducting molucca sea slab the mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy this study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems

magma genesis beneath northeast japan arc a new perspective on subduction zone magmatism abstract it is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrous minerals in the context of this dehydration embrittlement hypothesis we propose a new model to explain key features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basaltic systems determined by thermodynamic calculations and seismic structures of northeast japan arc revealed by latest seismic studies the model predicts that partial melting of basaltic crust in the subducting slab is an inevitable consequence of subduction of hydrated oceanic lithosphere aqueous fluids released from the subducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to just below overlying crust at volcanic front hydrous minerals in the mantle wedge are stable only in shallow

single crystal equation of state of phase d to lower mantle pressures and the effect of hydration on the buoyancy of deep subducted slabs 1 an understanding of the physical properties of the hydrous magnesium silicate phase d is important for the interpretation of the seismic anomalies observed in subducted slabs and to evaluate the effect of hydration on slab dynamics here we report the equation of state of phase d mg1 1si1 8h2 5o6 up to 65 gpa obtained from high precision single crystal x ray diffraction a single crystal of phase d was loaded in a diamond anvil cell using helium as pressure transmitting medium to ensure quasi hydrostatic conditions during the entire data collection the volume of phase d decreases smoothly over the entire pressure range without the anomalies in the compressibility reported at 40 gpa in previous powder diffraction studies if existing in phase d a hydrogen bond symmetrization transition as predicted by first principles calculation is therefore not associated with anomalies in the volume compression behavior the isothermal bulk modulus kt and its pressure derivative k t obtained from the fitting of the unit cell volumes using a third order birch murnaghan equation of state are kt 151 4 1 2 gpa and k t 4 89 0 08 respectively this bulk modulus is in good agreement with a recent single crystal brillouin scattering experiment the presence of 16 vol of phase d in hydrous peridotite lithologies reduces the density by up to 2 6 at upper most lower mantle pressure temperature conditions 1273 k and 30 gpa phase d is thus a potential candidate to influence the buoyancy of hydrated stagnant slabs below the transition zone

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

elasticity of phase d and implication for the degree of hydration of deep subducted slabs 1 seismic anomalies in subducted slabs including low velocity zones and shear wave splitting have often been related to hydrous regions phase d mgsi2h2o6 10 18 wt h2o may be the ultimate water carrier in hydrous subducted peridotite and its seismic properties are thus essential for interpreting observed anomalies in terms of hydration here we report the sound velocities and elasticity of mg and al fe bearing phase d single crystals measured by brillouin spectroscopy the room conditions adiabatic bulk and shear modulus are ks0 154 8 3 2 gpa and 104 3 2 1 gpa for mg phase d and ks0 158 4 3 9 gpa and 104 7 2 7 gpa for alfe phase d suggesting minor effect of cation substitution on the elasticity of phase d based on the seismic velocity data we found that 16 vol of alfe phase d in hydrous subducted peridotite with 1 2 wt h2o could provide a plausible explanation for the negative velocity anomalies of 3 observed in fragments of the tonga slab below the transition zone

the oh content of pyrope at high pressure the oh contents of pyrope garnets synthesised at 1000 c and pressures between 2 and 13 gpa have been measured by infrared spectroscopy we find that under the same conditions of pressure temperature ah2o and asio2 the oh content of pyrope is similar to that of grossular this shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular in the presence of excess sio2 the h2o content increases with pressure to 5 gpa 1000 ppm then decreases to below the detection limit at pressures above 7 gpa thus garnet becomes more hydrous with pressure to some critical value beyond which dehydration occurs even under h2o saturated conditions this observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study when corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle

phase transformations in serpentine and transportation of water into the lower mantle phase transformations in serpentine the major hydrous mineral in the upper part of the subducting slabs have been studied at pressures to 26 gpa and at temperatures between 500 and 1400 c while serpentine completely dehydrates at temperatures higher than 600 800 c and at pressures to 12 gpa phase assemblages including some dense hydrous magnesium silicates dhms such as phases a e superhydrous b and d were formed at higher pressures phase d was found to possess a stability field of particularly wide pressure and temperature regime suggesting that this phase may play an important role in transportation of water into the lower mantle via subduction of cold slabs

