High Mantle Temperatures Research Articles

Metasomatic processes in the lithospheric mantle beneath the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province, Russia)

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO2, Cr2O3, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.

The efficiency of plate tectonics and nonequilibrium dynamical evolution of planetary mantles

AbstractConsideration of the structure of dynamical equilibria in terrestrial planets using simplified descriptions of the relevant heat transport processes (rigid‐lid convection, plate tectonics, and heat pipe volcanism) reveals that if the efficiency of plate tectonic heat transport decreases at higher mantle temperature, then it cannot govern quasi‐equilibrium dynamical evolution, and the system is always evolving away from the plate tectonic regime. A planet on which plate tectonics is less efficient at higher temperature stays in heat pipe mode longer, spends less time undergoing plate tectonics, and has a low and ever‐decreasing Urey number during this phase. These conclusions are based solely on the structure of the equilibria in a system with less efficient plate tectonics in the past and are independent of the mechanisms leading to this behavior. Commonly used quasi‐equilibrium approaches to planetary thermal evolution are likely not valid for planets in which heat transport becomes less efficient at higher temperature.

Generation of felsic crust in the Archean: A geodynamic modeling perspective

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. Higher mantle temperatures and higher radiogenic heat production in the Archean Earth would have impacted the thickness and composition of the crust. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean gray gneiss complexes is the sodic tonalite–trondhjemite–granodiorite (TTG) suite. Melting of hydrated basalt at garnet amphibolite, granulite or eclogite facies conditions is considered to be the dominant process that generated the Archean TTGs. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical tectono-magmatic numerical model with initial conditions appropriate to the Eoarchean–Mesoarchean. Based on the result of our experiments, we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust: (1) delamination and dripping of the lower mafic crust into the mantle; (2) local thickening of the crust; and (3) small-scale crustal overturns. In the context of a stagnant-deformable lid tectono-magmatic geodynamic regime that is terminated by short-lived subduction, we identify two distinct types of continental crust. The first type is a pristine granite–greenstone-like crust with dome-and-keel geometry formed over delaminating–upwelling mantle which is mostly subjected to vertical tectonics processes. By contrast, the second type is a reworked (accreted) crust comprising strongly deformed granite–greenstone and subduction-related sequences and subjected to both strong horizontal shortening and vertical tectonics processes. Thus, our study has identified a possible spatial and temporal transition from pristine granite–greenstone-like crust with dome-and-keel geometry to reworked (accreted) crust forming more felsic gneiss terranes in the Archean. We suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the Archean geological record.

Iron and magnesium isotope fractionation in oceanic lithosphere and sub-arc mantle: Perspectives from ophiolites

We present high-precision Fe and Mg isotopic data for the Purang ophiolite, southwestern Tibet, representing the first combined Fe and Mg isotopic study of the oceanic lithosphere hitherto. The δ56Fe and δ26Mg values of the ophiolitic peridotite, dunite and gabbro vary from −0.209 to 0.187‰ and from −0.28 to −0.14‰, respectively. The average δ56Fe of the peridotites is −0.030±0.143‰ (2SD, n=17), a value indistinguishable from abyssal peridotites and chondrites, and lower than oceanic basalts. The average δ26Mg value of the peridotites is −0.20±0.10‰, a value slightly higher than both chondrites and oceanic basalts. Correlations between δ56Fe and indices of partial melting indicate fractionation of 0.323‰ in δ56Fe between the oceanic lithospheric mantle and the overlying mafic crust during an early episode of partial melting, presumably beneath a spreading centre. Subsequent metasomatism in a supra-subduction zone caused elevated oxygen fugacity and heavy Fe isotopic compositions in the oceanic lithospheric mantle. The dunite with high Ba/La, a proxy for oxygen fugacity, and high δ56Fe values was likely formed during this process of sub-arc mantle-melt interaction. The negatively coupled Fe–Mg isotopic variations of the Purang ophiolite indicate that Mg isotope fractionation may also occur during high-temperature mantle processes. The observed isotopic variations among different lithologies in the ophiolite may satisfactorily account for the isotopic differences between arc lavas and mantle peridotites with respect to oceanic basalts, thus providing implications for crust–mantle differentiation.

