On thermochemical mantle plumes with an intermediate thermal power that erupt on the Earth’s surface

The relative plume thermal power Ka = N/N₁ is used (N is the thermal power transferred from the plume base to its conduit and N₁ is the thermal power transferred from the plume conduit into the surrounding mantle in the steady-state heat conduction regime). Thermochemical mantle plumes with small (Ka < 1.15) and intermediate (1.15 < Ka < 1.9) thermal powers are formed at the core–mantle boundary beneath cratons in the absence of horizontal free-convection mantle flows beneath them, or in the presence of weak horizontal mantle flows. Thermochemical plumes reach the Earth’s surface when their relative thermal power is Ka > 1.15. The thermal and hydrodynamical structure of the plume conduit ascending from the core–mantle interface to the level from which the magmatic melt erupts on the Earth’s surface is presented. The model of two-stage eruption of the melt from the plume conduit to the surface is considered. The critical height of the massif above the plume roof, at which the eruption conduit supplying magmatic melt to the surface forms, is determined. The volume of melt erupting through the eruption conduit to the surface is estimated. The dependence of depth Δx from which the melt is transported to the surface on the plume diameter for a kinematic viscosity of ν = 0.5–2 m²/s is presented. In the case when the value Δx is larger than the depth starting from which diamond is stable (150 km), the melt from the plume conduit can transport diamonds to the Earth’s surface. The melt flow in the eruption conduit is considered as a turbulent flow in a cylindrical duct. The velocity of the melt flow in the eruption conduit and the time for the melt to be transported to the surface from a depth of Δx = 150 km for a kinematic viscosity of the melt in the eruption conduit ν_v = 0.01–1 m²/s are determined. Tangential stress on the eruption conduit sidewall is estimated in cases of melt flow both in smooth and rough conduits.     

Parameters of hotspots and thermochemical plumes during their ascent and eruption

Thermochemical plumes develop at the core-mantle boundary in the presence of a heat flow from the outer core and at local chemical doping that decreases the melting temperature near the bottom of the lower mantle (this dope triggers the melting of the mantle material and the ascent of the plume). The paper presents evaluations for the heat power of the Hawaiian and Iceland plumes and the results of the experimental modeling of a thermochemical plume. The diameter of a plume conduit was determined to remain virtually unchanging in the course of plume ascent. When the top of a plume reaches a “refractory” layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped head of the plume develops beneath the bottom of this layer. The analysis of geological and geophysical data and the results of experimental modeling are used to develop a thermal physical model for a thermochemical plume. The balance relations for the mass and thermal energy and systematic tendencies in the heat and mass transfer during free convection were utilized to derive a system of equations for the heat and mass transfer of a thermochemical plume. Parameters were determined for a thermochemical plume ascending from the core-mantle boundary. Geodynamic processes are considered that occur during the ascent of a plume before it reaches the surface. The effect of the P-T conditions on the shape and size of a plume roof is analyzed, and a model is proposed for mass transfer between a thermochemical plume and the lithosphere, when the plume reaches the bottom of a “refractory” layer in the lithosphere.     

Teleseismic imaging of the mantle beneath southernmost China: New insights into the Hainan plume

Intraplate volcanism during the Late Cenozoic in the Leiqiong area of southernmost China, with basaltic lava flows covering a total of more than 7000 km², has been attributed to an underlying Hainan plume. To clarify detailed features of the Hainan plume, such as the morphology of its magmatic conduits, the depth of its magmatic pool in the upper mantle and the pattern of mantle upwelling, we determined tomographic images of the mantle down to a depth of 1100 km beneath southern China using 18,503 high-quality arrival-time data of 392 distant earthquakes recorded by a dense seismic array. Our results show a mushroom-like continuous low-velocity anomaly characterized by a columnar tail with a diameter of 200–300 km extending down to the lower mantle beneath north of the Hainan hotspot and a head spreading laterally in and around the mantle transition zone, indicating a magmatic pool in the upper mantle. Further upward, the plume head is decomposed into smaller patches, and when reaching the base of the lithosphere, a pancake-like anomaly has formed to feed the Hainan hotspot. This result challenges the classical model of a fixed thermal plume that rises vertically to the surface. Hence we propose a new layering-style model for the magmatic upwelling of the Hainan plume. Our results indicate spatial complexities and variations of mantle plumes probably due to heterogeneous compositions and thermochemical structures of the deep mantle.     

