on the geometry of mantle flow beneath drifting lithospheric plates summary the convective flow pattern beneath the oceanic lithosphere is strongly influenced by the movement of the lithospheric plate on top assuming upper mantle convection the wavelengths geometric structure and time of onset for ridge perpendicular rolls as functions of the rayleigh number and the plate velocity have been investigated by numerical modelling the related gravity anomalies have been estimated and compared to observations from the se pacific which show ridge perpendicular anomalies with wavelengths of 150 220 km and amplitudes of about 10 mgal for constant viscosity the type of secondary circulation perpendicular to the diverging ridge depends on the relation between imposed plate velocity vs and maximum velocity in the free convection case vhfree if vs is less than vhfree the flow is dominated by off ridge up and downwellings if vs is about 10 times larger than vhfree the flow remains 2 d if vs is between these extrema a chain of ridge plumes develops from the lower thermal boundary layer with adjacent ridge perpendicular walls that lead to roll like circulation richter rolls throughout the model box the wavelength of these rolls is a function of the rayleigh number and the plate velocity for parameters realistic for the earth rolls with considerable amplitude first develop after more than 50 myr with wavelengths of at least 250 km for temperature and pressure depth dependent viscosity the wavelengths of the evolving rolls are determined by the rayleigh number related to the maximum viscosity in the interior of the convecting system and not to the average viscosity for models with a strong decrease in viscosity to values of about 1019 pa s in a 100 km wide asthenospheric channel below the lithosphere ridge perpendicular rolls which only fill the low viscosity layer appear after a few million years after a transitional stage of some tens of millions of years rolls in the whole upper mantle also evolve the asthenospheric rolls develop from instabilities of the upper thermal boundary layer and have wavelengths of about 220 280 km the time needed for alignment of these rolls to the direction of plate movement is a few million years and the distance from the ridge to the point of onset of the rolls is about 200 500 km both are functions of the plate velocity while the formation time for rolls in the whole upper mantle is quite large and their existence is related largely to two layered convection this type of flow might not be very relevant for the earth although it has been used to account for 3 d structures at oceanic ridges to explain the observed gravity anomalies in the se pacific roll like circulation in a low viscosity asthenospheric layer is the preferred candidate where the modelled amplitudes and wavelengths of gravity anomalies fit the observations

the effect of post pangea subduction on global mantle tomography and convection there is an excellent correlation between tomographic patterns and global tectonics in the upper 200 km of the mantle below this depth there is little relationship with present tectonic provinces most of the power in the tomography is in the longest wavelengths l 1 3 with l 2 generally dominant this is quite different from expectations based on mantle convection models the question then arises what causes the long wavelength variations mapped by seismic techniques we investigate the hypotheses that past locations of supercontinents and subduction may control temperature variations in the mantle we look for correlations between pangea and post pangeatic subduction and seismic velocity variations at various depths the best correlations with time integrated slab locations occur near the base of the uppermantle suggesting that slabs bottom out in the mesosphere the pangea hemisphere has a colder than average uppermantle probably due to circum and intra pangea subduction hotspots and ridges avoid regions cooled by subduction over the past 180 ma the presence of large continents at the surface and large areas of cold slab at depth explain the dominance of long wavelengths in the mantle s thermal structure despite the high rayleigh number

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

modeling the high frequency barotropic response of the ocean to atmospheric disturbances sensitivity to forcing topography and friction this study examines high frequency sea level variations forced by changes in surface atmospheric pressure and wind and their sensitivity to different forcing mechanisms bottom topography resolution and amount of friction in a barotropic ocean model optimal model performance defined in terms of the explained variance in satellite altimeter and bottom pressure data is found when using relatively strong friction equivalent to a damping timescale of only a few days over the deep ocean and topography with minimal smoothing spatial variations of the optimal friction parameter seem to reflect the roughness of bottom topography the model demonstrates skill in simulating the wind driven response as well as the nonequilibrium response to atmospheric pressure variations

