double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

transient hydrogeologic models for submarine flow in volcanic seamounts 2 comparison of the hawaiian canary and marquesas islands 1 large bathymetric gradients associated with volcanic seamounts can drive convective flow while thick sedimentary aprons that typically surround volcanic edifices host compaction driven flow in these submarine environments the interactions of compaction driven and buoyancy driven fluid flow lead to complex hydrogeologic regimes we apply transient numerical models of coupled fluid flow and heat transport to the hawaiian canary and marquesas islands to examine the role of volcanic architecture on the evolution of fluid flow and pore pressure during volcanic building lithospheric flexure sedimentation and compaction the islands differ in edifice size sedimentary apron structure amount of lithospheric flexure and sedimentation and volcanic growth rates by comparing these variations we examine how geometry and geologic history affect fluid flow and pore pressure patterns buoyancy driven flow is most influenced by edifice height and amount of lithospheric flexure compaction driven flow is altered primarily by thickness of prevolcanic sediment the sedimentation rate and the size of the volcanic edifice in hawaii the area with the highest edifice and most flexure flow velocities and excess pore pressures are greatest with darcy velocities of 30 mm yr and excess pressures of 7 mpa during the beginning of volcanic building high sedimentation rates in the marquesas sedimentary apron increase fluid velocities in the later portion of volcanic building with darcy velocities 12 mm yr in the canary islands compaction driven flow occurs for an extended time period because of long slower volcanic building resulting in darcy velocities 10 mm yr that endure over 5 myr

paleolatitudes of the kerguelen hotspot new paleomagnetic results and dynamic modeling abstract the kerguelen plateau a large igneous province in the southern indian ocean was formed as a product of the kerguelen hotspot in several eruptive phases during the last 120 myr we obtained new paleolatitudes for the central and northern kerguelen plateau from paleomagnetic investigations on basalts which were drilled during odp leg 183 to the kerguelen plateau broken ridge the paleolatitudes coincide with paleolatitudes from previous investigations at the kerguelen plateau and ninetyeast ridge the track of the kerguelen hotspot and indicate a difference between paleolatitudes and present position at 49 s of the kerguelen hotspot we show that true polar wander the global motion between the mantle and the rotation axis cannot explain this difference in latitudes we present numerical model results of plume conduit motion in a large scale mantle flow and the resulting surface hotspot motion a large number of models all predict southward motion between 3 and 10 for the kerguelen hotspot during the last 100 myr which is consistent with our paleomagnetic results

relations between interrill erosion processes and sediment particle size distribution in a semiarid mediterranean area of se of spain abstract erosion and sediment characteristics were measured using simulated rainfall on two cultivated soils of contrasting lithology quaternary calcareous colluvium and tertiary marls in a semiarid mediterranean area of se spain two rainfall intensities high 56 0 2 4 mm h 1 and medium 31 4 1 4 mm h 1 were used in order to know the mechanisms involved in each selected condition for each simulated event runoff and sediment were sampled at 1 min intervals on a 1 m wide by 2 m long erosion plot the erosion rate was calculated as the total amount of soil lost divided by the time period of the test the duration of the test was that needed to reach steady state runoff an average time of 24 5 min for quaternary calcareous colluvium and 17 7 min for tertiary marls the size distribution of the transported sediment in the field effective size distribution was compared with equivalent measurements of the same samples after chemical and mechanical dispersion ultimate size distribution to investigate the detachment and transport mechanisms involved in sediment mobilisation the results showed that the soil type determined the hydrological response regardless of the rainfall intensity the erosional response was however determined by the rainfall and soil surface characteristics in quaternary calcareous colluvial soils the predominant erosion process depended on the rainfall intensity with a prevailing detachment limited condition in high intensity events and prevailing transport limited conditions in those events of medium intensity these differences in the main erosion processes were reflected in the size of the transported material and in the change in sediment size within the storm thus a time dependent size distribution of the eroded material decreasing coarse fractions and increasing fine fractions with runoff time was observed for high intensity events in medium intensity events on the other hand the time independent size distribution of the eroded material indicated transport limited erosion due to the rapid surface crusting on the tertiary marl soil no differences in the main erosion processes or in the sediment size distribution occurred for the different rainfall intensities tested the erosion of marl soils was determined mainly by the limited quantity of available sediment the effective size of material was a more sensitive parameter than the ultimate size of the sediment to study the way in which the sediment was transported

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

volgo uralia the first u pb lu hf and sm nd isotopic evidence of preserved paleoarchean crust the crustal segment volgo uralia is the least known part of the east european craton its crystalline crust is hidden beneath a thick neoproterozoic to phanerozoic cover but disclosed by thousands of drill holes in conjunction with the recent tatseis reflection seismic profile we conducted the first isotopic study of the bakaly granitoid block in eastern volgo uralia which represents a subsurface section of the layered upper middle crust the study included whole rock sm nd and ion probe zircon u th pb sims and lu hf la icpms analyses of granitoids from seven drill cores the bakaly block was also targeted because its rocks have never been subjected to granulite facies metamorphism making it possible to date pristine pre metamorphic zircon our study showed that the four principal suites of granitoids in the bakaly block are different in age each corresponding to a particular stage of archean crustal evolution between 3 3 and 2 6 ga the tashliar monzonitic suite belonging to an alkaline series yielded zircon ages of 3 3 and 3 2 ga which are the oldest ages yet found in volgo urafia the epsilon hf t values of the dated zircon and the epsilon nd t values of their host rocks indicate that a paleo to eoarchean protolith with model t dm ages up to 3 8 ga had been involved in the formation of the tashliar melts three neoarchean rock suites one comprising quartz dioritic and tonalitic gneisses the bak 1 another k rich granodiorites granites and migmatites the bak 2 and the third monzonitic granitoids the aktanysh suite were formed sequentially between 2 72 and 2 60 ga the 2 72 ga bak 1 suite is chemically diverse it includes granitoids of the ttg type related to slab subduction melts as well as rocks formed by the re melting of older crust with whole rock nd t dm and hf t dm model ages of 3 4 to 3 2 ga the 2 69 to 2 65 ga bak 2 suite was probably associated with a major collisional event which defined the stacked structure of the archean crust in volgo uralia and its seismic layering our data suggest that the bak 2 melts originated partly from juvenile sources with epsilon hf t zircon values up to 4 8 as well as mixed crustal and juvenile mantle materials some crustal contamination of the melts appears to have occurred as evidenced by incorporated xenocrystic zircon the chemical compositions of bak 2 granitoids from the different plutons their zircon epsilon hf values and the hf and nd t dm ages all mirror a heterogeneous collisional crustal structure during post collisional extension at 2 6 ga the intrusion of aktanysh monzonitic granitoids took place these rocks also bear evidence of a long crustal pre history with nd and hf t dm model ages of 3 3 to 3 5 ga the aktanysh rocks are coeval with the tuymazy gabbro norite anorthosite intrusions which are widely distributed along post collisional shear zones in the bakaly block they could have provided the heat necessary to melt the crust at this stage altogether the isotopic evidence suggests several episodes of crustal growth and recycling possibly reaching back to 3 6 and 3 8 ga metamorphic zircon rims show that the archean crust in the bakaly block were subjected to several tectonothermal overprints in the paleoproterozoic between 2 4 and 1 9 ga ago less

