seismic detection of folded subducted lithosphere at the core mantle boundary seismic tomography of the d region beneath the caribbean reveals a 100 km vertical step in an otherwise flat d shear velocity discontinuity and localized low velocities at the edge of the slab material which may involve upwellings caused by the slab laterally displacing a thin hot thermal boundary layer seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the earth s lower mantle1 2 the fate of these slabs has remained unresolved but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle known as the d region relatively cold slab material may account for high seismic velocities imaged in d beneath areas of long lived plate subduction and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions3 4 the d discontinuity itself is probably the result of a phase change in relatively low temperature magnesium silicate perovskite5 6 here we present images of the d region beneath the cocos plate using kirchhoff migration of horizontally polarized shear waves and find a 100 km vertical step occurring over less than 100 km laterally in an otherwise flat d shear velocity discontinuity folding and piling of a cold slab that has reached the core mantle boundary as observed in numerical and experimental models can account for the step by a 100 km elevation of the post perovskite phase boundary due to a 700 c lateral temperature reduction in the folded slab we detect localized low velocities at the edge of the slab material which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer

incorporating self consistently calculated mineral physics into thermochemical mantle convection simulations in a 3 d spherical shell and its influence on seismic anomalies in earth s mantle 1 phase assemblages of mantle rocks calculated from the ratios of five oxides cao feo mgo al2o3 sio2 by free energy minimization were used to calculate the material properties density thermal expansivity specific heat capacity and seismic velocity as a function of temperature pressure and composition which were incorporated into a numerical thermochemical mantle convection model in a 3 d spherical shell the advantage of using such an approach is that thermodynamic parameters are included implicitly and self consistently obviating the need for ad hoc parameterizations of phase transitions which can be complex in regions such as the transition zone particularly if compositional variations are taken into account convective planforms for isochemical and thermochemical cases are however not much different from those computed using our previous simple parameterized reference state which means that our previous results are robust in this respect the spectrum and amplitude of seismic velocity anomalies obtained using the self consistently calculated material properties are more realistic than those obtained when seismic velocity is linearly dependent on temperature and composition because elastic properties are dependent on phase relationship of mantle minerals in other words pressure and temperature in all cases the spectra are dominated by long wavelengths spherical harmonic degree 1 to 2 similar or even longer wavelength than seismic tomographic models of earth which is probably due to self consistent plate tectonics and depth dependent viscosity in conclusion this combined approach of mantle convection and self consistently calculated mineral physics is a powerful and useful technique for predicting thermal chemical phase structures in earth s mantle however because of uncertainties in various parameters there are still some shortcomings in the treatment of the postperovskite phase transition additionally transport properties such as thermal conductivity and viscosity are not calculated by this treatment and are thus subject to the usual uncertainties

the relationships between large scale variations in shear velocity density and compressional velocity in the earth s mantle a large data set of surface wave phase anomalies body wave travel times normal mode splitting functions and long period waveforms is used to investigate the scaling between shear velocity density and compressional velocity in the earth s mantle we introduce a methodology that allows construction of joint models with various levels of scaling complexity d ln d lnvs d lnvs d lnvp in order to detect seismological signatures of chemical heterogeneity we demonstrate that the data sets considered cannot be fit concurrently with a uniform or a positive and uniform throughout the mantle the variance reductions to p wave travel times and vp sensitive modes are up to 40 higher with our preferred model of anisotropic shear and compressional velocity than the recent anisotropic shear velocity model s362ani m which was constructed assuming a uniform throughout the mantle several features reported in earlier tomographic studies persist after the inclusion of new and larger data sets anticorrelation between bulk sound and shear velocities in the lowermost mantle as well as an increase in with depth in the lower mantle are largely independent of the regularization scheme when correlations between density and shear velocity variations are imposed in the lowermost mantle variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40 recent measurements of the splitting of 0s2 in particular are largely incompatible with perfectly correlated shear velocity and density heterogeneity throughout the mantle a way to significantly improve the fits to various data sets is by allowing independent density perturbations in the lowermost mantle our preferred joint model consists of denser than average anomalies 1 peak to peak at the base of the mantle roughly coincident with the low velocity superplumes the relative variation of shear velocity density and compressional velocity in our study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle with implications for the long term stability and evolution of superplumes

