asymmetric phase effects and mantle convection patterns recent high pressure experiments and thermodynamic calculations have shown that the clapeyron slope of the spinel perovskite phase transition at a depth of 660 kilometers in the earth s mantle changes from negative to positive at temperatures above 1700 to 2000 c in numerical experiments that account for this phase behavior cold downwelling flows were impeded at the phase boundary but hot plumes ascended to the upper mantle with ease the resultant mantle convection was partially layered and strongly time dependent mantle layering was weaker when the mantle was hotter and when the rayleigh number was larger

three dimensional mantle dynamics with an endothermic phase transition three dimensional convection for the spinel to perovskite phase change has been simulated numerically results for rayleigh ra numbers of 0 106 show intermittent layering with a strong robust plume rising through the phase boundary many descending instabilities are deflected but merging cold sheets come together at a junction a pool of cold material accumulates underneath in the phase transition zone a strong gravitational instability results which precipitates a rapid and massive discharge of upper mantle material

initial pb sr nd isotopic heterogeneity in a single allanite epidote crystal implications of reaction history for the dating of minerals with low parent to daughter ratios we analyzed 17 fragments from a zoned allanite epidote crystal ca 2 2 mm 4 0 mm which had formed during different prograde and retrograde stages of ultra high pressure uhp and amphibolite facies metamorphism 240 230 ma sulu belt e china for the isotopic composition of pb nd and sr and contents of pb u and th sr and rb and nd and sm since most fragments had 238u 204pb and 232th 204pb values less than 1 corrections for in situ pb growth are small and uncertainties in the recalculation of the pb isotopic compositions to 240 ma are insignificant the recalculated pb falls on a linear trend in the 206pb 204pb vs 207pb 204pb diagram with the allanite defining the low 206pb 204pb end 17 07 of this trend and the epidote defining its high 206pb 204pb end 17 56 the recalculated data scatter in the 206pb 204pb vs 208pb 204pb diagram which implies that the initial pb isotopic variation reflects the involvement of at least three different pb sources the low 87rb 86sr values account for a change in 87sr 86sr by in situ 87sr growth of less than 0 0007 which implies that the isotopic heterogeneity of 87sr 86sr 0 70601 0 7200 is a primary feature the pb and sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product it is likely that such an initial isotopic heterogeneity also exists for nd but it could not be resolved in the present study initially heterogeneous pb and sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates especially for minerals with relatively low parent to daughter ratios

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

the interaction of plumes with the transition zone under continents and oceans abstract the dynamics of mantle flow with phase transitions can be influenced by the presence of a thick continental lithosphere especially for the 400 km phase transition we have studied the effects of the continental lithosphere on the mantle convection with the two major phase transitions at 400 km and 660 km a 2 d model with an aspect ratio 8 box was used in which the surface was divided into two domains oceanic and continental the continental lithosphere is modeled by a highly viscous 200 km thick lid which is 100 times stiffer than the oceanic part we also consider the consequences of plate reconfiguration by switching instantaneously the underlying rheological structure of the oceanic and continental domains a depth dependent viscosity with an additional jump at 660 km depth was employed other depth dependent properties employed are the thermal expansivity and the thermal diffusivity both internal and basal heating configurations were studied our results show that there are distinct differences in the mode of interaction of plumes with the transition zone under continents and oceans much larger stationary plumes are developed in the lower mantle under continents and erupt from time to time into the upper mantle creating a very hot continental upper mantle plumes under oceans are weaker but produce fast horizontal jets when the plumes impinge upon the surface very large plumes are formed in the internally heated cases because of the presence of depth dependent properties in the lower mantle creating a thick bottom boundary layer this thick thermal boundary layer is swept upward under the continents by a large scale flow in the lower mantle these results can be important for the interpretation of seismic tomography under africa and the central pacific

thermal expansion measurements at very high pressure systematics and a case for a chemically homogeneous mantle new data on thermal expansivity of iron to over 400 kbar in laser and electrically heated diamond anvil cells and on mgo and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats boehler 1982 imply that at high pressure may be described by a constant value of 1n 1nv t 5 5 0 5 this behavior is supported by previous measurements at lower pressures on a large variety of materials densities of magnesio wustite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and at high pressures using the above systematics match those in prem through the entire lower mantle making a strong case for a uniform mantle