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

interaction of small mantle plumes with the spinel perovskite phase boundary implications for chemical mixing abstract geochemical observations of ocean island basalts show a broad variety in the chemical composition of trace elements these observations cannot be explained by different temperature and pressure conditions at the melt production region only but also demand differences in the parental material at the base of the lithosphere here we add a new explanation for this finding based on the different interaction of small and large plumes with the spinel perovskite phase boundary spb in the earth s mantle we assume that mantle plumes detaching from the core mantle boundary cmb are of variable size and excess temperature while rising through the lower mantle they form an approximately spherical plume head when arriving at the spb they are retained from further rise depending on their size and excess temperature we have investigated the interaction of a plume with the spb using a numerical approach for viscous flow to focus particularly on this interaction we simply modelled the initial plume as a hot volume of variable cross section and excess temperature volumes with a radius exceeding about 150 km and an excess temperature of more than 100 c rapidly cross the spb with decreasing size and increasing excess temperature volumes are retarded at the spb and those with a radius smaller than about 80 km stagnate entirely below the spb these results are roughly independent of the particular rheology or two dimensional versus three dimensional geometry in the present study we investigated the degree of thermal entrainment of material at the spb for strongly retarded plumes we estimate the amount of chemical mixing by tracking passive markers from the original plume volume and the regions around the spb to the base of the lithosphere the amount of entrainment of material from close to the spb depends on the retardation time and the thermal growth of the plume head at the spb plume heads which reach the base of the lithosphere after a retardation time of more than 50 ma might entrain in the numerical model large amounts of material from the sbp but they may not be characteristic for the earth since they might be distorted by large scale mantle flow however even if we limit the retardation time to reach the base of the lithosphere to 50 ma plumes originally composed of material from the cmb and the adjacent lower mantle might easily entrain up to 15 of their volume of material originating from the spb

the importance of radiative heat transfer on superplumes in the lower mantle with the new post perovskite phase change abstract a new post perovskite phase has been identified recently from both x ray observations and first principles calculations this novel phase change occurs at a depth about 200 km above the core mantle boundary for temperatures of around 2800 k using a two dimensional extended boussinesq model we have studied the dynamical consequences of this deep mantle phase transition on mantle convection with particular emphasis on the effects on lower mantle plume structures we have employed a depth dependent viscosity with a viscosity maximum in the mid lower mantle and two phase transitions one at 670 km depth and the other at a depth of 2650 km the phase transition at 670 km is endothermic while we have examined both exothermic and endothermic possibilities for the new phase transition our results show the following situations favorable for the development of superplumes 1 endothermic phase transition for both constant and radiative thermal conductivities and 2 exothermic phase transition and radiative thermal conductivity smaller unstable plumes are found for exothermic phase transition and constant thermal conductivity extremely high lateral temperature increases exceeding 1500 degrees are found inside the lower mantle plume for constant thermal conductivity this lateral temperature contrast is mitigated by the presence of radiative thermal conductivity in order for superplumes to prevail in the lower mantle with a deep exothermic phase change we must invoke some form of radiative thermal conductivity for stabilizing and promoting large upwellings with a width exceeding at least 500 km strong lateral heterogeneities in both the seismic velocity and density fields can be produced in the plumes and the surrounding mantle for this new phase transition with a large clapeyron slope close to 10 mpa k

a typical calanchi landscape on the eastern apennine margin atri central italy geomorphological features and evolution abstract the surroundings of the town of atri abruzzo region central italy situated about 10 km from the adriatic shoreline can be considered as representative of most part of the italian seaward peninsular landscape the subject of this research the calanchi landscape often generally considered as badlands by non italian authors is in fact made up of many singular landforms each of which calanco corresponds to a more or less extended hydrographic unit the dynamics are mainly the result of the action of the particularly aggressive climatic factors such as the mediterranean rainfall regime on weak substrata clay silt and sand of the plio pleistocene marine cycles the morphogenetic activity is not only limited to channel erosion but it is also due to repeated superficial slides so that it should in fact be better to consider the calanco as the result of a combined erosion process sensu zachar d 1982 soil erosion elsevier amsterdam since the mechanism regulating this process remains unclear this work wants to offer a contribution to this research field multitemporal geomorphological analysis has helped us to distinguish the different areas in which the calanchi evolution has been more or less important in the past in such a way it has been possible to characterize the landscape evolution as a function of the parameters that influence soil loss especially in areas such as that of atri where calanchi like landforms are highly developed moreover this analysis coupled with the processing of 40 years rainfall data also taking into account the concurrent land use changes has clearly shown that the areas affected by calanchi erosion on the eastern apennine margin still tectonically active are progressively diminishing