phase relations and melt compositions in cmas pyrolite h2o system up to 25 gpa abstract phase relations and melt compositions in the cao mgo al 2 o 3 sio 2 pyrolite under hydrous 2 of h 2 o and anhydrous conditions have been determined at 10 25 gpa and temperatures from 1400 to 2400 c at the pressure of 16 and 25 gpa the apparent solidus temperatures for the hydrous system are about 200 and 100 c lower than the solidus temperatures for the dry system respectively the apparent solidus temperature changes drastically from 1600 c at 13 5 gpa to about 1850 c at 16 gpa majorite is a liquidus phase in the hydrous pyrolite from 10 to 25 gpa the liquidus phases in the dry pyrolite are olivine at 10 14 gpa majorite at 14 20 gpa and periclase at pressures above 20 gpa we observed expansion of the stability field of anhydrous phase b and its complex relations with the other phases in the cmas pyrolite system we suppose it is formed not only by incongruent melting of wadsleyite but also by the subsolidus reaction of wadsleyite and majorite in the dry condition compositions of partial melts formed by low degree of melting 10 20 have al 2 o 3 depleted and cao rich compositions up to 22 gpa and enriched in al 2 o 3 5 7 wt and cao at 25 gpa at 10 gpa dry and hydrous melts formed by low degree of melting have high sio 2 48 51 wt and relatively high al 2 o 3 3 5 5 2 wt their compositions are generally consistent with those of aluminum depleted komatiite at 13 22 gpa dry and hydrous melts have low sio 2 44 49 wt and al 2 o 3 1 7 3 0 wt and high cao contents 9 15 wt an essential depression of the apparent solidus temperature in the hydrous deep upper mantle compared to that in the transition zone provides the dehydration melting of the hydrous plume the present results support a model of komatiite genesis by dehydration melting of rising wet plumes at the base of the upper mantle

seismological evidence for the existence of serpentinized wedge mantle serpentinized peridotite is detected seismologically by mapping poisson s ratio beneath the kanto district central japan because serpentinite has a higher poisson s ratio than other rocks in the crust and upper mantle we determine three dimensional p and s wave velocity models using travel time tomography and then construct a three dimensional map of poisson s ratio the models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km in the low velocity region there is an area with poisson s ratio greater than 0 3 at depths of 20 45 km which we attribute to serpentinized peridotite the rest of the low velocity region is interpreted as descending oceanic crust with a poisson s ratio of about 0 25 the high poisson s ratio area is associated with low seismicity and weak interplate coupling which is consistent with the ductility of serpentinite

water partitioning between nominally anhydrous minerals in the mgo sio2 h2o system up to 24 gpa implications for the distribution of water in the earth s mantle multi anvil experiments have been conducted in the mgo sio2 h2o system at pressures of 15 24 gpa and temperatures of 1200 1600 c to investigate the partitioning of water between mantle phases the water contents were obtained from fourier transform infrared spectroscopy on doubly polished and clear crystals most of the mineral phases coexisted with a hydrous melt the results show that among all magnesium silicate phases 1 mgsio3 perovskite is the only phase which does not dissolve a detectable amount of water and 2 the high pressure polymorphs of mg2sio4 wadsleyite and ringwoodite dissolve about one order of magnitude more water than the polymorphs of mgsio3 clinoenstatite majorite and akimotoite the following partition coefficients of water were measured dwadsleyite clinoenstatite 3 8 at 15 gpa and 1300 c dringwoodite akimotoite 21 at 19 gpa and 1300 c dakimotoite perovskite 1 at 24 gpa and 1600 c and dpericlase perovskite 1 at 24 gpa and 1500 c these results suggest that most of the water in the earth s mantle is partitioned in the transition zone