three dimensional forward and backward numerical modeling of mantle plume evolution effects of thermal diffusion we investigate the effects of thermal diffusion on the evolution of mantle plumes by means of three dimensional numerical modeling forward and backward in time mantle plumes are fed by a hot low viscous material from the thermal boundary layer the material of the plumes is mainly advected toward the earth s surface with some effects of thermal diffusion however the feeding can become weaker with time and then thermal diffusion can take over and control the evolution of the plumes numerical experiments forward in time show that a week feeding of mantle plumes by the hot material from the boundary layer results in the diffusive disappearance of plume tails first and plume heads later this is the most likely explanation for the seismically detected low velocity mantle structures mantle plumes with prominent heads and almost invisible tails at midmantle depths we develop restoration models backward in time to recover strong features of mantle plumes in the geological past after they have dissipated due to thermal diffusion and analyze effects of thermal diffusion and temperature dependent viscosity on the reconstruction of the mantle plumes we investigate the impact of thermal diffusion on the performance of our restoration variational data assimilation algorithm for a given range of rayleigh number ra and two values of the viscosity ratio r between the upper and lower boundaries of the model domain we show that 1 the residuals between the temperature predicted by the forward model and that reconstructed by the backward modeling become larger and 2 the restoration process becomes poorer as ra decreases and r increases we assimilate temperature obtained from high resolution seismic tomography data for the southeastern carpathians and show that present diffused mantle structures can be restored to their prominent state in the miocene times we discuss the problems of smoothness of model input and output data errors associated with the modeling and some other challenges in the data assimilation for thermoconvective flow in the mantle

instabilities from phase transitions and the timescales of mantle thermal evolution we have investigated the effects of mantle phase transitions on the earth s thermal evolution by numerical modelling our results show that the tendency of layering grows with increasing vigor of convection a self consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed for a temperature dependent effective viscosity we found that the thermal history of the earth is characterized by two timescales one is associated with layered convection in the early period which is followed by a transition period with dramatic overturns as the rayleigh number decreases sufficiently low whole mantle convection prevails and a faster cooling timescale ensues

early paleozoic sedimentation in reelfoot rift abstract analysis of subsurface data from deep tests drilled in the northern mississippi embayment and southern mid continent suggests that earliest paleozoic sedimentation was dominated by the tectonic evolution of the reelfoot rift throughout most of the mid continent the upper cambrian lamotte mt simon sandstone rests nonconformably on precambrian basement and is overlain by the bonneterre eau claire formation however in the area of the reelfoot rift both the lamotte and bonneterre grade into thick basinal shales that locally display evidence of episodic deposition of coarse clastics perhaps on submarine fans moreover two major sedimentary units are present beneath the lamotte bonneterre basinal facies within the reelfoot rift immediately underlying the lamotte bonneterre shale is a carbonate stratum probably dolomite that thickens to more than 1 000 ft 300 m along the axis of the basin in eastern arkansas underlying this carbonate is a detrital unit that grades from arkosic sandstone near the northern terminus of the basin to a basinal shale southward this basinal shale is at least several hundred feet thick near the axis of the basin these two strata occupy the stratigraphic position of the conasauga middle cambrian and rome lower cambrian formations of the southern appalachians the axial and transverse distribution of these strata suggests that the reelfoot evolved as paired grabens or half grabens during the early and middle cambrian subsequently the reelfoot remained the axis for more widespread subsidence and sedimentation throughout much of the paleozoic end of article last page 1456