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

mesozoic evolution of the tisza mega unit the south eastern part of the basement of the pannonian basin is made up of variscan crystalline complexes and early mesozoic formations showing striking affinity with the corresponding formations in the southern margin of the european plate this large composite structural unit which is actually an exotic terrane of european plate origin has been named the tisza mega unit based upon relevant data of the pre tertiary basement of southern hungary the reconstruction of the position of the tisza terrane in the early alpine evolutionary stages the process of its separation and break off from the european plate and results of its eo alpine deformations are summarised in the present paper in the variscan and early alpine evolutionary stages the area of the later tisza mega unit was located at the margin of the european plate during variscan orogeny terrane accretion led to intensive deformation and metamorphism in this belt this was followed by transpressional tectonics and the development of molasse basins in the late and post variscan stages and passive margin evolution after the neotethys opening in the middle triassic the separation of the tisza mega unit began with incipient continental rifting along the axis of the later ligurian penninic vahic oceanic branch in the late triassic the end of terrigenous material deposition in the most external zones and a coeval change in fossil assemblage point to the separation of the tisza block from the european plate in the early bathonian significant rotation of the tisza mega unit and coeval paroxysm of alkaline rift type basalt volcanism took place in the early cretaceous in the mid cretaceous due to the northward motion of the adria block and the related closure of the westernmost neotethys basin the extensional regime changed to a compressional one leading to onset of the nappe stacking and low grade regional metamorphism within the tisza microplate in the foreland of the nappe systems flexural basins came into existence that are characterised by flysch type sedimentation in the early tertiary the north eastward motion of the alcapa and tisza dacia blocks led to the formation of the present day heterogeneous basement of the pannonian basin

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

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

simultaneous volume measurements of post perovskite and perovskite in mgsio3 and their thermal equations of state abstract simultaneous volume measurements of mgsio 3 post perovskite ppv and perovskite pv were performed in a diamond anvil cell dac combined with synchrotron x rays an externally heated dac was used in addition to a laser heated dac for the volume measurement experiment at high temperatures the volume data were collected in the stability field of post perovskite from 115 to 130 gpa the temperature generated in the externally heated and the laser heated dacs for the volume measurement were up to 832 and 2330 k respectively using two different but complementary heating techniques we collected the data at a wide temperature range from 300 to 2330 k the obtained p v t data for ppv and pv were fitted to a third ordered birch murnaghan equation of state eos for a precise comparison of the volume between the two phases the eoss were constructed based on the same pressure scale of mgo the simultaneous volume measurements and the volumes calculated from the determined eoss demonstrate that the volume difference between ppv and pv of about 1 5 is almost constant with increasing temperature to 4000 k at the transition at the base of the mantle this density difference corresponds to a temperature anomaly of 1300 k without the phase transition due to the very small thermal expansivity of minerals which has a significant effect on mantle dynamics the thermal expansivity contrast between the top and the bottom of the mantle is a factor of 3 6 from a mantle convection study this value suggests that huge and hot plumes are formed at the core mantle boundary

seismic evidence for earth s crusty deep mantle abstract seismic tomography resolves anomalies interpreted as oceanic lithosphere subducted deep into earth s lower mantle however the fate of the compositionally distinct oceanic crust that is part of the lithosphere is poorly constrained but provides important constraints on mixing processes and the recycling process in the deep earth we present high resolution seismic array analyses of anomalous p waves sampling the deep mantle and deterministically locate heterogeneities in the lowermost 300 km of the mantle spectral analysis indicates that the dominant scale length of the heterogeneity is 4 to 7 km the heterogeneity distribution varies laterally and radially and heterogeneities are more abundant near the margins of the lowermost mantle large low velocity provinces llvps consistent with mantle convection simulations that show elevated accumulations of deeply advected crustal material near the boundaries of thermo chemical piles the size and distribution of the observed heterogeneities is consistent with that expected for subducted oceanic crust these results thus suggest the deep mantle contains an imprint of continued subduction of oceanic crust stirred by mantle convection and modulated by long lasting thermo chemical structures the preferred location of the heterogeneity in the lowermost mantle is consistent with a thermo chemical origin of the llvps our observations relate to the mixing behaviour of small length scale heterogeneity in the deep earth and indicate that compositional heterogeneities from the subduction process can survive for extended times in the lowermost mantle

