Water and partial melting of Earth’s mantle

Water plays a crucial role in the melting of Earth’s mantle. Mantle magmatisms mostly occur at plate boundaries (including subduction zones and mid-ocean ridges) and in some intraplate regions with thermal anomaly. At oceanic subduction zones, water released by the subducted slab may induce melting of the overlying mantle wedge or even the slab itself, giving rise to arc magmatism, or may evolve into a supercritical fluid. The physicochemical conditions for the formation of slab melt and supercritical fluid are still under debate. At mid-ocean ridges and intraplate hot zones, water and CO₂ cause melting of the upwelling mantle to occur at greater depths and in greater extents. Low degree melting of the mantle may occur at boundaries between Earth’s internal spheres, including the lithosphere-asthenosphere boundary (LAB), the upper mantletransition zone boundary, and the transition zone-lower mantle boundary, usually attributed to contrasting water storage capacity across the boundary. The origin for the stimulating effect of water on melting lies in that water as an incompatible component has a strong tendency to be enriched in the melt (i.e., with a mineral-melt partition coefficient much smaller than unity), thereby lowering the Gibbs free energy of the melt. The partitioning of water between melt and mantle minerals such as olivine, pyroxenes and garnet has been investigated extensively, but the effects of hydration on the density and transport properties of silicate melts require further assessments by experimental and computational approaches.     

Effect of hydration on the elasticity of mantle minerals and its geophysical implications

Recent studies have shown that major nominally anhydrous minerals in the Earth’s mantle, such as olivine, pyroxene and garnet, can incorporate considerable amounts of water as structurally bound hydroxyl. Even a small amount of water is present in mantle minerals, it can strongly affect a number of physical properties, including density, sound velocity, melting temperature, and electrical conductivities. The presence of water can also influence the dynamic behavior, lead to lateral velocity heterogeneities, and affect the material circulation of the Earth’s deep interior. In particular, seismic studies have reported the existence of low-velocity zones in various locations of the Earth’s upper mantle and transition zone, which has been expected to be associated with the presence of water in the region. In the past two decades, the effect of water on the elasticity and sound velocities of minerals at relevant pressure-temperature (P-T) conditions of the Earth’s mantle attracted extensive interests. Combining the high P-T experimental and theoretical mineralogical results with seismic observations provides crucial constraints on the distribution of water in the Earth’s mantle. In this study, we summarize recent experimental and theoretical mineral physics results on how water affects the elasticity and sound velocity of nominally anhydrous minerals in the Earth’s mantle, which aims to provide new insights into the effect of hydration on the density and velocity profile of the Earth’s mantle, which are of particular importance in understanding of water distribution in the region.     

Hydration effects on crystal structures and equations of state for silicate minerals in the subducting slabs and mantle transition zone

There are potentially huge amounts of water stored in Earth’s mantle, and the water solubilities in the silicate minerals range from tens to thousands of part per minion (ppm, part per million). Exploring water in the mantle has attracted much attention from the societies of mineralogy and geophysics in recent years. In the subducting slab, serpentine breaks down at high temperature, generating a series of dense hydrous magnesium silicate (DHMS) phases, such as phase A, chondrodite, clinohumite, etc. These phases may serve as carriers of water as hydroxyl into the upper mantle and the mantle transition zone (MTZ). On the other hand, wadsleyite and ringwoodite, polymorphs of olivine, are most the abundant minerals in the MTZ, and able to absorb significant amount of water (up to about 3 wt.% H₂O). Hence, the MTZ becomes a very important layer for water storage in the mantle, and hydration plays important roles in physics and chemistry of the MTZ. In this paper, we will discuss two aspects of hydrous silicate minerals: (1) crystal structures and (2) equations of state (EoSs).     

On Earth’s Mantle Constitution and Structure from Joint Analysis of Geophysical and Laboratory-Based Data: An Example