solitary waves in mantle plumes numerical calculations in two dimensions show that solitary wave disturbances can propagate along thermal plumes in a homogeneous viscous fluid with a thermally activated rheology similar to the mantle comparison of the numerical results with analytic results from an idealized two fluid model indicates that the two fluid model can be applied to solitary waves in the thermally activated plume a two fluid model of solitary waves on cylindrical conduits is accordingly applied to mantle plumes to estimate the propagation speeds time durations and pulse lengths of solitary waves in mantle plumes as a function of background mantle viscosity plume flux plume density deficit plume viscosity and the volume of material transported by the solitary wave it is inferred that mantle plumes may be episodic via solitary waves and that these disturbances might account for observed 0 10 m y variations in the durations and spacings of episodes of enhanced hotspot volcanic activity solitary waves in mantle plumes could be generated by interactions among plumes and between plumes and the large scale time dependent mantle circulation

on the fate of mantle plumes at density interfaces abstract the motion of plumes through compositionally layered systems is investigated by laboratory experiments in order to understand the ascent modes of deep mantle plumes experimental plumes entrain the ambient heavier fluid and the average density of the plume head p t increases with time the degree of entrainment depends on the viscosity ratio of the matrix fluid matrix to the buoyant fluid buoyant and the mass flux two types of plume penetration through the density interface are possible the first which can be characterized as a pass through mode ptm exhibits relatively little entrainment here the whole plume head passes directly through the interface because p t is smaller than the density of the upper layer u in contrast in the second type of interaction named the rebirth mode rbm a large entrainment effect takes place the plume is now greatly disturbed as it passes through the interface between the two fluid layers and entrained material remains at the density interface since p t u in addition a new diapir emerges from the plume conduit and rises through the upper layer the difference between these two types of interaction is reflected in the spatial and the temporal characteristics of geochemical data obtained from mantle plume products such as ontong java plateau ojp

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

mantle convection with internal heating and pressure dependent thermal expansivity recent laboratory work 1 suggests that the thermal expansivity of the mantle decreases strongly with pressure determines the buoyancy of plumes and the rate at which plumes lose their thermal signature as they rise and so may be expected to have a strong influence on the temperature and velocity structure of the mantle numerical simulations were conducted on convection in an internally heated compressible mantle including constant and pressure dependent and thermal conductivityk they show that a pressure dependent allows the existence of thermal plumes rising from the core mantle boundary cmb however the plumes are weak and dissipate higher in the mantle

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

mantle plume interaction with an endothermic phase change high spatial resolution numerical simulations of mantle plumes impinging from below on the endothermic phase change at 660 km depth are used to investigate the effects of latent heat release on the plume phase change interaction both axisymmetric and planar upflows are considered and the strong temperature dependence of mantle viscosity is taken into account for plume strengths considered a clapeyron slope of 4 mpa k 1 prevents plume penetration of the phase change plumes readily penetrate the phase change for a clapeyron slope of 2 mpa k 1 and arrive in the upper mantle considerably hotter than if they had not traversed the phase change for the same amount of thermal drive i e the same excess basal temperature axisymmetric plumes are hotter upon reaching the upper mantle than are planar upwellings heating of plumes by their passage through the spinel perovskite endothermic phase change can have important consequences for the ability of the plume to thermally thin the lithosphere and cause melting and volcanism

topography of the 400 km discontinuity from observations of long period p400p phases summary new evidence is presented for lateral variation of the depth of upper mantle discontinuities this evidence is derived from an analysis of amplitude of p400p phases underside reflections from the 400 km discontinuity for great circle paths with bounce points in the upper mantle near the hawaii emperor hotspot chain the data are long period from wwssn sro and rstn stations the epicentral distance ranges from 95 to 115 which ensures that no other major phases arrive in the time window of interest observed p400p amplitudes are compared with synthetic amplitudes for both a laterally homogeneous model and models including 3 d topography on the discontinuities the observations permit models of topography that can be characterized by undulations of the 400 km discontinuity with wavelengths in the range 600 2000 km and peak to peak amplitude of the order of 30 40 km such long wavelength structure causes quite strong de focusing of pdp phases synthetic pdp waveforms reveal that topography not only affects its amplitude but also smears out the pdp wave causing overlap between different pdp phases this mechanism is proposed as the reason for the lack of well developed pdp phases reported in the literature the fact that the waveforms can be seriously distorted prohibits the use of pdp phases in a waveform inversion the curvature of the discontinuity can be explained as the signature of convection currents crossing the discontinuity although the data cover the hawaii hotspot no signature of a plume structure can be detected in the data probably because the resolving power of the data is too low

