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

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

the coupled effects of mantle mixing and a water dependent viscosity on the surface ocean abstract water content plays a vital role in determining mantle rheology and thus mantle convection and plate tectonics most parameterised convection models predict that earth initially underwent a period of rapid degassing and heating followed by a slow and sustained period of regassing and cooling however these models assume water is instantaneously mixed and homogeneously distributed into the mantle this is a limitation because the mixing time for water entering and leaving the mantle is a function of the rayleigh number which varies dramatically with water content temperature and through time here we present an adapted parametrised model crowley et al 2011 to include the coupled effects of the time scale of mixing with a water dependent viscosity we consider two mixing types first where the mixing time is constant throughout the model and second where mixing time varies as a response to an evolving rayleigh number we find that facilitated by the effects of water content in the melt region at mid ocean ridges a constant mixing time can induce long periods of degassing the inclusion of a variable mixing time dependent on the rayleigh number acts to limit the period of degassing and also results in more water being stored in the mantle and less at the surface than in both the constant and instantaneous mixing cases mixing time cannot be more than 2 billion years as large mixing times trap water in the mantle leaving a dry surface even small changes in the surface ocean induced by mixing times on the order of 0 1 gyrs can cause changes in the global mean sea level on the order of 10 s of metres these changes in sea level could easily uncover topographic highs in the bathymetry potentially aiding sub aerial erosion a process thought to be important in early earth evolution even in this relatively simple model the inclusion of a mixing time between water entering and leaving the mantle creates a more dynamic water cycle

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

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

the stability and structure of primordial reservoirs in the lower mantle insights from models of thermochemical convection in three dimensional spherical geometry large scale chemical lateral heterogeneities are inferred in the earth s lowermost mantle by seismological studies we explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time by using similar analysis in 3 d spherical geometry in this study we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle including the chemical density contrast between the primordial dense material and the regular mantle material buoyancy ratio thermal and chemical viscosity contrasts volume fraction of primordial dense material and the clapeyron slope of the phase transition at 660 km depth we find that most of the findings from the 3 d cartesian study still apply to 3 d spherical cases after slight modifications varying buoyancy ratio leads to different flow patterns from rapid upwelling to stable layering and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle chemical viscosity contrasts in a reasonable range have a second order role in modifying the stability of the dense anomalies the volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts but has significant effects on calculations with intermediate and small thermal viscosity contrasts the volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast an endothermic phase transition at 660 km depth acts as a filter allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle

evolution of terrestrial proto co2 atmosphere coupled with thermal history of the earth abstract the effect of volatile exchange between surface reservoirs and the mantle on the evolution of proto co2 atmosphere on the earth is investigated using a global carbon cycle model coupled with thermal evolution of the mantle carbon is assumed to circulate among five reservoirs atmosphere ocean continents seafloor and mantle and the carbon flux of each reservoir is calculated under varying conditions such as an increase in solar luminosity continental growth and a decrease in tectonic activity with time we consider processes such as continental weathering carbonate precipitation in the ocean carbonate accretion to the continents metamorphism of carbonates followed by co2 degassing through arc volcanism carbon regassing into the mantle and co2 degassing from the mantle the degassing rate of volatiles from the mantle is assumed to be proportional to the volatile concentration in the mantle multiplied by the mantle degassing volume the mantle degassing volume is determined by the seafloor spreading rate and the melt generation depth in the mantle we use a parameterized convection model to calculate the thermal evolution of the mantle from which we estimate the seafloor spreading rate and the melt generation depth in the mantle numerical simulations suggest that the amount of surface carbon at the present time would be in a steady state this is because the response time of the carbon cycle system against the perturbation for surface carbon is short being estimated at about 900 million years under the present conditions thus the present amount of surface carbon would not be affected by the initial amount of surface carbon the seafloor spreading rate would have been almost constant throughout the earth s history to explain apparent constancy of the carbon isotope ratio in the mantle after about 3 5 billion years ago the surface carbon might probably have been circulated between surface reservoirs and the mantle once or twice after the archean period in any event co2 in the proto atmosphere on the earth is suggested to have decreased with the growth of continents resulting in stabilization of the terrestrial environment against the increase in solar luminosity