Determining Earth’s structure is a fundamental goal of Earth science, and geophysical methods play a prominent role in investigating Earth’s interior. Geochemical, cosmochemical, and petrological analyses of terrestrial samples and meteoritic material provide equally important insights. Complementary information comes from high-pressure mineral physics and chemistry, i.e., use of sophisticated experimental techniques and numerical methods that are capable of attaining or simulating physical properties at very high pressures and temperatures, thereby allowing recovered samples from Earth’s crust and mantle to be analyzed in the laboratory or simulated computationally at the conditions that prevail in Earth’s mantle and core. This is particularly important given that the vast bulk of Earth’s interior is geochemically unsampled. This paper describes a quantitative approach that combines data and results from mineral physics, petrological analyses of mantle minerals, and geophysical inverse calculations, in order to map geophysical data directly for mantle composition (major element chemistry and water content) and thermal state. We illustrate the methodology by inverting a set of long-period electromagnetic response functions beneath six geomagnetic stations that cover a range of geological settings for major element chemistry, water content, and thermal state of the mantle. The results indicate that interior structure and constitution of the mantle can be well-retrieved given a specific set of measurements describing (1) the conductivity of mantle minerals, (2) the partitioning behavior of water between major upper mantle and transition-zone minerals, and (3) the ability of nominally anhydrous minerals to store water in their crystal structures. Specifically, upper mantle water contents determined here bracket the ranges obtained from analyses of natural samples, whereas transition-zone water concentration is an order-of-magnitude greater than that of the upper mantle and appears to vary laterally underneath the investigated locations.     

Deep electrical conductivity features in the transition zone from the Pacific to Eurasia

Understanding the processes that occur in the transition from the Pacific Ocean to Eurasia is key to constructing the tectonic models of the Earth’s shells and the convection models of the upper mantle. The electromagnetic methods permit estimating the temperature and fluid content (and/or carbon (graphite) content) in the Earth’s interior. These estimates are independent of the traditionally used estimates based on seismic methods because the dependence of electrical conductivity on the physical properties of the rock is based on different principles than the behavior of the elastic waves. The region is characterized by a complicated geological structure with intense three-dimensional (3D) surface heterogeneities, which significantly aggravate the retrieval of the information about the deep horizons in the structure of the Earth’s mantle from the observed electromagnetic (EM) fields. The detailed analysis of the nature of the deep electrical conductivity and structural features of the transition from the Pacific to Eurasia included numerical modeling of the typical two- and three-dimensional models has been carried out. Based on this analysis, the approaches that increase the reliability of the interpretation of the results of the EM studies are suggested.     

Experimental study of the electrical conductivity of hydrous minerals in the crust and the mantle under high pressure and high temperature

Hydrous minerals are important water carriers in the crust and the mantle, especially in the subduction zone. With the recent development of the experimental technique, studies of the electrical conductivity of hydrous silicate minerals under controlled temperature, pressure and oxygen fugacity, have helped to constrain the water distribution in the Earth’s interior. This paper introduces high pressure and temperature experimental study of electrical conductivity measurement of hydrous minerals such as serpentine, talc, brucite, phase A, super hydrous phase B and phase D, and assesses the data quality of the above minerals. The dehydration effect and the pressure effect on the bulk conductivity of the hydrous minerals are specifically emphasized. The conduction mechanism of hydrous minerals and the electrical structure of the subduction zone are discussed based on the available conductivity data. Finally, the potential research fields of the electrical conductivity of hydrous minerals is presented.     

First-principles calculations of elasticity of minerals at high temperature and pressure

The elasticity of minerals at high temperature and pressure(PT) is critical for constraining the composition and temperature of the Earth's interior and understand better the deep water cycle and the dynamic Earth. First-principles calculations without introducing any adjustable parameters, whose results can be comparable to experimental data, play a more and more important role in investigating the elasticity of minerals at high PT mainly because of(1) the quick increasing of computational powers and(2) advances in method. For example, the new method reduces the computation loads to one-tenth of the traditional method with the comparable precise as the traditional method. This is extraordinarily helpful because first-principles calculations of the elasticity of minerals at high PT are extremely time-consuming. So far the elasticity of most of lower mantle minerals has been investigated in detail. We have good idea on the effect of temperature, pressure, and iron concentration on elasticity of main minerals of the lower mantle and the unusual softening in bulk modulus by the spin crossover of iron in ferropericlase. With these elastic data the lower mantle has been constrained to have 10–15 wt% ferropericlase, which is sufficient to generate some visible effects of spin crossover in seismic tomography. For example, the spin crossover causes that the temperature sensitivity of P wave at the depth of ~1700 km is only a fraction of that at the depth of ~2300 km. The disruptions of global P wave structure and of P wave image below hotspots such as Hawaii and Iceland at similar depth are in consistence with the spin crossover effect of iron in ferropericlase. The spin crossover, which causes anomalous thermodynamic properties of ferropericlase, has also been found to play a control role for the two features of the large low shear velocity provinces(LLSVPs): the sharp edge and high elevation up to 1000 km above core-mantle boundary. All these results clearly suggest the spin crossover of iron in the lower mantle. The theoretical investigations for the elasticity of minerals at the upper mantle and water effect on elasticity of minerals at the mantle transition zone and subducting slab have also been conducted extensively. These researches are critical for understanding better the composition of the upper mantle and water distribution and transport in the Earth's mantle. Most of these were static calculations, which did not include the vibrational(temperature) effect on elasticity, although temperature effect on elasticity is basic because of high temperature at the Earth's interior and huge temperature difference between the ambient mantle and the subducting slab. Including temperature effect on elasticity of minerals should be important future work. New method developed is helpful for these directions. The elasticity of iron and iron-alloy with various light elements has also been calculated extensively. However, more work is necessary in order to meet the demand for constraining the types and amount of light elements at the Earth's core.     