Experimental modeling of the effect of relative thermal power on the shape of a plume conduit and the structure of free-convection flow in it

The shape of a plume conduit produced by melting solid paraffin block above a local heat source was studied experimentally as a function of the relative thermal power of the plume Ka= N/N₁, where N₁ is the power of the plume source and N₁ is the power corresponding to the amount of heat transferred by conduction through the plume conduit to the surrounding solid paraffin block. The limiting power of the plume source at which the plume erupts at the Earth’s surface (N_lim1= (1.35–1.60) × 10¹⁰ W) and the power at which the mushroom-shaped plume ^{head formed at the base of the refractory layer (N}_lim2= ^{(1.78–1.90) × 1010 W) with no horizontal mantle flow were determined. The dependence} of the diameter of the base of the plume on the Ka number was established. The Ka value and the diameter of the plume base were determined for the Hawaiian and Iceland plumes, for the plume responsible for the formation of the Tunguska syneclise and for the McKenzie and Central Atlantic continental plateau-basalt provinces and for the Ontong Java and Manihiki oceanic lava plateaus.     

High-MgO lavas associated to CFB as indicators of plume-related thermochemical effects: The case of ultra-titaniferous picrite–basalt from the Northern Ethiopian–Yemeni Plateau

A comprehensive petrological and geochemical dataset is reported in order to define the thermo-compositional characteristics of Ti (Fe)-enriched picrite–basalt lavas (HT2, TiO₂ 3–7 wt%), erupted close to the axial zone of the inferred Afar mantle plume, at the centre of the originally continuous Ethiopian–Yemeni CFB plateau (ca. 30 Ma) which is zonally arranged with progressively lower Ti basalts (HT1, TiO₂ 2–4 wt%; LT, TiO₂ 1–3 wt%) toward the periphery. Integrated petrogenetic modelling based on major and trace element analyses of bulk rocks, minerals, and melt inclusions in olivines, as well as Sr–Nd–Pb–He–O isotope compositional variations enables us to make several conclusions. 1) The phase equilibria constraints indicate that HT2 primary picrites were generated at ca. 1570 °C mantle potential temperatures (T_p) in the pressure range 4–5 GPa whereas the HT1 and LT primary melts formed at shallower level (< 2–3 GPa, T_p 1530 °C for HT1 and 1430 °C for LT). Thus, the Afar plume head was a thermally and compositionally zoned melting region with maximum excess temperatures of 300–350 °C with respect to the ambient mantle. 2) The HT2 primary melts upwelled nearly adiabatically to the base of the continental crust (ca. 1 GPa) where fractionation of olivine, followed by clinopyroxene, led to variably differentiated picritic and basaltic magmas. 3) Trace element modelling requires that the primary HT2 melts were generated—either by fractional or batch melting (F 9–10%)—from a mixed garnet peridotite source (85%) with 15% eclogite (derived from transitional MORB protoliths included in Panafrican terranes) that has to be considered a specific Ti–Fe and incompatible element enriched component entrained by the Afar plume. 4) The LT, HT1, and HT2 lavas have ¹⁴³Nd/¹⁴⁴Nd = 0.5131–0.5128, whereas Sr–Pb isotopes are positively correlated with TiO₂, varying from ⁸⁷ Sr/⁸⁶Sr 0.7032 and ²⁰⁶Pb/²⁰⁴Pb 18.2 in LT basalts to ⁸⁷Sr/⁸⁶Sr 0.7044 and ²⁰⁶Pb/²⁰⁴Pb 19.4 in HT2 picrite–basalts. High ³He/⁴He (15–20 R_A) ratios are exclusively observed in HT2 lavas, confirming earlier evidence that these magmas require a component of deep mantle in addition to eclogite, while the LT basalts may more effectively reflect the signature of the pre-existing mantle domains. The comparison between high-MgO (13–22%) lavas from several Phanerozoic CFB provinces (Karoo, Paranà–Etendeka, Emeishan, Siberia, Deccan, North Atlantic Province) shows that they share extremely high mantle potential temperatures (T_p 1550–1700 °C) supporting the view that hot mantle plumes are favoured candidates for triggering many LIPs. However, the high incompatible element and isotopic variability of these high-MgO lavas (and associated CFB) suggest that plume thermal anomalies are not necessarily accompanied by significant and specific chemical effects, which depend on the nature of mantle materials recycled during the plume rise, as well as by the extent of related mantle enrichments (if any) on the pre-existing lithospheric section.     

