Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite

The Earth's mantle transition zone could potentially store a large amount of water, as the minerals wadsleyite and ringwoodite incorporate a significant amount of water in their crystal structure. The water content in the transition zone can be estimated from the electrical conductivities of hydrous wadsleyite and ringwoodite, although such estimates depend on accurate knowledge of the two conduction mechanisms in these minerals (small polaron and proton conductions), which early studies have failed to distinguish between. Here we report the electrical conductivity of these two minerals obtained by high-pressure multi-anvil experiments. We found that the small polaron conductions of these minerals are substantially lower than previously estimated. The contributions of proton conduction are small at temperatures corresponding to the mantle transition zone and the conductivity of wadsleyite is considerably lower than that of ringwoodite for both mechanisms. The dry model mantle shows considerable conductivity jumps associated with the olivine-wadsleyite, wadsleyite-ringwoodite and post-spinel transitions. Such a dry model explains well the currently available conductivity-depth profiles obtained from geoelectromagnetic studies. We therefore conclude that there is no need to introduce a significant amount of water in the mantle transition to satisfy electrical conductivity constraints.     

The effect of water on the electrical conductivity of olivine

It is well known that water (as a source of hydrogen) affects the physical and chemical properties of minerals--for example, plastic deformation and melting temperature--and accordingly plays an important role in the dynamics and geochemical evolution of the Earth. Estimating the water content of the Earth's mantle by direct sampling provides only a limited data set from shallow regions (<200 km depth). Geophysical observations such as electrical conductivity are considered to be sensitive to water content, but there has been no experimental study to determine the effect of water on the electrical conductivity of olivine, the most abundant mineral in the Earth's mantle. Here we report a laboratory study of the dependence of the electrical conductivity of olivine aggregates on water content at high temperature and pressure. The electrical conductivity of synthetic polycrystalline olivine was determined from a.c. impedance measurements at a pressure of 4 GPa for a temperature range of 873-1,273 K for water contents of 0.01-0.08 wt%. The results show that the electrical conductivity is strongly dependent on water content but depends only modestly on temperature. The water content dependence of conductivity is best explained by a model in which electrical conduction is due to the motion of free protons. A comparison of the laboratory data with geophysical observations suggests that the typical oceanic asthenosphere contains approximately 10(-2) wt% water, whereas the water content in the continental upper mantle is less than approximately 10(-3) wt%.     

Electromagnetic detection of a 410-km-deep melt layer in the southwestern United States

A deep-seated melt or fluid layer on top of the 410-km-deep seismic discontinuity in Earth's upper mantle, as proposed in the transition-zone 'water filter' hypothesis, may have significant bearing on mantle dynamics and chemical differentiation. The geophysical detection of such a layer has, however, proved difficult. Magnetotelluric and geomagnetic depth sounding are geophysical methods sensitive to mantle melt. Here we use these methods to search for a distinct structure near 410-km depth. We calculate one-dimensional forward models of the response of electrical conductivity depth profiles, based on mineral physics studies of the effect of incorporating hydrogen in upper-mantle and transition-zone minerals. These models indicate that a melt layer at 410-km depth is consistent with regional magnetotelluric and geomagnetic depth sounding data from the southwestern United States (Tucson). The 410-km-deep melt layer in this model has a conductance of 3.0 x 10(4) S and an estimated thickness of 5-30 km. This is the only regional data set that we have examined for which such a melt layer structure was found, consistent with regional seismic studies. We infer that the hypothesized transition-zone water filter occurs regionally, but that such a layer is unlikely to be a global feature.     

Stability of hydrous melt at the base of the Earth's upper mantle

Seismological observations have revealed the existence of low-velocity and high-attenuation zones above the discontinuity at 410 km depth, at the base of the Earth's upper mantle. It has been suggested that a small amount of melt could be responsible for such anomalies. The density of silicate melt under dry conditions has been measured at high pressure and found to be denser than the surrounding solid, thereby allowing the melt to remain at depth. But no experimental investigation of the density of hydrous melt has yet been carried out. Here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410-km discontinuity. We infer that hydrous magma formed by partial melting above the 410-km discontinuity may indeed be gravitationally stable, thereby supporting the idea that low-velocity or high-attentuation regions just above the mantle transition zone may result from the presence of melt.     

