Mantle Plume Dynamics Research Articles

Zircon Xenocrysts From Easter Island (Rapa Nui) Reveal Hotspot Activity Since the Middle Jurassic

AbstractWe report the finding of mantle‐derived zircon grains retrieved from red soils, regoliths, and beach sands from Easter Island, that are much older than the island volcanism (0–2.5 Ma) and its underlying lithosphere (Pliocene, 3–4.8 Ma). A large population of 0–165 Ma old zircons have coherent oxygen (δ18O 3.8–5.9‰) and hafnium (εHf(t)+3.5–+12.5) mantle isotopic signatures. These results are consistent with the crystallization of zircon from plume‐related melts. In addition, a chemically diverse population with ages from the Paleozoic to the Archean was found. These older populations are enigmatic but they could represent remnants of ancient subducted sediments. Meanwhile, the ∼0–165 Ma population is interpreted as plume‐derived, suggesting that the hotspot started at least ∼165 Ma ago. A spike of ∼164–160 Ma zircons could represent a Large Igneous Province (LIP) stage upon the first arrival of the plume. We use plate reconstructions to show that such a LIP would have formed on the Phoenix Plate and would have subducted below the Antarctic Peninsula around 100–105 Ma. There, LIP subduction would offer a solution for the enigmatic Palmer Land deformation event, previously proposed to result from a collision with an unknown indenter. The here‐reported “ghost” of a prolonged hotspot activity suggests that fragments of the Easter plume and of the ambient sub‐lithospheric mantle stored and re‐sampled zircon xenocrysts due to convective (re)circulation at the scale of the plume head. Our study demonstrates how zircon geochronology and geochemistry provide novel insights into global‐scale geodynamics, offering new perspectives on the dynamics of mantle plumes and hotspot activity.

Pressure stabilizes ferrous iron in bridgmanite under hydrous deep lower mantle conditions

Earth’s lower mantle is a potential water reservoir. The physical and chemical properties of the region are in part controlled by the Fe3+/ΣFe ratio and total iron content in bridgmanite. However, the water effect on the chemistry of bridgmanite remains unclear. We carry out laser-heated diamond anvil cell experiments under hydrous conditions and observe dominant Fe2+ in bridgmanite (Mg, Fe)SiO3 above 105 GPa under the normal geotherm conditions corresponding to depth > 2300 km, whereas Fe3+-rich bridgmanite is obtained at lower pressures. We further observe FeO in coexistence with hydrous NiAs-type SiO2 under similar conditions, indicating that the stability of ferrous iron is a combined result of H2O effect and high pressure. The stability of ferrous iron in bridgmanite under hydrous conditions would provide an explanation for the nature of the low-shear-velocity anomalies in the deep lower mantle. In addition, entrainment from a hydrous dense layer may influence mantle plume dynamics and contribute to variations in the redox conditions of the mantle.

East European sedimentary basins long heated by a fading mantle upwelling

A strong negative anomaly of seismic wave velocities at the core-mantle boundary (the Perm Anomaly) beneath the East European platform is attributed to the remnant of a deep mantle upwelling. The interaction between the upwelling and the East European lithosphere in the geological past and its resulting surface manifestations are still poorly understood. Using mantle plume modelling and global plate motion reconstructions, we show here that the East European lithosphere is likely to have been situated over the weakening Perm Anomaly upwelling for about 150–200 million years. As the East European platform moved above the Perm Anomaly in post-Jurassic times, the vertical tectonic movements recorded in sedimentary hydrocarbon-rich basins show either hiatus/uplift or insignificant subsidence. Analytical modelling of heat conduction through the lithosphere demonstrates that the basins have been slowly heated for a long time by the Perm Anomaly upwelling, creating suitable conditions for hydrocarbon maturation. This suggests a profound relationship between mantle plume dynamics, basin evolution, and hydrocarbon generation.