investigating superswells and sea level changes caused by superplumes via normal mode relaxation theory 1 the superplume hypothesis particularly for one in the mid cretaceous has stimulated the search for evidence for or against such an event and the associated superswell an approach based on normal mode relaxation theory makes it possible to simulate the surface deformation caused by the rising plume using a spherical self gravitating and stratified viscoelastic earth model the results for the superswell are consistent with those of previous analyses using convection and experimental modeling the self consistent coupling of elastic and viscous properties based on maxwell rheology reveals that elasticity plays an important role contributing up to 30 to surface swelling when the plume head crosses the upper mantle we show that the deformation at great distances from the superswell is relatively small of the order of meters at most and opposite in sign with respect to the large uplift over the plume of the order of kilometers the global subsidence over a wide region of the planet surrounding the superswell counteracts its effects on sea level changes and triggers a eustatic signal of a few meters at most a negligible magnitude with respect to 250 m characteristic for the mid cretaceous previous arguments against the mid cretaceous superplume hypothesis which assume that the large amount of water displaced by the superswell causes a eustatic sea level rise of about 200 m thus competing with the displacement due to oceanic crust production are no longer tenable since such estimates did not account for the global behavior of the planet under internal loads

mulching effects on erosion from steep slopes and sediment particle size distributions of gully colluvial deposits abstract mulching is an effective soil conservation practice for permanent gullies in southern china knowledge of the sediment characteristics that occur in mulched soils of colluvial deposits could improve the utility of mulching for soil conservation a rainfall simulation experiment was designed to evaluate the effects of mulch on the runoff erosion and particle size distribution of eroded sediments straw mulch coverage of 0 25 50 75 and 95 was tested with simulated rainfall the effective particle size distribution of the sediment was compared with the ultimate particle size distribution to investigate the detachment and transport mechanisms involved in sediment mobilization mulching delayed the runoff initiation time and reduced the average runoff rate compared with bare soil the increased mulch coverage decreased the soil loss rate by 13 0 to 90 3 moreover the peak sediment concentration decreased from 80 to 200 g l 1 under the different mulch coverage conditions the optimal straw application rate was 1 5 to 3 0 mg ha 1 in the permanent gully s deposits the relationship between instantaneous kinetic energy of rainfall and the proportion of effective clay and sand sized particles was well represented using an exponential equation the effective clay sized sediments under the different mulch coverage conditions were 2 to 4 times more common than those of the original soil although there were only 13 9 sand sized particles in the sediment when the mulch coverage was 95 the silt sized sediment was transported as primary particles under the different mulch coverage conditions the effective to ultimate ratio of silt sized particles fluctuated around 1 there were depletions of clay and silt in the colluvial deposit soil with mulch cover and the enrichment ratios of clay and silt were larger than 1 while most of the enrichment ratios for sand were

timing of formation of beta regio and its geodynamical implications 1 recent evidence suggests that the beta regio equatorial highland generally considered to be a plume related structure was emplaced over a relatively short period of time in the recent venus history 100 400 m y we perform numerical simulations which use this evidence as a new constraint on geodynamic models and find that it is difficult to satisfy simultaneously the constraint on the uplift rate and the constraints on gravity topography and rheology one possible solution is that the effective viscosity contrast between the mantle and the lithosphere does not exceed about 104 if our simple models do capture the basic dynamics of formation of beta regio this value implies that the viscosity of the lithosphere is softer than olivine at subsolidus temperatures and that the venusian lithosphere may not be much stronger than the terrestrial one

the plume head continental lithosphere interaction using a tectonically realistic formulation for the lithosphere summary current debates on the existence of mantle plumes largely originate from interpretations of supposed signatures of plume induced surface topography that are compared with predictions of geodynamic models of plume lithosphere interactions these models often inaccurately predict surface evolution in general they assume a fixed upper surface and consider the lithosphere as a single viscous layer in nature the surface evolution is affected by the elastic brittle ductile deformation by a free upper surface and by the layered structure of the lithosphere we make a step towards reconciling mantle and tectonic scale studies by introducing a tectonically realistic continental plate model in large scale plume lithosphere interaction this model includes i a natural free surface boundary condition ii an explicit elastic viscous ductile plastic brittle rheology and iii a stratified structure of continental lithosphere the numerical experiments demonstrate a number of important differences from predictions of conventional models in particular this relates to plate bending mechanical decoupling of crustal and mantle layers and tension compression instabilities which produce transient topographic signatures such as uplift and subsidence at large 500 km and small scale 300 400 200 300 and 50 100 km the mantle plumes do not necessarily produce detectable large scale topographic highs but often generate only alternating small scale surface features that could otherwise be attributed to regional tectonics a single large wavelength deformation predicted by conventional models develops only for a very cold and thick lithosphere distinct topographic wavelengths or temporarily spaced events observed in the east african rift system as well as over french massif central can be explained by a single plume impinging at the base of the continental lithosphere without evoking complex asthenospheric upwelling

conditions for plumes to penetrate the mantle phase boundaries at a depth of 660 km in the earth s mantle the spinel perovskite phase boundary is a prominent barrier for mantle convection this is due to the negative clapeyron slope of the phase equilibrium curve which leads to an elevation of the phase boundary within hot upwellings causing negative buoyancy forces we have investigated the conditions for rising plumes either to penetrate and pass the spinel perovskite phase boundary or to stick and spread below it by studying the fundamental physics of this process the plume heads were simply modeled as hot three dimensional 3 d spheres or 2 d cylinders a simple calculation balancing the positive thermal and the negative phase bouyancy forces leads to a better parameterization using two dimensionless quantities in addition to the phase buoyancy parameter we defined a deflection parameter relating the elevation of the phase boundary to the plume head radius to account for the geometrical shape of a plume head this parameterization is further tested with numerical models that include the effects of thermal diffusion latent heat the olivine spinel phase boundary at a depth of 410 km and temperature and or phase dependent viscosity structure for laboratory estimates of the slope 3 mpa k and density increase at the spinel perovskite phase boundary 250 kg m 3 our models predict that plumes with excess temperatures of 50 600 c will stick at the top of the lower mantle if their head radii are less than 100 km plumes will penetrate into the upper mantle if plume head radii exceed 100 km while the style of plume penetration or spreading at the top of the lower mantle strongly depends on the viscosity structure the conditions for penetration do not all rising hot volumes with nonpenetrating conditions stick at the top of the lower mantle and spread laterally independent of their viscosity structure for weakly nonpenetrating conditions heat diffusion increases the radius of the hot volume and leads to penetration during a secondary stage for strongly nonpenetrating conditions spreading at the top of the lower mantle drives a mechanically coupled counterflow in the upper mantle which is stable for a very long time for volumes with an excess temperature of more than 350 c heat diffusion across the phase boundary will eventually inhibit this counterflow and stabilize a thermally coupled flow which might entrain some material of the hot volume however our results suggest that the spinel perovskite phase boundary is unlikely to inhibit the penetration of mantle plumes of the size thought to have generated many flood basalt provinces and hotspot chains