Earth science: a wet mantle conductor?

The suggestion that the transition zone of Earth's mantle (410-670 km in depth) is enriched in water is of great possible significance to the geodynamics and geochemistry of Earth's interior, as well as for the role of the mantle in the global water cycle. Huang et al. compare the effect of water on electrical conductivities of transition-zone phases to electromagnetic and magnetotelluric soundings of the mantle beneath the North Pacific and conclude that the transition zone contains between 1,000 and 2,000 p.p.m. of water, which is considerably more than the 50-200 p.p.m. present in the upper mantle. This conclusion is predicated on the assumption that the transition zone is relatively oxidized, but in fact fairly reduced conditions are more likely. Here I show that if the transition zone is reduced, high conductivities can be explained without the requirement for large enrichments of water.     

Low electrical resistivity associated with plunging of the Nazca flat slab beneath Argentina

Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30 degrees (ref. 1). A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28 degrees and 33 degrees S the subducted Nazca plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent. Above this 'flat slab', volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone 'water-filter' model of Bercovici and Karato.     

Multiple seismic discontinuities near the base of the transition zone in the Earth's mantle

The seismologically defined boundary between the transition zone in the Earth's mantle (410-660 km depth) and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma-spinel phase of olivine to magnesium-perovskite and magnesiowustite. Laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative Clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone. But a computational study has indicated that, because of the presence of garnet minerals, multiple seismic discontinuities might exist near a depth of 660 km (ref. 4), which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary. In particular, garnet minerals undergo exothermic transformations near this depth, acting to complicate the phase relations and possibly effecting mantle convection processes in some regions. Here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern California. The observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma-spinel phase of olivine. Temperature anomalies calculated from the imaged discontinuity depths--using Clapeyron slopes determined for the various transformations--generally match those predicted from an independent P-wave velocity model of the region.     

Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States

The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic 'receiver-function' observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.     

Whole-mantle convection and the transition-zone water filter

Because of their distinct chemical signatures, ocean-island and mid-ocean-ridge basalts are traditionally inferred to arise from separate, isolated reservoirs in the Earth's mantle. Such mantle reservoir models, however, typically satisfy geochemical constraints, but not geophysical observations. Here we propose an alternative hypothesis that, rather than being divided into isolated reservoirs, the mantle is filtered at the 410-km-deep discontinuity. We propose that, as the ascending ambient mantle (forced up by the downward flux of subducting slabs) rises out of the high-water-solubility transition zone (between the 660 km and 410 km discontinuities) into the low-solubility upper mantle above 410 km, it undergoes dehydration-induced partial melting that filters out incompatible elements. The filtered, dry and depleted solid phase continues to rise to become the source material for mid-ocean-ridge basalts. The wet, enriched melt residue may be denser than the surrounding solid and accordingly trapped at the 410 km boundary until slab entrainment returns it to the deeper mantle. The filter could be suppressed for both mantle plumes (which therefore generate wetter and more enriched ocean-island basalts) as well as the hotter Archaean mantle (thereby allowing for early production of enriched continental crust). We propose that the transition-zone water-filter model can explain many geochemical observations while avoiding the major pitfalls of invoking isolated mantle reservoirs.     

Geophysical evidence from the MELT area for compositional controls on oceanic plates

Magnetotelluric and seismic data, collected during the MELT experiment at the southern East Pacific Rise, constrain the distribution of melt beneath this mid-ocean-ridge spreading centre and also the evolution of the oceanic lithosphere during its early cooling history. Here we focus on structures imaged at distances approximately 100 to 350 km east of the ridge crest, corresponding to seafloor ages of approximately 1.3 to 4.5 million years (Myr), where the seismic and electrical conductivity structure is nearly constant and independent of age. Beginning at a depth of about 60 km, we image a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure, with higher conductivity in the direction of fast propagation for seismic waves. Conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Myr ago. We infer, however, that the structure of young oceanic plates is instead controlled by a decrease in water content above a depth of 60 km induced by the melting process beneath the spreading centre.     