stabilities and equations of state of dense hydrous magnesium silicates abstract the thermodynamic stability of high pressure phases in the ternary system mgo sio2 h2o is reviewed with special emphasis on hydrated phases including the alphabet phases on the basis of recent experimental data a stability diagram for these phases along a geotherm appropriate for a subducting slab is constructed this suggests that at realistic water contents the breakdown of antigorite will produce olivine plus phase a in the upper mantle but that the ability of wadsleyite to accommodate wt levels of water means that phases e and d are unlikely to occur in the upper portion of the transition zone for the lower part of the transition zone in which ringwoodite becomes stable the coexisting hydrous phase would be superhydrous phase b provided ringwoodite is not able to accommodate significant amounts of water at the top of the lower mantle phase d is stable but only at temperatures below 1300 c as the maximum solubility of h2o in magnesium silicate perovskite is also quite low slabs may dehydrate somewhat as they enter the lower mantle the available thermodynamic data for these phases is also reviewed from the most recent measurements of equations of state it is concluded that the elasticity of the phases in the mgo sio2 h2o ternary system is primarily dependent upon the density and not upon the water content it is therefore concluded that the presence of even significant amounts of hydrated phases in subducting slabs could not be unequivocally identified from seismological observations

a consistent model for fluid distribution viscosity distribution and flow thermal structure in subduction zone water plays crucial roles in the subduction zone dynamics affecting the thermal flow structure through the fluid processes we aim to understand what controls the dynamics and construct a model to solve consistently fluid generation fluid transport its reaction with the solid and resultant viscosity and thermal flow structure we highlight the effect of mechanical weakening of rocks associated with hydration the viscosity of serpentinite serp in subduction zones critically controls the flow thermal structure via extent of mechanical coupling between the subducting slab and overlying mantle wedge when serp is greater than 1021 pa s the thermal flow structure reaches a steady state beneath the volcanic zone and the melting region expands until cin initial water content in the subducting oceanic crust reaches 3 wt and it does not expand from 3 wt on the other hand when serp is less than 1019 pa s the greater water dependence of viscosity expressed by a larger fv confines a hot material to a narrower channel intruding into the wedge corner from a deeper part of the back arc region consequently the overall heat flux becomes less for a larger fv when ageba age of back arc basin as a rifted lithosphere 7 5 ma the increase in fv weakens but shifts the melting region toward the trench side because of the narrow channel flow intruding into the wedge corner where as it shuts down melting when ageba 20 ma several model cases particularly those with serp 1020 to 1021 pa s and a relatively large fv for cin 2 to 3 wt broadly account for the observations in the northeast japan arc i e location and width of volcanic chain extent of serpentinite surface heat flow and seismic tomography although the large variability of surface heat flow and seismic tomographic images does not allow us to constrain the parameter range tightly

high pressure fluids in the system mgo sio 2 h 2 o under upper mantle conditions fluids and melts have been trapped and analysed in high pressure experiments in the model mantle system mgo sio2 h2o at 6 to 10 5 gpa and 900 to 1 200 c the fluid melt traps consisted of a diamond layer that was added to the experimental charge and was separate from the silicate phases the recovered diamond traps were analysed by laser ablation icp ms starting materials were synthetic mixtures of brucite talc and silica with variable mg si containing 11 31 wt h2o experiments on a serpentine starting composition mg3si2o5 oh 4 result in mgo sio2 weight ratios in the subsolidus fluids close to 1 at 6 gpa and close to 2 at 9 gpa melt compositions at 6 and 9 gpa have mgo sio2 ratios close to that of forsterite at a single pressure the amount of dissolved silicate in the fluid increases steadily with increasing temperature up to 1 150 c where a sudden increase of both sio2 and mgo is observed this discrete step marks the solidus which is more clearly developed at 6 than at 9 gpa thus hydrous melts within the model mantle subsystem mg2sio4 mg2si2o6 h2o are chemically distinct from aqueous fluids up to at least 9 gpa corresponding to 300 km depth extrapolation of the current data set implies that total convergence between fluid and melt along the solidus probably occurs at 12 13 gpa 400 km i e close to the earth s mantle transition zone beneath cratons interactions of hydrous fluids with upper mantle lithologies cause relative silica depletion olivine enrichment at depths greater than 200 km and silica orthopyroxene enrichment at shallower depths

metastability of the 10 a phase in the system mgo sio2 h2o msh what about hydrous msh phases in subduction zones