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

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

compositional heterogeneity of archean mantle estimated from sr and nd isotopic systematics of basaltic rocks from north pole australia and the isua supracrustal belt greenland abstract compositional variability found in modern mid ocean ridge basalt morb and ocean island basalt oib reflects differentiation processes associated with material recycling in the mantle crust system to investigate the timing at which this recycling system was established and how it transformed into the present day system we present geochemical analyses of the archean basalts from north pole np in the east pilbara craton western australia and the isua supracrustal belt isb southern west greenland these rocks represent archean accretionary complexes with ages of 3 5 ga and 3 7 3 8 ga respectively we analyzed the trace element contents including rare earth elements rees and sr and nd isotopic compositions of the basalts which may represent morbs and oibs from np and isb their trace element compositions are broadly similar but show distinct geochemical characteristics particularly with respect to rees that probably reflect differences in both the source mantle and degree of melting such differences are also evident in their initial nd isotopic compositions which were estimated based on equilibrium partitioning of rees and well defined isochron ages in contrast the sr isotopic compositions of the np and isb basalts are highly variable and their isochron ages are inconsistent with previous studies furthermore the partitioning of rb and sr in the np basalts indicates disequilibrium suggesting that the rb sr system has been disturbed by post igneous alteration and metamorphism based on these observations we propose the following model to explain the temporal variations in the geochemical composition of the archean mantle i 3800 ma recycling of plate material and melting occurred quite readily and therefore morbs and oibs were produced from differentiated mantle sources ii 3460 ma to 3800 ma mantle crust mixing occurred as the result of an extreme event such as mantle overturning reducing the compositional variation of the mantle and iii after 3460 ma mantle heterogeneity gradually developed in the material recycling system re establishing the compositional differences between morbs and oibs this model requires an extreme event to drive the homogenization during stage ii which may provide new insights into the evolution of the crust mantle system

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

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

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

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

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

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

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

single crystal elasticity of wadsleyites β mg 2 sio 4 containing 0 37 1 66 wt h 2 o the presence of hydrogen can affect elastic properties and seismic velocities of minerals in the earth s upper mantle in this study the secondorder elastic constants of hydrous wadsleyites containing 0 37 0 84 and 1 66 wt h2o were determined by brillouin scattering at ambient conditions measurements were performed on at least three independent crystal planes for each composition the aggregate bulk modulus ks0 and shear modulus g0 were calculated using vrh voigt reuss hill averages the results are ks0 165 4 9 gpa g0 108 6 6 gpa for wadsleyite with 0 37 wt h2o ks0 160 3 7 gpa g0 105 3 6 gpa for 0 84 wt h2o ks0 149 2 6 gpa g0 98 6 4 gpa for 1 66 wt h2o we find that the bulk and shear moduli of hydrous wadsleyites decrease linearly with water content according to the following relations in gpa ks0 170 9 9 13 0 8 ch2o g0 111 7 6 7 8 4 ch2o where ch2o is the h2o weight percentage compared with anhydrous wadsleyite addition of 1 wt h2owillleadtoa7 6 decreaseinthebulkmodulus anda7 0 decreaseintheshearmodulus usingtheseresults weexaminethevelocitycontrast between hydrous olivine and wadsleyite at ambient conditions for an fe free system assuming an h2o partition coefficient between wadsleyite and olivine of 3 the velocity contrast in compressional and shear velocity between wadsleyite and olivine ranges from 12 13 for an h2o free system to7 8 forwadsleyitecontaining1 5wt h2o thus themagnitudeoftheseismicvelocitychangeat410 kmdepthcanbeexpectedtobesensitive

