Hot Mantle Research Articles

Asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern South Atlantic

The southern South Atlantic has often been considered a classic example of continental break-up in the presence of a starting mantle plume. Evidence for a mantle plume includes the Paranà-Etendeka continental flood basalts, which are associated with the Rio Grande Rise and Walvis Ridge, and the wide-spread presence of seaward dipping reflectors and high-velocity lower-crustal bodies along the conjugate margins. Observations from seaward dipping reflector distributions suggested that lithospheric segmentation played a major role in the pattern of volcanism during break-up in this region, and consequent numerical modelling was used to test this. We tested this hypothesis ourselves by measuring the thickness of the earliest oceanic crust generated. This was done through the use of 37 measurements of initial oceanic crustal thickness from wide-angle and multichannel seismic profiles collected along the conjugate margins. These measurements show that at 450kmsouth of the Paranà-Etendeka flood basalts the oceanic crust is thicker than the global average at 11.7km. Farther south the oceanic crust thins, reaching 6.1km at a distance of 2300km along-strike. Overall, the along-strike trend of oceanic crustal thickness is linear with a regression coefficient of 0.7 and little indication of segmentation. From numerical models representing extension of the lithosphere, we find that observed melt volumes are matched with the presence of a hot layer. If we assume this region of hot mantle has a thickness of 100km, its excess temperature relative to the asthenosphere has to decrease from 200 to 50°C, north to south. This decrease in temperature, also seen in published thermobarometry results, suggests that temperature was the primary control of volcanism during the opening of the southern South Atlantic.

The driving force and movement of materials in the Earth’s interior

The main connotation of geophysical research is the physics of the Earth’s interior whose core issue includes the Earth’s internal structure, the forces and responses for the motion of materials. Based on the researches of deep exchange process and dynamic source mechanism of material and energy in the Earth’s interior, we draw the following conclusions: (1) The layered structure and material composition of the Earth’s interior lay a solid foundation for the researches of the material movement, the properties and structures of the medium, and the deep processes in the Earth’s interior. (2) The important geoscience events, such as the continental drift, sea-floor spreading and plate motion indicate that the lithosphere is flowing on the asthenosphere. The eruption of continental flood basalt and the upwelling of hot mantle plume reflect the coupled responses and coexistences of the horizontal and vertical force systems. (3) The exchange process of material and energy in the Earth’s interior bring up many products, such as the strong earthquake, the eruption of volcano, the elevation of Tibet Plateau, metal mineral resources in the second deep space, and the formation of oil and gas field. (4) In related to the researches of mechanics properties of the Earth medium, although the driving source of material movement in the Earth’s interior become one of the key problems, a series of involved hypothesis cannot explain many geosciences events very well. The mantle convection theory acquire some consensus, however it is still a hypothesis due to the lack of reliable and high precision measured data. (5) The heat from radioactive decay and the thermodynamic boundary layer at core-mantle boundary may be the driving source of the deep processes in the Earth’s interior. Whether the dark matter and dark material recently proposed by in the studies of astronomical movement are analogously exist in the Earth’s interior, which needs to deepen the understanding of the Earth.

Lithospheric structure across the central Tien Shan constrained by gravity anomalies and joint inversions of receiver function and Rayleigh wave dispersion

Shear wave velocity structure across the central Tien Shan orogeny was generated by jointly inverting Rayleigh wave phase and group velocity with teleseismic P-wave receiver functions at 40 broadband seismic stations. The inferred seismic structure was validated by forward modeling of the complete Bouguer anomaly data. The joint inversion result reveals larger crust thicknesses beneath the Kokshaal (∼68–72km) and Kyrgyz ranges (∼62–64km), while other units have crustal thicknesses between 48 and 58km. A fast velocity layer (Vs=3.6–3.9km/s) in the upper crust is found in some seismic stations within the Kazakh Shield. Our models show the presence of high velocity and density layers in the lowermost crust throughout the region, consistent with the presence of mafic/ultramafic lithologies. The large crustal thickness is associated with a thickened mafic layer in the lower crust, indicating that the thickened crust may be partly caused by magmatic underplating. The low velocity and density anomaly in the middle crust, and low upper mantle velocity observed in our model beneath the middle Tien Shan reflect the presence of partial melt in the crust due to the intrusion of hot mantle material. The lack of correlation between Moho depth and topography, together with the gravity results, suggests that the topographic compensation in the central Tien Shan is not confined to the crust. This requires significant support from the mantle to account for the relative high elevation of the middle Tien Shan.