The Earth's mantle

Seismological images of the Earth's mantle reveal three distinct changes in velocity structure, at depths of 410, 660 and 2,700 km. The first two are best explained by mineral phase transformations, whereas the third-the D" layer-probably reflects a change in chemical composition and thermal structure. Tomographic images of cold slabs in the lower mantle, the displacements of the 410-km and 660-km discontinuities around subduction zones, and the occurrence of small-scale heterogeneities in the lower mantle all indicate that subducted material penetrates the deep mantle, implying whole-mantle convection. In contrast, geochemical analyses of the basaltic products of mantle melting are frequently used to infer that mantle convection is layered, with the deeper mantle largely isolated from the upper mantle. We show that geochemical, seismological and heat-flow data are all consistent with whole-mantle convection provided that the observed heterogeneities are remnants of recycled oceanic and continental crust that make up about 16 and 0.3 per cent, respectively, of mantle volume.     

Mapping the Hawaiian plume conduit with converted seismic waves

The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s(-1)) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of approximately 300 degrees C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100 degrees C higher temperature for the Hawaiian plume.     

Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data

The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa-235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th-238U data, the 231Pa-235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions--of the order of 0.1%--through the lithosphere, as might result from the coalescence of melt by compaction owing to melting-induced deformation. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (approximately 32.7 kyr). The 231Pa-230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km.     

Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling

Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ～250?kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ～660 and ～250 kilometres depth to form deep-seated melts in the Earth's mantle.     

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovici? discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.     

Density of hydrous silicate melt at the conditions of Earth's deep upper mantle

The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions. However, melts formed under deep mantle conditions are also likely to contain some water, which will reduce the melt density. Here we present data constraining the density of hydrous silicate melt at the conditions of approximately 410 km depth. We show that the water in the silicate melt is more compressible than the other components, and therefore the effect of water in reducing melt density is markedly diminished under high-pressure conditions. Our study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.     

A discontinuity in mantle composition beneath the southwest Indian ridge

The composition of mid-ocean-ridge basalt is known to correlate with attributes such as ridge topography and seismic velocity in the underlying mantle, and these correlations have been interpreted to reflect variations in the average extent and mean pressures of melting during mantle upwelling. In this respect, the eastern extremity of the southwest Indian ridge is of special interest, as its mean depth of 4.7 km (ref. 4), high upper-mantle seismic wave velocities and thin oceanic crust of 4-5 km (ref. 6) suggest the presence of unusually cold mantle beneath the region. Here we show that basaltic glasses dredged in this zone, when compared to other sections of the global mid-ocean-ridge system, have higher Na(8.0), Sr and Al2O3 compositions, very low CaO/Al2O3 ratios relative to TiO2 and depleted heavy rare-earth element distributions. This signature cannot simply be ascribed to low-degree melting of a typical mid-ocean-ridge source mantle, as different geochemical indicators of the extent of melting are mutually inconsistent. Instead, we propose that the mantle beneath approximately 1,000 km of the southwest Indian ridge axis has a complex history involving extensive earlier melting events and interaction with partial melts of a more fertile source.     

Core formation in planetesimals triggered by permeable flow

The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation. But core formation requires a mechanism for segregating metal, and the 'wetting' properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron-sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of short-lived radionuclides can increase temperature sufficiently above the iron-sulphur melting point (approximately 1,000 degrees C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of pre-formed cores.     

Melting in the Earth's deep upper mantle caused by carbon dioxide

The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03-0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of approximately 200-330 km in the upper mantle.     

矿物电导率Meyer-Neldel补偿规律及其应用研究

大量矿物电导率数据分析表明,矿物电导率存在Meyer-Neldel补偿规律,即同一矿物在不同物理、化学条件下测得电导率有很大差异,但其指前因子和活化能之间存在良好的线性关系.基于矿物电导率补偿规律,形成了从矿物电导率计算元素扩散系数的新方法,其计算结果与实验扩散数据符合较好.利用钙钛矿的电导率补偿关系,结合电磁测深结果,亦能获得更合理的地球深内部电导率结构.