Plume Conduit Research Articles

A Synoptic View of Mantle Plume Shapes Enabled by Virtual Reality

AbstractThe shapes of mantle plumes are sensitive to mantle viscosity, density structure, and flow patterns. Increasingly, global tomographic models reveal broad plume conduits in the lower mantle and highly tilting conduits in the mid and upper mantle. Previous studies mostly relied on 2D slices to analyze plume shapes, but fully investigating the complexity of 3D plume structures requires more effective visualization methods. Here, we use immersive headset‐based virtual reality (VR) to visualize the full‐waveform global tomographic models SEMUCB‐WM1 and GLAD‐M25. We develop criteria for the identification of plume conduits based on the relationship between the plume excess temperature and the VS anomaly (δVS). We trace 20 major plume conduits, measure the offsets of the conduits in azimuth and distance with respect to the hotspots, calculate the tilt angle, and evaluate the δVS along all traced conduits. We compare our traced conduits with the conduits predicted by global mantle convection models and vertical conduits. The wavespeed variations along conduits traced from each tomographic model are slower than modeled or vertical conduits, regardless of which tomographic model they are evaluated in. The shapes of traced conduits tend to differ greatly from modeled conduits. Plume ponding and the emergence of secondary plumes, which could result from a combination of compositional variations, phase transitions, small‐scale convection, and variations in viscosity, can contribute to the complex observed plume shapes. The variation of δVS along the traced conduits and complex plume shapes suggest a thermochemical origin of many plumes.

Seismic evidence of a plume conduit in a metasomatized lithosphere beneath the Barmer rift in northwestern India

Teleseismic tomography was done to investigate the lithosphere beneath northwestern India which was affected by several magmatic episodes since Neoproterozoic times. Broadband stations were deployed in phases at 29 locations in an area of 300 km × 300 km in northwestern India. About 8000 high-quality P phases corresponding to 934 events were used. Relative residuals estimated through adaptive stacking were inverted using the Fast Marching teleseismic tomography to obtain the 3D velocity perturbations. The tomograms revealed an anomalous ≈ 100 km wide, cylindrical zone of low velocities (−2% to −2.3%) extending from a depth of ≈ 60 km to ≈140 km beneath the Barmer rift. A broader zone of low velocities (−1%) encompasses the upper mantle down to a depth of at least 200 km. The anomaly lies beneath the localized, small-volume, eruptive centers of the earliest phase of Deccan volcanism within and on the flanks of the rift which falls on the projected extension of the Reunion hotspot track along the rift system of northwestern India. Explaining such large anomalies requires a significant increase in temperature, presence of melt and/or compositional changes. Geochemical studies indicate that the polychronous (early Cretaceous to Paleogene) alkaline complexes above the anomalous zone are derived from hydrated mantle peridotite and contain an abundance of phlogopites, suggesting a hydrated and metasomatized lithosphere beneath northwestern India. A thermal source impinging from below upon a metasomatized, weak, and rifted lithosphere may result in the observed signature akin to that of a plume conduit that overprints the older signatures of the Neoproterozoic Malani magmatic event.

The influence of uncertain mantle density and viscosity structures on the calculations of deep mantle flow and lateral motion of plumes

