Hot Mantle Plume Research Articles

Seismic evidence for a mantle plume beneath the Cape Verde hotspot

P- and S-wave tomography of the upper mantle beneath the Cape Verde hotspot is determined using arrival-time data measured precisely from three-component seismograms of 106 distant earthquakes recorded by a local seismic network. Our results show a prominent low-velocity anomaly imaged as a continuous column <100 km wide from the uppermost mantle down to about 500 km beneath Cape Verde, especially below the Fogo active volcano, which erupted in 1995. The low-velocity anomaly may reflect a hot mantle plume feeding the Cape Verde hotspot.

Hafnium and iron isotopes in early Archean komatiites record a plume-driven convection cycle in the Hadean Earth

Archean (>2.5 billion years) komatiites are considered expressions of mantle plumes that originate from and thereby sample the lowermost mantle overlying the Earth's core. Some komatiites have reported Hf isotope signatures that require a mantle source with a time-integrated Lu/Hf that is appreciably higher than average modern depleted mantle. The systematic study of the time and locus of parent–daughter fractionation of the mantle sources of these komatiites potentially constrains differentiation processes in the early Earth, and subsequent distribution and storage of early mantle reservoirs. We present radiogenic Hf and stable Fe isotopes for a series of komatiites from the Pilbara craton in Western Australia (aged 3.5 to 2.9 Ga). After careful evaluation of the effects of alteration, we find that pristine samples are characterised by a light Fe isotope mantle source and initial 176Hf/177Hf well above the age-corrected depleted mantle. Taken together these observations require a component of an old, melt-depleted reservoir in their mantle source. The Hf isotope signature of this component appears to be complementary to the first terrestrial crust, as preserved in Hadean (i.e., >4 Ga) detrital zircon cores, suggesting a causal relationship and a Hadean age for this depletion event. We propose that this Early Refractory Reservoir (ERR) is the residue formed by deep melting in hot Hadean mantle plumes, which then accumulated at the base of the first crust. Parts of this primordial lithosphere were destabilised and sank to the core–mantle boundary in cold drips and subsequently returned in hot mantle plumes, whose thermal capacity allows melting of such refractory mantle with its archetype isotope signature. The cycling of this material via cold drips and hot plumes suggests a plume-dominated convection prior to ∼3.9 Ga, which is then replaced by Archean-style plate tectonics.

Paleo-elevation and subsidence of [formula omitted] Shatsky Rise inferred from CO2 and H2O in fresh volcanic glass

Shatsky Rise, a large Mesozoic oceanic plateau in the northwest Pacific, consists of three massifs (Tamu, Ori, and Shirshov) that formed near a mid-ocean-ridge triple junction. Published depth estimates imply that Shatsky Rise has not subsided normally, like typical oceanic lithosphere. We estimated paleo-eruption depths of Shatsky Rise massifs on the basis of dissolved CO2 and H2O in volcanic glass and descriptions of cores recovered from five sites of Integrated Ocean Drilling Program Expedition 324. Initial maximum elevations of Shatsky Rise are estimated to be 2500–3500 m above the surrounding seafloor and the ensuing subsidence of Shatsky Rise is estimated to be 2600–3400 m. We did not observe the anomalously low subsidence that has been reported for both Shatsky Rise and the Ontong Java Plateau. Although we could not resolve whether Shatsky Rise originated from a hot mantle plume or non-plume fusible mantle, uplift and subsidence histories of Shatsky Rise for the both cases are constrained based on the subsidence trend from the center of Tamu Massif (∼2600 m) toward the flank of Ori Massif (∼3400 m). In the case of a hot mantle plume origin, Shatsky Rise may have formed on young (∼5 Ma) pre-existing oceanic crust with a total crustal thickness of ∼20 km. For this scenario, the center of Shatsky Rise is subsequently uplifted by later (prolonged) crustal growth, forming the observed ∼30 km thickness crust. For a non-plume origin, Shatsky Rise may have formed at the spreading ridge center as initially thick crust (∼30 km thickness), with later reduced subsidence caused by the emplacement of a buoyant mass—perhaps a refractory mantle residuum—beneath the center of Shatsky Rise.