The structure of mantle flows and stress fields in a two-dimensional convection model with non-Newtonian viscosity

The structure of mantle convection and spatial fields of superlitho static pressure and vertical and horizontal stresses in the Earth’s mantle are studied in a 2D numerical model with non-Newtonian viscosity and heat sources. The model demonstrates a jump-like motion of subduction zones and reveals abrupt changes in the stress fields depending on the stage of slab detachment. The stresses decrease dramatically in the areas without slabs. The horizontal stresses o_xx, superlitho static pressure, and vertical stresses o_zz in the part of the mantle lacking intense near-vertical flows are approximately equal, varying within ± 6, ± 8, and ± 10 MPa, respectively. However, these fields are stronger in the areas of descending slabs, where the values of the above parameters are about an order of magnitude higher (± 50 MPa).This result agrees with the current views of the oceanic slabs as the most important gent of mantle convection. We have found significant differences among the o_xx, o_zz, and pressure fields. The pressure field reveals both the vertical and horizontal features of slabs and plumes, clearly showing their long thermal conduits with broader heads. The distributions of o_xx are sensitive to the near-horizontal parts of the flows, whereas the o_zz fields reveal mainly their vertical substructures. The model shows the presence of relatively cold remnants of slabs in the lower mantle above the thermal boundary layer. Numerous hot plumes penetrating through these high-viscosity remnants, as well as the new descending slabs, induce intense stress fields in the lower mantle, which are strongly inhomogeneous in space and time.     

Origin of high-velocity anomalies beneath the Siberian craton: A fingerprint of multistage magma underplating since the Neoarchean

Despite the violent eruption of the Siberian Traps at ~ 250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m²) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high V_p (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (V_p = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.     

Interaction of a thermochemical plume with free convection mantle flows and its influence on mantle melting and recrystallization

We present a thermophysical model for interaction between the conduit of a thermochemical plume and horizontal free convection flows in the mantle: The mantle flow incident on the plume conduit melts at the conduit boundary (front part) and crystallizes at its back. Geological data on the intensity of plume magmatism over the last 150 Myr are used to estimate the total thermal power of mantle plumes. A possible scenario for plume-related mantle recrystallization is proposed. Over the lifespan of a thermochemical plume, mantle melts and recrystallizes owing to the motion of the plume source and interaction between the plume conduit and horizontal free convection flows. The plume conduits can melt and recrystallize the entire mantle over a certain period of time. The model for the interaction of drifting plume conduits with mantle flows and the estimated total thermal power of mantle plumes are used to estimate the duration of plume-related melting and recrystallization of the entire mantle. The influence of mantle plumes on the convective structure of the mantle through melting is judged from the model for plume interaction with horizontal mantle flows.     

Changing compositions in the Iceland plume; Isotopic and elemental constraints from the Paleogene Faroe flood basalts

Elemental and Sr, Nd, Hf and high precision Pb isotopic data are presented from 59 low-Ti and high-Ti lavas from the syn-break up part of the Faroe Flood Basalt Province. The depleted MORB-like low-Ti lavas erupted in the rift zone between the Faroe Islands and central East Greenland around the time of break up of the North Atlantic have isotopic end-member compositions different from the depleted Iceland lavas. We suggest that the main low-Ti mantle component is NAEM (North Atlantic End-Member (Ellam and Stuart, 2000, J. Petrol. 41, 919) and that the ²⁰⁷Pb/²⁰⁴Pb value of the component should be 15.35 and ε_Hf = + 16.5. NAEM is the main depleted component in the early Iceland plume. This is supported by high mantle potential temperatures (up to 1550 °C) calculated for the source of the low-Ti basalts. The unique mantle isotopic composition of NAEM with low ²⁰⁶Pb/²⁰⁴Pb (17.5) and Δ7/4Pb (? 3.8) precludes a derivation from recycled MORB lithosphere. Instead we suggest that NAEM represents a plume component of recycled depleted Archean lithospheric mantle that was further depleted ~ 500 Ma ago, possibly in connection with the recycling process. Two other isotopic end-members are required to explain the variation of the Faroe low-Ti basalts: (1) The Faroe depleted component (FDC), with ⁸⁷Sr/⁸⁶Sr = 0.7025, ε_Nd = + 11, ε_Hf = + 19.5, ²⁰⁶Pb/²⁰⁴Pb = 18.2, ²⁰⁷Pb/²⁰⁴Pb = 15.454 and ²⁰⁸Pb/²⁰⁴Pb = 37.75, which is similar in composition to some Atlantic MORB and is regarded as a local upper mantle source. (2) An enriched EM-type component similar in geochemistry to the Icelandic Öræfajökull lavas. This component is believed to be recycled pelagic sediments in the plume but it can alternatively be a local crustal or lithospheric mantle component. The enriched Faroe high-Ti lavas erupted inland from the rift have isotopic compositions very similar to the enriched Icelandic neo-volcanics and these lava suites apparently share the two enriched plume end-members IE1 and IE2 (Geochim. Cosmochim. Acta 68, 2, 2004). The lack of mixing between high and low-Ti melts at the time of break up, is explained by a zoned plume where only low-Ti sources were present beneath the rift zone surrounded by high-Ti sources on both sides of the rift. The enriched plume components in the high-Ti lava sequences on the Faroe Islands and central East Greenland changed rapidly on a ka-scale which implies, from geophysical modelling, that this area was positioned above the center of the plume, and that the Iceland plume was centered under the Atlantic ridge already from the Paleocene.     