SUMMARY Mantle plumes form from thermal boundary layers, such as Earth's core–mantle boundary. As plumes rise towards the surface, they are laterally deflected by the surrounding mantle flow that is governed by deep mantle density and viscosity structures. The lateral motions of mantle plumes carry information of deep mantle structure and dynamics and are used to setup reference frames by which absolute plate motions are reconstructed. In this study, we compare two methods to compute deep mantle flow and lateral motion of plumes. In mantle convection (MC) models, the mantle flow field and lateral motions of plumes are determined by solving conservation equations forward-in-time from given initial conditions. In plume advection (PA) models, approximate viscosity and present-day density structures are used to calculate present-day mantle flow which is then propagated backward-in-time assuming zero thermal diffusion, and plume conduits are represented by continuous lines and are passively advected within the background mantle flow. The question is how assumptions in PA models influence the predictions of deep mantle flow and plume lateral motions. Here, we perform purely thermal MC models and thermochemical MC models with intrinsically dense materials in the lowermost mantle. The deep mantle flow and plume lateral motions are determined accurately in each MC model. We also perform PA models using the approximated present-day viscosity and temperature structures in these MC models. We find that PA models without considering temperature-dependence of viscosity and/or only using long wavelength present-day temperature structure (up to degree 20) often lead to an average of ∼50–60 per cent and ∼60–200 per cent differences of present-day mantle flow velocities than purely thermal MC models and thermochemical MC models, respectively. By propagating inaccurate flow fields backward-in-time in PA models often cause even larger errors of mantle flow velocities in the past. Even using the same parameters and starting from the same present-day mantle flow fields as in MC models, the PA models still show an average of ∼10–30 per cent misfit of mantle flow velocities after ∼40 Ma. In addition, we show that errors of mantle flow fields in PA models can cause ∼100–600 per cent differences of plume lateral motions than that constrained in MC models in the past 60 Ma. Even we use the mantle flow in MC models to advected virtual plumes in PA models, the virtual plumes could still show ∼50–300 per cent difference of lateral motions than dynamic plumes in MC models if the virtual plumes do not start with the same locations and/or shapes as plumes in MC models. We also find virtual plumes in PA models initiated at different locations and/or with different shapes can be later advected to similar locations, suggesting that the lateral motions of plumes in PA models can be non-unique. Therefore, it is important to consider the build-in assumptions of PA models when interpreting their predictions on deep mantle flow field and plume lateral motions. The accuracy of PA models would improve as we gain better understanding on Earth's deep mantle structure and dynamics.

Temporal variations in plume flux: characterizing pulsations from tilted plume conduits in a rheologically complex mantle

SUMMARY Along age-progressive hotspot volcano chains, the emplacement rate of igneous material varies through time. Time-series analysis of changing emplacement rates at a range of hotspots finds that these rates vary regularly at periods of a few to several tens of millions of years, indicative of changing melt production within underlying mantle plumes. Many hotspots exhibit at least one period between ∼2 and 10 Myr, consistent with several proposed mechanisms for changing near-surface plume flux, and thus melting rate, such as small-scale convection, solitary waves and instability formation in tilted plume conduits. Here, we focus on quantifying instability growth within plumes tilted by overlying plate motion. Previous studies using fluids with constant or temperature-dependent viscosity suggest that such instabilities should not form under mantle conditions. To test this assertion, we use a modified version of the finite element code ASPECT to simulate 400 Myr of evolution of a whole-depth mantle plume rising through the transition zone and spreading beneath a moving plate. In a 2-D spherical shell geometry, ASPECT solves the conservation equations for a compressible mantle with a thermodynamically consistent treatment of phase changes in the mantle transition zone and subject to either a temperature- and depth-dependent linear rheology or a temperature-, depth- and strain-rate dependent non-linear rheology. Additionally, we examine plume evolution in a mantle subject to a range of Clapeyron slopes for the 410 km (1–4 MPa K–1) phase transitions. Results suggest that plume conduits tilted by >67° become unstable and develop instabilities that lead to initial pulses in the transition zone followed by repeated plume pulsing in the uppermost mantle. In these cases, pulse size and frequency depend strongly on the viscosity ratio between the plume and ambient upper mantle. Based upon our results and comparison with other studies, we find that the range of statistically significant periods of plume pulsing in our models (∼2–7 Myr), the predicted increase in melt flux due to each pulse (3.8–26 × 10−5 km3 km−1 yr−1), and the time estimated for a plume to tilt beyond 67° in the upper mantle (10–50 Myr) are consistent with observations at numerous hotspot tracks across the globe. We suggest that pulsing due to destabilization of tilted plume conduits may be one of several mechanisms responsible for modulating the melting rate of mantle plumes as they spread beneath the moving lithosphere.

Seismic Constraints on Crustal and Uppermost Mantle Structure Beneath the Hawaiian Swell: Implications for Plume‐Lithosphere Interactions