lateral variations in cmb heat flux and deep mantle seismic velocity caused by a thermal chemical phase boundary layer in 3d spherical convection article i nfo numerical simulations of thermo chemical multi phase compressible mantle convection in a three dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear wave velocity vs above the core mantle boundary cmb and lateral variations in heat flux across the cmb qcmb when compositional variations and the post perovskite phase transition are included for simple thermal convection the vs qcmb relationship is reasonably but not perfectly linear the post perovskite transition introduces a non linearity that amplifies fast vs anomalies in cold regions but there is still a unique mapping between vs and qcmb lateral variations in composition such as piles of dense material introduce another non linearity that affects hot upwelling regions and introduces a non uniqueness in vs qcmb if the dense material e g morb is seismically fast compared to the surrounding material in this case dense piles are ringed by sharp low vs anomalies if the cmb is covered by a global dense layer than variations in vs and qcmb are reduced but so is the mean value of qcmb in all cases the peak to peak lateral variation in qcmb is similar to or larger than twice the mean value which might create problems for generating a dynamo according to existing numerical dynamo simulations analytical scalings are developed to explain the observed trends

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

living dead slabs in 3 d the dynamics of compositionally stratified slabs entering a slab graveyard above the core mantle boundary abstract segregation of subducted mid ocean ridge basalt morb at the cmb has been identified as a potentially important mechanism in the long term evolution of the mantle and core here three dimensional 3 d and two dimensional 2 d simulations of a compositionally stratified slab reaching the cmb are presented with the goals of characterising the resulting thermo chemical structures for comparison with seismic studies and of quantifying the fraction of morb that is able to segregate and remain near the cmb compositional stratification of the slab results in a strong torque due to the relatively high density of basalt and low density of harzburgite which tends to rotate the slab such that the basalt side faces down slab cmb interaction is characterised by heating up of the slab followed by separation of the basalt and harzburgite layers with harzburgite rising in vigorous plumes plumes form at the edges and sides of slabs at the cmb as well as in their interiors as previously observed for purely thermal slabs with plume heads dominated by depleted harzburgitic material sometimes with small amounts of entrained basalt while plume tails entrain basaltic material segregation of basalt depends strongly on the presence or absence of a preexisting dense layer at the cmb and by dimensionality a preexisting dense layer greatly increases the fraction of basalt that segregates from the slab in 3 d this fraction is in the range 0 5 0 7 with a layer present compared to 0 25 0 45 with no preexisting layer two modes of basalt segregation are observed for slabs that land basalt side up i hot harzburgite extruding from the sides and edges and ii hot harzburgite rich plumes bursting through the basalt layer as previously observed in laboratory experiments whereas for a slab that lands basalt side down iii hot basalt can peel off from its underside displaying fingering instabilities in 3 d furthermore basalt harzburgite segregation is sometimes observed in slab segments that have already been heated at the cmb and risen a few hundred km by a folding mechanism a range of interesting structures are observed in the model cmb region which may be useful in interpreting seismic observations strong lateral gradients in composition and temperature are quite common structures include plumes next to vertical slab segments inverted slab sections perched above the cmb and harzburgite curtains the presence of a stratified layer above the cmb as observed in global simulations in which a layer builds up by basalt segregation strongly suppresses plume formation 2 d simulations give a good first order guide to the dynamics obtained in fully 3 d geometry but inherently 3 d structures are missed plumes versus sheets fingering and there is a quantitative difference in the fraction of basalt segregated

joint seismic tomography for bulk sound and shear wave speed in the earth s mantle high quality p and s travel times are now available from careful reprocessing of data reported to international agencies a restricted data set has been extracted for which comparable ray coverage is achieved for p and s and used for a joint inversion to produce a three dimensional model for shear and bulk sound velocities represented in terms of 2 2 cells and 18 layers in depth through the mantle about 106 times for each of p and s are combined to produce 312 549 summary rays for each wave type linearizing about the ak135 reference model 583 200 coupled tomographic equations are solved using an iterative partitioned scheme clear high resolution images are obtained for both bulk sound speed and shear wavespeed the bulk and shear moduli have differing sensitivity to temperature and mineral composition and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations different heterogeneity regimes can be recognised in the upper mantle the transition zone most of the lower mantle and the lowermost mantle in the upper mantle many features can be explained by thermal effects but in some orogenic zones e g western north america the opposite sense of the bulk sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects in the lower mantle pronounced narrow structures which may represent remnant subduction are most marked in shear the level of large scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km below this depth the variability of both wave speeds increases near the core mantle boundary the variations of the two wave speeds show little concordance suggesting the presence of widespread chemical heterogeneity