the role of deep subduction in supercontinent breakup abstract the breakup of continents and their subsequent drifting plays a crucial role in the earth s periodic plate aggregation and dispersal cycles while continental aggregation is considered the result of oceanic closure during subduction what drives sustained divergence in the following stages remains poorly understood in this study thermo mechanical numerical experiments illustrate the single contribution of subduction and coupled mantle flow to the rifting and drifting of continents we quantify the drag exerted by subduction induced mantle flow along the basal surface of continental plates comparing models of lithospheric slab stagnation above the upper lower mantle boundary with those where slabs penetrate into the lower mantle when subduction is upper mantle confined divergent basal tractions localise at distances comparable to the effective upper mantle thickness 500 km causing the opening of a marginal basin instead subduction of lithosphere in the lower mantle reorganises the flow into a much wider cell localising extensional stresses at greater distances from the trench 3000 km sub continental tractions are higher and more sustained over longer time periods in this case and progressively increase as the slab sinks deeper although relatively low basal shear stresses when integrated over large plates generate tension forces that may exceed the strength of the continental lithosphere eventually leading to breakup and opening of a distal basin the models illustrate the emergence of a similar mechanism which results in the formation of back arc basins above upper mantle confined subduction and scales to much larger distances for deeper subduction examples include the atlantic ocean formation and drifting of the south and north american plates during the mesozoic cenozoic farallon plate subduction

potash in a salt mushroom at hormoz island hormoz strait iran abstract increasing volumes of potash are currently being discovered in a cluster of diapirs of hormoz formerly hormuz salt near bandar abbas iran most of the potash beds studied so far occur in complex recumbent folds in a salt mountain that would be difficult to exploit safely however holocene marine erosion removed any salt mountains from a sub group of near shore zagros diapirs and exposed their deeper structural levels even though these diapirs are still active their potash deposits are likely more tractable to safe exploitation than in a salt mountain as we make clear here for hormoz island geochemical surveys on hormoz island reveal two separate potash anomalies that are valuable pseudo stratigraphic markers integrating field measurements of the attitudes of bedding with lineaments on air photos suggests that hormoz island consists of a mature bell or plume shaped mushroom diapir with potash beds wound around a toroidal axis of rotation near current exposure levels 2d numerical models simulate the salt mushroom on hormoz island and its internal circulation they also suggest that the diapir has a wide overhand above a narrow stem in this gas rich region we use the mushroom diapir model to outline a regional exploration strategy that has the potential of influencing the world potash market thereafter

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

hydrothermal seismicity beneath the summit of lucky strike volcano mid atlantic ridge abstract we present two years july 2007 july 2009 of earthquake locations from a local seismological network on the lucky strike volcano mid atlantic ridge whose summit hosts one of the most active and largest deep sea hydrothermal fields known two clusters of small m l

evolution of starting mantle plumes a comparison between numerical and laboratory models abstract laboratory experiments on the initiation and development of mantle plumes suggest the formation of a bulbous plume head that grows in time by entrainment of surrounding material followed by a thin feeder conduit this type of plume is generally not seen in numerical models of mantle convection to better determine the possible differences between numerical and laboratory experiments we have numerically reproduced a published laboratory experiment on plume initiation theoretical predictions for the growth of the plume head have been verified and the experiments allowed for a new determination of a similarity constant models with a constant viscosity fluid show much less entrainment simulations using a plume rising from a thermal boundary layer with an olivine type temperature dependence show that most of the source material is contained in the plume head and that the conduit is largely composed of entrained material simulations with a more realistic non newtonian rheology show that plumes can rise much faster through the mantle which will lead to higher temperatures of the plume head at the earth s surface compared to what experiments with newtonian rheology suggest

effect of lithospheric root on decompression melting in plume lithosphere interaction models summary global distribution of large igneous provinces lips shows that volcanic events associated with diapiric plume heads have often occurred near the margins of cratonic lithosphere previous numerical models of plume lithosphere interaction and decompression melting assumed the lithosphere as a rigid lid of mostly uniform thickness here we present a 3 d numerical model of a starting mantle plume consisting of a diapiric head and a trailing conduit which interacts with a moving lithospheric plate of non uniform thickness we study in particular the effect of a lithospheric root on plume flow and partial melting the plume is deflected away from the root preferentially in the upstream direction the presence of a root facilitates more than three fold enhancement in the melting rate by restricting the horizontal spreading of plume material and the diffusion of thermal anomaly in comparison to the model having no root the large melting rate is sustained for longer duration in the presence of depletion buoyancy of the residue our model does not require an excessively high plume temperature or compositionally heterogeneous plume head and reproduces the characteristics of melting rates and melt volumes seen in flood basalt volcanic provinces and the trailing hotspot tracks the model also supports occurrence of volcanism of different ages at a given location as a result of initial melting in the upstream region and its subsequent passage over the plume conduit due to plate motion inferences for the occurrence of such late stage volcanism are discussed for the deccan volcanic province cenozoic volcanism of eastern australia is also discussed as an example of deflection of a plume due to the presence of thick lithosphere