AbstractWe infer the lithospheric structure beneath the Hawaiian Swell based on a joint inversion of ambient noise and teleseismic Rayleigh waves collected during the PLUME experiment. These combined datasets let us use Rayleigh waves with a period range of 8–50 s that provides imaging resolution for the shallow lithosphere and constrains interactions between the upwelling plume and migrating plate. We find an elongated low‐velocity anomaly beneath the lithosphere along the island chain that connects to the plume conduit, consistent with the melting region associated with a restite hotspot swell root that has mechanically eroded the base of its overlying lithosphere. It could also be consistent with a plume refracted by the overriding plate motion if the plume could manage to deeply erode the lithospheric base, as only a more viscous restite root is seen to do in thermomechanical experiments. There is also a low‐velocity body beneath the North Arch that coincides with the location of recent off‐chain volcanic fields discovered there. Its location relative to the Molokai Fracture Zone (MFZ) supports the concept that swell‐root material has preferentially spread beneath the younger side of the MFZ. We also find a clear low‐velocity anomaly associated with the uppermost ∼50 km of the fracture zone lithosphere. The relatively well‐resolved shallow lithospheric structure determined here allows us to estimate the volumes of underplating and the total flux of the rising plume material that is added beneath the Hawaiian lithosphere, which we constrain to be in the range of 0.5–0.6 km3/yr.

Temporal Evolution of the Swell and Magmatic Fluxes Along the Louisville Hotspot Chain

AbstractHawaii and Louisville are the two long‐lived hotspot chains in the Pacific basin. Characterizing the swell flux, Qs, a measure of the plume buoyancy, and the flux of the magmatic material accumulated along the chain, Qm, brings important constraints on the plumes' temporal evolution and on their interaction with the lithosphere. Numerous studies focus on the temporal evolution of the Hawaiian chain, but no study investigates the temporal evolution along the Louisville chain. In the present study we constrain the temporal evolution of the swell and magmatism fluxes along the Louisville chain, from volcano ages and bathymetry data. The overall Louisville magmatism flux appears nearly constant, that is, Louisville plume activity has exhibited remarkably little variation over the past 30 m.y., in agreement with geochemical data. These results are also in agreement with recent studies along Hawaii, which show that the Qm increase is due to plume migration toward a more fertile mantle reservoir. This means that, in general, long term variations in Qm relate to variations in the deep source of the plume. Along Louisville, we find short‐term variations occurring with a periodicity of ∼5–10 m.y. in both Qm and Qs, similar to the short‐term variations observed along Hawaii by former studies. As these variations occur along hotspot chains associated with either a heterogenous or homogeneous chemical signature, they must be created by shallow phenomena. The tilt of the plume conduit by large‐scale mantle advection is our preferred model, as it can account for both the geophysical and geochemical observations.

Continental Magmatism: The Surface Manifestation of Dynamic Interactions Between Cratonic Lithosphere, Mantle Plumes and Edge‐Driven Convection

AbstractSeveral of Earth's intra‐plate volcanic provinces lie on or adjacent to continental lithosphere. Although many are believed to mark the surface expression of mantle plumes, our limited understanding of how buoyant plumes interact with heterogeneous continental lithosphere prevents further progress in identifying mechanisms at the root of continental volcanism. In this study, using a suite of 3‐D geodynamical models, we demonstrate that the magmatic expression of plumes in continental settings is complex and strongly sensitive to the location of plume impingement, differing substantially from that expected beneath more homogeneous oceanic lithosphere. Within Earth's continents, thick cratonic roots locally limit decompression melting. However, they deflect plume conduits during their ascent, with plume material channeled along gradients in lithospheric thickness, activating magmatism away from the plume conduit, sometimes simultaneously at locations more than a thousand kilometres apart. This magmatism regularly concentrates at lithospheric steps, where it may be difficult to distinguish from that arising through edge‐driven convection. At times, the flow field associated with the plume enhances melting at these steps long before plume material enters the melting zone, implying that differentiating geochemical signatures will be absent. Beneath regions of thinner lithosphere, plume‐related flow can force material downwards at lithospheric steps, shutting off pre‐existing edge‐related magmatism. Additionally, variations in lithospheric structure can induce internal destabilization of ponding plume material, driving intricate magmatic patterns at the surface. Our analysis highlights the challenges associated with linking continental magmatism to underlying mantle dynamics, motivating an inter‐disciplinary approach in future studies.