Picrites in central Hokkaido: Evidence of extremely high temperature magmatism in the Late Jurassic ocean recorded in an accreted oceanic plateau

The Sorachi-Yezo belt, central Hokkaido, Japan, is composed of voluminous tholeiitic basaltic volcanics, and has been thought to be accreted fragments of an oceanic plateau formed in the Late Jurassic Pacific Ocean. Picrites have been reported as pillow lava and hyaloclastite from the Sorachi-Yezo belt. These picrites are characterized by very magnesian olivine phenocrysts (up to Fo [=100* Mg/(Mg+Fe)] = 94.1), which indicate that the primary magma was unusually Mg rich. The estimated MgO of the primary magma and the mantle potential temperature are as high as 29 wt% and 1700 °C, respectively, comparable to those of the Neoarchean komatiites, and higher than those of the Gorgona komatiites and picrites. The rare earth element patterns of the Sorachi-Yezo picrites are divided into two groups that are chemically akin to the Neoarchean komatiites and Gorgona komatiites and picrites, indicating different melting regimes in an extremely hot mantle plume. The Sorachi-Yezo picrites provide evidence for extremely high temperature magmatism, like that of Archean komatiite, caused by melting of the hottest mantle plume among the Phanerozoic oceanic large igneous provinces.

Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts)

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

Oceanic Volcanism from the Low-velocity Zone – without Mantle Plumes

We develop a model for oceanic volcanism that involves fracturing of the seismic lithosphere to access melts from the partly melted seismic low-velocity zone. Data on global seismic shear-wave velocities are combined with major-element compositions of global mid-ocean ridge basalt glasses, Hawaiian basalt glasses, and phase relations in the CaO-MgO-Al2O3-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-Na2O-FeO systems at pressures from 1 atm to 6 GPa. We use these data to constrain the pressure–temperature conditions for melt extraction at Hawaii and mid-ocean ridges (including Iceland), and to evaluate the existence of hot mantle plumes. In the low-velocity zone, the maximum reduction and maximum anisotropy of seismic shear-wave velocity (maximum melt fraction) occurs at a depth of ∼140–150 km for crustal ages >∼50 Ma, and a depth of ∼65 km at the East Pacific Rise axis. Seismic data indicate a smooth depth transition between these extremes. Experimental phase-equilibrium data, when combined with natural glass compositions, show that pressure–temperature conditions of tholeiitic melt extraction at Hawaii (∼4–5 GPa, 1450–1500°C) and the global oceanic ridge system (∼1·2–1·5 GPa, 1250–1280°C) are an excellent match for the two depth ranges of maximum melting indicated by seismic shear-wave data. At Hawaii, magmas escape to the surface along a fracture system that extends through the lithosphere into the low-velocity zone. This allows eruption of progressively deeper melts from the low-velocity zone, which exist at equilibrium along a normal geotherm. No significant decompression melting occurs. This produces the characteristic sequence at each volcano of initial low-volume alkalic magmas, then voluminous tholeiitic magmas that show low-pressure olivine-controlled crystallization, and final low-volume alkalic magmas from extreme depths. At the East Pacific Rise, the more shallow depth of magma extraction is caused by a perturbed ridge geotherm that grazes the lherzolite solidus within the thermal boundary layer. This results in an absence of olivine-controlled crystallization. Hawaii is not a hot plume. Instead, it shows magmas characteristic of normal mature-ocean thermal conditions in the low-velocity zone. We find no evidence of anomalously high temperatures of magma extraction and no role for hot mantle plumes anywhere in the ocean basins.

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources, plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and 87Sr/86Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (206Pb/204Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.