Heterogeneous stress state of island arc crust in northeastern Japan affected by hot mantle fingers

AbstractBy considering a thermal structure based on dense geothermal observations, we model the stress state of the crust beneath the northeastern Japan island arc under a compressional tectonic regime using a finite element method with viscoelasticity and elastoplasticity. We consider a three‐layer structure (upper crust, lower crust, and uppermost mantle) to define flow properties. Numerical results show that the brittle‐viscous transition becomes shallower beneath the Ou Backbone Range compared with areas near the margins of the Pacific Ocean and the Japan Sea. Moreover, several elongate regions with a shallow brittle‐viscous transition are oriented transverse to the arc, and these regions correspond to hot fingers (i.e., high‐temperature regions in the mantle wedge). The stress level is low in these regions due to viscous deformation. Areas of seismicity roughly correspond to zones of stress accumulation where many intraplate earthquakes occur. Our model produces regions with high uplift rates that largely coincide with regions of high elevation (e.g., the Ou Backbone Range). The stress state, fault development, and uplift around the Ou Backbone Range can all be explained by our model. The results also suggest the existence of low‐viscosity regions corresponding to hot fingers in the island arc crust. These low‐viscosity regions have possibly affected viscous relaxation processes following the 2011 Tohoku‐oki earthquake.

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. 30Ma) 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 (Tp) in the pressure range 4–5GPa whereas the HT1 and LT primary melts formed at shallower level (<2–3GPa, Tp 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. 1GPa) 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 (15–20RA) 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 (Tp 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.

Themis Regio, Venus: Evidence for recent (?) volcanism from VIRTIS data

Themis Regio is interpreted to be a hotspot rise underlain by one or more mantle plumes. Many volcanic features in the Themis region have high emissivity anomalies in Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data. Other high emissivity anomalies have been found to correspond to volcanic flows interpreted to be relatively recent (Smrekar et al. [2010a] Science 328, 605–608). Similarly, at Themis the majority of the high emissivity anomalies closely correspond to flows associated with coronae and volcanic edifices, consistent with the interpretation that they represent recent volcanism. These volcanic features also have negative Bouguer gravity anomalies indicating low density at depth, consistent with hot mantle upwellings leading to pressure release melting. In addition, several volcanic features at Themis have low emissivity anomalies, suggestive of more evolved compositions. Combining geologic mapping data with gravity and emissivity data provide support that Themis is an active hotspot swell, with associated long-lived, volcanoes and coronae that are likely associated with small-scale upwellings.

Temperature fluctuation of the Iceland mantle plume through time

AbstractThe newly developed Al‐in‐olivine geothermometer was used to find the olivine‐Cr‐spinel crystallization temperatures of a suite of picrites spanning the spatial and temporal extent of the North Atlantic Igneous Province (NAIP), which is widely considered to be the result of a deep‐seated mantle plume. Our data confirm that start‐up plumes are associated with a pulse of anomalously hot mantle over a large spatial area before becoming focused into a narrow upwelling. We find that the thermal anomaly on both sides of the province at Baffin Island/West Greenland and the British Isles at ∼61 Ma across an area ∼2000 km in diameter was uniform, with Al‐in‐olivine temperatures up to above that of average mid‐ocean ridge basalt (MORB) primitive magma. Furthermore, by combining our results with geochemical data and existing geophysical and bathymetric observations, we present compelling evidence for long‐term (&gt;107 year) fluctuations in the temperature of the Iceland mantle plume. We show that the plume temperature fell from its initial high value during the start‐up phase to a minimum at about 35 Ma, and that the mantle temperature beneath Iceland is currently increasing.

Geochronological constraints on the metamorphic sole of the Semail ophiolite in the United Arab Emirates

The Semail ophiolite of Oman and the United Arab Emirates (UAE) provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and rutile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and rutile provide dominant single age populations that are 95–93 Ma, partially overlapping with the known age of oceanic crust formation (96.5–94.5 Ma), and partially overlapping with cooling ages of the metamorphic rocks (95–90 Ma). The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.

Plate tectonics and mantle plumes as a basis of deep-seated Earth’s tectonic activity for the last 2 Ga

The onset of the modern tectonic style that combines plate-related and plume-related mechanisms has been discussed. Such a tectonic style could have started on the Earth when all layers of our planet had formed. Analysis of available geological data shows that the solid inner core crystallized by 2.7 Ga. Typical geologic complexes appeared on the continents as a result of plate tectonics processes at that time. The layer D″ that accumulates lithospheric slabs, which do not remain at the upper-lower mantle interface but can go down to the core-mantle boundary, was finally produced by 2 Ga. At pressures and temperatures of the D″ layer perovskite, a principal mineral of the lower mantle transforms into postperovskite phase. The isotope data suggest the existence of mantle (EM-I and EM-II) domains, being the sources for oceanic island basalts (OIB) and the depleted upper mantle (asthenosphere), the source for mid-ocean ridge basalts (MORB), from at least 2 Ga. It is accepted that the recent tectonic style started on the Earth at that period. Deep-seated processes suggest the involvement of all Earth’s layers. The asthenosphere–lithosphere interaction is responsible for different types of large surface structures that develop as fold mountains, oceanic spaces, and subduction zones. The descending lithospheric slabs (cold mantle material) and ascending mantle material in hot mantle provinces or the so-called low shear velocity provinces are responsible for the lower mantle convection. The plume is produced in the layer D″, which accumulates the descending lithosphere slabs as well as light elements released from the outer liquid core, which are oxidized in this layer, thus resulting in thermochemical plume formation. At the same time the molten ferric iron penetrates the core. So, we see the interaction of all Earth’s layers. The article also considers the evolutionary history of the Siberian continent over 700 Myr as early as the breakdown of Rodinia and the formation of mountain folds and rifting structures and the associated metallogeny.

Formation of high-Al komatiites from the Mesoarchean Quebra Osso Group, Minas Gerais, Brazil: Trace elements, HSE systematics and Os isotopic signatures

We report highly siderophile element data combined with Re–Os isotopes and major and trace elements of the ca. 2.7–3.0Ga komatiites from the Quebra Osso Group, Minas Gerais, Brazil. These komatiites resemble the rare high Al-type, characterized by high Al2O3/TiO2 ratios (26.7–59.8). These geochemical similarities are shared with the 3.33Ga Commondale and 3.26Ga Weltevreden komatiites from the eastern Kaapvaal Craton pointing to a similar origin of these suites. While anhydrous melting in an unusually hot mantle was inferred for the Weltevreden komatiites, the Commondale komatiites were suggested to have formed by hydrous, multi-stage melting. Significant depletion in LREE is coupled with subchondritic Re/Os, unradiogenic to radiogenic 187Os/188Os and fractionated HSE, with enrichments in Ru, Pt, and Pd over Os and Ir. The combination of these signatures suggests minor late-stage crustal influence. Potential late-stage alteration overprint, assimilation of ambient mantle material during magma ascent and complex phase relationships of HSE-hosting phases make it difficult to estimate the composition of the source of the Quebra Osso komatiites and to place constraints on the nature of the late Archean mantle. However, the Quebra Osso komatiites are unlikely to have formed in a single-stage plume setting or in a supra-subduction zone setting. Instead we suggest a multi-stage melting history of the komatiite source to explain the origin of their peculiar geochemical characteristics, as has been suggested for other high-Al2O3/TiO2 komatiite suites.

Evidence for anomalous mantle upwelling beneath the Arabian Platform from travel time tomography inversion

We present a new model of P-velocity anomalies in the upper mantle beneath the Arabian Peninsula, Red Sea, and surrounding regions. This model was computed with the use of travel time data from the global catalogue of the International Seismological Center (ISC) for the years of 1980–2011. The reliability of the model was tested with several synthetic tests. In the resulting seismic model, the Red Sea is clearly associated with a higher P-velocity anomaly in the upper mantle at least down to 300km depth. This anomaly might be caused by upward deviation of the main mantle interfaces caused by extension and thinning of the lithosphere due to passive rifting. Thick lithosphere of the Arabian Platform is imaged as a high-velocity anomaly down to 200–250km depth. Below this plate, we observe a low-velocity structure that is interpreted as a hot mantle upwelling. Based on the tomography results, we propose that this upper mantle anomaly may represent hot material that migrates westward and play a major role in the formation of Cenozoic basaltic lava fields in western Arabia. On the northeastern side of the Arabian Plate, we clearly observe a dipping high-velocity zone beneath Zagros and Makran, which is interpreted as a trace of subduction or delamination of the Arabian Plate lithosphere.

A random‐walk algorithm for modeling lithospheric density and the role of body forces in the evolution of the Midcontinent Rift

AbstractThis paper develops a Monte Carlo algorithm for extracting three‐dimensional lithospheric density models from geophysical data. Empirical scaling relationships between velocity and density create a 3‐D starting density model, which is then iteratively refined until it reproduces observed gravity and topography. This approach permits deviations from uniform crustal velocity‐density scaling, which provide insight into crustal lithology and prevent spurious mapping of crustal anomalies into the mantle. We test this algorithm on the Proterozoic Midcontinent Rift (MCR), north‐central United States. The MCR provides a challenge because it hosts a gravity high overlying low shear‐wave velocity crust in a generally flat region. Our initial density estimates are derived from a seismic velocity/crustal thickness model based on joint inversion of surface‐wave dispersion and receiver functions. By adjusting these estimates to reproduce gravity and topography, we generate a lithospheric‐scale model that reveals dense middle crust and eclogitized lowermost crust within the rift. Mantle lithospheric density beneath the MCR is not anomalous, consistent with geochemical evidence that lithospheric mantle was not the primary source of rift‐related magmas and suggesting that extension occurred in response to far‐field stress rather than a hot mantle plume. Similarly, the subsequent inversion of normal faults resulted from changing far‐field stress that exploited not only warm, recently faulted crust but also a gravitational potential energy low in the MCR. The success of this density modeling algorithm in the face of such apparently contradictory geophysical properties suggests that it may be applicable to a variety of tectonic and geodynamic problems.

Effect of width, amplitude, and position of a core mantle boundary hot spot on core convection and dynamo action

Within the fluid iron cores of terrestrial planets, convection and the resulting generation of global magnetic fields are controlled by the overlying rocky mantle. The thermal structure of the lower mantle determines how much heat is allowed to escape the core. Hot lower mantle features, such as the thermal footprint of a giant impact or hot mantle plumes, will locally reduce the heat flux through the core mantle boundary (CMB), thereby weakening core convection and affecting the magnetic field generation process. In this study, we numerically investigate how parametrised hot spots at the CMB with arbitrary sizes, amplitudes, and positions affect core convection and hence the dynamo. The effect of the heat flux anomaly is quantified by changes in global flow symmetry properties, such as the emergence of equatorial antisymmetric, axisymmetric (EAA) zonal flows. For purely hydrodynamic models, the EAA symmetry scales almost linearly with the CMB amplitude and size, whereas self-consistent dynamo simulations typically reveal either suppressed or drastically enhanced EAA symmetry depending mainly on the horizontal extent of the heat flux anomaly. Our results suggest that the length scale of the anomaly should be on the same order as the outer core radius to significantly affect flow and field symmetries. As an implication to Mars and in the range of our model, the study concludes that an ancient core field modified by a CMB heat flux anomaly is not able to heterogeneously magnetise the crust to the present-day level of north--south asymmetry on Mars. The resulting magnetic fields obtained using our model either are not asymmetric enough or, when they are asymmetric enough, show rapid polarity inversions, which are incompatible with thick unidirectional magnetisation.

P-wave velocity structure beneath Mt. Melbourne in northern Victoria Land, Antarctica: Evidence of partial melting and volcanic magma sources

Mt. Melbourne is a late Cenozoic intraplate volcano located ∼30 km northeast of Jang Bogo Station in Antarctica. The volcano is quiescent with fumarolic activity at the summit. To monitor volcanic activity and glacial movements near Jang Bogo Station, a seismic network was installed during the 2010–11 Antarctic summer field season. The network is maintained during the summer field season every year, and the number of stations has been increased. We used continuous seismic data recorded by the network and an Italian seismic station (TNV) at Mario Zucchelli Station to develop a 3-D P-wave velocity model for the Mt. Melbourne area based on the teleseismic P-wave tomographic method. The new 3-D model presented a relative velocity structure for the lower part of the crust and upper mantle between depths of 30 and 160 km and revealed the presence of two low-velocity anomalies beneath Mt. Melbourne and the Priestley Fault. The low-velocity anomaly beneath Mt. Melbourne may be caused by the edge flow of hot mantle material at the lithospheric step between the thick East Antarctic Craton and thin Ross Sea crust. The other low-velocity anomaly along the Priestley Fault may have been beneath Mt. Melbourne and moved to the southern tip of the Deep Freeze Range, where the crustal thickness is relatively thin. The anomaly was trapped on the fault line and laterally flowed along the fault line in the northwest direction.

The role of mantle plumes in the evolution of the African segment of Pangea and the formation of the Atlantic Ocean

In this paper, we discuss a broad range of issues related to the formation of large igneous provinces in the African segment of Pangea on the basis of modern seismic tomography data. The formation of older igneous provinces (Central American and Karoo) is attributed to a prolonged phase of upwelling of hot mantle material or fluids in separate jets within a much larger area than the supposed plume head. Owing to its huge size and the thick, dense continental crust, Pangea acted as a shield promoting the accumulation and lateral channeling of heat energy beneath the lithosphere. The changes in global Earth dynamics and the generation of extensional stresses alone may have led to the breakup of Pangea, triggering the eruption of large volumes of magma over short period of time. The same factors led to the opening of the Atlantic Ocean. We provide arguments that the African superplume represents a Cenozoic structure not associated with the emplacement of the Karoo province. At the same time, the hot material brought under the lithosphere by this superplume synchronously with the start of magmatism in east Africa then spread out to the northwest to form local melting areas in Central and Northwestern Africa. We suggest that magmatic activity within the same region may have lasted, with interruptions, over tens of millions of years. Because of plate motion, these lowvelocity zones acting as heat sources appear to have lost their deep-seated roots, so that mantle reservoirs surviving at the base of the lithosphere may have fed magmatism and drifted together with the lithosphere.

South Atlantic opening: A plume-induced breakup?

Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.

The geological signature of a slab tear below the Aegean

In this paper we explore the geology of the Aegean region in the Miocene to identify the geological signature of a first-order slab tear observed in all tomographic models. From 15 to 8Ma, the tear is associated, spatially and timely, with a fast clockwise rotation of the External Hellenides, alkaline volcanism, high-temperature metamorphic domes with a predominance of north-dipping detachments, and south-westward migration of granitoid intrusions. These features suggest a warmer geodynamic environment during the rotation and the impact of a hot mantle flow associated with the tear. The ~8Ma duration between the first high-temperature metamorphic domes in the centre of the archipelago and the beginning of the fast rotation may correspond to the time needed for the slab to bend, stretch and finally tear.

The Early Proterozoic Matachewan Large Igneous Province: Geochemistry, Petrogenesis, and Implications for Earth Evolution

The Matachewan Large Igneous Province (LIP) is interpreted to have formed during the early stages of mantle plume-induced continental break-up in the early Proterozoic. When the Matachewan LIP is reconstructed to its original configuration with units from the Superior Craton and other formerly adjacent blocks (Karelia, Kola, Wyoming and Hearne), the dyke swarms, layered intrusions and flood basalts, emplaced over the lifetime of the province, form one of the most extensive magmatic provinces recognized in the geological record. New geochemical data allow, for the first time, the Matachewan LIP to be considered as a single, coherent entity and show that Matachewan LIP rocks share a common tholeiitic composition and trace element geochemistry, characterized by enrichment in the most incompatible elements and depletion in the less incompatible elements. This signature, ubiquitous in early Proterozoic continental magmatic rocks, may indicate that the Matachewan LIP formed through contamination of the primary magmas with lithospheric material or that the early Proterozoic mantle had a fundamentally different composition from the modern mantle. In addition to the radiating geometry of the dyke swarms, a plume origin for the Matachewan LIP is consistent with the geochemistry of some of the suites; these suites are used to constrain a source mantle potential temperature of c. 1500–1550°C. Comparison of these mantle potential temperatures with estimated temperatures for the early Proterozoic upper mantle indicates that they are consistent with a hot mantle plume source for the magmatism. Geochemical data from coeval intrusions suggest that the plume head was compositionally heterogeneous and sampled material from both depleted and enriched mantle. As has been documented with less ancient but similarly vast LIPs, the emplacement of the Matachewan LIP probably had a significant impact on the early Proterozoic global environment. Compilation of the best age estimates for various suites shows that the emplacement of the Matachewan LIP occurred synchronously with the Great Oxidation Event. We explore the potential for the eruption of this LIP and the emission of its associated volcanic gases to have been a driver of the irreversible oxygenation of the Earth.

Seismic evidence for a chemically distinct thermochemical reservoir in Earth's deep mantle beneath Hawaii

Nearly antipodal continent-sized zones of reduced seismic shear wave velocities exist at the base of Earth's mantle, one beneath the Pacific Ocean, the other beneath the South Atlantic Ocean and Africa. Geophysicists have attributed the low velocity zones to elevated temperatures associated with large-scale mantle convection processes, specifically, hot mantle upwelling in response to cooler subduction-related downwelling currents. Hypotheses have included superplumes, isochemical heterogeneity, and stable as well as metastable basal thermochemical piles. Here we analyze waveform broadening and travel times of S waves from 11 deep focus earthquakes in the southwest Pacific recorded in North America, resulting in 8500 seismograms studied that sample the deep mantle beneath the Pacific. Waveform broadening is referenced to a mean S-wave shape constructed for each event, to define a relative “misfit”. Large misfits are consistent with multipathing that can broaden wave pulses. Misfits of deep mantle sampling S-waves infer that the structure in the northeast part of the low velocity province beneath the Pacific has a sharp side as well as a sloping sharp top to the feature. This sharp boundary morphology is consistent with geodynamic predictions for a stable thermochemical reservoir. The peak of the imaged pile is below Hawaii, supporting the hypothesis of a whole mantle plume beneath the hotspot.

Early Precambrian Earth history: Plate and plume tectonics and extraterrestrial controls

The Hadean and Archean geologic history of the Earth is discussed in the context of available knowledge from different sources: space physics and comparative planetology; isotope geochronology; geology and petrology of Archean greenstone belts (GB) and tonalite-trondhjemite-granodiorite (TTG) complexes; and geodynamic modeling review to analyse plate-tectonic, plume activity, and impact processes. Correlation between the age peaks of terrestrial Hadean-Early Archean zircons and late heavy bombardment events on the Moon, as well as the Hf isotope composition of zircons indicating their mostly mafic sources, hint to an important role of impact processes in the Earth’s history between 4.4 and 3.8 Ga. The earliest continental crust (TTG complexes) formed at 4.2 Ga (Acasta gneisses), while its large-scale recycling left imprint in Hf isotope signatures after 3.75 Ga. The associations and geochemistry of rocks suggest that Archean greenstone belts formed in settings of rifting, ocean floor spreading, subduction, and plume magmatism generally similar to the present respective processes. The Archean history differed in the greater extent of rocks derived from mantle plumes (komatiites and basalts), boninites, and adakites as well as in shorter subduction cycles recorded in alternation of typical calc-alkaline andesite-dacite-rhyolite and adakite series that were generated in a hotter mantle with more turbulent convection and unsteady subduction. The Archean is interpreted as a transient period of small plate tectonics.