effects of multiple phase transitions in a three dimensional spherical model of convection in earth s mantle numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three dimensional flow pattern with cylindrical features and linear features that behave differently in their ability to penetrate the 670 km discontinuity the dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth resulting in frequent avalanches of upper mantle material into the lower mantle the effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670 km phase change resulting in smaller and more frequent avalanches and a wider range of morphologies large quantities of avalanched cold material accumulate above the coremantle boundary cmb resulting in a region of strongly depressed mean temperature at the base of the mantle the 670 km phase change has a strong effect on the temperature field with three distinct regions being visible 1 the upper mantle containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum 2 the midmantle containing quasi cylindrical avalanche conduits and characterized by a low amplitude broad spectrum and 3 the deep mantle containing large pools of cold avalanched material and characterized by a high amplitude ultra red i e long wavelength spectrum the effect on the velocity field is very different flow penetration across the 670 km phase change is strongly wavelength dependent with easy penetration at long wavelengths but strong inhibition at short wavelengths thus when comparing numerical models with long wavelength seismic tomography diagnostics based on the density field such as the radial correlation function are much more sensitive to the effects of phase transitions than those based on the velocity field the amplitude of the geoid is not significantly affected by the partial layering because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670 km discontinuity avalanches are associated with geoid lows however a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in earth

cascade model for fluvial geomorphology summary erosional landscapes are generally scale invariant and fractal spectral studies provide quantitative confirmation of this statement linear theories of erosion will not generate scale invariant topography in order to explain the fractal behaviour of landscapes we have introduced a modified fourier series that is the basis for a renormalization approach we have introduced a non linear dynamical model for the decay of the modified fourier series coefficients that yield a fractal spectra we argue that a physical basis for our approach is that a fractal or nearly fractal distribution of storms floods continually renews erosional features on all scales

various influences on plumes and dynamics in time dependent compressible mantle convection in 3 d spherical shell the effects on the dynamics of compressible mantle convection from increasing rayleigh number viscosity stratification and internal heating are investigated in a time dependent 3 d spherical shell model resolution of up to spherical harmonics of degree 192 and 150 higher order finite difference grid points unevenly distributed in the radial direction have been employed surface rayleigh number varies from 5 104 to o 107 the viscosity profiles used range from constant to a depth dependent viscosity profile the adams williamson adiabatic assumption is adopted for describing the equation of state both depth dependences of thermal expansivity and thermal conductivity are included we have focused on the viscosity stratification in the lower mantle the mean temperature profiles are very sensitive to the input parameters such as the amount of internal heating and the dissipation number in the isoviscous model the number of hot plumes decreases for low rayleigh number 5 104 and increases dramatically with higher rayleigh number 106 in the model with a high viscosity zone in the lower mantle the number of plumes is small and does not change too much when the rayleigh number varies from 3 106 to 8 106 in the purely base heated viscously stratified models well organized triple junctions of the downwelling sheets are developed near the surface and interconnected cold walls of downwellings are produced around upwelling plumes the presence of internal heating increases the number of upwelling plumes and breaks up the interconnection of downwelling sheets strong viscous heating in the compressible solutions is found near the top and bottom at the higher values of rayleigh number the laterally averaged value of viscous dissipation is comparable with the chondritic strength of internal heating whether the maximum viscous heating is found in upwelling plumes or downwelling flows depends on the presence of internal heating and the mechanical boundary conditions the effects of compressibility on local structures and plume morphologies can be significant however these compressible effects are moderate on the global patterns

melting temperature of the earth s mantle and core earth s thermal structure abstract although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs the temperature distribution in the lower mantle is poorly known models depend strongly on the assumptions of the amount of internal heating and the viscosity profile new melting data on iron to pressures of the outer core 2 mbar and the observed strong decrease of eutectic melting depression in the fe feo fes system with increasing pressure however tightly constrain the temperature at the inner core boundary to slightly less than 5000 k this estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements the resulting temperature at the core mantle boundary of about 4000 k then requires a large temperature gradient at the bottom of the lower mantle of about 1500 k recent findings of the very high melting temperatures o