Paleogene buried landscapes and climatic aberrations triggered by mantle plume activity

The Icelandic plume, a major mantle upwelling located beneath the North Atlantic Ocean, plays a dominant role in controlling periodic uplift of continental margins and of oceanographic gateways during Cenozoic times. Independent evidence shows that plume activity fluctuates on timescales of ≤1 Myr. Here, we describe and analyze a spectacular sequence of Paleogene buried landscapes imaged on seismic reflection surveys from the continental margin of northwest Europe. These transient landscapes recur on intervals of 2–3 Myrs, coincide with volcanic activity, and exhibit topographic relief of hundreds of meters. They probably represent the surficial expression of thermal fluctuations that spread radially away from the plume conduit. Five landscapes are coeval with global hyperthermal aberrations characterized by negative excursions of δ13C and by positive excursions of Fe intensity, which is a proxy for carbonate dissolution. We propose a causal relationship whereby thermal fluctuations of the mantle plume generate transient uplift and volcanic activity, which act to release significant quantities of greenhouse gases and thus trigger hyperthermal aberrations. A longer period oscillation of δ13C values is superimposed upon these aberrations that is attributable to a combination of silicate weathering and volcanic degassing caused by the interaction of plume activity and plate spreading.

On the Relation Between Basal Erosion of the Lithosphere and Surface Heat Flux for Continental Plume Tracks

AbstractWhile hotspot tracks beneath thin oceanic lithosphere are visible as volcanic island chains, the plume‐lithosphere interaction for thick continental or cratonic lithosphere often remains hidden due to the lack of volcanism. To identify plume tracks with missing volcanism, we characterize the amplitude and timing of surface heat flux anomalies following a plume‐lithosphere interaction using mantle convection models. Our numerical results confirm an analytical relationship in which surface heat flux increases with the extent of lithosphere thinning, which is primarily controlled by the viscosity structure of the lower lithosphere and the asthenosphere. We find that lithosphere thinning is greatest when the plate is above the plume conduit, while the maximum heat flux anomaly occurs about 40–140 Myr later. Therefore, younger continental and cratonic plume tracks can be identified by observed lithosphere thinning, and older tracks by an increased surface heat flux, even if they lack extrusive magmatism.

ВЛИЯНИЕ ПЛЮМОВ, НЕ ВЫШЕДШИХ НА ПОВЕРХНОСТЬ, НА ОБРАЗОВАНИЕ ПОДНЯТИЙ

The thermochemical plume originates at the core-mantle boundary in an area of increased concentration of light components that lower the melting point. The object of the study is mantle thermochemical plumes that have not reached the surface and create surface uplifts due to superlithostatic pressure on the plume roofs. The objectives of the study are to present the structure of the plume channel that has not reached the surface, the mechanism of daytime surface uplifts formation and to determine the influence of the plume roof depth and the influence of the plume group on the structure of daytime surface uplift above them. Research methodology and methods are to study the influence of mantle thermochemical plumes formed at the core-mantle boundary on the height and structures of surface rises, the method of geodynamic modeling is used: the motion in the high-viscosity massif above the plume roof, occurring under superlithostatic pressure is analyzed. Based on geological and geophysical data, a geodynamic model of surface rises is created which satisfies the three laws of conservation: energy, matter and momentum. The plume conduit is a melt in the mantle massif. Based on the available experimental modeling data, the cellular structure of the plume conduit is presented. Depending on the location depth of the roof of the plume that has not reached the surface, the thermal power on the plume base, plume diameter, and the superlithostatic pressure on the plume roof are determined. Movement in the high-viscosity block above the plume roof occurs under the influence of superlithostatic pressure. To determine the velocity field in the block above the plume roof, the solution obtained for the sphere moving in a highly viscous liquid with a constant velocity is used. When the day surface rises, the driving force due to the superlithostatic pressure decreases. When the superlithostatic pressure at the plume roof is equal to the pressure caused by elevation, the movement in the block above the plume stops. The maximum elevation height hmax = 4.5 ... 6 km was found. Elevation profiles were found for different values of the location depth of the plume roof X. The dependence of the horizontal size of the main part of the elevation y1 on the location depth of the plume roof is found. Elevation profiles were obtained for a group of five plumes, the roofs of which are at a depth of 30 km and the distance between the plume axes is y = 150 km. The elevation profiles were obtained for a group of three plumes for y = 400 km as well. At y > y1, the height of the main ridge has a saw-toothed character. Ridges whose axes are perpendicular to the axis of the main ridge are formed during the formation of uplift. The number of such ridges is equal to the number of plumes responsible for the formation of uplift. The uplift formed by a group of plumes at X = 30 km refers to the uplift of the Caucasus type, at X = 100 km refers to the uplift of the Tibet type

High 3He/4He in central Panama reveals a distal connection to the Galápagos plume

It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high 3He/4He (up to 8.9RA) in low-temperature (≤50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source 3He/4He >10.3RA (and potentially up to 26RA, similar to Galápagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1RA). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high 3He/4He values in central Panama are likely derived from the infiltration of a Galápagos plume-like mantle through a slab window that opened ∼8 Mya. Two potential transport mechanisms can explain the connection between the Galápagos plume and the slab window: 1) sublithospheric transport of Galápagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galápagos plume material blown by a "mantle wind" toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galápagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits.