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

experiments on the interaction of thermal convection and compositional layering at the base of the mantle accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids two models of heterogeneity in the d layer are considered 1 a reservoir of dense material above the core mantle boundary and 2 continuous recharge of the d layer by subduction of cold negatively buoyant high viscosity slabs of oceanic lithosphere survival of a dense reservoir depends on rp the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy rapid mixing occurs for rp 1 for rp 1 mixing occurs by slow entrainment only the reservoir model predicts d layer heterogeneity is concentrated beneath lower mantle upwellings differentiation of composite slabs consisting of dense crustal and light depleted components occurs in a hot thermal boundary layer composite slabs pile up in the basal thermal boundary layer and upon heating the light component is ejected by rayleigh taylor instabilities the dense component accumulates in the boundary layer arrayed in a pattern of interconnected spokes and is slowly entrained into thermal plumes according to these experiments the time required for conversion of slabs to plumes in d is 1 2 gyr the correlation between lateral variations in d layer thickness and lateral variations in lower mantle seismic velocity suggests that d may consist of dense material perhaps former oceanic lithosphere which has sunk through the lower mantle

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

seismic evidence links the subducted mongol okhotsk slab to deformation in d near the northeastern margin of the perm anomaly abstract numerical simulations have indicated that the large low shear velocity provinces llsvps dynamically interact with subducted slabs in d and slabs in turn play an important role in influencing the shape and location of the llsvps in this study we find evidence for the existence of the d discontinuity near the northeastern margin of the perm anomaly a smaller llsvp beneath eurasia by analyzing the scd the lower mantle triplication phases generated from two deep events occurring in the northwestern pacific subduction zone and recorded at stations in europe and turkey the d discontinuity in the north is characterized with a 2 velocity increase occurring at a height of 300 km above the core mantle boundary cmb whereas the discontinuity in the south is characterized with a 3 velocity increase occurring at a height of 260 km above the cmb the d discontinuity is most likely attributed to a phase transition from perovskite to post perovskite in conjunction with the presence of relic slab materials likely related to the subducted mongol okhotsk slab our observations along with previous seismic anisotropic detections provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the llsvp

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

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

geodynamic evolution and palaeogeography of the polish carpathians and adjacent areas during neo cimmerian and preceding events latest triassic earliest cretaceous

thermal conductivity of mgo periclase at high pressure implications for the d region abstract the thermal conductivity of the lowermost mantle determines the rate at which heat flows across the core mantle boundary and consequently plays a central role in the thermal evolution of both the mantle as well as the core in an effort to test and improve model estimates of thermal conductivity in the deep mantle we perform first principles computations of the lattice conductivity for mgo periclase the method uses the peierls boltzmann transport equation via a combination of first principles molecular dynamics and first principles lattice dynamics phonon lifetimes are found to be inversely proportional to temperature and increases 3 fold as the density is increased from 3 37 to 5 49 g cm 3 thermal conductivities increase nearly 6 fold over the same density interval and show excellent agreement with the available experimental measurements we use these results to assess how conductivity measurements of mantle minerals can be reliably extrapolated to conditions characteristic of the lowermost mantle and find debye theory to be superior for this task we then apply this insight to estimate the conductivity of mgsio 3 perovskite in the lowermost mantle and combine this with our computational results for mgo periclase to construct a model estimate for a representative lower mantle assemblage we find the mantle conductivity to be 5 9 0 6 w m k at the top of the thermal boundary layer and 4 0 0 5 w m k at its base implying core mantle boundary heat flux values at the lower end of geophysical estimates

shape and origin of the east alpine slab constrained by the alpass teleseismic model abstract during the last two decades teleseismic studies yielded valuable information on the structure of the upper mantle below the alpine mediterranean area subducted oceanic lithosphere forms a broad anomaly resting on but not penetrating the 670 km discontinuity more shallow slabs imaged below the alpine arc are interpreted as subducted continental lower lithosphere substantial advances in our understanding of past and active tectonic processes have been achieved due to these results however important issues like the polarity of subduction under the eastern alps and the slab geometry at the transition to the pannonian realm are still under debate the alpass teleseismic experiment was designed to address these open questions teleseismic waveforms from 80 earthquakes recorded at 75 temporary and 79 permanent stations were collected during 2005 and 2006 from these data a tomographic image of the upper mantle was generated between 60 km and 500 km depth crustal corrections additional station terms and ray bending caused by the velocity perturbations were considered a steeply to vertically dipping shallow slab below the eastern alps is clearly resolved down to a depth of 250 km it is interpreted as european lower lithosphere detached from the crust and subducted during post collision convergence between adria and europe below the pannonian realm low velocities or high mantle temperatures prevail down to 300 km depth consistent with the concept of a pannonian lithospheric fragment which underwent strike slip deformation relative to the european plate and extension during the post collision phase of the alpine orogeny between 350 km and 400 km depth a deep slab extends from below the central eastern alps to under the pannonian realm it is interpreted as subducted lithosphere of the alpine tethys at greater depth there is a continuous transition to the high velocity anomaly above the 670 km discontinuity