benggang erosion the influencing factors abstract in the tropical and southern subtropic granite areas of south china a special kind of small scale erosional landform is widely distributed called benggang by the local people many natural factors are responsible for the development of benggang such as lithology precipitation relief vegetation etc which are discussed in detail in this paper study shows that the development of benggangs is the outcome of a joint operation of natural factors and human activities lithology especially the characteristics of the granite weathering mantle provides material in which benggangs can develop while intense rainstorms in a tropical monsoon climate act as a powerful force to cause benggang erosion the slope characteristics such as steepness and exposure control the spatial difference of the distribution of benggangs the development of benggangs results from the reverse succession of vegetation induced by human activities in this region and the density of benggangs is closely related to forest cover and unit timber growing stock

geoid and topographic swells over temperature dependent thermal plumes in spherical axisymmetric geometry geoid and topographic profiles over constant viscosity thermal plumes in spherical axisymmetric geometry at moderate rayleigh numbers are compared with results from previous studies in other geometries plumes in spherical axisymmetric models produce larger geoid anomalies than plumes in either cartesian or cylindrical axisymmetric geometries however the topographic anomalies over the plumes are smaller than the corresponding anomalies in a cylindrical axisymmetric geometry but larger than those in a cartesian geometry for plumes in a temperature dependent viscosity fluid the geoid and topographic profiles are reduced by a factor of two when compared to a constant viscosity fluid with the same background viscosity profile in this case the radially averaged temperature profile around the plume creates a low viscosity layer beneath the lithosphere

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

numerical modelling of a mantle plume the plume head lithosphere interaction in the formation of an oceanic large igneous province abstract the thermomechanical processes associated with formation of large igneous provinces lips remain poorly understood owing to fundamental difficulties in simulating plume lithosphere interactions in current numerical models these models which aim to simulate the rise of mantle plume and the spread of plume head material imply a mechanically over simplified lithosphere and commonly a flat lithosphere zero vertical displacement as the upper boundary condition we propose a new numerical model derived from lithospheric scale models it has a high numerical resolution in the lithospheric domain and explicitly accounts for 1 free upper surface boundary condition 2 elastic plastic ductile lithospheric rheology including surface faulting and 3 vertical strength variations in the lithosphere we study the final stages of plume ascent and we focus on surface and lithospheric evolution and intra plate strain localisations the experiments predict that the first surface elevation occurs in less than 0 2 ma after plume initiation at 400 km depth variation of rheological parameters results in different surface elevations 500 2500 m ascent 2 10 m yr and base plate strain rates 10 12 10 15 s 1 fast 0 2 0 3 m yr plume head flattening starts at the moment when the plume head reaches the base of the lithosphere it leads to large scale extension and deep normal faulting at the centre of the plateau and to strong thermomechanical erosion at its base the erosion is maximal not under the plume centre as was predicted before but in two large bordering zones our study locally is the igneous province of the caribbean plate where the pre existing farallon lithosphere has been affected by the galapagos hotspot activity that generated thermal perturbations and crustal thickening with two main episodes of volcanism and underplating

wavefront healing renders deep plumes seismically invisible summary since w j morgan proposed that intraplate volcanism at some pacific hotspots is caused by hot plumes rising from the lower mantle geophysicists have been actively pursuing physical evidence for mantle plumes several seismic studies have mapped low velocity anomalies below a number of hotspots however the association of low velocity structures with plume tails has remained controversial given the debate on whether lower mantle plumes impart observable traveltime or amplitude perturbations on seismic waves using high resolution numerical simulations of plume ascent through the mantle and their effects on waveforms we demonstrate that the delay of shear waves by plume tails at depths larger than 1000 km are immeasurably small 0 2 s at seismic periods commonly used in waveform analysis therefore we conclude that narrow lower mantle plumes are not detectable

assessing the sources of sediment transported in gully systems using a fingerprinting approach an example from south east china abstract eroded sediment is an important feature of permanent gully s development however obtaining the specific characteristics of the sediment transport in the gully can be difficult thus a composite fingerprinting technique incorporating uncertainty analysis has been employed to investigate the mean relative contribution of sediment sources in permanent gullies in this study 31 tracers were measured at 62 different sampling sites from three layers of a gully wall surface layer sl sandy soil layer ssl and semi weathering rock layer swl and 36 sediment samples from the slumping deposit and alluvial fan of the gullies the sediment source tracing procedure was therefore used to assess the relative contribution of each source the mean relative contribution from the sl to the sediment of the slumping deposit is higher in the active gully g1 and is relatively lower in the potentially stable g2 and stable gullies g3 thus there was no obvious gully bank retreat in g2 and g3 in different places of the g1 alluvial fan the mean relative contribution of the sl ssl and swl ranged from 9 11 36 43 and 46 55 respectively however the sediment contribution of the sl increased in g2 and g3 due to the changes in the erosion type the mean relative contributions of the sl were 33 5 g2 and 30 3 g3 these findings have important implications for the establishment of a scientific basis for permanent gully sediment transport management and control policies

rheological conditions for emplacement of ural alaskan type ultramafic complexes article i nfo ural alaskan or alaskan type complexes correspond to a particular class of ultramafic intrusions that attract particular attention due to their deep mantle origin and their platinum group element pge mineralization when defined as massifs of dunite clinopyroxenite only forty six complexes are reported in the literature these large scale dunite pipe like structures are rarely isolated and they even can appear in clusters to better understand genesis of these relatively young b460 ma complexes a worldwide compilation has been built and three categories have been defined single circular or elliptical bodies twin bodies with similar shapes and dismembered dunite bodies pge enrichment in alaskan type complexes is highest for the second category where twin bodies are interpreted as horizontal sections of y shaped dunite pipes to constrain mechanical properties of the lithosphere allowing emplacement of the alaskan type complexes the forceful diapiric ascent hypothesis is investigated through numerical thermo mechanical models one hundred high resolution experi ments accounting for realistic phase changes and softening mechanisms have been performed the experiments show that with no rheological softening of the host rock and in case of a relatively weak ductile lower crust the uprising magma tends to spread laterally without reaching the surface to account for the forceful ascent of deep magmas it is hence necessary to assume a strong lower crust rheology and strong local softening mechanisms besidesreproducingtheclustereddistributionoftheweaknesszonesrepresentingmagmapathways theselatter experiments reproduce large scale pipe like cylindrical structures y shaped and funnel shaped bodies and laterally shifted structures interestingly zones of highest strain rates are located at the bottom parts of the inclined edges of y shaped and funnel shaped bodies the restricted age range of alaskan type complexes b460 ma would mean that prior to this time the lower crust was less resistant due to the hotter geotherm prohibiting the possibility of alaskan type magmatism