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

geology and petrology of jasper seamount fifteen dredges on the summit and upper flanks of jasper seamount 122 44 w 30 27 n recovered a wide variety of lithologies including pillow lavas vesicular lapillistones from shallow submarine explosive volcanism and a range of xenoliths on the basis of dredge locations geochemical characteristics and 40ar 39ar age data three distinct phases of volcanism can be distinguished a shield building tholeiitic transitional phase flank transitional series fts followed by a flank alkalic series fas and a late stage summit alkalic series sas all three series consist exclusively of differentiated mg 54 to 21 mg mg2 mg2 fe2 compositions the fts represents a low pressure differentiation trend from tholeiitic transitional basalts to quartz normative residual liquids and probably accounts for more than 90 of the volume of jasper 40ar 39ar age data the dominant reversed polarity of jasper and a plausible duration 1 m y for shield construction suggest fts volcanism began about 11 ma and ended about 10 ma fts lavas probably erupted from a nw trending hotspot track parallel rift system the intermediate alkalinity fas lavas which probably comprise 3 8 of the volume of jasper erupted from 8 7 to 7 5 ma possibly after a brief volcanic hiatus or period of reduced eruptive activity normative projections suggest the fas lavas are the product of fractionation or equilibration at elevated pressures the hawaiites and mugearites of the sas erupted between 4 8 and 4 1 ma after a probable 2 7 m y period of volcanic quiescence and probably constitute 1 of the seamount volume a suite of xenoliths incorporated in sas lavas includes 1 tholeiitic basalt fragments from either the ocean crust or seamount interior 2 a range of differentiated gabbros largely derived from the ocean crust 3 residual mantle spinel lherzolites and 4 pyroxenite and peridotite cumulates the abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the sas suggests that these lavas probably fractionated in a magma chamber at the crust mantle boundary the occurrence of orthopyroxene bearing alkalic cumulate xenoliths in these lavas however is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates the sas vents of jasper define a ne sw volcanic trend which is orthogonal to the fts rift the pattern of volcanic activity including periods of volcanic quiescence and the general increase in alkalinity as well as the structural reorganization of magmatic feeder systems of jasper seamount is strikingly similar to the patterns observed on hawaiian volcanoes thus our data from jasper 690 km3 extend the concepts of structural and petrological evolution of hotspot volcanoes based on hawaii to moderate sized seamounts

seismic imaging of transition zone discontinuities suggests hot mantle west of hawaii the hawaiian hotspot is often attributed to hot material rising from depth in the mantle but efforts to detect a thermal plume seismically have been inconclusive to investigate pertinent thermal anomalies we imaged with inverse scattering of ss waves the depths to seismic discontinuities below the central pacific which we explain with olivine and garnet transitions in a pyrolitic mantle the presence of an 800 to 2000 kilometer wide thermal anomaly tmax 300 to 400 kelvin deep in the transition zone west of hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward hawaii and perhaps other islands this implies that geochemical trends in hawaiian lavas cannot constrain lower mantle domains directly

mantle upwellings above slab graveyards linked to the global geoid lows the global geoid is characterized by a semi continuous belt of lows that surround the pacific ocean including isolated minima in the indian ocean ross sea and northeast pacific and west atlantic oceans these geoid lows have been attributed to mesozoic subduction geodynamic models that include slab graveyards in the lower mantle as inferred from seismic topography or from plate reconstructions correctly predict the general trend of geoid minima however these models fail to accurately reproduce localized geoid lows in the indian ocean ross sea and northeast pacific ocean here we show that the geoid lows are correlated with high velocity anomalies near the base of the mantle and low velocity anomalies in the mid to upper mantle our mantle flow models reproduce the geoid minima if the mid to upper mantle upwellings are positioned above the inferred locations of ancient subducted slabs we find that the long wavelength trough in the geoid is linked to high density slab graveyards in the lower mantle whereas upwelling regions in the mantle above 1 000 km depth cause discrete lows within the larger trough we suggest that this mode of upwelling in the mid to upper mantle is caused by buoyant hydrated mantle that was created by processes around and above subducted slabs

remarkable fossil locality crinoid stems from migmatite of the coast plutonic complex british columbia well preserved unstrained casts of crinoid stem fragments have been discovered in a calc silicate boudin in deformed migmatitic gneiss of the coast plutonic complex west central british columbia this is among the first reports of macrofossils in rock of such high metamorphic grade