dynamic origins of seismic wavespeed variation in d the d discontinuity is defined by a seismic velocity increase of 1 3 about 250 km above the core mantle boundary cmb and is mainly detected beneath locations of inferred paleosubduction a phase change origin for the interface can explain a triplicated arrival observed in seismic waveform data and is supported by the recent discovery of a post perovskite phase transition we investigate the interaction of slabs plumes and the phase change within d in 2 d compressible convection calculations and predict waveform complexity in synthetic seismic data the dynamic models produce significant thermal and phase heterogeneity in d over small distances and reveal a variety of behaviors including 1 distinct ppv blocks separated by upwellings 2 notches at the top of a ppv layer caused by plume heads 3 regions of pv embedded within a ppv layer due to upwellings advected isotherms produce complicated thermal structure that enable multiple crossings of the phase boundary perturbations to s sds and scs arrivals distances 80 degrees

the difficulty for subducted oceanic crust to accumulate at the earth s core mantle boundary 1 seismic tomography has revealed two large low shear velocity provinces llsvps in the lowermost mantle beneath the central pacific and africa the llsvps are further shown to be compositionally different from their surroundings among several hypotheses put forth in recent years to explain the cause of the llsvps one postulates that they are thermochemical piles caused by accumulation of subducted oceanic crust at the core mantle boundary cmb mineral physics experiments indicate that oceanic crust becomes denser than the surrounding mantle at lower mantle pressures in addition seismic observations provide evidence of subducted slabs arriving at the cmb however a major question pertains to whether subducted oceanic crust can survive viscous stirring associated with mantle plumes and accumulate into piles with the same spatial scale as llsvps we perform a set of high resolution convection calculations to examine this hypothesis by investigating the interaction of thin oceanic crust 6 km with mantle plumes our results show that as subducted oceanic crust is swept toward upwelling plume regions the majority of it is viscously stirred into the surrounding mantle only a small amount of oceanic crust may accumulate at the base of plumes but it is consistently entrained away into the plume at a rate equal to or greater than it is accumulated we find that it is difficult for subducted oceanic crust to accumulate into large thermochemical piles at the cmb

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

jurassic microplate movements and brachiopod migrations in the western part of the tethys abstract the paleobiogeography of the jurassic brachiopods of the alpine carpathian region and adjacent areas is discussed on the basis of the distribution of distinctive taxa the jurassic microplates of the western part of tethys and the present day terranes of the alpine carpathian region are outlined and their relationships are discussed the migration possibilities of the brachiopods in the jurassic tethys were controlled mainly by plate microplate movements and by changes in the oceanic current system the mediterranean microcontinent isolated from the european and african shelves by oceanic deep sea belts was the homeland of the mediterranean brachiopod province in the course of the jurassic the mediterranean microcontinent moved as part of the african plate away from europe the widening alboran ligurian penninic oceanic belt became a barrier preventing migration of brachiopods by the end of the middle jurassic the tisza microplate detached from europe and formed a stepping stone for brachiopod dispersal at about the same time the hesperian strait opened between the basins of the tethys and the central atlantic the opening of this strait resulted in a reorganization of the tethyan current system the westward flowing equatorial currents which made a turn in the western corner of tethys in the first half of the mesozoic now ran to the west through the hesperian strait via the central atlantic to the pacific this change produced a new pattern in brachiopod distribution the mediterranean fauna successfully invaded the european shelf at the same time the mediterranean province became impoverished

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

geochemistry and petrogenesis of the early cretaceous continental rift type volcanic rocks of the mecsek mountains south hungary abstract early cretaceous volcanic rocks basanite to phonolite from the mecsek mountains south hungary represent the products of late mesozoic extension related alkaline magmatism at the southern margin of the european plate two mafic groups have been distinguished ankaramite alkali basalt and na basanite phonotephrite phonolites could have been formed from the na basanitic magma by low pressure fractionation the major and trace element characteristics of the mecsek basalts are similar to those of alkaline basalts of other intraplate areas and have a st helena type oib affinity the mantle source of the mecsek volcanics could be similar to that proposed by wilson and downes 1991 as one of the mantle endmembers for extension related tertiary quaternary alkaline basalts in europe geochemical modelling indicates that the primary magmas of the na basanite series were formed by about 4 partial melting whereas ankaramites and alkali basalts originated by about 6 partial melting of a garnet peridotite source