Permian doleritic dikes in the Beishan Orogenic Belt, NW China: Asthenosphere–lithosphere interaction in response to slab break-off

Extensive Permian mafic–ultramafic intrusions crop out within the Central Asian Orogenic Belt (CAOB). Because the magmatic activity is spatially and temporally associated with the Tarim Large Igneous Province (LIP), a mantle plume has been invoked to explain the petrogenesis of these mafic–ultramafic rocks. In order to test this hypothesis, we present new geochronological and geochemical data for doleritic dike swarms in the Beishan area, southern CAOB. Zircon U–Pb dating of the Yinaoxia and Gubaoquan doleritic dikes indicates that they were emplaced during the Early Permian (ca. 282Ma). The Yinaoxia doleritic dikes have higher Mg# (52–74) and εNd(t) values (9.0–9.1) than the Gubaoquan dikes (Mg#=35–56; εNd(t)=5.8–7.4), indicating that the former are more primitive than the latter. The Gubaoquan dikes are characterized by higher Ba/Nb (15–276), Ba/La (6–103), Th/Yb (0.12–0.24) and Th/Nb (0.20–0.36) ratios, suggesting that their parental magmas contained more subduction-related material. Calculated parental magmatic compositions in equilibrium with Cr-rich clinopyroxenes within the dikes have LREE-depleted to flat REE patterns ((La/Yb)N=0.57–1.28) and negative to slightly positive Nb anomalies ((Nb/La)PM=0.33–0.91). These data suggest that their parental magmas may have been generated by interaction between depleted asthenospheric mantle and metasomatized lithospheric mantle. Using clinopyroxene–liquid thermobarometers, we estimate a temperature range of 1142°C–1276°C, indicative of an upper asthenospheric mantle source. Although the doleritic dikes in Beishan are coeval with the Tarim LIP, their petrological and geochemical compositions do not suggest the abnormally high mantle temperatures or OIB-like mantle signatures that characterize the mantle source of the Tarim LIP magmas. This means that the Permian Tarim mantle plume cannot account for the formation of the doleritic dikes in the Beishan area, and that the Tarim LIP does not extend to the Beishan area. Instead, the doleritic dikes were probably derived from magmas generated by partial melting of upwelling depleted asthenospheric mantle that was subsequently contaminated to various degrees by metasomatized lithospheric mantle in a slab break-off scenario.

Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle

Silver abundances in mantle peridotites and the behavior of Ag during high temperature mantle processes have received little attention and, as a consequence, the abundance of Ag in the bulk silicate Earth (BSE) has been poorly constrained. In order to better understand the processes that fractionate Ag and other chalcophile elements in the mantle, abundances of Ag and Cu in mantle peridotites from different geological settings (n=68) have been obtained by isotope dilution ICP-MS methods. In peridotite tectonites and in a few suites of peridotite xenoliths which display evidence for variable extents of melt depletion and refertilization by silicate melts, Ag and Cu abundances show positive correlations with moderately incompatible elements such as S, Se, Te and Au. The mean Cu/Ag in fertile peridotites (3500±1200, 1s, n=38) is indistinguishable from the mean Cu/Ag of mid ocean ridge basalts (MORB, 3600±400, 1s, n=338) and MORB sulfide droplets. The constant mean Cu/Ag ratios indicate similar behavior of Ag and Cu during partial melting of the mantle, refertilization and magmatic fractionation, and thus should be representative of the Earth’s upper mantle. The systematic fractionation of Cu, Ag, Au, S, Se and Te in peridotites and basalts is consistent with sulfide melt–silicate melt partitioning with apparent partition coefficients of platinum group elements (PGE)>Au⩾Te>Cu≈Ag>Se⩾S.Because of the effects of secondary processes, the abundances of chalcophile elements, notably S, Se, but also Cu and the PGE in many peridotite xenoliths are variable and lower than in peridotite massifs. Refertilization of peridotite may change abundances of chalcophile and lithophile elements in peridotite massifs, however, this seems to mostly occur in a systematic way. Correlations with lithophile and chalcophile elements and the overlapping mean Cu/Ag ratios of peridotites and ocean ridge basalts are used to constrain abundances of Ag and Cu in the BSE at 9±3(1s)ng/g and 30±6μg/g(1s), respectively. The very different extent of depletion of Ag and Cu in the BSE cannot be explained by low pressure–temperature core formation if currently available metal–silicate partitioning data are applied.

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

We investigate the upper-mantle transition zone thickness beneath the Ibero-Maghrebian region using teleseismic events recorded at 258 seismic stations from the IberArray of the Spanish TopoIberia project. For this purpose, the 410km and 660km depth discontinuities are mapped through the detection of P-to-s conversions. In order to add consistency and robustness to the detections, the final results are based on a joint analysis of receiver functions and two different cross-correlation functionals. Discontinuity depths are determined using time corrections obtained from a 3-D velocity model. This study presents the first high-resolution topography maps for the 410 and 660 discontinuities, which show lateral variations in the transition zone thickness beneath the study area. The results are discussed to add new constraints on temperature and composition to seismic velocity anomalies observed in the transition zone beneath the controversial Ibero-Maghrebian region. We obtained a thickened transition zone beneath the Alboran Sea and the north Balearic Sea suggesting low transition zone temperatures, and coinciding with the position of the high-velocity anomalies in the tomographic images related to the Betic-Alboran slab and the Alpine-Tethys remnant slab, respectively. A dominantly thinned transition zone beneath the Gulf of Cadiz and its surroundings suggest high-mantle temperatures. We also found evidences of the presence of a low-velocity layer atop the 410 discontinuity beneath the study area.

Initiation of plate tectonics from post‐magma ocean thermochemical convection

AbstractLeading theories for the presence of plate tectonics on Earth typically appeal to the role of present day conditions in promoting rheological weakening of the lithosphere. However, it is unknown whether the conditions of the early Earth were favorable for plate tectonics, or any form of subduction, and thus, how subduction begins is unclear. Using physical models based on grain‐damage, a grainsize‐feedback mechanism capable of producing plate‐like mantle convection, we demonstrate that subduction was possible on the Hadean Earth (hereafter referred to as proto‐subduction or proto‐plate tectonics), that proto‐subduction differed from modern day plate tectonics, and that it could initiate rapidly. Scaling laws for convection with grain‐damage show that though either higher mantle temperatures or higher surface temperatures lead to slower plates, proto‐subduction, with plate speeds of ≈1.75 cm/yr, can still be maintained in the Hadean, even with a CO2 rich primordial atmosphere. Furthermore, when the mantle potential temperature is high (e.g., above ≈2000 K), the mode of subduction switches to a “sluggish subduction” style, where downwellings are drip like and plate boundaries are diffuse. Finally, numerical models of post‐magma ocean mantle convection demonstrate that proto‐plate tectonics likely initiates within ∼100 Myr of magma ocean solidification, consistent with evidence from Hadean zircons. After the initiation of proto‐subduction, non‐plate‐tectonic “sluggish subduction” prevails, giving way to modern style plate tectonics as both the mantle interior and climate cool. Hadean proto‐subduction may hasten the onset of modern plate tectonics by drawing excess CO2 out of the atmosphere and cooling the climate.

The evolution of Hadean–Eoarchaean geodynamics

Geodynamic modelling of Hadean/Eoarchaean tectonics typically requires higher rates of internal heat production, and higher mantle temperatures, in models that possess temperature-dependent viscosity and a yield criterion. In such models under Hadean conditions, for a wide range of geodynamic configurations and modelling approaches, subduction has a propensity to fail. This has led to the suggestion that the predominant tectonic regime in the Hadean was stagnant-lid convection, with intermittent recycling events. Various lines of evidence support this suggestion, from i) the long mixing time of mantle isotopic anomalies or compositional heterogeneities, such as 142Nd, 182W, and platinum group elements, to ii) the long residence time of the Hadean protolith to the Jack Hills zircons, and iii) thermal evolution models, which typically require lower heat flux in the past to avoid the “Archaean thermal catastrophe”. The framework provided by stagnant lid, or episodic overturn, convection, iv) provides an explanation for the formation of early Archaean TTGs and greenstones, and v) explains the interleaving arc-plume sequence observed in many Archaean terranes, suggesting subduction initiation events may have been common, increasing their preservation potential. Implications include a low magnetic field strength in the Hadean, which is consistent with emerging paleointensity data from these times.