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

origin of the oceanic lithosphere in a global examination of mid ocean ridge basalt morb glass compositions we find that na8 fe8 depth variations do not support modeling of morbs as aggregates of melt compositions generated over a large range of temperature and pressure however the na 8 fe8 variations are consistent with the compositional systematics of solidus melts in the plagioclase spinel lherzolite transition in the cao mgo al2o3 sio2 na2o feo cmasnf system for natural compositions the p trange for melt extraction is estimated to be 1 2 1 5 gpa and 1250 12808c this p trange is a close match with the maximum p t conditions for explosive pressurerelease vaporization of carbonate bearing melts it is proposed that fracturing of the lithosphere induces explosive formation and escape of co2 vapor this provides the vehicle for extraction of morbs at a relatively uniform tand p the upper portion of the co2 bearing and slightly melted seismic low velocity zone flows toward the ridge rises at the ridge axis to lower lithosphere depths melts much more extensively during this rise and releases morb melts to the surface driven by explosively escaping co2 vapor the residue and overlying crust produced by this melting then migrate away from the ridge axis as new oceanic lithosphere the entire process of oceanic lithosphere creation involves only the upper 140 km when lithospheric stresses shift fracture formation to other localities escape of co2 ceases the vehicle for transporting melt to the surface disappears and ridges die inverse correlations of na8 vs fe8 for morb glasses are explained by mantle heterogeneity and positive variations superimposed on the inverse variations are consistent with progressive extraction of melts from short ascending melting columns the uniformly low temperatures of morb extraction are not consistent with the existence of hot plumes on or close to ocean ridges in this modeling the southern atlantic mantle from bouvet to about 268n is relatively homogeneous whereas the atlantic mantle north of about 268 ns hows significant long range heterogeneity the mantle between the charlie gibbs and jan mayen fracture zones is strongly enriched in feo mgo perhaps by a trapped fragment of basaltic crust iceland is explained as the product of this enrichment not a hot plume the east pacific rise galapagos ridge gorda ridge and juan de fuca ridge sample mantle that is heterogeneous over short distances the mantle beneath the red sea is enriched in feo mgo relative to the mantle beneath the northern indian ocean

the scales of mantle convection seismic topographic and gravity data show that there are two important scales of mantle convection these are associated with spherical harmonic degrees l 2 and 6 the l 2 pattern corresponds to the pattern of subduction cooling since the breakup of pangea most hotspots and ridges occur in the half of the globe unaffected by this cooling and all large igneous provinces were generated over this part of the mantle the l 2 distribution of upwellings and downwellings is likely to be a long lived feature of the earth cold regions of the mantle repeatedly attract continents and subduction reinforces the coldness an l 1 pattern of convection is probably related to supercontinents and their breakup degree 6 convection shows up in the spectrum of hotspots and upper mantle tomography and in the correlation of tomography and topography with the geoid cratons with their deep cold keels control the l 6 pattern and may even cause it by their role in establishing lateral temperature gradients and relief at the top of the convecting mantle subduction zones reinforce the craton pattern downwellings preferentially occur under cratons upwellings and hotspots occur at complementary locations supercontinents and their associated subduction zones l 1 constantly assemble and reassemble in the african atlantic hemisphere and the continental fragments in the dispersed state periodically settle into the polar band of geoid lows l 2 and 6 that now includes the americas antarctica australia and india if cratons control the l 6 pattern of convection and many patterns of l 6 are possible e g sectoral zonal checker board then upper mantle convection may reorganize roughly every 30 ma as cratons move about the hotspot spectrum is probably related to lithospheric extension as well as to broad upwellings a third and smaller scale of convection order 400 1000 km in dimension is just below the resolution of global tomography but shows up in the gravity field geoid and topography this scale is controlled by the depth of an endothermic phase change which tends to stratify mantle convection and the thickness of the upper mantle low viscosity zone convective domains of this dimension are also implied by the scales of chemical homogeneity lengths of rifts ridges and seamount chains fracture zone spacing and mid ocean ridge segmentation the temperature geochemistry and fertility of these upper mantle scale domains is controlled by their previous history of subduction continental insulation or refrigeration and processing by ridges fertile or volatile rich or hot cells can be mistaken for plumes hotspot swells are typically of the dimension that we argue is a characteristic upper mantle scale rather than a deep mantle plume scale geochemical domains of various sizes exist in the upper mantle they are broad scale features rather than point sources as in plume theories lithospheric dynamics and geometric focusing not mantle dynamics control the dimensions of so called hotspot eruptives

effects of the core cooling on the internal dynamics and thermal evolution of terrestrial planets the effects of thermal coupling of the core and mantle on the thermal evolution and mantle dynamics of terrestrial planets are investigated using three dimensional thermal convection models in a spherical shell three pairs of models are presented to study mantle models of different viscosities and internal heating in each pair one model adopts a constant temperature at the core mantle boundary and the other allows the core to thermally couple to the mantle and cool in the first and second pairs the mantle is heated only from below whereas in the third pair internal heating is due to radioactive elements the mantle viscosity of the first and third pairs is 1021 pa s while the mantle of the second pair has a viscosity of 1022 pa s the thermal coupling of the core and mantle has strong effects on the thermal boundary layer near the core mantle boundary but only minor effects on the thermal boundary layer near the surface it cools the core substantially weakens the thermal boundary layer near the core mantle boundary and thus reduces the vigor of convection in the mantle the models with cooling cores result in a substantially colder and relatively less dynamic mantle than the models with a fixed temperature at the core mantle boundary which overestimates temperatures in the mantle