Crustal accretion at high temperature spreading centres: Rheological control of crustal thickness

New determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as Iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides ( Bjarnason and Schmeling, 2009). To understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass, momentum and energy with melting, melt extraction, and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume. The dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust. Four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone, the accretional heating rate, which can be associated with the thickness of the surface layer in which magmatic emplacement takes place, and the spreading rate. Mode 1: zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone. Mode 2: strongly or moderately crustal thickening away from the axis develops in case of warm (deep reaching) accretion and wide emplacement zones. Mode 3: nearly constant crustal thickness develops in case of warm (deep reaching) accretion but narrow emplacement zones. Dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis. Modes 2 or 3 may be identified with the situation in Iceland. Mode 4: a stagnating central crustal block evolves for cool accretion and narrow emplacement. This mode disappears for increasing spreading rates. No accretional mode with maximum crustal thickness above the plume at the rift axis has been found. The absence of mode 1 accretion (with zero crust at ridge axis) on earth may be an indication that in general crustal accretion is not cold (and shallow). The model is also applied to other hotspot-ridge settings (Azores, Galapagos) and suggests modes 2–3 accretion.

Constructing a Craton

Geophysics![Figure][1] CREDIT: USGS The cores of continents, or cratons, make up the stable lithosphere. Their formation probably occurred billions of years ago, though the mechanisms responsible remain unclear. One way to seek further insight is to look below the present-day continents for clues, such as where the lithosphere ends and the asthenosphere begins. Miller and Eaton used receiver function seismic analysis to study a transect across the width of Canada, revealing the lithosphere-asthenosphere boundary in addition to tilted discontinuities in the mid-lithosphere. These discontinuities may be the scars of an ancient collision of several smaller microcontinents that ultimately fused together to form the North American craton. In a related study, Yuan and Romanowicz modeled the anisotropy of seismic waves across nearly the entire North American continent and observed not only a continuous lithosphere-asthenosphere boundary, but another shallower, laterally variable boundary within the middle of the lithosphere. Based on its thickness and a correlation with geochemical evidence, this new boundary may separate the older part of the craton from a relatively younger layer. Although the two studies present a contrasting view of the evolution of the lithosphere beneath North America, they suggest that accretion processes, and not hot mantle plumes, were primarily responsible for its formation. Geophys. Res. Lett. 10.1029/2010GL044366 (2010); Nature 466 , 1063 (2010). [1]: pending:yes

Lithospheric drift on early Mars: Evidence in the magnetic field

The crustal magnetic anomalies on Mars may represent hot spot tracks resulting from lithospheric drift on ancient Mars. As evidence, an analysis of lineation patterns derived from the ΔBr magnetic map is presented. The ΔBr map, largely free of external magnetic field effects, allows excellent detail of the magnetic anomaly pattern, particularly in areas of Mars where the field is relatively weak. Using cluster analysis, we show that the elongated anomalies in the martian magnetic field form concentric small circles (parallels of latitude) about two distinct north pole locations. If these pole locations represent ancient spin axes, then tidal force on the early lithosphere by former satellites in retrograde orbits may have pulled the lithosphere in an east–west direction over hot mantle plumes. With an active martian core dynamo, this may have resulted in the observed magnetic anomaly pattern of concentric small circles. As further evidence, we observe that, of the 15 martian giant impact basins that were possibly formed while the core dynamo was active, seven lie along the equators of our two proposed paleopoles. We also find that four other re-magnetized giant impact basins lie along a great circle about the mean magnetic paleopole of Mars. These 11 impact basins, likely the result of fallen retrograde satellite fragments, indicate that Mars once had moons large enough to cause tidal drag on the early martian lithosphere. The results of this study suggest that the magnetic signatures of this tidal interaction have been preserved to the present day.

Variety of plumes and the fate of subducted basaltic crusts

Subducted basaltic crusts induce various types of plumes in a recently developed self-consistent numerical model of mantle convection with tectonic plates and mantle magmatism. Subducted basaltic crusts segregate from the convecting mantle by their compositionally induced high density. The basaltic crusts are formed into a continuous layer along the core–mantle boundary, if there is no influence of subducting slabs; the layer induces classic hot mantle plumes by basal heating. When influenced by subducting slabs, the subducted basaltic crusts are formed into hot accumulations similar to the Earth's superplumes, provided that the internal heating is weaker than a threshold. Chemical discontinuities often develop within and along the boundaries of the basaltic accumulations, and hot plumes ascend from the uppermost parts of the accumulations. Some of the plumes hit plates, cause hot spot magmatism, and induces new spreading centers. The rest of the plumes often stagnate on the 660km phase boundary to form basaltic pools. Occasionally, secondary plumes grow from the pools to induce hot spot magmatism. The pools are also sucked up by the ridges that happen to be above the pools, and activate the ridge magmatism. Hot plumes ascend from basaltic accumulations more frequently as the internal heating becomes stronger. When the internal heating becomes too strong, however, the accumulations of subducted basaltic crusts become convectively unstable and no longer work as the sources of plumes. The various types of the plumes observed here are compared to the plumes in the Earth's mantle.

Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark: Four Episodes of magmatism on the margin of the Baltic Shield

More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively. The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated. The Kelseaa type magmas were derived at shallow depth from a fluid-enriched, relatively depleted, mantle source,but some have a component derived from mantle with residual garnet. They are suggested to have formed in a back-arc environment. The more than 200 narrow dykes are olivine tholeiites (some picritic), alkali basalts, trachybasalts, basanites and a few phonotephrites. The magmas evolved by olivine and olivine + clinopyroxene fractionation. They have trace element characteristics which can be described mainly by mixing of two components: one is a typical OIB-magma (La/Nb &lt; 1, Zr/Nb = 4, Sr/Nd = 16) and rather shallowly derived from spinel peridotite; the other is enriched in Sr and has La/Nb = 1.0 - 1.5, Zr/Nb = 9, Sr/Nd = 30 and was derived at greater depth, probably from a pyroxenitic source. Both sources were probably recycled material in a mantle plume. A few of these dykes are much more enriched in incompatible elements and were derived from garnet peridotite by a small degree of partial melting. The Kaas and Listed dykes (20-40 m) and related dykes are evolved trachybasalts to basaltic trachyandesites. They are most likely related to the Blekinge Dalarne Dolerite Group. The few NW-trending dykes are quartz tholeiites, which were generated by large degrees of rather shallow melting of an enriched mantle source more enriched than the source of the older Bornholm dykes. The source of the NW-trending dykes was probably a very hot mantle plume.

Archean mafic–ultramafic volcanic landmasses and their effect on ocean–atmosphere chemistry

Ancient sediments that precipitated from seawater contain elemental and isotopic records that have yielded mutually exclusive conclusions regarding the supply of elements. Certain isotopic data (e.g. Sr and Nd) appear to imply a preponderance of hydrothermal flux while a number of elemental abundances (e.g. Eu and Y) apparently require a much greater flux from land. Refined insight into the origin of the marine rare earth element pattern is shown to strengthen the requirement for a significant elemental flux from the early Earth landmass. Here it is proposed that the Archean landmass included a significant fraction of emerged oceanic plateaus in addition to the continents. Alternatively, repeated volcanic resurfacing of the continents with mantle-derived basalt increased continental element flux. Model calculations for the Eo- to Neo-Archean marine rare earth element pattern require that ca. 20% of the planet was occupied by volcanic plateaus (compared to the 3% of today). Not only were the plateaus more extensive but they were also thicker. A tectonic picture emerges in which the Archean ocean basins may have consisted of lithosphere of lesser (similar to present day) thickness populated by relatively short-lived (several 100 Ma at most) plateaus of considerable thickness. One important feature that distinguishes the Archean from the modern land surface is the preponderance of freshly erupted volcanic rocks. Due to the instability of high-Mg rocks under weathering, a substantial land surface of this type would have had importance both for the supply of nutrients to the ocean, as well as a potential sink for water and atmospheric gases. For example, the disappearance of high-MgO land towards the end of the Archean may have reduced the supply of Ni to the ocean and brought to an end the supremacy of methanogens, ultimately leading to the great oxidation event. Furthermore, both the submarine and subaerial volcanic resurfacing of the continents and oceanic plateaus may have led to high 18O/ 16O alteration products and isotopically light seawater, removing the need for an unreasonably hot early Earth surface temperature. Hot mantle plumes capable of producing vast volcanic plateaus may thus have played a major role in regulating the composition of the early hydrosphere and biosphere and indirectly also the evolution of the atmosphere.