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

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

chemical stratification in a two dimensional convecting mantle with magmatism and moving plates 1 a self consistent numerical model is presented for magmatism in a convecting mantle with moving plates mantle convection is modeled as a thermal chemical convection of binary eutectic material with newtonian rheology in a two dimensional rectangular box internally heated by radioactive elements viscosity is assumed to depend on stress history as well as temperature and pressure to self consistently reproduce moving plates the barrier effect of the solid solid phase transitions at depths around 660 km to convective flow across the 660 km phase boundary is also taken into account magmatism is modeled as a permeable flow of basaltic melt produced upon decompression melting of the convecting material through the coexisting matrix magmatism makes mantle chemically stratified with the shallower part occupied by chemically buoyant residue of magma and the deeper part occupied by hot but chemically dense materials enriched in basaltic component even under the influence of moving plates and subducting slabs the magmatism is induced by moving plates ridge volcanism mantle overturn and hot plumes uprising from the 660 km phase boundary or the lower mantle depending upon the internal heating rate and the strength of the 660 km barrier a weaker 660 km barrier leads to more continuous plate tectonics when plates move subducting slabs penetrate deep into the lower mantle and induce broad thermal and chemical heterogeneity at depth in the lower mantle the chemically stratified mantle with moving plates is compared to the earth s mantle

deformation crystal preferred orientations and seismic anisotropy in the earth s d layer abstract we use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms dislocation glide twinning in mgsio3 post perovskite ppv and periclase mgo at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations cpo development and seismic anisotropy in d we model the cpo evolution in aggregates of 70 ppv and 30 mgo submitted to simple shear axial shortening and along corner flow streamlines which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core mantle boundary cmb within d or between cmb parallel flow and upwelling at the borders of the large low shear wave velocity provinces llsvp in the lowermost mantle axial shortening results in alignment of ppv 010 axes with the shortening direction simple shear produces ppv cpo with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant 100 010 slip system and the macroscopic shear these predictions differ from mgsio3 post perovskite textures formed in diamond anvil cell experiments but agree with those obtained in simple shear and compression experiments using cairo3 post perovskite development of cpo in ppv and mgo results in seismic anisotropy in d for shear parallel to the cmb at low strain the inclination of scs sdiff and skks fast polarizations and delay times vary depending on the propagation direction at moderate and high shear strains all s waves are polarized nearly horizontally downwelling flow produces sdiff scs and skks fast polarization directions and birefringence that vary gradually as a function of the back azimuth from nearly parallel to inclined by up to 70 to cmb and from null to 5 change in the flow to shear parallel to the cmb results in dispersion of the cpo weakening of the anisotropy and strong azimuthal variation of the s wave splitting up to 250 km from the corner transition from horizontal shear to upwelling also produces weakening of the cpo and complex seismic anisotropy patterns with dominantly inclined fast scs and skks polarizations over most of the upwelling path models that take into account twinning in ppv explain most observations of seismic anisotropy in d but heterogeneity of the flow at scales

equations of state of na k al host phases and implications for morb density in the lower mantle abstract we report equations of state eos of two important silicate phases known to store na k and al at pressures and temperatures prevailing in the earth s lower mantle using laser heated diamond anvil cell we produced the ca ferrite type cf and the new aluminum rich nal phases using different synthetic and natural starting materials at pressures ranging from 23 to 65 gpa unit cell volumes were investigated by means of angle dispersive x ray diffraction at the id30 beamline of the esrf synchrotron ring grenoble france cf phases were synthesized in naalsio4 mgal2o4 and naalsio4 mg fe 2sio4 systems and for a natural mid ocean ridge basalt morb the insertion of mgal2o4 in the naalsio4 based lattice has reduced effects on the cf phase eos whereas that of mg fe 2sio4 is mainly to increase the room pressure volume cf phases most relevant to the earth s mantle show compression behavior comparable to the pure naalsio4 cf phase with bulk moduli values close to k0 190 gpa and k0 4 we also synthesized nal phases in naalsio4 mgal2o4 system and using cana and k rich silicate glasses as starting materials all nal compounds investigated show similar compression behavior except for one anomalously naalsio4 rich composition for nal phase with the most natural conditions relevant composition we refine bulk moduli of k0 182 gpa and k0 6 we then model the density profiles expected for the cf and nal phases using our newly determined eos and the most accurate chemical compositions reported in the literature these two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 k however the density profiles calculated for mineral assemblages corresponding to typical morb compositions point out to denser morbs compared to the pyrolite this is due to the high fe content in morb yielding a densest fe rich perovskite that counterbalances the lower densities of the cf and nal phases the density gap is calculated between 0 4 and 4 0 depending on the starting morb composition and pressure