Evidence of Mid-ocean ridge and shallow subduction forearc magmatism in the Nagaland-Manipur ophiolites,northeast India: constraints from mineralogy and geochemistry of gabbros and associated mafic dykes

We discuss here the mineralogical and geochemical characteristics of mafic intrusive rocks from the Nagaland-Manipur Ophiolites (NMO) of Indo-Myanmar Orogenic Belt, northeast India to define their mantle source and tectonic environment. Mafic intrusive sequence in the NMO is characterized by hornblende-free (type-I) and hornblende-bearing (type-II) rocks. The type-I is further categorized as mafic dykes (type-Ia) of tholeiitic N-MORB composition, having TiO₂ (0.72–1.93 wt.%) and flat REE patterns (La_N/Yb_N = 0.76–1.51) and as massive gabbros (type-Ib) that show alkaline E-MORB affinity, having moderate to high Ti content (TiO₂ = 1.18 to 1.45 wt.%) with strong LREE-HREE fractionations (La_N/Yb_N = 4.54–7.47). Such geochemical enrichment from N-MORB to E-MORB composition indicates mixing of melts derived from a depleted mantle and a fertile mantle/plume source at the spreading center. On the other hand, type-II mafic intrusives are hornblende bearing gabbros of SSZ-type tholeiitic composition with low Ti content (TiO₂ = 0.54 wt.%–0.86 wt.%) and depleted LREE pattern with respect to HREE (La_N/Yb_N = 0.37–0.49). They also have high Ba/Zr (1.13–2.82), Ba/Nb (45.56–151.66) and Ba/Th (84.58–744.19) and U/Th ratios (0.37–0.67) relative to the primitive mantle, which strongly represents the melt composition generated by partial melting of depleted lithospheric mantle wedge contaminated by hydrous fluids derived from subducting oceanic lithosphere in a forearc setting. Their subduction related origin is also supported by presence of calcium-rich plagioclase (An_16.6–32.3). Geothermometry calculation shows that the hornblende bearing (type-II) mafic rocks crystallized at temperature in range of 565°–625 °C ± 50 (at 10 kbar). Based on these available mineralogical and geochemical evidences, we conclude that mid ocean ridge (MOR) type mafic intrusive rocks from the NMO represent the section of older oceanic crust which was generated during the divergent process of the Indian plate from the Australian plate during Cretaceous period. Conversely, the hornblende-bearing gabbros (type-II) represent the younger oceanic crust which was formed at the forearc region by partial melting of the depleted mantle wedge slightly modified by the hydrous fluids released from the subducting oceanic slab during the initial stage of subduction of Indian plate beneath the Myanmar plate.     

Melting phase relations of the Udachnaya-East Group-I kimberlite at 3.0–6.5 GPa: Experimental evidence for alkali-carbonatite composition of primary kimberlite melts and implications for mantle plumes

Experiments on the origin of the Udachnaya-East kimberlite (UEK) have been performed using a Kawai-type multianvil apparatus at 3–6.5 GPa and 900–1500 °C. The studied composition represents exceptionally fresh Group-I kimberlite containing (wt.%): SiO₂ = 25.9, TiO₂ = 1.8, Al₂O₃ = 2.8, FeO = 9.0, MgO = 30.1, CaO = 12.7, Na₂O = 3.4, K₂O = 1.3, P₂O₅ = 1.0, Cl = 0.9, CO₂ = 9.9, and H₂O = 0.5. The super-solidus assemblage consists of melt, olivine (Ol), Ca-rich (26.0–30.2 wt.% CaO) garnet (Gt), Al-spinel (Sp), perovskite (Pv), a CaCO₃ phase (calcite or aragonite), and apatite. The low pressure assemblage (3–4 GPa) also includes clinopyroxene. The apparent solidus was established between 900 and 1000 °C at 6.5 GPa. At 6.5 GPa and 900 °C Na–Ca carbonate with molar ratio of (Na + K)/Ca ≈ 0.44 was observed. The UEK did not achieve complete melting even at 1500 °C and 6.5 GPa, due to excess xenogenic Ol in the starting material. In the studied P–T range, the melt has a Ca-carbonatite composition (Ca# = molar Ca/(Ca + Mg) ratio = 0.62–0.84) with high alkali and Cl contents (7.3–11.4 wt.% Na₂O, 2.8–6.7 wt.% K₂O, 1.6–3.4 wt.% Cl). The K, Na and Cl contents and Ca# decrease with temperature. It is argued that the primary kimberlite melt at depths > 200 km was an essentially carbonatitic (< 5 wt.% SiO₂), but evolved toward a carbonate–silicate composition (up to 15–20 wt.% SiO₂) during ascent. The absence of orthopyroxene among the run products indicates that xenogenic orthopyroxene was preferentially dissolved into the kimberlite melt. The obtained subliquidus phase assemblage (Ol + Sp + Pv + Ca-rich Gt) at P–T conditions of the UEK source region, i.e. where melt was in the last equilibrium with source rock before magma ascent, differs from the Opx-bearing peridotitic mineral assemblage of the UEK source region. This difference can be ascribed to the loss of substantial amounts of CO₂ from the kimberlite magma at shallow depths, as indicated by both petrological and experimental data. Our study implies that alkali-carbonatite melt would be a liquid phase within mantle plumes generated at the core–mantle boundary or shallower levels of the mantle, enhancing the ascent velocity of the plumes. We conclude that the long-term activity of a rising hot mantle plume and associated carbonatite melt (i.e. kimberlite melt) causes thermo-mechanical erosion of the subcontinental lithosphere mantle (SCLM) roots and creates hot and deformed metasomatic regions in the lower parts of the SCLM, which corresponds to depths constrained by P–T estimates of sheared Gt-peridotite xenoliths. The sheared Gt-peridotites undoubtedly represent samples of these regions.     