compressible thermochemical convection and application to lower mantle structures a new finite element code for compressible thermochemical convection is developed to study the stability of a chemical layer at the base of the mantle using composition dependent compressibility and a density difference between compositions at a reference pressure a composition dependent density profile is derived together with depth dependent thermal expansion this combination of parameters yields a wide range of dynamic evolutions for the chemical layer the chemical structures are classified into five major categories classical plumes mushroom shaped plume domes ridges and continuous layers and a few abnormal cases such as hourglass shaped plumes and columnar plumes several models have a chemical structure morphologically similar to the african low v s structure in the lower mantle at least at a single time guided by our models several dynamic scenarios are proposed for the dynamic nature of the lower mantle low velocity structures a k a superplumes including plumes at an early stage plume clusters ridges passive piles sluggish domes and high bulk modulus domes we predict seismic velocity anomalies from these dynamic models the thermoelastic parameters used in the conversion are additional constraints we compare the density structure with normal mode inversion the predicted seismic signature observations and the required thermoelastic parameters with mineral physics data among the proposed scenarios only the scenario of high bulk modulus domes satisfies all constraints simultaneously the implication on the geochemistry and mineralogy of lower mantle chemical structures is discussed

mantle updrafts and mechanisms of oceanic volcanism convection in an isolated planet is characterized by narrow downwellings and broad updrafts consequences of archimedes principle the cooling required by the second law of thermodynamics and the effect of compression on material properties a mature cooling planet with a conductive low viscosity core develops a thick insulating surface boundary layer with a thermal maximum a subadiabatic interior and a cooling highly conductive but thin boundary layer above the core parts of the surface layer sink into the interior displacing older colder material which is entrained by spreading ridges magma characteristics of intraplate volcanoes are derived from within the upper boundary layer upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets their morphology aspect ratio inferred ascent rate and temperature show that they are passively responding to downward fluxes as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating response to doward flux is the inverse of the heat pipe mantle plume mode of planetary cooling shear driven melt extraction from the surface boundary layer explains volcanic provinces such as yellowstone hawaii and samoa passive upwellings from deeper in the upper mantle feed ridges and near ridge hotspots and others interact with the sheared and metasomatized surface layer normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism

ponded melt at the boundary between the lithosphere and asthenosphere the boundary between earth s rigid lithosphere and ductile asthenosphere is marked by a seismic discontinuity laboratory experiments on basaltic magmas show that melts should pond at pressures that correspond to the lithosphere asthenosphere boundary thus magma ponding could explain the observed seismic discontinuity

origin of volcanic seamounts at the continental margin of california related to changes in plate margins volcanic samples collected with the monterey bay aquarium research institute s rov tiburon from eight seamounts at the continental margin offshore central to southern california comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite all samples experienced complex crystal fractionation probably near the crust mantle boundary based on the presence in some of mantle xenoliths incompatible trace elements poorly correlated with isotopic compositions suggest variable degrees of partial melting of compositionally heterogeneous mantle sources ranging from morb like to relatively enriched oib high precision 40ar 39ar ages indicate episodes of volcanic activity mainly from 16 to 7 ma but document one eruption as recent as 2 8 ma at san juan seamount synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland collectively the samples from these seamounts have age ranges and chemical compositions similar to those from davidson seamount identified as being located atop an abandoned spreading center these seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction dominated to a transform margin they differ in composition age and origin from other more widespread near ridge seamounts which commonly have circular plans with nested calderas and from age progressive volcanoes in linear arrays such as the fieberling guadalupe chain that occur in the same region each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric the process may be aided by transtensional tectonics related to continued faulting along the continental margin

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

modeling of residual spheres for subduction zone earthquakes 1 apparent slab penetration signatures in the nw pacific caused by deep diffuse mantle anomalies we have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the pacific using regional and global mantle velocity models from tomographic inversions the mantle models explain much of the observed residual sphere data and to a certain extent suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns for most deep events considered the fast slablike residual sphere anomalies are caused by diffuse heterogeneities mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab the region immediately below the deepest earthquakes depths of 650 1500 km has an effect usually smaller than or comparable to the effect of other regions of the mantle without a proper account of the teleseismic effect attributing the long wavelength anomalies of the residual sphere to near source slab effects alone or even primarily is not valid the fast bands in many observed residual spheres agree with seismicity trends once the deep mantle and receiver mantle effects are removed these may give the approximate orientation but not the depth extent of near source fast velocities for most deep earthquakes under japan the predominant fast band is subhorizontal rather than near vertical this type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data