thermochemical structures within a spherical mantle superplumes or piles 1 heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology in particular two regions in the earth s lower mantle under the pacific and africa are often interpreted as being either piles or superplumes of dense material we perform numerical modeling of thermochemical convection in a three dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated rounded piles or superplumes of material we study the effect of temperature and compositionally dependent viscosity and we find two different modes of dense layer deformation temperature dependent rheology leads to a low viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle the addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material forming large isolated superplumes our results indicate that piles and superplumes are separate features which in general do not occur together and in order for isolated rounded superplumes to form an intrinsic compositional viscosity increase is required for the dense material

the mid hungarian line a zone of repeated tectonic inversions abstract the mid hungarian line is a major tectonic feature of the intra carpathian area separating two terranes of different origin and tectonic structure although this tectonic line was known from borehole records it has not been described in seismic sections the study presents interpreted seismic lines crossing the supposed trace of the mid hungarian line these seismic sections show north dipping normal faults and thrust faults as well as cross cutting young strike slip faults a complex tectonic history is deduced including intra oligocene early miocene thrusting middle miocene extension local late miocene inversion and late miocene pliocene normal faulting and left lateral wrenching in the light of our seismic study we think that the best candidate for the mid hungarian line is a north dipping detachment fault beneath large masses of neogene volcanics the auxiliary structures to the north seen on seismic sections suggest that it moved as a south vergent thrust fault during the palaeogene early miocene which later was reactivated as a set of normal faults the northern alcapa unit overrode the southern tisza dacia unit along this fault zone the same relative positions are observed in the northern termination of the line other structures along the supposed trace of the line are north dipping normal or strike slip faults which frequently were reactivated as smaller thrust faults during the late neogene palaeogene early miocene thrusting along the line might be the result of the opposite tertiary rotations of the two major units as suggested by palaeomagnetic measurements and earlier models

radial 1 d seismic structures in the deep mantle in mantle convection simulations with self consistently calculated mineralogy 1 numerical thermo chemical mantle convection simulations in a spherical annulus geometry with self consistently calculated mineralogy and mineral physics are used to predict detailed deep mantle seismic structures particularly local radial profiles of shear wave velocity vs and bulk sound velocity vb the predicted structures are compared to seismological observations and used to guide the interpretation of seismic observations and to test the model the mantle composition is described as a mixture of morb mid oceanic ridge basalt and harzburgitic end members in the na2o cao feo mgo al2o3 sio2system to assess the influence of chemical variability four different sets of end member compositions are evaluated results confirm that the post perovskite ppv phase causes anti correlated s wave and bulk sound velocities in the deep mantle due to ppv being fast in vs but slow in bulk sound velocity local 1 d seismic profiles display great lateral variability and often have multiple discontinuities in the deep mantle due to morb layers in folded slabs with a positive vs anomaly and negative bulk sound anomaly or the perovskite ppv phase transition the ppv transition is not visible inside piles of segregated morb because of the high temperature and small velocity contrast of ppv in morb piles of segregated morb are seismically slow in both vs and bulk sound despite being expected to be fast in vs because they are hotter than the surrounding material anelasticity has a significant influence on profiles of vs only in the lower thermal boundary layer which corresponds to below 2600 to 2800 km depth depending on region where temperatures are higher than the extrapolated adiabat these results indicate the importance of using a joint geodynamical mineralogical approach to predict and aid in the interpretation of deep mantle seismic structure because interpretations based on seismology and mineral physics alone may be misleading and do not capture the strong lateral variability in 1 d structure obtained here for example multiple reflections arising from folded slabs and the precise balance between thermal and compositional influences on seismic structure