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

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

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

northeastern atlantic benthic foraminifera during the last 45 000 years changes in productivity seen from the bottom up we studied benthic foraminifera from the last 45 kyr in the 63 mu m size fraction in biogeochemical ocean flux studies bofs cores 5k 50 degrees 41 3 n 21 degrees 51 9 w depth 3547 m and 14k 58 degrees 37 2 n 19 degrees 26 2 w depth 1756 m at a time resolution of several hundreds to a thousand years the deepest site showed the largest fluctuations in faunal composition species richness and benthic foraminiferal accumulation rates the fluctuations resulted from changes in abundance of epistominella exigua and alabaminella weddellensis in the present oceans these species bloom opportunistically when a spring plankton bloom results in seasonal deposition of phytodetritus on the seafloor the phytodetritus species had very low relative abundances and accumulation rates during the last glacial maximum a strong increase in absolute and relative abundance of e exigua and a weddellensis during deglaciation paralleled the decrease in abundance of the polar planktonic foraminifer neogloboquadrina pachyderma s and the increase in abundance of warmer water planktonic species such as globigerina bulloides this strong increase in relative abundance of the phytodetritus species and the coeval increase in benthic foraminiferal accumulation rate were thus probably caused by an increase in the deposition of phytodetritus to the seafloor and thus probably of surface productivity when the polar front retreated to higher latitudes the abundance of phytodetritus species decreased during the younger dryas but not to the low levels of fully glacial conditions during heinrich events periods of excessive melt water formation and ice rafting benthic accumulation rates were very low as were the absolute and relative abundances of the phytodetritus species supporting suggestions that surface productivity was very low during these events in both cores pullenia and cassidulina species were common during isotope stages 2 3 and 4 as were bolivinid buliminid and uvigerinid species high relative abundances of these species have been interpreted as indicative either of sluggish deep water circulation or of high organic carbon fluxes to the seafloor in our cores relative abundances of these species are negatively correlated with benthic foraminiferal accumulation rates and we can thus not interpret them as indicative of increased productivity during glacials the percentage of these low oxygen species calculated on a phytodetritus species free basis decreased slightly at deglaciation at 5k but not at 14k this suggests that decreased production of north atlantic deep water during the last glacial might have slightly affected benthic foraminiferal faunas in the eastern north atlantic at 3547 m depth but not at 1756 m in conclusion major changes in deep sea benthic foraminiferal faunas over the last 45 000 years in our cores from the northeastern atlantic were the result of changes in surface water productivity not of changes in deep water circulation productivity was lower during the glacial probably because of extensive ice cover

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

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

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

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

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

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

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

a reassessment of the proposed lairg impact structure and its potential implications for the deep structure of northern scotland the lairg gravity low may represent a buried impact crater c 40 km across that was the source of the 1 2 ga stac fada member ejecta deposit but the gravity anomaly is too large to represent a simple crater and there is no evidence of a central peak reanalysis of the point bouguer gravity data reveals a ring of positive anomalies around the central low suggesting that it might represent the eroded central part of a larger complex crater the inner or peak rings of complex craters show a broadly consistent 2 1 relationship between ring diameter and total crater diameter implying that the putative lairg crater may be as much as 100 km across this would place the crater rim within a few kilometres of the stac fada member outcrop a location inconsistent with the thickness and clast size of the ejecta deposit we propose that the putative impact crater originally lay further east substantially further from the stac fada member than today and was translocated westwards to its present location beneath lairg during the caledonian orogeny this model requires that a deep seated thrust fault analogous to the flannan and outer isles thrusts exists beneath the moine thrust in north central scotland

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

a free plate surface and weak oceanic crust produce single sided subduction on earth 1 earth s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection subduction zones on present day earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle while global self consistent numerical models of mantle convection have reproduced some aspects of plate tectonics the assumptions behind these models do not allow for realistic single sided subduction here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates 1 the presence of a free deformable upper surface and 2 the presence of weak hydrated crust atop subducting slabs we show that assuming a free surface rather than the conventional free slip surface allows the dynamical behavior at convergent plate boundaries to change from double sided to single sided a weak crustal layer further improves the behavior towards steady single sided subduction by acting as lubricating layer between the sinking and the overriding plate this is a first order finding of the causes of single sided subduction which by its own produces important features like the arcuate curvature of subduction trenches citation crameri f p j tackley i meilick t v gerya and b j p kaus 2012 a free plate surface and weak oceanic crust produce single sided subduction on earth geophys res lett 39 l03306 doi 10 1029 2011gl050046