Frontier of Understanding Earth's Dynamics

Frontier of Understanding Earth's Dynamics

Mantle plume dynamics at the rear of a retreating slab

SUMMARY In this paper, we present 3-D numerical simulations in which a compositional mantle plume rises through a shallow mantle layer at the rear of a retreating slab. The slab–plume buoyancy flux ratio Bs/Bp is varied from 3 to 30 over nine slab–plume simulations. The plume causes an overall decrease of the slab retreat rate from 15 to 7 % in the corresponding range $B_s/B_p=[3,30]$. The retreat rate decrease occurs in two stages: the first decrease occurs remotely when plume and slab are hundreds of kilometres apart; the second decrease is linked to the slab–plume-head impact. Continuous tracking of key positions along the plume head and conduit together with velocity profiling further shows a very close interplay between the conduit and plume head dynamics. In particular, the combination of the slab– and plume(head)–induced flows at the rear of the slab increases the advection and tilt of the conduit and causes its flaring with height in the direction parallel to the trench. As a result, the conduit source slowly drifts away from the slab by hundreds of kilometres and flares by one and a half times its original radius over 30 Myr of plume head spreading. The conduit tilt and flaring result in an increasingly unbalanced azimuthal distribution of the incoming plume flux from the feeding conduit into the head. These changes in the feeding conditions coupled with the mantle flow at the level of the plume head lead to the asymmetric spreading of plume material beneath the plate in the preferential direction that is parallel to the trench. Upon its arrival against the slab, it can be a front of buoyant material set to subduct along the slab width that has widened up to two and a half times its initial dimension. In nature, it is expected to extend from hundreds to thousands of kilometres depending on the slab–plume buoyancy flux ratio. To our knowledge, this study is the first to highlight the inter-relationships between a plume head and its feeding conditions in the plume source region.

The combined effects of post-spinel and post-garnet phase transitions on mantle plume dynamics

Mineralogical studies indicate that two major phase transitions occur near the depth of 660 km in the Earth's pyrolitic mantle: the ringwoodite (Rw) to perovskite (Pv) + magnesiowüstite (Mw) and the majorite (Mj) to perovskite (Pv) phase transitions. Seismological results also show a complicated phase boundary structure at this depth in plume regions. However, previous geodynamical modeling has mainly focused on the effects of the Rw–Pv+Mw phase transition on plume dynamics and has largely neglected the effects of the Mj–Pv phase transition. Here, we develop a 3-D regional spherical geodynamic model to study the combined influence of these two phase transitions on plume dynamics. Our results show the following: (1) A double phase boundary occurs in the high-temperature center of the plume, corresponding to the double reflections in seismic observations. Other plume regions feature a single, flat uplifted phase boundary, causing a gap of high seismic velocity anomalies. (2) Large amounts of relatively low-temperature plume materials can be trapped in the transition zone due to the combined effects of phase transitions, forming a complex truncated cone shape. (3) The Mj–Pv phase transition greatly enhances the plume penetration capability through 660-km phase boundary, which has a significant influence on the plume dynamics. Our results provide new insights which can be used to better constrain the 660-km discontinuity variations, seismic wave velocity structure and plume dynamics in the mantle transition zone. The model can also help to estimate the mantle temperature and Clapeyron slopes at the 660 km phase boundary.

Early signs of geodynamic activity before the 2011–2012 El Hierro eruption

The potential relation between mantle plume dynamics, regional tectonics and eruptive activity in the Canary Islands has not been studied yet through the analysis of long-time series of geophysical observational data. The existence of highly reliable seismic and geodetic data has enabled us to study from 1996 to 2014 the geodynamic evolution of the North Atlantic Azores-Gibraltar region (including the NW African margin) and its relationship with recent volcanic activity in El Hierro (Canary Islands). We compiled a new and unified regional seismic catalog and used long time-series of digital 3D surface displacements recorded by permanent GPS stations in the region. A joint regional- and local-scale analysis based on these data enabled us to identify signs of anomalous tectonic activity from 2003 onwards, whose intensity increased in 2007 and finally accelerated three months before the onset of the volcanic eruption on El Hierro in October 2011. Activity included the occurrence of regional extension and an uplift process affecting the southern Iberian Peninsula, NW Africa, and the Canary Islands. We interpret these observations as early signs of the geodynamic activity, which led to El Hierro eruption and the subsequent episodes of magma intrusion. Results point to the significant contribution of the mantle plume dynamics (i.e. external forces) in this renewed volcanic activity in the Canary Islands and emphasize the role of mantle dynamics in controlling regional tectonics.