the effects of temperature dependent viscosity on mantle convection with the two major phase transitions abstract we have studied the consequences of including temperature dependent viscosity in mantle convection with both of the major phase transitions included we have employed a two dimensional cartesian model in a box of aspect ratio four the time dependent equations have been cast within an extended boussinesq framework in which each of the phase changes has been simulated by an enhanced thermal expansivity both latent heat release and viscous dissipation have been considered two different forms of temperature dependent viscosity the arrhenius and the frank kamenetzky forms have been employed we have considered both models with a constant temperature equilibrium boundary condition at the core mantle boundary and models with a time varying non equilibrium temperature boundary condition from cooling of the core the effects of temperature dependent viscosity are to enhance the tendency of the convective system to be layered because of the influences of the latent heat release and the mechanical heating on creating a sharp viscosity drop across the transition zone we have found that there is a non linear coupling between latent heat release from the interaction of the hot plume with the endothermic phase transition and the temperature dependent viscosity this positive feedback process can give rise to the production of very high temperatures in the transition zone which may cause melting in the region between the two major phase transitions flushing events produced by the non equilibrium boundary condition are much more dramatic than those generated by the equilibrium boundary condition the non linear coupling between latent heat release and temperature dependent viscosity is also stronger in the non equilibrium models comparison with models without the presence of latent heat release and viscous dissipation shows that the models without the heating terms produce a greater degree of mass flux across the endothermic phase boundary because of the lack of the viscosity stratification in the interior of the convective flow results of this moded can be applied to other types of endothermic phase transition found in the lithosphere or to the spinel perovskite transition in the deep portion of the martian mantle

mechanism for generating stagnant slabs in 3 d spherical mantle convection models at earth like conditions abstract seismic tomography reveals the natural mode of convection in the earth is whole mantle with subducted slabs clearly seen as continuous features into the lower mantle however simultaneously existing alongside these deep slabs are stagnant slabs which are if only temporarily trapped in the upper mantle previous numerical models of mantle convection have observed a range of behavior for slabs in the transition zone depending on viscosity stratification and mineral phase transitions but typically only exhibit flat lying slabs when mantle convection is layered or trench migration is imposed we use 3 d spherical models of mantle convection which range up to earth like conditions in rayleigh number to systematically investigate three effects on mantle dynamics 1 the mineral phase transitions 2 a strongly temperature dependent viscosity with plastic yielding at shallow depth and 3 a viscosity increase in the lower mantle first a regime diagram is constructed for isoviscous models over a wide range of rayleigh number and clapeyron slope for which the convective mode is determined it agrees very well with previous results from 2 d simulations by christensen and yuen 1985 suggesting present day earth is in the intermittent convection mode rather than layered or strictly whole mantle two calculations at earth like conditions ra and ra h 2 i 10 7 and 5 i 10 8 respectively which include effects 2 and 3 are produced with and without the effect of the mineral phase transitions the first calculation without the phase transition successfully produces plate like behavior with a long wavelength structure and surface heat flow similar to earth s value while the observed convective flow pattern in the lower mantle is broader compared to isoviscous models it basically shows the behavior of whole mantle convection and does not exhibit any slab flattening at the viscosity increase at 660 km depth the second calculation which includes the phase transitions successfully exhibits the coexisting state of stagnant and penetrating slabs within the transition zone these results indicate the importance of both a viscosity increase and mineral phase transitions for generating the structure of stagnant slabs observed by seismic tomography