laboratory convection experiments effect of lateral cooling and generation of instabilities in the horizontal boundary layers convection experiments are carried out in a tank with two isothermal heat sinks the top plate and one of the sidewalls heating is supplied either by an isothermal bottom plate or by generation within the fluid this situation is similar to that of the earth s subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere there the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab the thermal structure of the convective fluid is observed by a variety of techniques including differential interferometry and strioscopy two parameters characterize the observations the usual vertical rayleigh number and a newly defined lateral rayleigh number the lateral cooling induces a large roll with axis parallel to the cold wall the variation of its width relative to the values of the two rayleigh numbers has been determined an application to the earth s upper mantle would predict rolls 5 times wider than high the circulation in the large roll remains two dimensional for rayleigh numbers up to about 3 104 beyond this value boundary layer instabilities are observed within the persisting large roll their period has been determined the interferometric method is shown to be very useful for visualizing other time dependent processes such as the growth of the induced large rolls this growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break up

pulsating diapiric flows consequences of vertical variations in mantle creep laws abstract recent laboratory work has suggested that the rheology of the lower mantle may be newtonian we have studied the time dependent dynamics of plumes interacting with a rheological interface separating an upper non newtonian mantle and a newtonian lower mantle pulsating diapiric structures with fast time scales are promoted by the interaction of the rising plumes with this rheological boundary surface heat flow signals are discernible as pulses which remain relatively stationary they correlate well with the localized upwellings just below the surface the presence of a mobile lithosphere from increasing the non newtonian power law index helps to produce a large scale circulation in the upper mantle which draws large hot patches away from nearby upwellings in our calculations the resultant averaged effective viscosity of the non newtonian upper mantle is about two orders of magnitude lower than that of the newtonian lower mantle a viscously stratified newtonian model produces more incoherent and broad scale diapiric structures and is less efficient for generating sharply varying time dependent thermal signatures

controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics differences and similarities summary mantle plume activity and lithospheric folding by far field stresses exerted from plate boundaries are two important end members as mechanisms for continental intraplate deformation the topographic expression of mantle plume impingement on continental lithosphere and lithospheric folding has some striking similarities observations from a number of areas in europe s intraplate lithosphere demonstrate that these mechanisms commonly interact in space and time we present the results of thermomechanical modelling addressing the role of factors such as the presence of a hot upper mantle the spatial dimensions of the plume and the time constants involved in the temporal succession of plume activity and lithospheric folding by stress accumulation in intraplate continental lithosphere the results demonstrate that both the processes plume lithosphere interactions and folding may interact resulting either in strong amplification attenuation or modification of their surface expression these inferences are compatible with a number of key observations on the nature and the temporal succession of topography evolution in the alpine foreland the pannonian basin the scandinavian continental margin and the iberian peninsula

shape of thermal plumes in a compressible mantle with depth dependent viscosity the mantle plume model has been invoked to explain the formation of large igneous provinces lip and associated age progressive hotspot tracks the shape of mantle plumes should be significantly altered by physical properties of the mantle and will influence how plume theory is used to interpret observational constraints based on theoretical analysis and numerical modeling we explore the parameters that control the shape of thermal plumes in a compressible mantle a theoretical analysis shows that the ambient mantle viscosity plays a dominant role in determining the radius of thermal plumes this analysis is verified by numerical solutions a continuously decreasing mantle viscosity from the cmb to the lithosphere can effectively reduce the radius of both plume head and tail a low viscosity zone between 100 and 660 km depths where viscosity decreases by a factor of 100 reduces the radius of a plume conduit by approximately a factor of 3 such a low viscosity zone can reduce the plume head radius impinging the lithosphere from larger than 500 km to 200 km when the low viscosity zone is confined to between 100 and 410 km depths the plume head size becomes even smaller to form large igneous provinces a small plume head implies time progressive volcanism from lip center to lip edge

the 40ar 39ar age dating of the madeira archipelago and hotspot track eastern north atlantic the 40ar 39ar ages for 35 volcanic rocks and 14c ages for two charcoal samples from the madeira archipelago and ampere seamount eastern north atlantic are presented the volcanic evolution of madeira can be divided into a voluminous shield stage 4 6 0 7 ma and a subsequent low volume posterosional stage 70 myr old hotspot which formed porto santo island 11 1 14 3 ma seine ampere 31 ma corral patch and ormond 65 67 ma feraud et al 1982 1986 seamounts and the serra de monchique 70 72 ma mcintyre and berger 1982 complex in southern portugal age and spatial relationships result in a calculated absolute african plate motion above the hotspot of 1 2 cm yr around a rotation pole located at 43 36 n 24 33 w

flood basalts and ocean island basalts a deep source or shallow entrainment abstract basalts from continental flood basalts cfbs and intraplate or hotspot ocean islands are found to have distinct geochemical signatures this diversity in composition is generally believed to result from the upwelling plume entraining overlying reservoirs of shallow and intermediate depth mantle material during its ascent from the deep mantle here we present laboratory experiments and numerical model calculations which clarify that for a strongly temperature dependent viscosity like that of the mantle a rising plume head should be expected to bring up a surrounding sheath of deep mantle from its source region mixing between the central core of the plume and this sheath produces the whorl like structures noted in previous studies where they were typically attributed to thermal entrainment of surrounding ambient mantle but this is mainly the product of intermixing between neighboring parts of the plume s deep source material these results imply that the popular idea that mantle plumes should typically mix small fractions of deep primitive mantle material with much larger fractions of shallower depleted mantle needs to be critically reexamined

multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes the hypothesis that a single mushroom like mantle plume head can generate a large igneous province within a few million years has been widely accepted the siberian traps at the permian triassic boundary and the deccan traps at the cretaceous tertiary boundary were probably erupted within one million years these large eruptions have been linked to mass extinctions but recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years this observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal single stage plume model suggests here we present numerical experiments to demonstrate that the entrainment of a dense eclogite derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces the characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations such a process may contribute to multiple episodes of large igneous provinces

dynamics of a laminar plume in a cavity the influence of boundaries on the steady state stem structure 1 the steady state structure of thermal plumes rising from a small heater is studied in high prandtl number fluids we show good agreement between laboratory experiments and numerical simulations we study the effect of the boundaries on the plume development by numerically simulating the plume rise in very large geometries the thermal structure of the plume axis is similar for all box sizes considered but the velocity structure changes strongly as box sizes are increased we show that the effect of the side boundaries becomes unimportant for large aspect ratio but that the free slip top boundary has a strong influence on the velocity structure under all conditions we show that the use of an outflow boundary condition significantly reduces the influence of the top boundary under these conditions we recover to good precision the theoretical predictions for plumes rising in an semi infinite half space the strong influence of the boundaries in high prandtl number fluids is important in the interpretation of laboratory experiments and numerical simulation for the dynamics of the earth s mantle

mantle plumes pinched in the transition zone abstract we have employed an axisymmetric spherical shell model with the two major phase transitions and variable viscosity to study the dynamical processes associated with both regular mantle plumes and superplumes which can develop spontaneously in strongly time dependent situations this model is heated both from below and from within and the viscosity varies according to the temperature pressure and the phase boundaries we have studied the physical processes due to the interactions between the different types of plumes and superplumes with the transition zone which can shed light on some new understanding of intraplate volcanism the interaction between the plume and the two major phase transitions gives rise to the production of a reservoir of plume material localized in the transition zone the spinel to olivine phase transition induces the ejection of very fast plume material within a very thin vertical channel towards the surface the spreading of this plume is controlled by the upper mantle circulation stable plumes which extend throughout the mantle for several hundred million years can produce volcanism with an age progression of the seamounts other plumes which are bent and move relative to the global circulation may generate chaotic surface volcanism with a very limited age progression of the seamounts plume plume collision produces a superplume growing from the core mantle boundary cmb the interaction between superplumes and the perovskite to spinel phase transition generates a thick thermal boundary layer in which the upper mantle can serve as the site for the launching of new smaller plumes stress fields associated with the superplume have the largest magnitude at the cmb and just below the 670 km discontinuity they may be associated with seismic anisotropy found there

flood basalts and large igneous provinces from deep mantle plumes fact fiction and fallacy abstract flood basalts large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature in the starting plume model flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively and the deccan flood basalt province of india and the laccadives maldives chagos reunion island volcanic chain in the indian ocean are widely thought to be an excellent example supporting this model however the existing geodynamic thermal petrological geochemical and geochronological constraints on deccan volcanism do not require or support a mantle plume origin a new interpretation made here rules out any role for the reunion plume in deccan volcanism and even questions the notion that the reunion hotspot is the manifestation of an underlying reunion plume the laccadives reunion hotspot track is viewed here as caused by a southward propagating fracture based on the published radiometric age data and the ultimate cause of deccan volcanism is argued to be mega scale lithospheric rifting the appreciation of the great original areal extent of the deccan nearly as far as the southern tip of peninsular india solves the hitherto unsolved problem of the cause of the uplift of the 1500 km long western ghats escarpment this cause being combined surface erosion and magmatic underplating hypothesized mantle plumes do not appear responsible for most large igneous provinces instead their very existence is questionable no geological evidence of any kind geochemical petrological thermal topographic requires mantle plumes the self contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts based on various grounds that a globe encircling enriched mantle layer the perisphere may overlie a deeper depleted mantle which is the source of morb the systematic geochemical variations along the laccadive reunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding morb mantle as in the current plume model hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere as originally proposed decades ago the locations of large igneous provinces hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history they are not placed randomly on the globe indifferent to the surface geology neither are large igneous provinces sudden chance events within the vastness of geological time which plume theories typically allege

the mesoproterozoic thermal evolution of the musgrave province in central australia plume vs the geological record abstract the 1090 to 2 of western and central australia it also included one of the most voluminous additions of juvenile felsic material to earth s crust with the development of one of the world s longest lived rhyolitic centres including the talbot supervolcano previous suggestions that the event was the result of a deep mantle plume cannot adequately account for the 50 m y duration of mantle derived magmatism or the fact that isolated localities such as the talbot sub basin preserve the entire magmatic record with no discernible regional age progressive spatial trend for at least 100 m y before the giles event the musgrave region experienced high to ultra high crustal temperatures possibly as an ultra hot orogen born from a c 1300 ma back arc granitic magmatism prior to the giles event also involved a significant mantle derived component and was accompanied by mid crustal ultra high temperature 1000 c metamorphism reflecting a thin and weak lithosphere this magmatism also resulted in a mid crustal 25 km deep layer greatly enriched in radiogenic heat producing elements that strongly augmented the already extreme crustal geotherms over a prolonged period the giles event may have been triggered when this regional musgrave thermal anomaly was displaced and again significantly destabilised along the mundrabilla shear zone a continent scale structure that juxtaposed the musgrave province against the easterly extension of the capricorn orogen where pre existing orogen scale structures were in extension these orogen scale structures funnelled the magmas that produced the warakurna large igneous province and the intersection of the musgrave thermal anomaly and the mundrabilla shear zone was the site of the talbot supervolcano although previously thought to be a result of a deep mantle plume the giles event was more likely the product of intra plate tectonic processes involving an anomalous and prolonged thermal pre history a magma focussing lithospheric architecture and large scale tectonic movements

