Lower Mantle Plume Research Articles

Conditions of Rock Formation and Magma Sources of the Late Cenozoic Udokan Volcanic Plateau

The Udokan volcanic plateau (UVP), like other areas of the Late Cenozoic volcanic province of Central Asia, formed in the interval from the Middle Miocene to the Pleistocene. Its products have high alkalinity and vary in composition from alkaline picrobasalts and basanites to alkaline trachytes. Compositional changes were controlled by two differentiation trends, which corresponded to different conditions of the original magmas generation. Rocks with low SiO2 contents (45 wt. %) were formed from melts of low melting degrees that arose under conditions of elevated pressures and temperatures. Formation of rocks corresponding to the composition range 45–61 wt. %. % SiO2 was associated with the differentiation of basalt melts, which arose at shallower depths and at lower temperatures. The geochemical characteristics of the UVP basaltoids make them similar to OIB-type basalts. They are also close in Sr, Nd, and Pb isotopic composition, corresponding to the parameters of a moderately depleted mantle, which is close to the composition of oceanic basalt sources corresponding to the mantle of deep mantle plumes. The corresponding mantle component is present in the sources of other volcanic regions of the Late Cenozoic intraplate volcanic province of Central Asia, which indicates the involvement of the lower mantle plume in the formation of these regions.

Phase Relations in Spinel Lherzolite KLB-1 According to Results of Thermodynamic Modeling up to 30 GPa: Peculiarities of Mineral Assemblages and Geodynamic Effects

The composition of spinel lherzolite xenolith KLB-1 from the Kilbourne Hole volcanic crater, United States, which is close to the composition of the Earth’s primitive mantle, was used for thermodynamic modeling of phase relations in the Na2O–CaO–FeO–MgO–Al2O3–SiO2 system (NCFMAS) using the Perple_X software package in the temperature range of 900–2000 °C and pressures of 0.0001–30 GPa. The calculated phase diagram is in good agreement with published thermodynamic data on KLB-1 composition and reveals the peculiarities of mantle mineral assemblages at P–T parameters on which experimental data are insufficient or absent. The results showed that the mineral assemblage of garnet wehrlite (garnet + olivine + clinopyroxene), the least common type of mantle peridotite on the Earth’s surface, prevails in the upper mantle since the Archean. Mineral assemblage of garnet lherzolite (garnet + olivine + clinopyroxene + orthopyroxene), which is a variety of mantle peridotites most widely found on the Earth’s surface, is formed in the lithospheric mantle because its temperatures are lower than those of the convecting mantle. Thermodynamic modeling reveals a ringwoodite-free field in the P–T diagram (located at the bottom of the mantle transition zone), which is crosscut by Archean adiabats and by the geotherms of Archean and the hottest Phanerozoic plumes. This area causes a change, from negative to positive, in the slope of the boundary between the lower mantle and the mantle transition zone. A positive slope of the boundary in the Archean should have stimulated the ascend of lower mantle plumes through the transition zone. Conversely, this boundary has a negative slope for most Phanerozoic plumes, rising from the lower mantle, and as a result, the plumes either slow down or stop.

Dynamic topography and the nature of deep thick plumes

Deep mantle plumes imaged by seismic tomography have much larger radii (∼400 km) than predicted by conventional geodynamic models (∼100 km). Plume buoyancy fluxes estimated from surface topography concur with narrow plumes with low viscosities expected from their high temperatures. If plumes are thick as imaged by tomography, buoyancy flux estimates may require very viscous or thermochemical plumes. Here we assess the dynamical plausibility of an alternative model, a ponding plume, which has been suggested to explain thick plumes as well as buoyancy fluxes estimated from surface topography. In the ponding plume model, a thick conduit in the lower mantle narrows significantly after passing through the mantle transition zone, below which excess material from the thick lower-mantle plume, which cannot be accommodated by the narrow upper-mantle plume, spreads laterally. Such excess material in the mid-mantle, however, should still manifest itself in surface topography, the amplitude of which can be quantified via topography kernels. We find that the ponding of a purely thermal plume would lead to unrealistic excess topography, with the scale of ponding material large enough to be detected by seismic tomography. If mantle plumes are as thick as indicated by seismic tomography, it appears to be necessary to deviate from either conventional temperature-dependent viscosity or the assumption of purely thermal origins.