ON HEAT SOURCE IN SUBDUCTION ZONE

The subduction of an oceanic plate is studied as the motion of a high-viscosity Newtonian fluid. The subducting plate spreads along the 670-km depth boundary under the influence of oppositely directed horizontal forces. These forces are due to oppositely directed horizontal temperature gradients. We consider the flow structure and heat transfer in the layer that includes both the oceanic lithosphere and the crust and moves underneath a continent. The heat flow is estimated at the contact between the subducting plate and the surrounding mantle in the continental limb of the subduction zone. Our study results show that the crustal layer of the subducting plate can melt and a thermochemical plume can form at the 670-km boundary. Our model of a thermochemical plume in the subduction zone shows the following: (1) formation of a plume conduit in the crustal layer of the subducting plate; (2) formation of a primary magmatic chamber in the area wherein the melting rate equals the rate of subduction; (3) origination of a vertical plume conduit from the primary chamber melting through the continent; (4) plume eruption through the crustal layer to the surface, i.e. formation of a volcano. Our experiments are aimed to model the plume conduit melting in an inclined flat layer above a local heat source. The melt flow structure in the plume conduit is described. Laboratory modeling have revealed that the mechanisms of melt eruption from the plume conduit differ depending on whether a gas cushion is present or absent at the plume roof.

Unsteady conductive heat transfer on the cylindrical surface of a plume conduit

Unsteady conductive heat transfer on the cylindrical surface of a plume conduit

Multi‐Mode Waveform Tomography of the Indian Ocean Upper and Mid‐Mantle Around the Réunion Hotspot

AbstractRéunion Island in the western Indian Ocean is well known as one of the most active volcanic hotspots on Earth. Its birth, Ma ago, created the Deccan volcanic traps in India (almost 2 million ), associated with the Cretaceous‐Tertiary boundary and with the extinction of about 90% of life on the Earth, including dinosaurs. However, the deep structure of the underlying mantle, the potential presence of a rising plume and its exact geometry in the lower and in the upper mantle are still subjects of debates. The use of seismic data acquired by the French‐German RHUM‐RUM experiment in the Indian Ocean around the Réunion volcanic hotspot (2012–2013) and the collection of broadband seismic data from temporary experiments and from the FDSN (Federation of Digital Seismograph Networks) data center make it possible to investigate the deep structure of the Réunion mantle plume along its complete track, from its birth to its present stage, with a lateral resolution of km. So far, global seismic tomography models cannot provide such high resolution images of the transition zone or lower mantle in this region. In this study, we used the spectral element method (SEM) to perform waveform forward modeling for several thousand paths beneath the Indian Ocean, and normal mode perturbation theory to compute the gradient and the Hessian for the inverse part of the tomography. Using this hybrid method, we derived a regional tomographic model (including teleseismic and regional events) beneath the Indian Ocean, down to km depth, from simultaneous inversion of fundamental and higher mode three components waveforms down to 40 s period. Our model retrieves a low‐velocity channel extending from West to East in the western side of the Central Indian Ridge, in the depth range of 150–250 km. It also reveals a plume conduit with a broad head in the upper mantle and narrow tail anchored in the lower mantle at 1,200 km depth or deeper. The connection between the Réunion hotspot and the South‐Africa Large Low‐Shear Velocity Province (LLSVP) is also brought to light. Our findings suggest a long‐lived Réunion hotspot, since the lower part of the conduit appears to be anchored in the lower mantle, likely fed by the African LLSVP. Our results will guide further geochemical and geodynamic studies on the interaction between the lower transition zone (660–1,000 km) and the deep lower mantle beneath the Réunion hotspot.