ultrafast mantle plumes and implications for flood basalt volcanism in the northern atlantic region abstract recent geochemical and geochronological data on paleocene flood basalts from west greenland se greenland and the british isles show that volcanic activity at these widely separated locations commenced nearly simultaneously around 61 62 ma ago and that the duration of this initial phase of flood basalt magmatism was of the order of a few million years or less a small fast moving upper mantle plume which rapidly spreads out horizontally on encountering the base of the lithosphere appears to be a viable mechanism to explain these observations however in order to reconcile the idea of an ultrafast plume with the observed plate velocities a physical mechanism is needed for inducing a separation of timescales between the plume speed and the surrounding mantle circulation thermal convection with a non newtonian temperature and depth dependent rheology provides such a mechanism wherein extremely fast plumes ascending at velocities between one to tens of meters per year can be produced in an otherwise slowly convecting mantle moving at cm yr this transport mechanism is capable of bringing up very hot matter from the transition zone to the lithosphere the fast upward velocities lead to the production of high viscous heating rates surrounding the plume upon impinging the lithosphere it is thus possible to explain the near simultaneous onset of magmatism in west greenland se greenland and the british isles in the early tertiary by a fast moving mantle plume spreading out horizontally with a velocity of around 0 5 m yr for resolving numerically the thermal mechanical state of these strongly time dependent mantle flows extremely high spatial resolution on the kilometer scale is required finally we suggest the possibility that the source of the fast upper mantle plume under iceland may be rooted in the lower mantle this is consistent with the recent findings by seismic tomography of a deep mantle plume under iceland

fast plumeheads temperature dependent versus non newtonian rheology in a series of two dimensional cartesian simulations with high resolution on the order of 2 km we have compared for the same effective rayleigh number around 106 the dynamics between newtonian and non newtonian rheologies a viscosity contrast due to temperature of 1200 has been employed throughout with surface dissipation numbers varying between 0 05 and 0 2 peak velocities for non newtonian plumes are at least 50 times faster than the peak newtonian velocities and reach magnitudes in excess of m yr upon impinging the surface plume head structures of the non newtonian rheology contain much greater complexity in the thermal mechanical structure than for newtonian plumes

pin pricking the elephant evidence on the origin of the ontong java plateau from pb sr hf nd isotopic characteristics of odp leg 192 basalts age corrected pb sr and nd isotope ratios early aptian basalt from four widely separated sites on the ontong java plateau that were sampled during ocean drilling program leg 192 cluster within the small range reported for three earlier drill sites for outcrops in the solomon islands and for the nauru and east mariana basins hf isotope ratios also display only a small spread of values a vitric tuff with end t 4 5 that lies immediately above basement at site 1183 represents the only probable example from leg 192 of the singgalo magma type flows of which comprise the upper 46 750 m of sections in the solomon islands and at leg 130 site 807 on the northern flank of the plateau all of the leg 192 lavas including the high mgo 8 10 wt kroenke type basalts found at sites 1185 and 1187 have end t between 5 8 and 6 5 they are isotopically indistinguishable from the abundant kwaimbaita basalt type in the solomon islands and at previous plateau nauru basin and east mariana basin drill sites the little fractionated kroenke type flows thus indicate that the uniform isotopic signature of the more evolved kwaimbaita type basalt with 5 8 wt mgo is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers but instead must reflect the signature of an inherently rather homogeneous relative to the scale of melting mantle source in the context of a plume head model the kwaimbaita type magmas previously have been inferred to represent mantle derived largely from the plume source region our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event e g a small amount of partial melting approximately 3 ga or earlier and subsequently evolved in nearly closed system fashion until being tapped by plateau magmatism in the early aptian these results are consistent with current models of a compositionally distinct lower mantle and a plume head origin for the plateau however several other key aspects of the plateau are not easily explained by the plume head model the plateau also poses significant challenges for asteroid impact icelandic type and plate separation perisphere models at present no simple model appears to account satisfactorily for all of the observed first order features of the ontong java plateau the geological society of london 2004

plume related oceanic plateaus as a potential source of gold mineralization we propose that the formation of anomalously gold enriched provinces on opposite sides of the pacific basin at ca 120 ma can be traced back to the formation of a mantle plume related oceanic plateau in the permian fertile fragments of this oceanic plateau travelled across the developing pacific basin and arrived at the accretionary margin of the north china and american cratons respectively some time earlier than 120 ma formation of the oceanic plateau proximal to a midoceanic rise or triple junction implies a long cooling history and reduced buoyancy by the time it arrived at an active subduction zone consequently in this model at least a part of the plateau material could have subducted like normal oceanic crust metamorphic devolatilization caused the release of gold into the overlying crust thereby fertilizing the advective hydrothermal system which gave rise to among others gold provinces in the sierra nevada foothills california and jiaodong peninsula china this process could be a mechanism for the complete temporal and spatial decoupling of the initial mantle plume activity formation of a fertile primitive oceanic source rock and the establishment of a highly endowed gold province at an accretionary margin

the influences of surface temperature on upwellings in planetary convection with phase transitions abstract the importance of surface temperature for mantle convection appears with the presence of adiabatic heating and cooling and the release and consumption of latent heat in the presence of phase transitions for some planetary bodies these effects cannot be neglected the dimensionless surface temperature t 0 which is the ratio between the temperature at the top of the convective region and the temperature drop across the mantle is close to one for mars and venus for the earth t 0 lies between 0 2 and 0 5 the dynamical influence of t 0 is especially poignant for internally heated convection with temperature dependent viscosity there is a tight coupling between the magnitude of the temperature field and the viscosity itself we have studied temperature dependent viscosity convection for both low t 0 0 2 and high t 0 1 2 situations and with internal heating in mantle convection with two upper mantle phase transitions our results show that within this range of t 0 there exist two regimes for the evolution of upwellings in the mantle in transient situations plume plume collisions lead to the formation of megaplumes for high t 0 regimes but are less likely to do so for low t 0 in the long term regime plumes with low t 0 are prone to develop from the transition zone with a supply of hot material coming from the shallow lower mantle in systems with high t 0 however long lived plumes tend to have deeper mantle origins in quasi layered situations high t 0 may act as a positive feed back mechanism in inducing powerful hot upwellings into the upper mantle