the source location of mantle plumes from 3d spherical models of mantle convection abstract mantle plumes are thought to originate from thermal boundary layers such as earth s core mantle boundary cmb and may cause intraplate volcanism such as large igneous provinces lips on the earth s surface previous studies showed that the original eruption sites of deep sourced lips for the last 200 myrs occur mostly above the margins of the seismically observed large low shear velocity provinces llsvps in the lowermost mantle however the mechanism that leads to the distribution of the lips is not clear the location of the lips is largely determined by the source location of mantle plumes but the question is under what conditions mantle plumes form outside at the edges or above the middle of llsvps here we perform 3d geodynamic calculations and theoretical analyses to study the plume source location in the lowermost mantle we find that a factor of five decrease of thermal expansivity and a factor of two increase of thermal diffusivity from the surface to the cmb which are consistent with mineral physics studies significantly reduce the number of mantle plumes forming far outside of thermochemical piles i e llsvps an increase of mantle viscosity in the lowermost mantle also reduces number of plumes far outside of piles in addition we find that strong plumes preferentially form at near the edges of piles and are generally hotter than that forming on top of piles which may explain the observations that most lips occur above llsvp margins however some plumes originated at pile edges can later appear above the middle of piles due to lateral movement of the plumes and piles and morphologic changes of the piles 65 70 strong plumes are found within 10 degrees from pile edges in our models although plate motion exerts significant controls over the large scale mantle convection in the lower mantle mantle plume formation at the cmb remains largely controlled by thermal boundary layer instability which makes it difficult to predict geographic locations of most mantle plumes however all our models show consistently strong plumes originating from the lowermost mantle beneath iceland supporting a deep mantle plume origin of the iceland volcanism

hadean age for a post magma ocean zircon confirmed by atom probe tomography the only physical evidence from the earliest phases of earth s evolution comes from zircons ancient mineral grains that can be dated using the u th pb geochronometer 1 oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2 3 chemical homogenization of earth s crust and the existence of a magma ocean have not been dated directly but must have occurred earlier 4 however the accuracy of the u pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline pb mobility 5 7 here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from earth a 4 4 gyr old hadean zircon with a high temperature overgrowth that formed about 1gyrafterthemineral score isolatednanoclusters measuring about 10nm and spaced 10 50nm apart are enriched in incompatible elements including radiogenic pb with unusually high 207 pb 206 pbratios wedemonstratethatthelengthscales of these clusters make u pb age biasing impossible and that they formed during the later reheating event our tomography data thereby confirm that any mixing event of the silicate earth must have occurred before 4 4gyr ago consistent with

dispersion of very long period rayleigh waves along the east pacific rise evidence for s wave velocity anomalies to 450 km depth the phase dispersion of fundamental mode rayleigh waves from the second gazli earthquake has been measured at periods between 40 and 400 s between los angeles and the south pole over most of its length the path between the two stations is parallel and close to the east pacific rise the phase velocities we have observed are significantly lower than the global averages the inversion for the oceanic part of the path gives a shear wave velocity model with 1 unusually low velocities down to a depth of about 200 km with a minimum of 4 1 km s around 100 km 2 normal or slightly less than normal velocities between depths of 200 km and about 400 km and 3 a depression of the olivine spinel phase boundary by about 50 km or an equivalent velocity reduction around a depth of 450 km in a model without mantle discontinuities we can make no definitive assertions regarding deeper anomalies in the mantle the velocity anomalies below a depth of 200 km are consistent with the assumption that the temperature of the upper mantle between the low velocity zone and the spineloxide transition at 671 km in model 1066b is elevated by 140 k relative to the average earth corrections to customary data processing procedures are offered that are designed to eliminate bias in estimates of dispersion at periods greater than 240 s

search for deep slabs in the northwest pacific mantle a residual sphere is formed by projecting seismic ray travel time anomalies relative to a reference earth model onto an imaginary sphere around an earthquake any dominant slab like fast band can be determined with spherical harmonic expansion the technique is useful in detecting trends associated with high velocity slabs beneath deep earthquakes after deep mantle and near receiver effects are removed two types of corrections are used the first uses a tomographic global mantle model the second uses teleseismic station averages of residuals from many events over a large area centered on the events of interest under the mariana izu bonin and japan trenches the dominant fast bands are generally consistent with seismicity trends the results are unstable and differ from the seismicity trend for kurile events the predominant fast band for most deep earthquakes under japan is subhorizontal rather than near vertical we find little support for the deep slab penetration hypothesis