spatial and temporal distributions of exotic and local obsidians in central western patagonia southernmost south america abstract central western patagonia cwp is a key area for assessing long distance procurement of high quality obsidians throughout the holocene given that almost all relevant types represented in the archaeological record are exotic to this region by using surface and stratigraphic obsidian artifacts from archaeological sites compared to standards from known sources in patagonia this paper discusses the spatial and temporal distribution of this lithic material sampling was oriented to assemblages from deposits with radiocarbon based time frames 10 700 300 cal bp this paper presents geochemical icp ms analyses of 178 samples from 58 archaeological sites at 11 surveyed areas located along the pacific coast the andean forest and eastern steppe out of six potential sources the chaiten volcano source los lagos region chile dominates exclusively the occurrence of obsidians along the coastal fringe while the pampa del asador source pda santa cruz province argentina largely dominates 86 of samples obsidian in the eastern steppe and the forest steppe ecotone this broad distribution is explained by the presence of the densely forested andean mountain range acting as a biogeographical barrier east of the andes we recorded an absolute dominance of pda south of 45 30 s while more variability prevailed north of this point the highest diversity of obsidians was recorded in the cisnes river valley probably because it is located closer to other alternative northern sources telsen sierra negra sacanana and angostura blanca all in chubut province argentina and because it also hosts a local low quality obsidian type based on this distribution we discuss obsidian procurement behaviors by considering obsidian frequency and tool debitage class representation with increasing distance we use the analysis of fall off curves based on the distance of studied locations from the sources and include the use of least cost paths for providing the most likely procurement routes no obsidian diversification was recorded during the holocene hence the main driver for its procurement seems to be the distance from the source rather than the antiquity of its knowledge alternative procurement behaviors are discussed specifically direct acquisition exchange and or sporadic visits as mechanisms for explaining the archaeological patterns throughout the holocene

problems of the pannonian median massif and the plate tectonic concept contributions based on the distribution of late paleozoic early mesozoic isopic zones after a short historical review of conceptions about the pannonian median massif or tisia the distribution of middle upper carboniferous and lower triassic liassic facies is briefly examined on the two sides of the zagreb zemplin line dividing into two main segments the basement of the carpathian basin it shows on the one hand that the igal bukk zone igal bukk eugeosyncline ofwein 1969 thought to establish a connection between the nw dinarides and the dinaric type bukkium is only a tectonic zone and could never have been a paleogeographical unit on the other hand the northern marginal part of tisia mecsek bihor crystalline belt and its sedimentary cover dank b it was rather part of the northern northeastern marginal complex of tethys therefore allochthonous models suggesting largerscale horizontal movements give a much more plausible explanation the style of late hercynian development and the distribution of lower triassic liassic facies zones suggest that tisia was part of the northern northeastern shelf of the tethys until the end of the liassic in accordance with the opinion ofbleahu then split off and was wedged in by horizontal movements between the nw dinarides and the dinaric type bukkium by the end of the jurassic beginning of the cretaceous

the interaction between the post perovskite phase change and a thermo chemical boundary layer near the core mantle boundary numerical simulations of thermo chemical mantle convection with the newly discovered post perovskite ppv phase change are used to characterize its dynamical consequences for the dq region which has frequently been interpreted as being thermo chemical in origin a strong lateral variation in the appearance of the ppv region is observed with high cmb temperatures leading to bdouble crossingq of the phase boundary in average or cold regions or no crossing in hot regions lower cmb temperatures result in a single crossing everywhere but with highly variable depth the strongly exothermic nature of the post perovskite phase change induces an anti correlation between regions containing a thick ppv layer and piles of chemically dense material formed by the segregation of the crustal component from subducted slabs the ppv phase change destabilizes chemically dense material allowing subducted slabs to easily penetrate into a dense layer forming isolated patches of the post perovskite phase whereas the lateral spectrum of thermal anomalies in the deep mantle is dominated by long wavelengths the lateral spectrum of composition is relatively flat and is the dominant contributor to seismic velocity variations at all wavelengths except the very longest ones d 2005 elsevier b v all rights reserved

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

box modeling the chemical evolution of geophysical systems case study of the earth s mantle geochemical measurements have been widely used for understanding geophysical dynamic systems fluid mechanics and box models are quantitative tools for testing the reliability of these interpretations we present here the connection between these two methods especially in the case of the earth s mantle we show that box models implicitly assume a chemical diffusivity inversely proportional to the number of boxes from fluid dynamics considerations we suggest that at least 15 boxes should be used to model the mantle then we compare the results of a simple convective geochemical model with box models to illustrate a way to incorporate dynamical constraints in them