Quantifying Periodic Variations in Hotspot Melt Production

AbstractFluctuations in estimated mantle plume melt production rates suggest complex dynamics in the upper mantle including changes in plume upwelling, compositional heterogeneity within plume stems, entrainment of anomalous mantle, or other processes. Despite the implications for mantle dynamics, differences between previous studies including the methods, study areas, and types of melt volumes estimated, limit inter‐hotspot comparisons and assessment of global variability. Here, we use a consistent methodology to calculate igneous melt production through time across a suite of 12 hotspot chains located in the Pacific, Indian, and Atlantic Ocean basins. Using global data sets of topography, gravity, sediment cover, and seafloor age along with spectral methods for calculating elastic plate flexure and gravity anomalies, we estimate the total volume of igneous material at each hotspot in three components: (a) volcanic edifices, (b) flexural infill, and (c) intrusions (underplating) within the upper mantle and lower crust. Results indicate time‐averaged melt production rates across all studied hotspots range from 3.44 m3 s−1 (Cobb) to 14.75 m3 s−1 (Kerguelen), with underplating making up the largest component. Through time, melt production rates vary from 0.1 to 2.5 times average values with statistically significant periodic fluctuations of 1.1–71.5 Myr. Spectral analysis of estimated melt volumes shows total melt production varies periodically at all 12 hotspots. Excepting Galápagos, the studied hotspots exhibit multiple periodicities of melt production variability, implying complex dynamic, thermal, and/or chemical processes within the plume conduit. Moreover, we find several groups of hotspots exhibiting similar periods of melt production variability.

A tree of Indo-African mantle plumes imaged by seismic tomography

Mantle plumes were conceived as thin, vertical conduits in which buoyant, hot rock from the lowermost mantle rises to Earth’s surface, manifesting as hotspot-type volcanism far from plate boundaries. Spatially correlated with hotspots are two vast provinces of slow seismic wave propagation in the lowermost mantle, probably representing the heat reservoirs that feed plumes. Imaging plume conduits has proved difficult because most are located beneath the non-instrumented oceans, and they may be thin. Here we combine new seismological datasets to resolve mantle upwelling across all depths and length scales, centred on Africa and the Indian and Southern oceans. Using seismic waves that sample the deepest mantle extensively, we show that mantle upwellings are arranged in a tree-like structure. From a central, compact trunk below ~1,500 km depth, three branches tilt outwards and up towards various Indo-Austral hotspots. We propose that each tilting branch represents an alignment of vertically rising blobs or proto-plumes, which detached in a linear staggered sequence from their underlying low-velocity corridor at the core–mantle boundary. Once a blob reaches the viscosity discontinuity between lower and upper mantle, it spawns a ‘classical’ plume-head/plume-tail sequence.

Geomagnetic evidence for a continuously connected plume conduit extending to at least the 660-km discontinuity below Hawaii

Do plumes exist? If so, do plume tails connect hotspots to the core-mantle boundary (CMB), or only to the 660-km discontinuity (between the upper and the lower mantle)? Are they continuous or fragmented? Here, we demonstrate quantitatively that the Hawaiian plume distorts 24-h-period magnetovariational data by providing a continuous electrical connection between the Earth's surface (via the ocean) and the 660-km mid-mantle discontinuity. We extend our analysis to predict how a plume ascending from the CMB should distort magnetic secular variation observed locally at the surface. Hence, the Hawaiian plume is demonstrated to be a continuous feature with a deep-seated (at least mid-mantle) source. Our analysis provides a novel technique for determining whether the Hawaiian plume originates at the mid-mantle transition zone or CMB subject to magnetic secular variation data being acquired locally, as well as allowing the depth of genesis of other mantle plumes to be constrained should further geomagnetic data become available in their vicinity.