Mantle plume or slab window?: Physical and geochemical constraints on the origin of the Caribbean oceanic plateau

The Caribbean oceanic plateau formed in the Pacific realm when it erupted onto the Farallon plate from the Galapagos hotspot at ～ 90 Ma. The plateau was subsequently transported to the northeast and collided with the Great Arc of the Caribbean thus initiating subduction polarity reversal and the consequent tectonic emplacement of the Caribbean plate between the North and South American continents. The plateau represents a large outpouring of mafic volcanism, which has been interpreted as having formed by melting of a hot mantle plume. Conversely, some have suggested that a slab window could be involved in forming the plateau. However, the source regions of oceanic plateaus are distinct from N-MORB (the likely source composition for slab window mafic rocks). Furthermore, melt modelling using primitive (high MgO) Caribbean oceanic plateau lavas from Curaçao, shows that the primary magmas of the plateau contained ～ 20 wt.% MgO and were derived from 30 to 32% partial melting of a fertile peridotite source region which had a potential temperature (T_p) of 1564–1614 °C. Thus, the Caribbean oceanic plateau lavas are derived from decompression melting of a hot upwelling mantle plume with excess heat relative to ambient upper mantle. Extensional decompression partial melting of sub-slab asthenosphere in a slab window with an ambient mantle T_p cannot produce enough melt to form a plateau. The formation of the Caribbean oceanic plateau by melting of ambient upper mantle in a slab window setting, is therefore, highly improbable.     

Flood basalts and metallogeny: The lithospheric mantle connection

Continental flood basalts, derived from mantle plumes that rise from the convecting mantle and possibly as deep as the core–mantle boundary, are major hosts for world-class Ni–Cu–PGE ore deposits. Each plume may have a complex history and heterogeneous composition. Therefore, some plumes may be predisposed to be favourable for large-scale Ni–PGE mineralisation (“fertile”).Geochemical data from 10 large igneous provinces (LIPs) have been collected from the literature to search for chemical signatures favourable for Ni–PGE mineralisation. The provinces include Deccan, Kerguelen, Ontong Java, Paraná, Ferrar, Karoo, Emeishan, Siberia, Midcontinent and Bushveld. Among these LIPs, Bushveld, Siberia, Midcontinent, Emei Mt and Karoo are “fertile”, hosting magmatic ore deposits or mineralisation of various type, size and grade. They most commonly intruded through, or on the edges of, Archaean–Paleoproterozoic cratonic blocks. In contrast, the “barren” LIPs have erupted through both continental and oceanic crustal terranes of various ages.Radiogenic isotopic signatures indicate that almost all parental LIP magmas are generated from deep-seated mantle plumes, and not from the more widespread depleted asthenospheric mantle source: this confirms generally accepted plume models. However, several important geochemical signatures of LIPs have been identified in this study that can discriminate between those that are “fertile” or “barren” in terms of their Ni–PGE potential.The fertile LIPs generally contain a relatively high proportion of primitive melts that are high in MgO and Ni, low in Al₂O₃ and Na₂O, and are highly enriched in most of the strongly incompatible elements such as K, P, Ba, Sr, Pb, Th, Nb, and LREE. They have relatively high Os contents (≥ 0.03 to 10 ppb) and low Re/Os (< 10). The fertile LIP basalts display trends of Sr–Nd–Pb isotopic variation intermediate between the depleted plume and an EM1-type mantle composition (and thus could represent a mixing of these two source types), and have elevated Ba/Th, Ba/Nb and K/Ti ratios. These elemental and isotopic signatures suggest that interaction between plume-related magmas and ancient cratonic lithospheric mantle with pre-existing Ni- and PGE-rich sulfide phases may have contributed significantly to the PGE and Ni budget of the fertile flood basalts and eventually to the mineralisation. This observation is consistent with the location of fertile LIPs adjacent to deep old lithospheric roots (as inferred from tectonic environment and also seen in global tomographic images) and has predictive implications for exploration models.Barren LIPs contain fewer high-MgO lavas. The barren LIP lavas in general have low Os contents (mostly ≤ 0.02 ppb) with high Re/Os (10–≥ 200). They show isotopic variations between plume and EM2 geochemical signatures and have high Rb/Ba ratios. These signatures may indicate involvement of deep recycled material in the mantle sources or crustal contamination for barren LIPs, but low degrees of interaction with old lithospheric-type roots.     