Reconstructing the Cenozoic evolution of the mantle: Implications for mantle plume dynamics under the Pacific and Indian plates

The lack of knowledge of the initial thermal state of the mantle in the geological past is an outstanding problem in mantle convection. The resolution of this problem also requires the modelling of 3-D mantle evolution that yields maximum consistency with a wide suite of geophysical constraints. Quantifying the robustness of the reconstructed thermal evolution is another major concern. To solve and estimate the robustness of the time-reversed (inverse) problem of mantle convection, we analyse two different numerical techniques: the quasi-reversible (QRV) and the backward advection (BAD) methods. Our investigation extends over the 65 Myr interval encompassing the Cenozoic era using a pseudo-spectral solution for compressible-flow thermal convection in 3-D spherical geometry. We find that the two dominant issues for solving the inverse problem of mantle convection are the choice of horizontally-averaged temperature (i.e., geotherm) and mechanical surface boundary conditions. We find, in particular, that the inclusion of thermal boundary layers that yield Earth-like heat flux at the top and bottom of the mantle has a critical impact on the reconstruction of mantle evolution. We have developed a new regularisation scheme for the QRV method using a time-dependent regularisation function. This revised implementation of the QRV method delivers time-dependent reconstructions of mantle heterogeneity that reveal: (1) the stability of Pacific and African ‘large low shear velocity provinces’ (LLSVP) over the last 65 Myr; (2) strong upward deflections of the CMB topography at 65 Ma beneath: the North Atlantic, the south–central Pacific, the East Pacific Rise (EPR) and the eastern Antarctica; (3) an anchored deep-mantle plume ascending directly under the EPR (Easter and Pitcairn hotspots) throughout the Cenozoic era; and (4) the appearance of the transient Reunion plume head beneath the western edge of the Deccan Plateau at 65 Ma. Our reconstructions of Cenozoic mantle evolution thus suggest that mantle plumes play a key role in driving surface tectonic processes and large-scale volcanism.

Time-dependent convection models of mantle thermal structure constrained by seismic tomography and geodynamics: implications for mantle plume dynamics and CMB heat flux

SUMMARY One of the outstanding problems in modern geodynamics is the development of thermal convection models that are consistent with the present-day flow dynamics in the Earth’s mantle, in accord with seismic tomographic images of 3-D Earth structure, and that are also capable of providing a time-dependent evolution of the mantle thermal structure that is as ‘realistic’ (Earth-like) as possible. A successful realization of this objective would provide a realistic model of 3-D mantle convection that has optimal consistency with a wide suite of seismic, geodynamic and mineral physical constraints on mantle structure and thermodynamic properties. To address this challenge, we have constructed a time-dependent, compressible convection model in 3-D spherical geometry that is consistent with tomography-based instantaneous flow dynamics, using an updated and revised pseudo-spectral numerical method. The novel feature of our numerical solutions is that the equations of conservation of mass and momentum are solved only once in terms of spectral Green’s functions. We initially focus on the theory and numerical methods employed to solve the equation of thermal energy conservation using the Green’s function solutions for the equation of motion, with special attention placed on the numerical accuracy and stability of the convection solutions. A particular concern is the verification of the global energy balance in the dissipative, compressible-mantle formulation we adopt. Such validation is essential because we then present geodynamically constrained convection solutions over billion-year timescales, starting from present-day seismically constrained thermal images of the mantle. The use of geodynamically constrained spectral Green’s functions facilitates the modelling of the dynamic impact on the mantle evolution of: (1) depth-dependent thermal conductivity profiles, (2) extreme variations of viscosity over depth and (3) different surface boundary conditions, in this case mobile surface plates and a rigid surface. The thermal interpretation of seismic tomography models does not provide a radial profile of the horizontally averaged temperature (i.e. the geotherm) in the mantle. One important goal of this study is to obtain a steady-state geotherm with boundary layers which satisfies energy balance of the system and provides the starting point for more realistic numerical simulations of the Earth’s evolution. We obtain surface heat flux in the range of Earth-like values : 37 TW for a rigid surface and 44 TW for a surface with tectonic plates coupled to the mantle flow. Also, our convection simulations deliver CMB heat flux that is on the high end of previously estimated values, namely 13 TW and 20 TW, for rigid and plate-like surface boundary conditions, respectively. We finally employ these two end-member surface boundary conditions to explore the very-long-time scale evolution of convection over billion-year time windows. These billion-year-scale simulations will allow us to determine the extent to which a ‘memory’ of the starting tomography-based thermal structure is preserved and hence to explore the longevity of the structures in the present-day mantle. The two surface boundary conditions, along with the geodynamically inferred radial viscosity profiles, yield steady-state convective flows that are dominated by long wavelengths throughout the lower mantle. The rigid-surface condition yields a spectrum of mantle heterogeneity dominated by spherical harmonic degree 3 and 4, and the plate-like surface condition yields a pattern dominated by degree 1. Our exploration of the time-dependence of the spatial heterogeneity shows that, for both types of surface boundary condition, deep-mantle hot upwellings resolved in the present-day tomography model are durable and stable features. These deeply rooted mantle plumes show remarkable longevity over very long geological time spans, mainly owing to the geodynamically inferred high viscosity in the lower mantle.