intershield geochemical differences among hawaiian volcanoes implications for source compositions melting process and magma ascent paths as hawaiian volcanoes develop their lavas systematically change in composition and isotopic ratios of sr nd and pb these trends provide important constraints for understanding plume related volcanism as a volcano migrates away from the hotspot there are also geochemical differences between hawaiian shields in particular lavas from adjacent shields such as kilauea and mauna loa on hawaii and koolau and waianae on oahu have significant differences in abundances of some major and incompatible elements and isotopic ratios of sr nd and pb some incompatible element abundance ratios such as zr nb and sr nb are correlated with intershield differences in sr and nd isotope ratios but these isotopic ratios are not correlated with intershield differences in major element composition or even parent daughter abundance ratios such as rb sr and sm nd moreover at kilauea and mauna loa the intershield differences have apparently persisted for a relatively long time perhaps 100 ka these intershield geochemical differences provide important constraints on plume volcanism specifically i each volcano must have distinct magma ascent paths from the region of melt segregation ii the 25 50 km distance between adjacent but geochemically distinct shields requires that the sources vary on a similar scale and that the melt production region is similarly restricted the absence of correlations between lava compositions and radiogenic isotope ratios provides evidence for significant differences in melting process such as each shield forming by a different mean extent of melting with melt segregation at different mean pressures two types of models are consistent with the intershield geochemical differences i a relatively large radius ca 40 km plume conduit with a systematic spatial distribution of geochemical heterogeneities or ii a small radius less than 20 km plume conduit composed of geochemically distinct diapirs because relatively small radius diapirs of limited vertical extent are too small to create the large hawaiian shields a possible alternative is a continuous conduit containing solitary waves which transport geochemically distinct packets of material

the effects of shallow rheological boundaries in the upper mantle on inducing shorter time scales of diapiric flows the authors report on a study of pulsating diapiric flow in the upper mantle they study the effect of introducing new phase boundaries in the mantle on such flow basic models have an interface at a depth of 670 km in the rheological properties the authors study the effect of introducing another phase boundary at a depth close to 400 km

the effects of adiabatic and viscous heatings on plumes we have assessed the influence of viscous and adiabatic heatings on the transient development of plumes by employing the time dependent convection equations in the extended boussinesq limit both temperature dependent and temperature and pressure dependent viscosities are employed for constant thermal expansion of coefficient we find the vigor of plume flows is diminished with larger magnitudes of the thermal expansivity this effects is caused by the local domination of adiabatic decompression over viscous heating inside the developing plume for both the cartesian and axisymmetric domains for deep mantle flows a thermal expansivity decreasing with depth is required for thin plume structures to exist values of the thermal expansivity contrast which can promote plume growth are in accord with estimates from recent studies in seismic equation of state copyright american geophysical union 1987

thermally induced phase changes lateral heterogeneity of the mantle continental roots and deep slab anomalies pressure induced solid solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle lateral variations in temperature cause a similar effect abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion temperature induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure induced phase changes in cold slabs the equilibrium mineral assemblage contains high density high velocity phases which are not stable in hotter mantle in particular the ilmenite form of mgsio 3 and the spinel form of mg 2sio 4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling recent arguments for slab penetration into the lower mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature induced phase changes when these effects are taken into account the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection the seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions and this is due to the presence of a melt phase and lower velocity high temperature phase assemblages deep 200 km long lived continental roots differing in chemistry from normal mantle are not required when isobaric phase changes are taken into account high velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high temperature phase assemblages the whole mantle convection thick continental root and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition the lack of correlation of the geoid with ridges shields heat flow and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle consistent with layered convection

interaction of the upwelling plume with the phase and chemical boundary at the 670 km discontinuity effects of temperature dependent viscosity abstract numerical simulations have been performed to investigate the interaction of upwelling plumes with the 670 km discontinuity temperature dependent rheology is employed to introduce the low viscosity and strength of the hot plume the 670 km discontinuity is taken as a phase and or chemical boundary the condition for penetration of the plume into the upper mantle is examined by varying the value of the clapeyron slope of the phase transition and the compositional density difference of the chemical boundary possible styles of the mantle convection are classified into six types depending on the nature of the 670 km discontinuity the plume in the lower mantle can penetrate into the upper mantle and reach the bottom of the lithosphere when the clapeyron slope is greater than 3 to 4 mpa k for the pure phase boundary and when the compositional density difference is smaller than 0 5 1 5 for the pure chemical boundary the plume can barely penetrate into the upper mantle when the 670 km discontinuity possesses even a small amount of the compositional density contrast less than 0 5 1 0 together with the phase transition of the experimentally determined clapeyron slope of 3 1 mpa k this result suggests that the mantle may be of uniform composition or at most has a weak compositional layering if the origin of the plume is in the deep mantle