Spatial Characteristics of Recycled and Primordial Reservoirs in the Deep Mantle

AbstractThe spatial distribution of the geochemical domains hosting recycled crust and primordial (high‐3He/4He) reservoirs, and how they are linked to mantle convection, are poorly understood. Two continent‐sized seismic anomalies located near the core‐mantle boundary—called the Large Low Shear Wave Velocity Provinces (LLSVPs)—are potential geochemical reservoir hosts. It has been suggested that high‐3He/4He hotspots are spatially confined to the LLSVPs, hotspots sampling recycled continental crust are associated with only one of the LLSVPs, and recycled continental crust shows no relationship with latitude. We reevaluate the links between LLSVPs and isotopic signatures of hotspot lavas using improved mantle flow models including plume conduit advection. While most hotspots with the highest‐3He/4He can indeed be traced to the LLSVP interiors, at least one high‐3He/4He hotspot, Yellowstone, is located outside of the LLSVPs. This suggests high‐3He/4He is not geographically confined to the LLSVPs. Instead, a positive correlation between hotspot buoyancy flux and maximum hotspot 3He/4He suggests that it is plume dynamics (i.e., buoyancy), not geography, which determines whether a dense, deep, and possibly widespread high‐3He/4He reservoir is entrained. We also show that plume‐fed EM hotspots (enriched mantle, with low‐143Nd/144Nd), signaling recycled continental crust, are spatially linked to both LLSVPs, and located primarily in the southern hemisphere. Lastly, we confirm that hotspots sampling HIMU (“high‐μ,” or high 238U/204Pb) domains are not spatially limited to the LLSVPs. These findings clarify and advance our understanding of deep mantle reservoir distributions, and we discuss how continental and oceanic crust subduction is consistent with the spatial decoupling of EM and HIMU.

The interaction between mantle plumes and lithosphere and its surface expressions: 3-D numerical modelling

SUMMARY The rise of mantle plumes to the base of the lithosphere leads to observable surface expressions, which provide important information about the deep mantle structure. However, the process of plume–lithosphere interaction and its surface expressions remain not well understood. In this study, we perform 3-D spherical numerical simulations to investigate the relationship between surface observables induced by plume–lithosphere interaction (including dynamic topography, geoid anomaly and melt production rate) and the physical properties of plume and lithosphere (including plume size, plume excess temperature, plume viscosity, and lithosphere viscosity and thickness). We find that the plume-induced surface expressions have strong spatial and temporal variations. Before reaching the base of the lithosphere, the rise of a plume head in the deep mantle causes positive and rapid increase of dynamic topography and geoid anomaly at the surface but no melt production. The subsequent impinging of a plume head at the base of the lithosphere leads to further increase of dynamic topography and geoid anomaly and causes rapid increase of melt production. After reaching maximum values, these plume-induced observables become relatively stable and are more affected by the plume conduit. In addition, whereas the geoid anomaly and dynamic topography decrease from regions above the plume centre to regions above the plume edge, the melt production always concentrates at the centre part of the plume. We also find that the surface expressions have different sensitivities to plume and lithosphere properties. The dynamic topography significantly increases with the plume size, plume excess temperature and plume viscosity. The geoid anomaly also increases with the size and excess temperature of the plume but is less sensitive to plume viscosity. Compared to the influence of plume properties, the dynamic topography and geoid anomaly are less affected by lithosphere viscosity and thickness. The melt production significantly increases with plume size, plume excess temperature and plume viscosity, but decreases with lithosphere viscosity and thickness.

СТРУКТУРА КАНАЛА ПЛЮМА, ФОРМИРУЮЩЕГОСЯ В ПЛОСКОМ СЛОЕ (МОДЕЛИРОВАНИЕ ПЛЮМА В ЗОНЕ СУБДУКЦИИ)

Experimental studies of a plume forming in a flat paraffin layer above a local heat source are presented. The experiments were performed on an installation with a transparent front wall. The flat payer is inclined two degrees with respect to the gravity vector. The thermal power transferred from the plume conduit under conditions of stationary thermal conduction is determined. The structure of the plume conduit and the structure of free-convective flows are determined for different plume thermal power values. Free-convection flows along the conduit height are cellular ones. The cellular structure occurs due to unstable stratification along the height of the plume conduit. The ascending flow in the plume conduit is a localized jet stream; the descending flow exists along the perimeter of the conduit. The cell boundaries correspond to narrowings of the plume conduit, formed due to cooling of the descending flow to a temperature close to the melting point. The cell parameters are determined for different values of relative thermal power (Ka criterion).The free-convection flow structure in the cells of the plume conduit is elucidated. The modes of reaching the surface for plumes forming in a flat layer are determined in relation to Ka criterion. When the relative power Ka exceeds 1.14, the model plume reaches the surface with the formation of a mushroom-shaped head. This case may correspond to plumes responsible for the formation of batholiths in the Andean subduction zone. Experiments have shown that the structure of the plume conduit in a flat layer corresponds to the plume structure formed in a solid massif above a local heat source. Distinctions in the thermal and hydrodynamic structure for a flat layer are observed due to the solid surfaces bounding the layer.