South Pacific hotspot swells dynamically supported by mantle flows

The dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics. Here we construct a numerical model of the mantle flow beneath the French Polynesia region. Our study is based on a new regional seismic tomography model, which high resolution allows obtaining information at the scale of plumes. We find excellent correlations between the observed and the modeled dynamic swells, between the modeled flow pattern and the active volcanism and between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology. These outstanding fits reveal for the first time that a direct link exists between the surface observations and mantle flows.

Surface subsidence caused by mantle plumes and volcanic loading in large igneous provinces

Large igneous provinces form with massive flood basalts being erupted over a large region within a few million years. A prevailing model for their formation is a mantle plume model in which a plume head originates from the core–mantle boundary, ascends through the mantle, and produces topographic uplift and eventually massive melting at the surface. However, many geological observations indicate surface subsidence in the central region before or during flood basalt eruptions. In this study, we demonstrate that a plume head temporarily ponding below the 660-km phase change boundary causes significant subsidence at the Earth's surface over an extended period before the eruption, and that the loading from erupted basalts causes surface subsidence at the periphery of the eruption area that affects the environment for subsequent episodes of basalt eruptions. Our studies therefore demonstrate that the observed subsidence history in many flood basalt provinces is characteristic of the dynamics of mantle plumes and may be used as diagnostics for identifying plume-induced flood basalt events.

Evidence for mantle plumes?

Geophysical hotspots have been attributed to partially molten asthenosphere, fertile blobs, small-scale convection and upwellings driven by core heat. Most are short-lived or too close together to be deeply seated, and do not have anomalous heat flow or temperature; many are related to tectonic features. Bourdon et al. investigate the dynamics of mantle plumes from uranium-series geochemistry and interpret their results as evidence for thermal plumes. Here we show why alternative mechanisms of upwelling and melting should be considered.

Insights into the dynamics of mantle plumes from uranium-series geochemistry

The long-standing paradigm that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumes--hot, buoyant upwelling regions beneath the Earth's lithosphere--has recently been the focus of controversy. Whether mantle plumes exist or not is pivotal for our understanding of the thermal, dynamic and compositional evolution of the Earth's mantle. Here we show that uranium-series disequilibria measured in hotspot lavas indicate that hotspots are indeed associated with hot and buoyant upwellings and that weaker (low buoyancy flux) hotspots such as Iceland and the Azores are characterized by lower excess temperatures than stronger hotspots such as Hawaii. This direct link between buoyancy flux and mantle temperature is evidence for the existence of mantle plumes.

Dynamics of mantle plumes and their implications for the heat budget and composition of the mantle

Dynamics of mantle plumes and their implications for the heat budget and composition of the mantle

Dynamics of thermochemical plumes: 2. Complexity of plume structures and its implications for mapping mantle plumes

The mantle plume hypothesis provides explanations for several major observations of surface volcanism. The dynamics of plumes with purely thermal origin has been well established, but our understanding of the role of compositional variations in the Earth's mantle on plume formation is still incomplete. In this study we explore the structures of plumes originating from a thermochemical boundary layer at the base of the mantle in an attempt to complement fluid dynamical studies of purely thermal plumes. Our numerical experiments reveal diverse characteristics of thermochemical plumes that frequently deviate from the classic features of plumes. In addition, owing to the interplay between the thermal and compositional buoyancy forces, the morphology, temperature, and flow fields in both the plume head and plume conduit are strongly time‐dependent. The entrainment of the dense layer and secondary instabilities developed in the boundary layer contribute to lateral heterogeneities and enhance stirring processes in the plume head. Our models show that substantial topography of the compositional layer can develop simultaneously with the plumes. In addition, plumes may be present in the lower mantle for more than 70 million years. These features may contribute to the large low seismic velocity provinces beneath the south central Pacific, the southern Atlantic Ocean, and Africa. Our model results support the idea that the dynamics of mantle plumes is much more complicated than conventional thinking based on studies of purely thermal plumes. The widely used criteria for mapping mantle plumes, such as a vertically continuous low seismic velocity signature and strong surface topography swell, may not be universally applicable. We propose that the intrinsic density contrast of the distinct composition may reduce the associated topography of some large igneous provinces such as Ontong Java.

Transpressional collision tectonics and mantle plume dynamics: the Variscides of southwestern Iberia

Abstract In southwestern Iberia, three continental domains (the South Portuguese Zone (SPZ), Ossa-Morena Zone (OMZ) and Central Iberian Zone (CIZ) collided in Devonian-Carboniferous time. The collision was transpressional, with left-lateral kinematics, and was interrupted by extensional tectonics during the earliest Carboniferous, when bimodal magmatism (with associated mineral deposits) and basin development were the dominant orogenic features. Transpression was renewed in Visean time, and persisted until the end of the Carboniferous. The IBERSEIS deep seismic reflection profile helps to define the 3D geometry of transpressional structures: out-of-section displacements concentrate in bands, which bound wedges of upper crust; this crustal wedging strongly modifies the geometry of the sutures between continental blocks. A mid-crustal strongly reflective thick band (the Iberseis Reflective Body, IRB) is interpreted as a huge body of basic rocks. The IRB magma trapped in the middle crust was linked to the Early Carboniferous mantle-derived magmatism that crops out in the SPZ, OMZ and CIZ. Magmatism at the surface and trapped in the crust, high thermal gradients and basin development reflect a thermal anomaly in the underlying mantle, influencing both the thermal and the stress state of the orogen at that time. A mantle plume is inferred to have existed in the Early Carboniferous, the transpressional tectonic regime dominating again after its decay.

Dynamics of thermal plumes in three-dimensional isoviscous thermal convection

SUMMARY The dynamics of mantle plumes are important for understanding intraplate volcanism and heat transfer in the mantle. Using 3-D numerical models and scaling analyses, we investigated the controls of convective vigour or Ra (Rayleigh number) on the dynamics of thermal plumes in isoviscous and basal heating thermal convection. We examined the Ra dependence of plume number, plume spacing, plume vertical velocity and plume radius. We found that plume number does not increase monotonically with Ra. At relatively small Ra(≤106), plume number is insensitive to Ra. For 3 × 106≤Ra≤ 3 × 107, plume number scales as Ra0.31 and plume spacing λ∼Ra−0.16∼δ1/2, where δ is the thickness of the thermal boundary layer. However, for larger Ra(∼108) plume number and plume spacing again become insensitive to Ra. This indicates that the box depth poses a limit on plume spacing and plume number. We demonstrate from both scaling analyses and numerical experiments that the scaling exponents for plume number, n, heat flux, β, and average velocity on the bottom boundary, v, satisfy n= 4β− 2v. Our scaling analyses also suggest that vertical velocity in upwelling plumes Vup∼Ra2(1−n+β/2)/3 and that plume radius Rup∼Ra(β−1−n/2)/3, which differ from the scalings for the bottom boundary velocity and boundary layer thickness.