Modelling of thermochemical plumes and implications for the origin of the Siberian traps

Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t₁ between plume formation and eruption. These relationships are used to determine thermal power and time t₁ for the Tunguska syneclise and the Siberian traps as a whole.

The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts.     

Shrimp U–Pb zircon geochronology,geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province,SW China

The Cida A-type granitic stock (∼ 4 km²) and Ailanghe I-type granite batholith (∼ 100 km²) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO^⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (ε_Nd(t) = − 0.25 to + 1.35 and (⁸⁷Sr/⁸⁶Sr)_i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε_Nd(t) ratios (− 6.34 to − 6.26) and high (⁸⁷Sr/⁸⁶Sr)_i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks.In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.     

Enhancement of in situ biodegradation of organic compounds in groundwater by targeted pump and treat intervention

This study demonstrates the value of targeted pump and treatment (PAT) to enhance the in situ biodegradation of organic contaminants in groundwater for improved restoration. The approach is illustrated for a plume of phenolic compounds in a sandstone aquifer, where PAT is used for hydraulic containment and removal of dissolved phase contaminants from specific depth intervals. Time-series analysis of the plume hydrochemistry and stable isotope composition of dissolved species (δ³⁴S-SO₄, δ¹³C-CH₄, δ¹³C-TDIC (TDIC = Total Dissolved Inorganic Carbon)) in groundwater samples from high-resolution multilevel samplers were used to deduce changes in the relative significance of biodegradation processes and microbial activity in the plume, induced by the PAT system over 3 years. The PAT system has reduced the maximum contaminant concentrations (up to 6800 mg L⁻¹ total phenols) in the plume by 50% to ∼70% at different locations. This intervention has (i) stimulated in situ biodegradation in general, with an approximate doubling of contaminant turnover based on TDIC concentration, which has increased from <200 mg L⁻¹ to >350 mg L⁻¹, (ii) enhanced the activity of SO₄-reducing microorganisms (marked by a declining SO₄ concentration with corresponding increase in SO₄-δ³⁴S to values >7–14‰_V-CDT relative to background values of 1.9–6.5‰_V-CDT), and (iii) where the TDIC increase is greatest, has changed TDIC-δ¹³C from values of −10 to −15‰_V-PDB to ∼−20‰_V-PDB. This indicates an increase in the relative importance of respiration processes (including denitrification and anaerobic methane oxidation, AMO) that yield ¹³C-depleted TDIC over fermentation and acetoclastic methanogenesis that yield ¹³C-enriched TDIC in the plume, leading to higher contaminant turnover. The plume fringe was found to be a zone of enhanced biodegradation by SO₄-reduction and methanogenesis. Isotopically heavy methane compositions (up to −47.8‰_V-PDB) and trends between δ¹³C-TDIC and δ¹³C-CH₄ suggest that AMO occurs at the plume fringe where the contaminant concentrations have been reduced by the PAT system. Mass and isotope balances for inorganic carbon in the plume confirm the shift in spatial dominance of different biodegradation processes and significant increase in contribution of anaerobic respiration for contaminant biodegradation in zones targeted by the PAT system. The enhanced in situ biodegradation results from a reduction in organic contaminant concentrations in the plume to levels below those that formerly suppressed microbial activity, combined with increased supply of soluble electron acceptors (e.g. nitrate) into the plume by dispersion. An interruption of the PAT system and recovery of the dissolved organic contaminant concentrations towards former values highlights the dynamic nature of this enhancement on restoration and relatively rapid response of the aquifer microorganisms to changing conditions induced by the PAT system. In situ restoration using this combined engineered and passive approach has the potential to manage plumes of biodegradable contaminants over shorter timescales than would be possible using these methods independently. The application of PAT in this way strongly depends on the ability to ensure an adequate flux of dissolved electron acceptors into the plume by advection and dispersion, particularly in heterogeneous aquifers.     

Petrology and geochemistry of Abyssal Peridotites from the Manipur Ophiolite Complex,Indo-Myanmar Orogenic Belt,Northeast India: Implication for melt generation in mid-oceanic ridge environment

The Manipur Ophiolite Complex (MOC) located in the Indo-Myanmar Orogenic Belt (IMOB) of Northeast India forms a section of the Tethyan Ophiolite Belt of the Alpine–Himalayan orogenic system. Whole rock compositions and mineral chemistry of mantle peridotites from the MOC show an affinity to the abyssal peridotites, characterized by high contents of Al₂O₃ (1.28–3.30 anhydrous wt.%); low Cr# of Cr-spinel (0.11–0.27); low Mg# of olivine (∼Fo₉₀) and high Al₂O₃ in pyroxenes (3.71–6.35 wt.%). They have very low REE concentrations (∑REE = 0.48–2.14 ppb). Lherzolites display LREE-depleted patterns (La_N/Sm_N = 0.14–0.45) with a flat to slightly fractionated HREE segments (Sm_N/Yb_N = 0.30–0.65) whereas Cpx-harburgites have flat to upward-inflected LREE patterns (La_N/Sm_N = 0.13–1.23) with more fractionated HREE patterns (Sm_N/Yb_N = 0.13–0.65) than the lherzolite samples. Their platinum group elements (PGE) contents (<50 ppb) and distinct mantle-normalised PGE patterns with the Pd/Ir values (1.8–11.9) and Pt/Pt^* values (0.2–1.1) show an affinity to the characteristic of the residual mantle material. Evaluation of mineralogical and petrological characteristics of these peridotites suggests that they represent the residues remaining after low degree of partial melting (∼2–12%) in the spinel stability field of a mid-oceanic ridge environment. The well-preserved mid-oceanic ridge characteristics of these peridotites further suggest that the mantle section was subsequently trapped in the forearc region of the subduction zone without undergoing significant modification in their chemistry by later subduction-related tectonic and petrological processes before its emplacement to the present crustal level.     

Enriched crustal and mantle components and the role of the lithosphere in generating Paleoproterozoic dyke swarms of the Ungava Peninsula,Canada

Paleoproterozoic mafic dyke swarms (2.5–2.0 Ga) of the Ungava Peninsula can be divided in three chemical groups. The main group has a wide range of Fe (10–18 wt.% Fe₂O₃) and Ti (0.8–2.0 wt.% TiO₂) contents, and the most magnesian samples have compositions consistent with melting of a fertile lherzolitic mantle at ~ 1.5 GPa. Dykes of a low-LREE (light rare earth element) subgroup (La/Yb ≤ 4) display decreasing Zr/Nb with increasing La/Yb ratios and positive εNd_2.0 Ga values (+ 3.9 to + 0.2) that trend from primitive mantle towards the composition of Paleoproterozoic alkaline rocks. In contrast, dykes of a high-LREE subgroup (La/Yb ≥4) display increasing Zr/Nb ratios and negative εNd_2.0 Ga values (? 2.3 to ? 6.4) that trend towards the composition of Archean crust. A low Fe–Ti group has low Fe (< 11 wt.% Fe₂O₃), Ti (< 0.8 wt.% TiO₂), high field strength elements (HFSE; < 6 ppm Nb) and heavy rare earth elements (HREE; < 2 ppm Yb) contents, but are enriched in large ion lithophile elements (LILE; K/Ti = 0.7–3) and LREE (La/Yb > 4). These dykes are interpreted as melts of a depleted harzburgitic mantle that has experienced metasomatic enrichment. A positive correlation of Zr/Nb ratio and La/Yb ratio, negative εNd_2.0 Ga values (? 14 to ? 6), and the presence of inherited Archean zircons further suggest the incorporation of a crustal component. A high Fe–Ti group has high Fe (> 14 wt.% Fe₂O₃) and Ti (> 1.4 wt.% TiO₂) contents, along with higher Na contents relative to the main group dykes. Dykes of a high-Al subgroup (> 12 wt.% Al₂O₃) share Fe contents, εNd_2.0 Ga values (? 2.3 to ? 3.4), La/Yb and Th/Nb ratios with Archean ferropicrites, and may represent evolved ferropicrite melts. A low-Al subgroup (< 12 wt.% Al₂O₃) has relatively lower Yb contents (< 2 ppm) and fractionated HREE patterns that indicate the presence of garnet in their melting residue. A comparison with ~ 5 GPa experimentally-derived melts suggests that these dykes may be derived from garnet-bearing pyroxenite or peridotite. The εNd_2.0 Ga values (? 0.3 to ? 2.0) of these dykes lie between the compositions of Archean granitoids and Paleoproterozoic alkaline rocks, signifying their petrogenesis involved both crustal and mantle components.Paleoproterozoic dykes containing a crustal component occur within, or close to, an isotopically enriched Archean terrane (T_DM 4.3–3.1 Ga), whereas dykes without this component occur in an isotopically juvenile terrane (T_DM < 3.1 Ga). The lack of a crustal component and the positive εNd_2.0 Ga values of dykes intruding the latter suggest that the crust they intruded was either too cold to be assimilated, or that its lower crust and/or lithosphere were Paleoproterozoic in age. In contrast, the ubiquitous presence of a crustal component and the diversity of mantle sources for dykes intruding the enriched terrane (lherzolite, harzburgite, pyroxenite) suggest a warmer crust with underlying heterogeneous lithospheric mantle.     