Melting and metasomatism in West Eifel and Siebengebirge Sub-Continental Lithospheric Mantle: Evidence from concentrations of volatiles in fluid inclusions and petrology of ultramafic xenoliths

The possibility of constraining the composition and evolution of specific portions of the Sub-Continental Lithospheric Mantle (SCLM) by means of an integrated study of petrography, mineral chemistry, and concentrations of volatiles in fluid inclusions (FI) is a novel approach that can provide clues on the recycling of volatiles within the lithosphere. This approach is even more important in active or dormant volcanic areas, where the signature of the gaseous emissions at the surface can be that of the underlying lithospheric mantle domains. In this respect, the ultramafic xenoliths brought to the surface in West Eifel (~0.5–0.01 Ma) and Siebengebirge (~30–6 Ma) volcanic fields (Germany) are ideal targets, as they provide direct information on one of the most intriguing portions of SCLM beneath the Central European Volcanic Province (CEVP). Five distinct populations from these localities were investigated using petrographic observations, mineral phase analyses and determination of He, Ne, Ar and CO2 contents in olivine-, orthopyroxene-, and clinopyroxene-hosted FI. The most refractory Siebengebirge rocks have highly forsteritic olivine, high-Mg#, low-Al pyroxene, and spinel with high Cr#, reflecting high extents (up to 30%) of melt extraction. In contrast, xenoliths from West Eifel are modally and compositionally heterogeneous, as indicated by the large forsterite range of olivine (Fo83–92), the Cr# range of spinel (0.1–0.6), and the variable Al and Ti contents of pyroxene. Equilibration temperatures vary from 870 °C to 1070 °C in Siebengebirge, and from ⁓900 °C to ⁓1190 °C in West Eifel xenoliths, at oxygen fugacity values generally between −0.5 and + 1.3 ΔlogƒO2 [FMQ]. In both areas, the FI composition was dominated by CO2, with clinopyroxene, and most of the orthopyroxene had the highest concentrations of volatiles, while olivine was gas-poor. The noble gas and CO2 distributions suggest that olivine is representative of a residual mantle that experienced one or more melt extraction episodes. The 3He/4He ratio corrected for air contamination (Rc/Ra values) varied from 6.8 Ra in harzburgitic lithotypes to 5.5 Ra in lherzolites and cumulate rocks, indicating that the original MORB-like mantle signature was progressively modified by interaction with crustal-related components and melts having 3He/4He and 4He/40Ar* values consistent with those published for magmatic gaseous emissions. The Ne and Ar isotope systematics indicated that most of the data were consistent with mixing between a recycled atmospheric component and a MORB-like mantle, which does not necessarily require the involvement of a lower mantle plume beneath this portion of the CEVP. The major element distribution in mineral phases from West Eifel and Siebengebirge, together with the systematic variations in FI composition, the positive correlation between Al enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease with increasing temperature, suggest that the SCLM beneath Siebengebirge represented the Variscan lithosphere in CEVP prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. In contrast, West Eifel xenoliths reflect multiple heterogeneous metasomatism/refertilisation events that took place in the regional SCLM between ~6 and ~ 0.5 Ma.

Helium isotopes and olivine geochemistry of basalts and mantle xenoliths in Jeju Island, South Korea: Evaluation of role of SCLM on the Cenozoic intraplate volcanism in East Asia

Jeju Island, South Korea, is a Cenozoic intraplate volcano located in East Asia. The Jeju basalts display ocean island basalt (OIB)-like trace element patterns and enriched mantle type 2 (EM2) radiogenic isotope compositions, but the source that enriched the mantle remains unclear. Here we report new geochemical compositions of basalts and xenoliths, including the first helium isotope and element analysis of olivine phenocrysts to constrain the source of basalt. Olivines in the Jeju basalts have 3He/4He ratios of 3.5 to 7.3 Ra while olivines in the mantle xenoliths are 2.9 to 6.3 Ra. They provide no evidence of a lower mantle plume but overlap the range of the regional subcontinental lithospheric mantle (SCLM) and OIBs with low 3He/4He values. Olivine phenocrysts included in the basalts show variations in Mn, Ni, and Ca concentrations with forsterite (Fo) contents, similar to the trend for olivines crystallized from the pyroxenite-derived melts. Elemental ratios of Ca-Fe-Ni-Mg-Mn for the olivines also indicate the pyroxenite contribution. Considering our results, it is suggested that the main source of the Jeju basalts is the SCLM containing pyroxenite. In addition, the tomography image shows the low-velocity zone extending to the asthenosphere mantle beneath the central Jeju Island. This implies that the localized asthenospheric upwelling is caused by edge-driven convection, and the high-velocity zones exist around Jeju island indicating a thick cratonic lithosphere. Therefore, we propose that the interaction between the SCLM component containing pyroxenite and the rising asthenosphere is the main mechanism to generate enriched basaltic magmas for the Cenozoic intraplate volcanism in East Asia, including Jeju Island.