Ar-Ar and U-Pb geochronol-ogy of Late Paleozoic basalts in western Guangxi and its constraints on the eruption age of Emeishan basalt mag-matism

The Late Paleozoic layered or stratoid-layered basalts in western Guangxi have similar elemental and isotopic compositions to Emeishan high-Ti basalts. Whole-rock 40Ar/39Ar and SHRIMP zircon U-Pb dating were carried out for the representative basalt samples in three typical profiles in the area. Three basalts from the upper segment of Yangxu profile and lower segment of Yufeng and Min’an profiles yield the 40Ar/39Ar plateau ages of 253.6/+-0.4 Ma (20BS-71), 255.4±0.4 Ma (20BS-99) and 256.2±0.8 Ma (20BS-119), respectively. Twenty-three analyses on 23 zircons of the basalt from the upper segment of Yangxu profile give a weighted mean 206Pb/238U age of 253.7 ±6.1 Ma with an MSWD = 2.8. These new and published geochronological data for Emeishan large igneous province (LIP) indicate that the Emeishan LIP was initiated at ∼260 Ma, voluminously erupted between 253 and 256 Ma, and possibly ended at ∼251–253 Ma: The age (251–260 Ma) is generally consistent with that of the associated environmental deterioration and mass extinction events at the end-Guadalupian and Permo-Triassic boundary. These precise geochronological data provide important constraints on the dominantly eruptive time of the Emeisban LIP and understanding of the distribution of Emeishan high-Ti basalts and its mantle plume dynamics.

Phase relation and mineral chemistry in pyrolite to 2200°C under the lower mantle pressures and implications for dynamics of mantle plumes

Phase relations in pyrolite were studied at pressures of 24 and 30 GPa and temperatures up to 2200°C. At 24 GPa and 1600°C, an assemblage of Mg‐perovskite, magnesiowüstite, Ca‐perovskite, and garnet was observed, with the proportion of garnet increasing continuously with temperature. At 30 GPa, only the two perovskites and magnesiowüstite were observed, and no significant changes in mineral proportions with temperature were observed. Iron was almost equally partitioned between Mg‐perovskite and magnesiowüstite at temperatures below about 1800°C at 24 and 30 GPa. With increasing temperature, however, iron was concentrated more in magnesiowüstite at both of these pressures. The decrease of Al content in Mg‐perovskite with temperature, which is caused by an increase in the proportion of garnet, is likely to be responsible for the iron partitioning behavior at 24 GPa, while the minor elements (Na, Al, and Cr) in magnesiowüstite may play an important role in Fe partitioning at 30 GPa. The density difference between ascending hot plumes (ΔT = 200 and 400°C) and the surrounding mantle was calculated using the present experimental data. The density difference (ΔT = 400°C) at 30 GPa (806‐km depth) is 0.06 g/cm3 but increases to 0.22 g/cm3 at 24 GPa (670‐km depth). The large increase in the density difference that occurs in a hot plume at the uppermost part of the lower mantle originates from an increase of the proportion of garnet and may be a trigger for the plume's passage through the boundary between the lower mantle and the mantle transition zone.

Constraints on the dynamics of mantle plumes from uplift of the Hawaiian Islands

The ∼0.2 mm/yr uplift of Hawaiian islands Lanai and Molokai and Hawaiian swell topography pose important constraints on the structure and dynamics of mantle plumes. We have formulated 3-D models of mantle convection to investigate the effects of plume–plate interactions on surface vertical motions and swell topography. In our models, the controlling parameters are plume radius, excess plume temperature, and upper mantle viscosity. We have found that swell height and swell width constraints limit the radius of the Hawaiian plume to be smaller than 70 km. The additional constraint from the uplift at Lanai requires excess plume temperature to be greater than 400 K. If excess plume temperature is 400 K, models with plume radius between 50 and 70 km and upper mantle viscosity between 1020 and 3×1020 Pa s satisfy all the constraints. Our results indicate that mantle plume in the upper mantle may be significantly hotter than previously suggested. This has important implications for mantle convection and mantle melting. In addition to constraining plume dynamics, our models also provide a mechanism to produce the observed uplift at Lanai and Molokai that has never been satisfactorily explained before.