Geodynamics of rapid voluminous felsic magmatism through time

Two end member geodynamic settings produce the observed examples of rapid voluminous felsic (rhyolitic) magmatism through time. The first is driven by mantle plume head arrival underneath a continent and has operated in an identifiable and regular manner since at least 2.45 Ga. This style produces high temperature (≤ 1100 °C), low aspect ratio rheoignimbrites and lavas that exhibit high SiO₂/Al₂O₃ ratios, high K₂O/Na₂O ratios, and where available data exists, high Ga/Al₂O₃ ratios (> 1.5) with high F (in thousands of parts per million) and low water content. F concentration is significant as it depolymerizes the silicate melt, influencing the magmas' physical behavior during development and emplacement. These rhyolites are erupted as part of rapidly emplaced (10–15 Myr) mafic LIPs and are formed primarily by efficient assimilation-fractional crystallization processes from a mafic mantle parent. The second is driven by lithospheric extension during continental rifting or back arc evolution and is exclusive to the Phanerozoic. SLIPs (silicic large igneous provinces) develop over periods < 40 Myr and manifest in elongate zones of magmatism that extend up to 2500 km, contrasting with the mafic LIP style. Some of the voluminous felsic magmas within SLIPs appear to have a very similar geochemistry and petrogenesis to that of the rhyolites within mafic LIPs. Other voluminous felsic magmas within SLIPs are sourced from hydrous lower crust, and contrast with those sourced from the mantle. They exhibit lower temperatures (< 900 °C), explosive ignimbrites with lower SiO₂/Al₂O₃ ratios, and lower K₂O/Na₂O ratios. Rapid voluminous felsic magmatism represents both extreme examples of continental growth since the Archean, and also dramatic periods of crustal recycling and maturation during the Phanerozoic.     

Neoproterozoic ultramafic–mafic-carbonatite complex and granitoids in Quruqtagh of northeastern Tarim Block,western China: Geochronology,geochemistry and tectonic implications

U–Pb zircon and baddeleyite ages, and geochemical and Nd isotopic data, are reported for a ultramafic–mafic-carbonatite complex and granites in Quruqtagh of northeastern Tarim Block, NW China. The carbonatite and plagioclase-bearing pyroxenite from the Qiganbulake mafic–ultramafic-carbonatite ring complex (QMC), the Xingdi granodiorite and the Taiyangdao granite were emplaced at 810 ± 6, 818 ± 11, 820 ± 10 and 795 ± 10 Ma (95% confidence level), respectively. The QMC is composed of dunite, apatite- and/or feldspar-bearing pyroxenite, pyroxenite, phlogopitelite and carbonatite. Petrography, geochemistry and mineral chemistry suggest that the QMC rocks were generated by partial melting of a CO₂-metasomatized mantle in a rifting environment. The Xingdi and Taiyangdao granitoids possess high LREE, Na₂O/K₂O, Sr/Y, (La/Yb)_N ratios and low HREE and HFSE contents, similar to modern adakites. However, they have lower MgO (or Mg^#), Cr and Ni contents and unradiogenic Nd isotopes (pronounced negative ɛNd(t) value of −12.7 to −17.3 and Neoarchaean Nd model ages) than slab-derived adakites. Thus, they were likely formed by partial melting of Neoarchaean mafic protoliths in the lower crust, leaving behind a granulite residue. The QMC and the granitoids in Quruqtagh constitute a bimodal intrusive suite in a Neoproterozoic continental rift setting, possibly related to mantle plume activities beneath the Rodinian supercontinent.