Trace element and helium isotope geochemistry of the Cenozoic intraplate volcanism in the East Sea (Sea of Japan): Implications for lithosphere-asthenosphere interaction

Extensive intraplate volcanism in the East Asia regions, including Dokdo and Ulleungdo (DU), occurred due to the extensional stress field during the Cenozoic era. However, the origin of magmatism is still controversial. Here we report new results of helium isotopes in olivine and clinopyroxene phenocrysts as well as major and trace element compositions from the DU basalts in order to determine the origin of the DU magmas and provide insights into Cenozoic mantle dynamics in East Asia. The DU volcanic islands were formed by magmatism after opening of the East Sea (Sea of Japan), a back-arc basin behind the Japanese arc, and are currently located in the Ulleung basin, one of sub-basins of the East Sea. The 3He/4He ratios range from 5.7 to 5.9 Ra for Dokdo and 4.5 to 6.0 Ra for Ulleungdo, respectively, similar to the sub-continental lithospheric mantle (SCLM) range of East Asia (4.6 to 7.7 Ra). Helium isotope compositions indicate that the depleted asthenosphere or the lower mantle plume might not be the direct source of the DU magmas. Also, there is no geochemical evidence that the DU magmas were influenced by subduction-derived components or the HIMU mantle source. Hence, we argue that SCLM provided enriched geochemical features in the DU basalts, contrary to basalts from other back-arc basins composed of the oceanic crust. In this respect, it is likely that the extended SCLM may still remain beneath the Ulleung basin. Our helium isotope and trace element mixing model shows the mixing trend between the metasomatized lithospheric melts and the asthenospheric melts. In addition, the presence of the low-velocity zone beneath the surface of the DU volcanoes in seismic tomography implies that the magmas were formed through the lithospheric melting due to the hot asthenospheric upwelling. Therefore, our study shows that the DU volcanism was mainly contributed by SCLM and that the lithosphere-asthenosphere interaction was the main mechanism that led to the Cenozoic magmatism in East Asia.

Late Mesozoic–Cenozoic Stages of Volcanism and Geodynamics of the Sea of Japan and Sea of Okhotsk

A model of the geological evolution of the Japan and Okhotsk seas was developed based on radioisotope age, mineralogical, and isotope-geochemical study of the Late Mesozoic–Cenozoic volcanic rocks. The geodynamic settings of volcanic stages were determined as follows: (1) the Late Cretaceous continental-margin (calc-alkaline), (2) the Eocene transform-margin (adakite) in the Sea of Okhotsk, (3) the Miocene–Pliocene marginal sea (alkaline basaltic) in the Sea of Japan, and (4) the Pliocene–Pleistocene island arc (calc-alkaline) in the southern Sea of Okhotsk. It has been established that parental magmas were derived from subcontinental lithosphere, oceanic asthenosphere, and lower-mantle plume-continental (CAB) and plume-oceanic (OIB) sources. Geodynamic settings changed from the Late Cretaceous subduction to the Maastrichtian–Pliocene transform margin. The transform margin setting involved destruction and extension, maximum oceanic-margin spreading (end of the Early Miocene–beginning of the Middle Miocene), and post-spreading lower mantle plume upwelling (the Middle Miocene – Pliocene). It was completed by the resumption of the Pliocene–Pleistocene subduction of the Pacific plate beneath the Eurasian continent.