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

three dimensional spherical models of layered and whole mantle convection we present numerical calculations of three dimensional spherical shell thermal convection for constant viscosity and stratified viscosity models of whole layer and two layer mantle convection these four examples are intended to provide theoretical guidance for determining the style of convection that is occurring in earth s mantle an impermeable interface between the upper and lower converting shells in the two layer solutions is placed at a depth of 670 km to coincide with the mantle seismic discontinuity that divides the upper and lower mantle the interface results in an internal thermal boundary layer that raises the mean temperature in the lower shell by about 1400 k compared to the whole layer solutions the patterns of convection in the upper part of the whole layer solutions are dominated by narrow arcuate sheetlike downflows in a background of weak upflow in contrast the upper shells of the two layer solutions have complicated networks of convective rolls with the upflows and downflows having very similar structure the structure of convection in the lower shells is similar to that in the lower part of the whole layer solutions an increase decrease in the viscosity in the lower upper shell decreases increases the convective velocity in the lower upper shell and increases decreases the horizontal scale in the lower upper shell the upper and lower shells are viscously coupled at the 670 km interface when viscosity is the same in the two shells however when viscosity in the lower shell is 30 times greater than that in the upper shell the coupling is partially viscous and partially thermal based on the horizontal structure of subduction zones on earth s surface and on tomographic images of temperature variations in earth s mantle we conclude that the style of convection in earth s mantle is more like that of the whole mantle models

development of diapiric structures in the upper mantle due to phase transitions solid state phase transitions in time dependent mantle convection can induce diapiric flows in the upper mantle when a deep mantle plume rises toward phase boundaries in the upper mantle the changes in the local thermal buoyancy local heat capacity and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir this mechanism may explain episodic hot spot volcanism the nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes allowing hot plumes to go through and retarding the tepid ones

three dimensional modelling of upper mantle structure under the pacific ocean and surrounding area summary long period 75 250 s fundamental mode love and rayleigh waves are inverted for the seismic velocity variations in the upper mantle under the pacific ocean and surrounding area the data are inverted to map the regional variation of phase velocity and then the velocity structure of the upper mantle no a priori regionalization is used in this study and the lateral heterogeneity is expanded horizontally into blocks lo x 10 to estimate the reliability of the velocity distribution we have calculated the resolving kernels in the central nw ne and sw regions the resolving kernels have fairly sharp peaks in the se region however resolution is not so good the results show good correlation with surface tectonics in the velocity map shields and ridges are the major features in the depth interval 50 100km anomalies associated with the east pacific rise and the shields persist to 300 km at 400 and 500 km high velocity anomalies appear under the eastern and southern pacific ridges and a low velocity anomaly appears beneath australia the present study provides better resolution than previous studies because of the better path coverage for example a large low velocity anomaly emerges under hawaii down to the depth of 200 km for sh velocity the horizontal extent of this anomaly is comparable to the sue of the hawaiian swell this feature supports that the hawaii hotspot is an isolated truly anomalous feature in the pacific ocean

the dependence of convection planform on mode of heating a fundamental property of a convecting fluid is its planform the distribution in the horizontal plane of hot rising regions and cold sinking regions for the earth s mantle the planform might he visualized as a map of subduction zones hotspots and possibly ocean ridges here i report numerical experiments of convection at high rayleigh number which show a strong dependence of planform on heating mode when heat generation is distributed uniformly through the box the preferred planform consists of an ensemble of time dependent cold axial sinkers distributed in a hot diffuse upward flow when half of the heat is generated within the box and the other half is input through the base the preferred planform consists of an array of hot axial plumes and elongated cold sheets in the former case the mean horizontal wavelength is about equal to the layer depth for the latter it is about twice the layer depth

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs