Elevated Mantle Temperatures Research Articles

High-Frequency Shear-Wave Propagation across the Hellenic Subduction Zone

Abstract This study investigates shear-wave propagation across the Hellenic subduction zone using high-quality waveforms of 71 intermediate-depth events (40–160-km depth). The data have been recorded by a permanent seismic network consisting of 22 broadband, three-component stations that cover most parts of Greece. Shear waves can be efficiently transmitted with low attenuation ( Q s ∼1100) through the descending slab, which extends laterally over much of the forearc area with a northwest–southeast strike. The slab also appears to be continuous downdip in the depth range 60–160 km, even though it was found that a gap smaller than 10 km would not affect shear-wave efficiency significantly. Two highly attenuating areas have been detected, one in the central/southeast Aegean corresponding to the mantle wedge and the other in the northern Aegean, probably caused by elevated upper mantle temperatures owing to the ongoing extension in that area. These results provide additional constraints for the geometry of the Hellenic subduction zone and the geodynamic setting of the Aegean region.

Liquidus surfaces of ultracalcic primitive melts: formation conditions and sources

CaO-rich, Al2O3-poor ultracalcic primitive melts occur at mid-ocean-ridges, back-arc basins, ocean islands and volcanic arcs. They are subdivided into a “nepheline-normative” alkaline-rich, silica-poor group uniquely found in arcs and in “hypersthene-normative” fairly refractory melts which occur in all of the above environments. The high CaO contents (to 19.0 wt%) and CaO/Al2O3 ratios (to 1.8) exclude an origin from fertile lherzolites at volatile-absent conditions. Experimental investigation of the liquidus of a hypersthene-normative and a nepheline-normative ultracalcic melt results in quite distinct pressure-temperature conditions of multiple saturation: whereas the hypersthene-normative liquid saturates in olivine + clinopyroxene at 1.2 GPa and 1,410°C, this occurs at 0.2 GPa and 1,220°C for the nepheline-normative ultracalcic liquid. Our results in combination with melting experiments from the literature suggest that hypersthene-normative melts result from melting of a refractory olivine + clinopyroxene ± orthopyroxene source at elevated mantle temperatures. Contrasting, nepheline-normative ultracalcic melts form from wehrlitic cumulates in the arc crust; to account for the high alkaline and low silica contents, and the relatively low temperatures, source wehrlites must have contained amphibole.

Observational and theoretical studies of the dynamics of mantle plume–mid‐ocean ridge interaction

Hot spot–mid‐ocean ridge interactions cause many of the largest structural and chemical anomalies in Earth's ocean basins. Correlated geophysical and geochemical anomalies are widely explained by mantle plumes that deliver hot and compositionally distinct material toward and along mid‐ocean ridges. Compositional anomalies are seen in trace element and isotope ratios, while elevated mantle temperatures are suggested by anomalously thick crust, low‐density mantle, low mantle seismic velocities, and elevated degrees and pressures of melting. Several geodynamic laboratory and modeling studies predict that the width over which plumes expand along the ridge axis increases with plume flux and excess buoyancy and decreases with plate spreading rate, plume viscosity, and plume‐ridge separation. Key aspects of the theoretical predictions are supported by observations at several prominent hot spot–ridge systems. Still, many basic aspects of plume‐ridge interaction remain enigmatic. Outstanding problems pertain to whether plumes flow toward and along mid‐ocean ridges in narrow pipe‐like channels or as broad expanding gravity currents, the origin of geochemical mixing trends observed along ridges, and how mantle plumes alter the geometry of the mid‐ocean ridge plate boundary, as well as the origin of other ridge axis anomalies not obviously related to mantle plumes.

高圧地球科学 地球マントル中の水

High-pressure experiments on the stability of hydrous minerals likely to be present in the Earth's mantle provide constraints on the distribution of water in the mantle, and the form in which it is stored. However, there has been confusion about compositions and structures between hydrous minerals. In regions of elevated mantle temperature, water may be stored not in hydrous minerals but in anhydrous minerals, melts, and fluids. Such water is important to understand the dynamics and evolution of the mantle, volcanism, and metasomatism. In this article, the state and migration of water in the mantle are reviewed.

The influence of pre-existing structures on the evolution of the southern Kenya Rift Valley — evidence from seismic and gravity studies

The Kenya Rift is an active continental rift that has developed since the Late Oligocene. Although a thermal origin for the rifting episode is indicated by the scale of volcanism and its relative timing with uplift and faulting, the influence of pre-existing lithospheric structural controls is poorly understood. The interpretation of a 430-km-long seismic refraction and gravity line across the southern part of the Kenya Rift shows that the rift is developed across a transition zone, thought to represent the sheared Proterozoic boundary between the Archaean Nyanza Craton and the mobile Mozambique Belt. This zone of weakness has been exploited by the recent thermal rifting event. The Moho is at a depth of 33 km beneath the Archaean craton in the western part of the profile, and 40 km beneath the Mozambique Belt in the east. A few kilometres of localised crustal thinning has developed across the transition from thin to thick crust. At the surface, brittle faulting has formed an asymmetric rift basin 3.6 km deep, filled with low-velocity volcanic rocks. Basement velocities show a transition across the same area from low velocities (6.0 km s −1) in the Archaean, to high velocities (6.35 km s −1) in the Proterozoic. Mid-crustal layers show no deformation that can be attributed to the rifting event. Poorly constrained upper mantle velocities of 7.8 km s −1 beneath the southern rift confirm the continuation of the axial low-velocity zone imaged in previous seismic experiments. This is interpreted as the effect of small degrees of partial melt caused by elevated mantle temperatures. Gravity modelling suggests a contribution to the Bouguer anomaly from below the Moho, invoking the need for deep density contrasts. The regional gravity gradient necessary to model the Bouguer anomaly is used as supporting evidence for mantle-plume type circulation beneath the uplifted East African Plateau to the west of the Kenya Rift.

Färoe‐Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland

Results from the Färoe‐Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Mantle temperature and lithospheric thinning beneath the Midcontinent rift system: evidence from magmatism and subsidence

The tectono-magmatic history of the Midcontinent rift system can be explained by the presence of a mantle plume bringing elevated-temperature mantle beneath the rift system at about 1110 Ma. Huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift. Geochemical and isotopic data from the Keweenawan volcanics show that the earliest melts were derived from small-degree melting of primitive plume mantle, coupled with enriched metasomatic melts derived from the continental lithosphere. As rifting progressed, the main bulk of the volcanics was generated primarily from the plume mantle, with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphère at 45 km depth. Elevated mantle temperatures of 1500–1560 °C, approximately 150–200 °C above normal, are inferred from the rare earth element concentrations in the volcanic rocks. Further constraints on the mantle temperature come from combined subsidence and melt-generation modelling. I assume that rifting occurred in two main periods, during 1110–1105 and 1100–1094 Ma, with a reduced rate of stretching and greatly decreased melt production during the intervening period, 1105–1100 Ma. At the centre of the rift, production of more than 15 km of volcanic rocks close to, or above, sea level was followed by the accumulation of up to 8 km of mainly coarse terrigenous sediments in the postrift subsidence phase. This can be explained by lithospheric thinning by a factor of approximately 6 above mantle with a potential temperature of about 1550 °C. Subsequently, the mantle cooled to a normal potential temperature of 1350 °C as the plume thermal anomaly died away.

Effects of planetary thermal structure on the ascent and cooling of magma on Venus

Magellan radar images of the surface of Venus show a spatially broad distribution of volcanic features. Models of magmatic ascent processes to planetary surfaces indicate that the thermal structure of the interior significantly influences the rate of magmatic cooling and thus the amount of magma that can be transported to the surface before solidification. In order to understand which aspects of planetary thermal structure have the greatest influence on the cooling of buoyantly ascending magma, we have constructed magma cooling profiles for a plutonic ascent mechanism, and evaluated the profiles for variations in the surface and mantle temperature, surface temperature gradient, and thermal gradient curvature. Results show that, for a wide variety of thermal conditions, smaller and slower magma bodies are capable of reaching the surface on Venus compared to Earth, primarily due to the higher surface temperature of Venus. Little to no effect on the cooling and transport of magma are found to result from elevated mantle temperatures, elevation-dependent surface temperature variations, or details of the thermal gradient curvature. The enhanced tendency of magma to reach the surface on Venus may provide at least a partial explanation for the extensive spatial distribution of observed volcanism on the surface.

The northern Mid‐Atlantic and Reykjanes Ridges: Spreading center morphology between 55°50′N and 63°00′N

SeaMARC II and Hydrosweep acoustic imagery and bathymetry reveal that the northern Mid‐Atlantic Ridge (MAR) and Reykjanes Ridge between 55°50′N and 63°00′N are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from Iceland. The northern 100 km of the MAR (55°50′N‐56°47′N) is oriented 005° (perpendicular to the spreading azimuth) and contains four third‐order spreading segments separated along strike by right‐ and left‐stepping axial discontinuities. The boundary between MAR and Reykjanes Ridge occurs at 56°47′N, where the Bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092°. This is the only transform fault between 55°50′N and 63°00′N and is inferred to have generated the Bight Fracture Zone. The Reykjanes Ridge lies between the Bight transform and Iceland's Reykjanes Peninsula and is oriented ∼30° oblique to the spreading‐orthogonal direction. Axial faults and volcanic systems on the Reykjanes Ridge are arranged in a right‐stepping en echelon pattern and display strikes intermediate between the spreading‐orthogonal direction and the strike of the ridge, which is compatible with experimental models of oblique extension. Spreading segments on the northern MAR and Reykjanes Ridge are defined structurally and bathymetrically. At the smallest scale, the volcanic axis is segmented into individual volcanic systems 4–45 km long, which are separated along strike by intervolcano gaps or offsets. These features are analogous to fourth‐order spreading segments documented elsewhere on the MAR. Axial volcanic systems are superimposed on intermediate‐wavelength (13–65 km) axial topographic highs that constitute third‐ or second‐order spreading segments. Segment boundaries occur at axial depth maxima, which correspond to offsets in the volcanic axis. Four primary characteristics of the spreading axis (axial valley depth, axial boundary fault throw, relief along the neovolcanic axis, and degree of inter‐segment structural discontinuity) decrease toward the north. This variation is consistent with higher crustal temperatures near Iceland, due to either thicker crust or elevated mantle temperatures.

Development of the Rockall Trough and the northwest European continental margin

During the Cretaceous and early Tertiary a series of sedimentary basins opened by rifting on the northwest margin of Europe. None of these developed into a fully-fledged ocean until the northern North Atlantic opened at 55 Ma between NW Europe and Greenland. The subsidence, magmatism, and thermal history of these basins and of the continental margin depended critically on the amount and style of stretching and on the temperature of the underlying asthenospheric mantle. Small variations in the mantle temperature can have dramatic effects on both the magmatism produced during rifting and on the subsidence of the basin. An increase of as little as 10% in the mantle temperature can cause uplift above sea-level rather than the more common subsidence when continental crust undergoes large amounts of extension. The initiation of the Iceland mantle plume at about 62 Ma caused abnormally elevated mantle temperatures across a 2000 km diameter region and had a dramatic effect on the subsidence and rifting history of sedimentary basins west of Britain. Their thermal history can be documented from the subsidence and magmatic record in the sedimentary basins and the continental margin. Using normal incidence and wide-angle seismic data and drilling results from the continental margin and from basins of NW Europe, their subsidence and thermal history can be modelled as a function of the amount of stretching and the temperature of the underlying mantle. The Rockall Trough itself is underlain by 5 km of sediments intruded by Tertiary volcanics. The basement imaged on seismic . . .

Water in the Earth's upper mantle

Experiments on the stability of hydrous minerals likely to be present in the Earth's mantle provide constraints on the distribution of water in the mantle, and the form in which it is stored. In regions of elevated mantle temperature, water may be stored not in minerals but in melts: such hydrous melts are important metasomatizing agents, and can induce volcanism beneath thick cratonic lithosphere.

Shear wave splitting and subcontinental mantle deformation

We have made measurements of shear wave splitting in the phases SKS and SKKS at 21 broadband stations in North America, South America, Europe, Asia, and Africa. Measurements are made using a retrieval scheme that yields the azimuth of the fast polarization direction ϕ and delay time δt of the split shear wave plus uncertainties. Detectable anisotropy was found at most stations, suggesting that it is a general feature of the subcontinental mantle. Delay times range from 0.65 s to 1.70 s and average about 1 s. Somewhat surprisingly, the largest delay time is found in the 2.7 b.y.‐old Western Superior Province of the Canadian Shield. The splitting observations are interpreted in terms of the strain‐induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anisotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of ϕ. For stable continental regions, this is interpreted as “fossil” anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present‐day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent “underplating” or conductive growth of the thermal boundary layer. One of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes. Most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed (by convective instability or delamination) and replaced by warm asthenosphere. These models, however, are incompatible with the evidence for preserved coherent lithospheric deformation; rather, the deformed mantle appears to have been heated in place. We suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation.

Basalts, water, or shear zones in the lower continental crust?

Bright, sub-horizontal, laterally extensive, multi-cyclic layered reflections have now been recorded in the lower continental crust from many areas. In contrast, this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere. With the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering.Qualitative explanations for lower crustal layering abound and include igneous, metamorphic, structural and fluid related models. Measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models. Structurally induced anisotropy, metamorphic layering, or free fluids cannot realistically produce sufficiently bright reflections. Igneous layering or the juxtaposition of pre-existing heterogeneities by shear zones can produce strong reflections.Results from crustal xenoliths, helium-isotope ratios, palaeo-geothermal gradients and chemistry of flood basalts,together with numerical modelling of lithospheric extension and asthenospheric melting, suggest that underplating and intrusion of the lower crust with mantle-derived mafic igneous rocks is common. The apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections, suggest that underplating, related to extension and/or elevated mantle temperatures, is a good explanation for lower crustal layering.