The tungsten-182 record of kimberlites above the African superplume: Exploring links to the core-mantle boundary

Many volcanic hotspots are connected via ‘plume’ conduits to thermochemical structures with anomalously low seismic velocities at the core-mantle boundary. Basaltic lavas from some of these hotspots show anomalous daughter isotope abundances for the short-lived 129I-129Xe, 146Sm-142Nd, and 182Hf-182W radioactive decay systems, suggesting that their lower mantle sources contain material that dates back to Earth-forming events during the first 100 million years in solar system history. Survival of such ‘primordial’ remnants in Earth's mantle places important constraints on the evolution and inner workings of terrestrial planets.Here we report high-precision 182W/184W measurements for a large suite of kimberlite volcanic rocks from across the African tectonic plate, which for the past 250 million years has drifted over the most prominent thermochemical seismic anomaly at the core-mantle boundary. This so-called African LLSVP, or ‘large low shear-wave velocity province’, is widely suspected to store early Earth remnants and is implicated as the ultimate source of global Phanerozoic kimberlite magmatism. Our results show, however, that kimberlites from above the African LLSVP, including localities with lower mantle diamonds such as Letseng and Karowe Orapa A/K6, lack anomalous 182W signatures, with an average μ182W value of 0.0 ± 4.1 (2SD) for the 18 occurrences studied.If kimberlites are indeed sourced from the African LLSVP or superplume, then the extensive 182W evidence suggests that primordial or core-equilibrated mantle materials, which may contribute resolvable μ182W excesses or deficits, are only minor or locally concentrated components in the lowermost mantle, for example in the much smaller ‘ultra-low velocity zones’ or ULVZs. However, the lack of anomalous 182W may simply suggest that low-volume kimberlite magmas are not derived from hot lower mantle plumes. In this alternative scenario, kimberlite magmas originate from volatile-fluxed ambient convecting upper mantle domains beneath relatively thick and cold lithosphere from where previously ‘stranded’ lower mantle and transition zone diamonds can be plucked.

Gas Flows in the Sea of Okhotsk Resulting from Cretaceous-Cenozoic Tectonomagmatic Activity

A model of the geological evolution of the Sea of Okhotsk is presented based on data from radioisotope age determinations and the mineral and isotope-geochemical composition of Late Mesozoic–Cenozoic volcanic rocks. We discuss the probable interrelationship between gas geochemical manifestations of gas fluxes with anomalous methane concentrations and the formation of gas hydrates, along with the occurrence of volcanic processes in the Sea of Okhotsk, fault zones, and different geological structures of basement and sedimentary deposits, landslides, and earthquakes. As a result of the studies, the nature of each volcanic stage has been determined. These include the Late Cretaceous continental-marginal belt stage (calc-alkaline), the Eocene transform-marginal (adakite) stage, and the Pliocene-Pleistocene island arc stage in the southern part of the Sea of Okhotsk. The sources of magma generation have been recognized, namely, the lithosphere subcontinental, asthenosphere oceanic, and plume-oceanic (OIB). The change in geodynamic regimens was traced back from the Late Cretaceous subduction regimen to the Maastrichtian–Datian transform-marginal, which continued as far as the Pliocene and ended at the resumption of the Pliocene–Pleistocene subduction of the Pacific Plate beneath the Eurasian continent. This involved the processes of destruction of the subduction plate, lithosphere and asthenosphere diapirism, and lower mantle plume upwelling. In the periods of geodynamic, seismotectonic and volcanic activities, gas migrated along the fault zones from the depth to the surface, together with different volcanic substrates ascending through the upper and lower mantle. Gas contained CO2, CH4, H2, He, N2, O2, and superheated steam (Н2О). Gas plays an important dynamic and physicochemical role in the evolution of the Sea of Okhotsk. At the present stage, gas fluxes migrating from the depth to the surface manifest as gas bubbles penetrating from the bottom sediments into the water column, with some portion of gas penetrating into the atmosphere. In the areas with gas fluxes, the fields containing anomalous concentrations of hydrocarbons, gas hydrate, carbon dioxide, hydrogen, and helium result in the formation of gas hydrates and associations of authigenic minerals and various geochemical elements.

Plate Tectonics vs. Plume Tectonics Interplay: Possible Models and Typical Cases

The interplay of plume and plate tectonics is discussed with reference to well-documented late Paleozoic and Mesozoic–Cenozoic structures. The considered issues include models of lower mantle plumes; the typical case of the Hawaii plume; plate motions in the Pacific basin under the effect of plume activity and subduction processes; the role of plumes in the breakup of continents and rifting for the cases of the East African rifts and Gondwana breakup; large continental igneous provinces of Siberia and Tarim, with meimechites and kimberlites; and the formation of large granitic batholiths and related metallogeny. The study contains several points of novelty: integrated use of lower mantle seismic tomography and satellite altimetry data on gravity patterns; correlation of plume heat with plate velocities assuming that some plumes may dissolve in the asthenosphere; and correlation of rifting with compression and extension zones.

Three‐Dimensional S Velocity Structure of the Mantle Transition Zone Beneath Central America and the Gulf of Mexico Inferred Using Waveform Inversion

AbstractTravel time tomography studies have reported various modes of subduction of slabs around the mantle transition zone, but the causes of this variability are still controversial. In order to place additional constraints on the style of subduction in and near the mantle transition zone, we apply waveform inversion to S wave triplications due to the 410‐ and 660‐km discontinuities, and infer the regional three‐dimensional S velocity structure in the depth range 370–820 km beneath Central America and the Gulf of Mexico. We use ~3,400 transverse component records at epicentral distances 17–30° from 20 intermediate focus earthquakes beneath Central America recorded at stations of the USArray and other smaller networks. We filter the records between 20 and 100 s, and use the portions of the waveforms from 10 s before to ~40–80 s after the arrival of the direct S wave, thereby including the S wave triplications while excluding the sS depth phase. We correct the data for the structure above our target region using previous S velocity models, and we show that the results of the inversion do not depend heavily on which correction is used. Resolution tests confirm that we can resolve small‐scale structure in the target region. Our inferred models show that the Cocos slab penetrates into the lower mantle, but with important variations along the Middle America Trench in the subduction style of the slab. We suggest that this variability could be due to the thermal structure of the Cocos slab, and to the interaction of the slab with a possible lower mantle plume.

Lower-mantle plume beneath the Yellowstone hotspot revealed by core waves

The Yellowstone hotspot, located in North America, is an intraplate source of magmatism the cause of which is hotly debated. Some argue that a deep mantle plume sourced at the base of the mantle supplies the heat beneath Yellowstone, whereas others claim shallower subduction or lithospheric-related processes can explain the anomalous magmatism. Here we present a shear wave tomography model for the deep mantle beneath the western United States that was made using the travel times of core waves recorded by the dense USArray seismic network. The model reveals a single narrow, cylindrically shaped slow anomaly, approximately 350 km in diameter that we interpret as a whole-mantle plume. The anomaly is tilted to the northeast and extends from the core–mantle boundary to the surficial position of the Yellowstone hotspot. The structure gradually decreases in strength from the deepest mantle towards the surface and if it is purely a thermal anomaly this implies an initial excess temperature of 650 to 850 °C. Our results strongly support a deep origin for the Yellowstone hotspot, and also provide evidence for the existence of thin thermal mantle plumes that are currently beyond the resolution of global tomography models. The Yellowstone hotspot could be fed by a thin, thermal mantle plume that extends from the core–mantle boundary to the surface position of the hotspot, according to analyses of seismic data.

Evaluating the Resolution of Deep Mantle Plumes in Teleseismic Traveltime Tomography

AbstractThe strongest evidence to support the classical plume hypothesis comes from seismic imaging of the mantle beneath hot spots. However, imaging results are often ambiguous and it is questionable whether narrow plume tails can be detected by present‐day seismological techniques. Here we carry out synthetic tomography experiments based on spectral element method simulations of seismic waves with period T > 10 s propagating through geodynamically derived plume structures. We vary the source‐receiver geometry in order to explore the conditions under which lower mantle plume tails may be detected seismically. We determine that wide‐aperture (4,000–6,000 km) networks with dense station coverage (<100–200 km station spacing) are necessary to image narrow (<500 km wide) thermal plume tails. We find that if uncertainties on traveltime measurements exceed delay times imparted by plume tails (typically <1 s), the plume tails are concealed in seismic images. Vertically propagating SKS waves enhance plume tail recovery but lack vertical resolution in regions that are not independently constrained by direct S paths. We demonstrate how vertical smearing of an upper mantle low‐velocity anomaly can appear as a plume originating in the deep mantle. Our results are useful for interpreting previous plume imaging experiments and guide the design of future experiments.

Volcanism and Tectonics of the Central Deep Basin, Sea of Japan

The paper presents the results of a study on the geomorphic structure, tectonic setting, and volcanism of the volcanoes and volcanic ridges in the deep Central Basin of the Sea of Japan. The ridges rise 500–600 m above the acoustic basement of the basin. These ridges were formed on fragments of thinned continental crust along deep faults submeridionally crossing the Central Basin and the adjacent continental part of the Primorye. The morphostructures of the basin began to submerge below sea level in the Middle Miocene and reached their contemporary positions in the Pliocene. Volcanism in the Central Basin occurred mostly in the Middle Miocene–Pliocene and formed marginal-sea basaltoids with OIB (ocean island basalt) geochemical signatures indicating the lower-mantle plume origin of these rocks. The OIB signatures of basaltoids tend to be expressed better in the eastern part of the Central Basin, where juvenile oceanic crust has developed. The genesis of this crust is probably related to rising and melting of the Pacific superplume apophyse.

Timing and genesis of the Karoo-Ferrar large igneous province: New high precision U-Pb data for Tasmania confirm short duration of the major magmatic pulse

The Karoo-Ferrar igneous province is one of the largest igneous provinces on Earth. It extends from South Africa, along the Trans-Antarctic Mountains to Tasmania and South Australia. Reconstruction of the continents back to the Gondwana configuration in the Early Jurassic reveals a total length of the Karoo-Ferrar province of >5000km. New isotope dilution thermal ionization mass spectrometry (ID-TIMS) single grain U-Pb ages for zircon and baddeleyite from Tasmanian dolerites combined with ID-TIMS literature single grain U-Pb ages from the Ferrar and Karoo suites are consistent with the major pulse of synchronous magmatism throughout the province lasting about 1Ma or less for the major pulse of magmatism at the time of the Toarcian mass extinction event. We argue that the mechanism of synchronization of magmatism over such a short period of time along such a long distance is the major question which has to be answered in search of the correct model for the origin of the Karoo-Ferrar large igneous province. It cannot be reconciled with the lower mantle plume head model with the plume impingement beneath the Karoo. Plume material could not spread beneath the lithosphere at a rate of ~5–10m/yr (5000km per 0.5–1Myr), at least based on the current knowledge of the mantle physical properties. It seems unlikely that the entire Karoo-Ferrar large igneous province formed due to long distance magma migration through dykes from the same mantle plume irrespective on the proposed plume centre location. In such case, magma would have had to cross the boundaries (and thus weakness zones) between three future continents. In the framework of the dyke propagation model we would expect dykes to follow these weakness zones, not cross them. In addition to this, the Karoo and Ferrar contain geochemically different igneous rocks, which were not formed from the same magma source, preventing interpretations based on one single plume. Both the Karoo and Ferrar contain low-Ti tholeiites, which are similar by their trace element patterns to modern arc analogues – the Central Andes and Kamchatka, respectively. Thus, our preferred model for the origin of the Karoo-Ferrar large igneous province is associated with subduction of the Phoenix plate beneath the southern Gondwana. Probably, deep slab dehydration at the depth of the mantle transition zone modulated surface volcanism or the Toarcian tectonic event triggered voluminous but short-term melting of mantle, which was metasomatized by subduction-derived fluids.

Multiple mantle upwellings in the transition zone beneath the northern East‐African Rift system from relative P‐wave travel‐time tomography

AbstractMantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low‐shear‐velocity province beneath Southern Africa, the so‐called African Superplume, to multiple lower‐mantle sources along the rift. We present a new P‐wave travel‐time tomography model below the northern East‐African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of ∼100 km length‐scale down to depths of 700–800 km beneath the study region. Our images provide evidence of two clusters of low‐velocity structures consisting of features with diameter of 100–200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off‐rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 ± 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low‐velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep‐plume material below the transition zone may be spawning these upper mantle upwellings.

The deep water cycle and flood basalt volcanism

Experimental data combined with numerical simulations suggest that fast-subducting oceanic slabs are cold enough to carry a significant amount of H2O into the deep mantle in antigorite; with increasing depth, this mineral undergoes solid–solid transitions to phase A and then to phase E and/or wadsleyite. Ice VII and clathrate hydrates can also be stable under pressure and temperature (PT) conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Cold slabs are expected to stagnate in the mantle transition zone. With time, they are heated to the temperature of the ambient transition zone and release excess water as a H2O-bearing fluid. This may cause voluminous melting of overlying upper mantle rocks. If such a process operates in nature, magmas geochemically similar to island-arc basalts are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the southeastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with the fast (cold) subduction of the Mongolia–Okhotsk oceanic slab, and originated by the dehydration of the stagnant slab in the transition zone. We show that the influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually diminished with increasing distance from the subduction zone. Thus, the unique size and volume of the Siberian flood basalt province could have originated due to long-term underflow beneath Siberia with or without the existence of a lower mantle plume.

Upper‐mantle tomography and dynamics beneath the North China Craton

A high‐resolution tomographic model of the upper mantle beneath the North China Craton (NCC) is determined using a large number of precisely hand‐picked teleseismic P wave arrival times. The results are generally consistent with previous results but high‐quality arrivals provide new insights into the dynamics beneath the NCC. Obviously north‐south trending low‐velocity (low‐V) zones are revealed down to ∼300–400 km depth under the Shanxi rift and Tanlu fault zone, while a north‐south trending high‐velocity (high‐V) zone representing the remainder of detached lithosphere is visible down to ∼200 km depth under the western portion of eastern NCC. High‐V anomalies representing the detached lithosphere are detected at 200–400 km depth under central and eastern NCC. Under the Ordos block high‐V anomalies are visible above ∼400 km depth, indicating intact lithosphere. Broad high‐V anomalies representing the stagnant Pacific slab are imaged with a low‐V anomaly from Datong volcano to the edge of Bohai Sea in the mantle transition zone beneath eastern and central NCC, suggesting that the Pacific slab has subducted to central NCC but with a gap. A continuously Y‐shaped low‐V structure is clearly imaged under Datong volcano and Bohai Sea from the lower mantle through this gap in the mantle transition zone to the upper mantle, indicating the existence of a lower mantle plume. These results suggest that in addition to the subduction of the Pacific plate, the plume has also played an important role in lithospheric destruction by thermal erosion of the asthenosphere and detachment of the lithosphere beneath the NCC.

Bent-shaped plumes and horizontal channel flow beneath the 660 km discontinuity

Recent high-resolution seismic imaging of the transition zone topography beneath the Hawaiian archipelago shows strong evidence for a 1000 to 2000 km wide hot thermal anomaly ponding beneath the 660 km boundary west of Hawaii islands [Q. Cao et al. Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii. Science (2011), 332, 1068–1071]. This scenario suggests that Hawaiian volcanism may not be caused by a stationary narrow plume rising from the core–mantle boundary but by hot plume material first held back beneath the 660 km discontinuity and then entrained under the transition zone before coming up to the surface. Using a cylindrical model of iso-chemical mantle convection with multiple phase transitions, we investigate the dynamical conditions for obtaining this peculiar plume morphology. Focusing on the role exerted by pressure-dependent thermodynamic and transport parameters, we show that a strong reduction of the coefficient of thermal expansion in the lower mantle and a viscosity hill at a depth of around 1800 km allow plumes to have enough focused buoyancy to reach and pass the 660 km depth interface. The lateral spreading of plumes near the top of the lower mantle manifests itself as a channel flow whose length is controlled by the viscosity contrast due to temperature variations ∆η T. For small values of ∆η T, broad and highly viscous plumes are generated that tend to pass through the transition zone relatively unperturbed. For higher values (10 2 ≤ ∆η T ≤ 10 3), we obtain horizontal channel flows beneath the 660 km boundary as long as 1500 km within a timescale that resembles that of Hawaiian hotspot activity. This finding could help to explain the origin of the broad hot anomaly observed west of Hawaii. For a normal thermal anomaly of 450 K associated with a lower mantle plume, we obtain activation energies of about 400 kJ/mol and 670 kJ/mol for ∆ ηT = 10 2 and 10 3, respectively, in good agreement with values based on lower mantle mineral physics. If an increase of the thermal conductivity with depth is also included, our model can predict both long channel flows beneath the 660 km discontinuity and also values of the radial velocity of local blobs sinking in the lower mantle of around 1 cm/yr, in good agreement with those inferred for the sinking rate of cold remnants of the subducted lithosphere.

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.