Siberian meimechites: origin and relation to flood basalts and kimberlites

Here we combine petrological-geochemical and thermomechanical modeling techniques to explain origin of primary magmas of both Maimecha–Kotui meimechites and the Gudchikhinskaya basalts of Norilsk region, which represent, respectively, the end and the beginning of flood magmatism in the Siberian Trap Province. We have analyzed the least altered samples of meimechites, their olivine phenocrysts, and melt inclusions in olivines, as well as samples of dunites and their olivines, from boreholes G-1 and G-3 within the Guli volcanoplutonic complex in the Maimecha–Kotui igneous province of the northern Siberian platform. The Mn/Fe and Ni/MgO ratios in olivines indicate a mantle peridotite source of meimechites. Meimechite parental magma that rose to shallow depths was rich in alkalis and highly magnesian (24 wt.% MgO), largely degassed, undersaturated by sulfide liquid and oxidized. At greater depths, it was, likely, high in CO 2 (6 wt.%) and H 2O (2 wt.%) and resulted from partial melting of initially highly depleted and later metasomatized harzburgite some 200 km below the surface. Trace-element abundances in primary meimechite magma suggest presence of garnet and K-clinopyroxene, in the mantle source and imply for genetic link to the sources of the early Siberian flood basalts (Gudchikhinskaya suite) and kimberlites. The analyzed dunite samples from the Guli complex have chemistry and mineralogy indicating their close relation to meimechites. We have also computed thermomechanical model of interaction of a hot mantle plume with the shield lithosphere of variable thickness, using realistic temperature- and stress-dependent visco-elasto-plastic rocks rheology and advanced finite element solution technique. Based on our experimental and modeling results we propose that a Permian–Triassic plume, with potential temperature of about 1650 °C transported a large amount of recycled ancient oceanic crust (up to 15%) as SiO 2-supersaturated carbonated eclogite. Low-degree partial melting of eclogite at depths of 250–300 km produced carbonate-silicate melt that metasomatized the lithospheric roots of the Siberian shield. Further rise of the plume under relatively attenuated lithosphere (Norilsk area) led to progressive melting of eclogite and formation of reaction pyroxenite, which then melted at depths of 130–180 km. Consequantly, a large volume of melt (Gudchikhinskaya suite) penetrated into the lithosphere and caused its destabilization and delamination. Delaminated lithosphere that included fragments of locally metasomatized depleted harzburgite subsided into the plume and was heated to the temperatures of the plume interior with subsequent generation of meimechite magma. Meimechites showed up at the surface only under thicker part of the lithosphere aside from major melting zone above because otherwise they were mixed up in more voluminous flood basalts. We further suggest that meimechites, uncontaminated Siberian flood basalts and kimberlites all shear the same source of strongly incompatible elements, the carbonated recycled oceanic crust carried up by hot mantle plume.

The onset of the North Atlantic Igneous Province in a rifting perspective

AbstractThe processes that led to the onset and evolution of the North Atlantic Igneous Province (NAIP) have been a theme of debate in the past decades. A popular theory has been that the impingement on the lower lithosphere of a hot mantle plume (the ‘Ancestral Iceland’ plume) initiated the first voluminous outbursts of lava and initiated rifting in the North Atlantic area in Early Palaeogene times. Here we review previous studies in order to set the NAIP magmatism in a time–space context. We suggest that global plate reorganizations and lithospheric extension across old orogenic fronts and/or suture zones, aided by other processes in the mantle (e.g. local or regional scale upwellings prior to and during the final Early Eocene rifting), played a role in the generation of the igneous products recorded in the NAIP for this period. These events gave rise to the extensive Paleocene and Eocene igneous rocks in W Greenland, NW Britain and at the conjugate E Greenland–NW European margins. Many of the relatively large magmatic centres of the NAIP were associated with transient and geographically confined doming in Early Paleocene times prior to the final break-up of the North Atlantic area.