Theoretical models and experimental determination methods for equations of state of silicate melts: A review

Silicate melts are very active in the interior of the Earth and other terrestrial planets, and are important carriers for the transport of material and energy. The determination of the equation of state(EOS) for silicate melts and the acquisition of a precise quantitative relationship between molar volume(or density) and temperature, pressure, and composition is essential for simulating the generation, migration, and eruption processes of magmas and the evolution of the magma ocean stage during the early formation of the Earth and other terrestrial planets, for calculating and modeling the phase equilibria involving silicate melts, and for revealing the variation of the microstructure of silicate melts with pressure. However, it is experimentally challenging to determine the volumetric properties of silicate melts and the accumulated density data at high pressure are still very limited due to a series of problems such as: the high liquidus temperature of silicate rocks; proneness for silicate melts to react with sample capsules to change the melt composition; and proneness for melts to flow and leak during the high pressure and high temperature experiments. In recent years, there is rapid progress in the high pressure and high temperature experimental techniques, in terms of not only the extension of temperature and pressure ranges but also the improvement on the accuracy of measurements, and the emergence of new methods for in-situ measurements. Here, we review the widely-used theoretical models of ambient-pressure and high-pressure EOS for silicate melts, and illustrate some problems that need to be solved urgently:(1) the room pressure EOS for iron-and titanium-bearing silicate melts needs to be improved;(2) the partial molar properties of the H2 O and CO2 components in silicate melts containing volatile components may vary markedly with the melt composition, which need to be addressed in high-pressure EOS;(3) how the formulation and applicable range of EOS correspond to changes in melt structure and compression mechanism requires further study. We highlight the basic principle and applicable range of various methods for determining the EOS for silicate melts, and compare the advantages and disadvantages of doublebob Archimedes method, fusion curve analysis, shock compression experiments, sink-float method, X-ray absorption, X-ray diffraction and ultrasonic interferometry. Future trends in this field are to develop experimental techniques for in situ measurements on melt density or sound velocity at high temperature and high pressure and to accumulate more experimental data,and on the other hand, to improve the theoretical models of the EOS for silicate melts by a combination of research on the microstructure and compression mechanisms of silicate melts.     

Temperature and distribution of radioactive matter in the upper mantle

Temperatures of the Earth’s upper mantle, derived from Gutenberg’s seismic velocities, have been revised, using some recent determinations of the clastic constants of dunites. For depths greater than about 50 km the temperature obtained is sufficient to melt the basaltic fraction of a periodotitic mantle (if this low-melting fraction still exists at those depths). The distribution of the radioactive heat sources appears to be fairly uniform down to about 110 km; below this level the radioactive matter is probably absent. This is what would be expected for the upper mantle below the oceans; therefore, Gutenberg’s velocities seem to correspond to oceanic areas, rather than to the mantle below a continental shield.     

Heterogeneous accretion,composition and core–mantle differentiation of the Earth

A model of core formation is presented that involves the Earth accreting heterogeneously through a series of impacts with smaller differentiated bodies. Each collision results in the impactor's metallic core reacting with a magma ocean before merging with the Earth's proto-core. The bulk compositions of accreting planetesimals are represented by average solar system abundances of non-volatile elements (i.e. CI-chondritic), with 22% enhancement of refractory elements and oxygen contents that are defined mainly by the Fe metal/FeO silicate ratio. Based on an anhydrous bulk chemistry, the compositions of coexisting core-forming metallic liquid and peridotitic silicate liquid are calculated by mass balance using experimentally-determined metal/silicate partition coefficients for the elements Fe, Si, O, Ni, Co, W, Nb, V, Ta and Cr. Oxygen fugacity is fixed by the partitioning of Fe between metal and silicate and depends on temperature, pressure and the oxygen content of the starting composition. Model parameters are determined by fitting the calculated mantle composition to the primitive mantle composition using least squares minimization. Models that involve homogeneous accretion or single-stage core formation do not provide acceptable fits. In the most successful models, involving 24 impacting bodies, the initial 60–70% (by mass) of the Earth accretes from highly-reduced material with the final 30–40% of accreted mass being more oxidised, which is consistent with results of dynamical accretion simulations. In order to obtain satisfactory fits for Ni, Co and W, it is required that the larger (and later) impactor cores fail to equilibrate completely before merging with the Earth's proto-core, as proposed previously on the basis of Hf-W isotopic studies. Estimated equilibration conditions may be consistent with magma oceans extending to the core–mantle boundary, thus making core formation extremely efficient. The model enables the compositional evolution of the Earth's mantle and core to be predicted throughout the course of accretion. The results are consistent with the late accretion of the Earth's water inventory, possibly with a late veneer after core formation was complete. Finally, the core is predicted to contain ~ 5 wt.% Ni, ~ 8 wt.% Si, ~ 2 wt.% S and ~ 0.5 wt.% O.     

Chemical fractionation in the silicate vapor atmosphere of the Earth

Despite its importance to questions of lunar origin, the chemical composition of the Moon is not precisely known. In recent years, however, the isotopic composition of lunar samples has been determined to high precision and found to be indistinguishable from the terrestrial mantle despite widespread isotopic heterogeneity in the Solar System. In the context of the giant-impact hypothesis, this level of isotopic homogeneity can evolve if the proto-lunar disk and post-impact Earth undergo turbulent mixing into a single uniform reservoir while the system is extensively molten and partially vaporized. In the absence of liquid–vapor separation, such a model leads to the lunar inheritance of the chemical composition of the terrestrial magma ocean. Hence, the turbulent mixing model raises the question of how chemical differences arose between the silicate Earth and Moon. Here we explore the consequences of liquid–vapor separation in one of the settings relevant to the lunar composition: the silicate vapor atmosphere of the post-giant-impact Earth. We use a model atmosphere to quantify the extent to which rainout can generate chemical differences by enriching the upper atmosphere in the vapor, and show that plausible parameters can generate the postulated enhancement in the FeO/MgO ratio of the silicate Moon relative to the Earth's mantle. Moreover, we show that liquid–vapor separation also generates measurable mass-dependent isotopic offsets between the silicate Earth and Moon and that precise silicon isotope measurements can be used to constrain the degree of chemical fractionation during this earliest period of lunar history. An approach of this kind has the potential to resolve long-standing questions on the lunar chemical composition.     

Reaction experiments between tonalitic melt and mantle olivine and their implications for genesis of high-Mg andesites within cratons

High-Mg (Mg#>45) andesites (HMA) within cratons attract great attention from geologists. Their origin remains strongly debated. In order to examine and provide direct evidence for previous assumptions about HMA’s genesis inferred from petrological and geochemical investigations, we performed reaction experiments between tonalitic melt and mantle olivine on a six-anvil apparatus at high-temperature of 1250–1400°C and high-pressure of 2.0–5.0 GPa. Our experiments in this work simulated the interaction between the tonalitic melt derived from partial melting of eclogitized lower crust foundering into the Earth’s mantle and mantle peridotite. The experimental results show that the reacted melts have very similar variations in chemical compositions to the HMAs within the North China Craton. Therefore, this interaction is probably an important process to generate the HMAs within crations.     

The immediate environmental effects of tephra emission

The Earth’s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere. These tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents. The interaction of tephra with or in receiving environments may induce an array of physical, chemical and biological effects. The consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent, small eruptions, while larger and rarer events may have acute, regional-scale impacts. It is, therefore, necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems. We collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post-eruptive effects of tephra emission into the atmosphere, onto vegetation, soil or ice/snow surfaces and in aquatic systems. We further consider the repercussions that may be induced in the weeks to years afterwards. In the atmosphere, tephra can influence cloud properties and air chemistry by acting as ice nuclei (IN) or by offering sites for heterogeneous reactions, respectively. Tephra on vegetation causes physical damage, and sustained coverage may elicit longer-term physiological responses. Tephra deposits on soils may alter their capacity to exchange gas, water and heat with the atmosphere or may have a specific chemical effect, such as nutrient input or acidification, on sensitive soils. Tephra deposition onto snow or ice may affect ablation rates. Rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged (months or years) erosion from the tephra-covered catchment. In the first weeks after deposition, tephra leaching may affect river chemistry. The abundance and speciation of phytoplankton populations in lakes may be altered by tephra-induced changes in water chemistry or sediment–water nutrient cycling. In the oceans, tephra deposition may fertilise Fe-limited waters, with potential impacts on the global carbon cycle. Embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies, combined with consideration of the interconnectivity of induced impacts within and between different receiving environments.     

Irrigation Effects on Hydro-Climatic Change: Basin-Wise Water Balance-Constrained Quantification and Cross-Regional Comparison

Hydro-climatic changes driven by human land and water use, including water use for irrigation, may be difficult to distinguish from the effects of global, natural and anthropogenic climate change. This paper quantifies and compares the hydro-climatic change effects of irrigation using a data-driven, basin-wise quantification approach in two different irrigated world regions: the Aral Sea drainage basin in Central Asia and the Indian Mahanadi River Basin draining into the Bay of Bengal. Results show that irrigation-driven changes in evapotranspiration and latent heat fluxes and associated temperature changes at the land surface may be greater in regions with small relative irrigation impacts on water availability in the landscape (here represented by the Mahanadi River Basin) than in regions with severe such impacts (here represented by the Aral region). Different perspectives on the continental part of Earth’s hydrological cycle may thus imply different importance assessments of various drivers and impacts of hydro-climatic change. Regardless of perspective, however, actual basin-wise water balance constraints should be accounted to realistically understand and accurately quantify continental water change.     

Mechanisms of metal–silicate equilibration in the terrestrial magma ocean

It has been proposed that the high concentrations of moderately siderophile elements (e.g. Ni and Co) in the Earth’s mantle are the result of metal–silicate equilibration at the base of a deep magma ocean that formed during Earth’s accretion. According to this model, liquid metal ponds at the base of the magma ocean and, after equilibrating chemically with the overlying silicate liquid at high pressure (e.g. 25–30 GPa), descends further as large diapirs to form the core. Here we investigate the kinetics of metal–silicate equilibration in order to test this model and place new constraints on processes of core formation. We investigate two models: (1) Reaction between a layer of segregated liquid metal and overlying silicate liquid at the base of a convecting magma ocean, as described above. (2) Reaction between dispersed metal droplets and silicate liquid in a magma ocean. In the liquid-metal layer model, the convection velocity of the magma ocean controls both the equilibration rate and the rate at which the magma ocean cools. Results indicate that time scales of chemical equilibration are two to three orders of magnitude longer than the time scales of cooling and crystallization of the magma ocean. In the falling metal droplet model, the droplet size and settling velocity are critical parameters that we determine from fluid dynamics. For likely silicate liquid viscosities, the stable droplet diameter is estimated to be ∼1 cm and the settling velocity ∼0.5 m/s. Using such parameters, liquid metal droplets are predicted to equilibrate chemically after falling a distance of <200 m in a magma ocean. The models indicate that the concentrations of moderately siderophile elements in the mantle could be the result of chemical interaction between settling metal droplets and silicate liquid in a magma ocean but not between a segregated layer of liquid metal and overlying silicate liquid at the base of the magma ocean. Finally, due to fractionation effects, the depth of the magma ocean could have been significantly different from the value suggested by the apparent equilibration pressure.     

Observing,Modeling, and Interpreting Magnetic Fields of the Solid Earth

Many Earth system processes generate magnetic fields, either primary magnetic fields or in response to other magnetic fields. The largest of these magnetic fields is due to the dynamo in the Earth’s core, and can be approximated by a geocentric axial dipole that has decayed by nearly 10% during the last 150 years. This is an order of magnitude faster than its natural decay time, a reflection of the growth of patches of reverse flux at the core–mantle boundary. The velocity of the North magnetic pole reached some 40 km/yr in 2001. This velocity is the highest recorded so far in the last two centuries. The second largest magnetic field in the solid Earth is caused by induced and remanent magnetization within the crust. Controlled in part by the thermo-mechanical properties of the crust, these fields contain signatures of tectonic processes currently active, and those active in the distant past. Recent work has included an estimate of the surface heat flux under the Antarctic ice cap. In order to understand the recent changes in the Earth’s magnetic field, new high-quality measurements are needed to continue those being made by Ørsted (launched in 1999), CHAMP and the Ørsted-2 experiment onboard SAC-C (both launched in 2000). The present paper is motivated by the advent of space surveys of the geomagnetic field, and illustrates how our way of observing, modeling, and interpreting the Earth’s magnetic field has changed in recent years due to the new magnetic satellite measurements.     

V. V. Beloussov and Geophysics: A Tribute on His 110th Birth Anniversary

The contribution made by V.V. Beloussov (1907–1990), an outstanding Earth scientist in the former Soviet Union and Russia, to the development of planetary geophysics is considered. Beloussov was a brilliant coordinator of international cooperation and direct inspirer of international scientific programs of paramount importance. He took up one of the key positions in organizing and holding the International Geophysical Year (IGY) in 1957–1958. In 1960, Beloussov was elected President of the International Union of Geodesy and Geophysics and proposed the project “The upper mantle and its influence on the Earth’s crust,” which subsequently became known worldwide as the Upper Mantle Project. The project underlined that the experience of the IGY should be extended to studies of the deep structure of the Earth and the processes taking place in the Earth’s interior. The fulfillment of this and the subsequent Geodynamic project resulted in a breakthrough in the knowledge about the deep structure of the Earth, particularly the structure of the oceans. Beloussov actively advocated integrating science of the Earth, geonomy, and in his scientific research sought a geonomic approach incorporating the entire complex of geological, geophysical, and geochemical data. Beloussov’s scientific heritage contains propositions that are of current importance and can be involved in modern developments of the Earth sciences.     

Core cooling by subsolidus mantle convection

Although vigorous mantle convection early in the thermal history of the Earth is shown to be capable of removing several times the latent heat content of the core, we are able to construct a thermal evolution model of the Earth in which the core does not solidify. The large amount of energy removed from the model Earth's core by mantle convection is supplied by the internal energy of the core which is assumed to cool from an initial high temperature given by the silicate melting temperature at the core-mantle boundary. For the smaller terrestrial planets, the iron and silicate melting temperatures at the core-mantle boundaries are more comparable than for the Earth, and the cores of these planets may not possess enough internal energy to prevent core solidification by mantle convection. Our models incorporate temperature-dependent mantle viscosity and radiogenic heat sources in the mantle. The Earth models are constrained by the present surface heat flux and mantle viscosity. Internal heat sources produce only about 55% of the Earth model's present surface heat flow.     

Energy cycle of geodynamics and the seismic process

Estimates are obtained for the energy cycle of geodynamics, namely, the contributions of the geothermal flux to the generation rate of the kinetic energy of convective motions in the mantle and the generation of stresses in the crust that are later released during earthquakes. As follows from theoretical considerations, about half of the geothermal flux is spent on the generation of motions in the mantle, and the fraction of the total geothermal flux from the Earth’s interior that is spent on the seismic process is estimated at 0.5%. This estimate is obtained with the use of data of global earthquake catalogs. For volcanic processes, this fraction is smaller by two orders of magnitude. The energies required for the formation of the Earth’s surface topography and for seismic activity are comparable.     

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

Measurements of the splitting or birefringence of seismic shear waves that have passed through the Earth’s mantle yield constraints on the strength and geometry of elastic anisotropy in various regions, including the upper mantle, the transition zone, and the D″ layer. In turn, information about the occurrence and character of seismic anisotropy allows us to make inferences about the style and geometry of mantle flow because anisotropy is a direct consequence of deformational processes. While shear wave splitting is an unambiguous indicator of anisotropy, the fact that it is typically a near-vertical path-integrated measurement means that splitting measurements generally lack depth resolution. Because shear wave splitting yields some of the most direct constraints we have on mantle flow, however, understanding how to make and interpret splitting measurements correctly and how to relate them properly to mantle flow is of paramount importance to the study of mantle dynamics. In this paper, we review the state of the art and recent developments in the measurement and interpretation of shear wave splitting—including new measurement methodologies and forward and inverse modeling techniques,—provide an overview of data sets from different tectonic settings, show how they help us relate mantle flow to surface tectonics, and discuss new directions that should help to advance the shear wave splitting field.