Effect of temperature-dependent viscosity on mantle convection

Two-dimensional numerical calculations in cylindrical shell geometry have been carried out to investigate the effect of the temperature-dependent viscosity on the pattern and the characteristic parameters of the thermal convection occurring in the Earth’s mantle. Systematic model runs established that the viscosity decreasing with the temperature is reduced around the hot core-mantle boundary (CMB) which facilitates ‘the heat transport’ from ‘the core to the mantle’. On the other hand, the viscosity increases near the cold surface which hinders the heat outcome and results in higher mantle temperature and lower surface velocity. A power law relation was revealed between the strength of the temperature-dependence and the observed parameters, such as the velocity, surface mobility, heat flow, average temperature and viscosity. Two additional ‘mantle-like’ models were built up with extra strong temperature-dependent viscosity to imitate the flow in the Earth’s mantle. In model 1, in which the viscosity decreases seven orders of magnitude with the temperature increase, a highly viscous stagnant lid evolves along the cold surface which does not participate in the convection. The existence of the stagnant surface lid reduces the surface heat flow and generates a low viscosity asthenosphere beneath the lid with vigorous small-scale convection. In model 2, in which the viscosity decreases only six orders of magnitude with the temperature and the pressure-dependent viscosity is stronger, does not form a surface stagnant lid, highly viscous ‘slabs’ submerge to the CMB and effectively influence the hot upwelling plumes. Based on our numerical results it is necessary to implicate the yield stress into the simulations in order to obtain a highly viscous, ‘rigid’ surface lid, the lithosphere which can be broken up and subduct down to the mantle.

Seismic constraints of the formation process on the back‐arc basin in the southeastern Japan Sea

AbstractTo clarify the formation process of the back‐arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa‐shima Island. The crust beneath the northern Yamato Basin off Awa‐shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P wave velocity of 5.4–6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high‐velocity lowermost crust in the northern Yamato Basin with 7.2–7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25–26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.

Geochronology and geochemistry of the Parashi granitoid, NE Colombia: Tectonic implication of short-lived Early Eocene plutonism along the SE Caribbean margin

The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U–Pb LA-MC-ICP-MS and K–Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al2O3, Na2O and Ba–Sr contents. Initial 87Sr/86Sr isotopic values range between 0.7050 and 0.7054, with 143Nd/144Nd = 0.51235–0.51253, εNd and εHf values from −0.81 to −4.40 and −4.4 and −5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean–South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust.

Dynamic recrystallization and phase mixing in experimentally deformed peridotite

Ductile shear zones evolve through complex feedbacks between microstructure and rheology. In the mantle, shear zones often display mylonitic or ultramylonitic microstructures, characterized by extensive grain-size reduction and well-mixed polymineralic domains. The feedback between the formation of these microstructures and grain-size sensitive deformation is often considered an important weakening mechanism in high-temperature mantle shear zones. To understand better the processes that transform coarse-grained tectonites into fine-grained and well-mixed mylonites, we have performed triaxial deformation experiments on synthetic peridotite samples comprised of mm-scale olivine and orthopyroxene clasts. Experiments were conducted in a Griggs apparatus at a confining pressure of ∼1 GPa, temperatures of 1400 to 1550 K and strain rates of 10−5–10−6 s−1 under nominally dry conditions. Experiments yield deformed samples with macroscopic natural strains ranging from 0.31 to 0.74. Samples are partially recrystallized, with neoblasts ranging in grain size from 2–25 μm. At these deformation conditions, unrecrystallized mm-sized clasts deform by grain-size insensitive dislocation creep, while recrystallized grains deform by grain-size sensitive dislocation accommodated grain boundary sliding (disGBS). Using electron-backscatter diffraction (EBSD) we investigate the interfaces between recrystallized olivine and orthopyroxene domains. At the majority of these interfaces no mixing is observed. However, many boundaries are serrated, which is interpreted to be the result of surface tension driven phase boundary migration. In a few cases we observe small degrees of mixing, with enclaves of one phase completely surrounded by the other phase. This mixing occurs only within a few recrystallized grain length scales of the interface. These results demonstrate that the serial processes of dynamic recrystallization, phase boundary migration and disGBS deformation are a plausible mechanism for solid-state phase mixing. However, more extensive deformation may be needed to reproduce the near steady-state microstructure observed in highly deformed mantle shear zones.

Melting during late-stage rifting in Afar is hot and deep

Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin.

Permian hornblende gabbros in the Chinese Altai from a subduction-related hydrous parent magma, not from the Tarim mantle plume

In the Chinese Altai, on the northern side of the Erqis fault, the ~10-m-wide Qiemuerqieke gabbro is composed almost entirely of hornblende and plagioclase. Its relative crystallization sequence is olivine, hornblende, plagioclase, and it shows a narrow compositional range in SiO 2 (48.7–50.2 wt%), MgO (6.33–8.54 wt%), FeO (5.27–6.46 wt%), Na 2 O (3.06–3.71 wt%), and K 2 O (0.26–0.37 wt%). These contents result in a high Mg# value (68–72) and a low K 2 O/Na 2 O ratio of ~0.1. It has ( 87 Sr/ 86 Sr) i ratios of 0.70339–0.70350, e Nd(t) values of 4.8–6.0, and zircon e Hf(t) of 11.4–15.8; these values demonstrate a mantle-derived source, and a whole-rock δ 18 O of ~6.7‰ suggests a mantle wedge origin. The presence of magmatic hornblende suggests a relatively high water fugacity, and the crystallization temperature (715–826 °C) calculated using Ti-in-zircon thermometry is considerably lower than that of a normal mafi c melt but consistent with an origin from a water-bearing magma. The gabbro has a secondary ion mass spectrometry zircon U-Pb age of 276.0 ± 2.1 Ma, which is coeval with the 275 Ma mantle plume in the northern Tarim craton, but the Qiemuerqieke petrological and geochemical data do not indicate an abnormally high mantle temperature or a deep mantle signature, which would commonly characterize a mantle plume source. Our results integrated with published data support a model of juvenile crustal growth by a subduction-related process.

High mantle temperatures following rifting caused by continental insulation

The continents are thought to insulate and heat the underlying mantle. Geochemical analyses of lava samples formed at a mid-ocean ridge in the Atlantic Ocean immediately after continental breakup show that the mantle was up to 150 °C hotter than today and took about 70 million years to cool. The distribution of continents is thought to influence the temperature of the underlying mantle. Over geological timescales, insulation effects generate a build-up of heat that may cause increased magmatism1,2,3,4, such as flood basalt volcanism5,6, and ultimately rift the continents, causing them to break apart and new ocean basins to form. Here we use analyses of the major element geochemistry of lava samples collected from oceanic crust in the Atlantic and Pacific oceans to quantify the effect of continental insulation. The lavas formed at mid-ocean ridges following continental rifting and break up, and preserve a record of upper mantle temperatures over the past 170 Myr. We find that samples from the Pacific Ocean—formed more than 2,000 km from the nearest continental margin—do not record raised mantle temperatures. In contrast, samples from the Atlantic Ocean that formed close to the margin of the rifted continent reveal an upper mantle temperature immediately after continental rifting that was up to 150 °C higher than the present-day average; mantle temperatures remained high for 60–70 Myr. We conclude that the Atlantic thermal anomaly was created by continental insulation, and persisted in the mantle beneath the Atlantic Ocean long after the continental fragments had dispersed.

Eclogite-facies fluid infiltration: constraints from δ18O zoning in garnet

In situ analysis reveals that eclogite-facies garnets are zoned in δ18O with lower values in the core and rims that are ~1.5 to 2.5 ‰ higher. This pattern is present in 9 out of 12 garnets analyzed by SIMS from four orogenic eclogite terranes, and correlates with an increase in the mole fraction of pyrope and Mg/Fe ratio from core to rim, indicating prograde garnet growth. At the maximum temperatures and the time-scales experienced by these garnets, calculated intragranular diffusion distances for oxygen are small ( 5 ‰) from two kimberlites are homogeneous in δ18O, possibly due to diffusive exchange, which is possible for prolonged periods at higher mantle temperatures. Homogeneity of δ18O in garnets outside the normal mantle range (5–6 ‰) may be common in kimberlitic samples.

Olivine Pseudomorphs after Serpentinized Orthopyroxene Record Transient Oceanic Lithospheric Mantle Dehydration (Leka Ophiolite Complex, Norway)

We examine the partial survival of high-temperature mantle microstructures throughout multi-stage hydration-and dehydration-mediated pseudomorphism at differing pressure-temperature-fluid condition ...

An integrated geophysical study of Vestbakken Volcanic Province, western Barents Sea continental margin, and adjacent oceanic crust

This paper describes results from a geophysical study in the Vestbakken Volcanic Province, located on the central parts of the western Barents Sea continental margin, and adjacent oceanic crust in the Norwegian-Greenland Sea. The results are derived mainly from interpretation and modeling of multichannel seismic, ocean bottom seismometer and land station data along a regional seismic profile. The resulting model shows oceanic crust in the western parts of the profile. This crust is buried by a thick Cenozoic sedimentary package. Low velocities in the bottom of this package indicate overpressure. The igneous oceanic crust shows an average thickness of 7.2 km with the thinnest crust (5–6 km) in the southwest and the thickest crust (8–9 km) close to the continent-ocean boundary (COB). The thick oceanic crust is probably related to high mantle temperatures formed by brittle weakening and shear heating along a shear system prior to continental breakup. The COB is interpreted in the central parts of the profile where the velocity structure and Bouguer anomalies change significantly. East of the COB Moho depths increase while the vertical velocity gradient decreases. Below the assumed center for Early Eocene volcanic activity the model shows increased velocities in the crust. These increased crustal velocities are interpreted to represent Early Eocene mafic feeder dykes. East of the zone of volcanoes velocities in the crust decrease and sedimentary velocities are observed at depths of more than 10 km. The amount of crustal intrusions is much lower in this area than farther west. East of the Knolegga Fault crystalline basement velocities are brought close to the seabed. This fault marks the eastern limit of thick Cenozoic and Mesozoic packages on central parts of the western Barents Sea continental margin.

Crustal structure of Mexico and surrounding regions from seismic ambient noise tomography

SUMMARY Using continuous seismic data from newly available broadband stations in Mexico and Central America we have obtained group and phase velocity maps of fundamental mode Rayleigh wave for the region. These new maps have been calculated for periods between 8 and 60 s from cross-correlations of seismic ambient noise between 100 broadband stations, and stacked for 30 months from 2006 to 2008. The tomographic inversion of the obtained dispersion measurements has been carried out on a 1°× 1° grid, resulting in maps with resolution better than 250 km in the well-sampled regions of the model. For short periods (8–16 s) dispersion maps show good correlation with surface structural features. Low-velocity anomalies correlate with sedimentary basins around the Gulf of Mexico and Colorado embayment. High-velocity anomalies at short periods correlate with mountain ranges and regions of thin, extended crust such as the Gulf of California. Both the lowest and highest group velocity anomalies at these periods reach values of up to 15 per cent. For periods between 25 and 40 s, velocity anomalies are related to variations in crustal thickness and temperature. The most prominent low velocity anomaly correlates with thick crust and high mantle temperatures associated with the Trans-Mexican Volcanic Belt and the Central American volcanic arc. The most remarkable features for longer periods (40–60 s) are the differences between high-velocity anomalies beneath the Mayan block, the Gulf of Mexico and the cratonic part of the United States and low-velocity anomalies beneath the Chortis block and northern Mexico. The long wavelength features of our model agree well with previous global and continental scale studies. However, because of the increased station density of the data set used, we are able to obtain reliable dispersion maps for shorter periods, and to image smaller scale features.