Magma productivity and early seafloor spreading rate correlation on the northern Vøring Margin, Norway — Constraints on mantle melting

Continental rifting at the Vøring Margin off mid-Norway was initiated during the earliest Eocene (~ 54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, a marine survey collecting ocean bottom seismometer, single-channel reflection, and magnetic data was conducted on the Norwegian Margin to constrain continental breakup and early seafloor spreading processes. The profile described here crosses the northern part of the Vøring Plateau, and the crustal velocity model was constructed through a combination of ray-tracing and forward gravity modeling, the latter corrected for the thermal effects remaining from the seafloor spreading. We found a maximum igneous crustal thickness of 18 km, decreasing to 6.5 km over the first ~ 6 M.y. after continental breakup. Both the volume and the duration of excess magmatism are about twice as large as that of the Møre Margin south of the East Jan Mayen Fracture Zone, which offsets the two margin segments by ~ 170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~ 150 km to the Lofoten Margin, but without a margin offset. Both the geochemical data and the mean P-wave velocity indicate that there is active mantle upwelling combined with a moderate temperature increase during the earliest mantle melting at the Vøring Margin. The mean P-wave velocity versus crustal thickness also indicates that there is a transition from convection dominated to temperature dominated magma production ~ 2 M.y. after breakup. The magnetic data were used to derive plate half-spreading rates for the Northern Vøring Margin, which are very similar to that obtained at the Møre Margin. There is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. Proposed end-member hypotheses comprise elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Edge-driven convection does not easily explain these observations, but a mantle plume model in which buoyant plume material flows laterally to pond in the rift-topography at the base of the lithosphere close to breakup time is promising: When the continents break apart, the hot and buoyant plume-material can flow up into the rift zone from surrounding areas as the rift transits to drift, and the excess temperature of this material will then cause excess magmatism which dies off as the rift-restricted material is spent. The buoyancy of the plume-material may in addition cause active upwelling which can increase the melting furthermore, and also increase the force on the plate boundaries to enhance plate spreading rate. This conceptual model explains how both excess magmatism and spreading rate will be reduced similarly with time as the plume material is consumed by plate spreading, and thus correlate.

Geology, Geochronology, and Geodynamics of the Khan Bogd alkali granite pluton in southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

Deep origin of the Hawaiian tilted plume conduit derived from receiver functions

We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3-D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection.

Differences between Archean and Proterozoic lithospheres: Assessment of the possible major role of thermal conductivity

We study heat transfer through the conductive lithosphere and convective mantle on the basis of a 2‐D convection model in order to better understand the differences between Archean and Proterozoic lithospheres. The original improvement in the modeling consists of a precise track of the cutoff temperature between conduction and convection. The conductive lithosphere is undeformable, and the convective mantle has a constant viscosity. The conductive lithosphere in Archean cratons is assumed to have a lower radiogenic heat production in the crust and/or the conductive mantle, and/or a higher cutoff temperature (attributed to a stiffening of the more depleted mantle) relative to Proterozoic terrains. We also investigate the effect of a fourth factor never considered before: an enhancement of the vertical thermal conductivity in the conductive Archean mantle due to a vertical lineation. Our model successfully predicts the observations on the Canadian and South African shields. A high vertical thermal conductivity in the Archean lithospheric mantle possibly associated to a radiogenic crust depletion explains many observations: i.e., (1) the development of a thick Archean cold root, (2) a dipping cold convective plume beneath Archean cratons, (3) a uniform mantle heat flux along Archean and Proterozoic terrains, and (4) strong high seismic velocity anomalies over Archean cratons. In addition, a lower radiogenic heat production of the lithospheric mantle or a larger cutoff temperature in Archean cratons is not favored by our models. Finally, we investigate how a cold root and a sub‐Archean cold mantle plume react to the shear induced by large‐scale mantle flow and to the collision with a hot mantle plume. Models show that shear removes the cold plume from below the Archean craton, but cold dipping instabilities are periodically generated inside the convective boundary layer below the Archean cratonic lid. However, the thick Archean lithospheric root may survive several orogeneses. In addition, the models show that it takes at least 200 Myr for a hot mantle plume to erode the thick lithospheric root. This suggests that an Archean root may only disappear if the plate remains above a hot plume for a very long time.

The chemical structure of the Hawaiian mantle plume

The Hawaiian-Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes--even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume.