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Astronomical surveys have discovered thousands of transiting exoplanets, revealing that rocky planets are common in the galaxy. A planet's interior chemistry is frequently inferred by average density, described by mass‐radius (M‐R) relationships. However, M‐R relationships give rise to non‐unique interpretations of a planet's interior composition, an issue that limits our ability to characterize far‐away worlds. We present experimental and density functional theoretical results addressing the influence of an ultra‐reducing (oxygen‐poor) interior chemistry on rocky mantle phases and discuss the possible implications for atmospheric observables. We show that silicon carbide (SiC) and molecular nitrogen (N2) react to form solid silicon nitride (γ‐Si3N4) at high pressures and high temperatures in a laser‐heated diamond‐anvil cell, consistent with ab initio computations. Si3N4 remains stable under extreme conditions and when quenched to ambient conditions. As SiC is a common compound found under very reducing conditions, these results indicate that nitrogen may form solid phases in an oxygen‐poor rocky planet. If, by sequestering nitrogen in a planet's mantle, the distribution of nitrogen between a planet's interior and atmosphere is altered (i.e., a nitrogen‐rich mantle and nitrogen‐poor atmosphere), these results indicate that there may be atmospheric observables connected to the mantle‐redox state of a rocky planet besides the oxygen‐containing phases ubiquitous in exoplanet literature. Plain Language Summary: Identifying the interior composition of a planet is a challenging task, particularly when the planet resides outside of the Solar System. This is due to limited and difficult observations combined with countless non‐unique interpretations. Here we present the results of mineral physics experiments exploring nitrogen chemistry at mantle conditions in an oxygen‐poor environment. We show that silicon carbide (SiC) and molecular nitrogen (N2) react to form solid silicon nitride (γ‐Si3N4) at high pressures and high temperatures. This suggests that solid nitrogen phases, which are rare on Earth, may be common within an oxygen‐poor planet's mantle. The presence of these solid nitrogen phases may influence the distribution of nitrogen between a planet's interior and atmosphere, possibly providing observable clues to the interior redox state of a planet's mantle, a property tied to geophysical behavior and surface chemistry. Key Points: SiC and N2 react to form solid γ‐Si3N4 at pressures between ∼15 and 30 GPa and temperatures in excess of ∼1,700 KOnce formed, γ‐Si3N4 remains stable while heating to temperatures up to ∼3,000 KOur results indicate that nitrogen distribution in planets with reduced mantles may be different from that of planets with oxidized mantles [ABSTRACT FROM AUTHOR]

The combination of seismic observations and mineral physics data represents a unique tool to understand the structure and evolution of the deep Earth's interior. However, to date, elasticity data on both compressional (vP) and shear (vS) wave velocities of MgSiO3 bridgmanite are limited to shallow mantle conditions, hampering the resolution of mineral physics models. Here, we report the first single‐crystal measurements of vP and vS of MgSiO3 bridgmanite up to ∼79 GPa using high‐pressure Brillouin scattering and single‐crystal X‐ray diffraction in a diamond anvil cell. At shallow lower mantle pressures, the elastic anisotropy of MgSiO3 bridgmanite was found to be similar, albeit smaller than that of Fe,Al‐bearing bridgmanite of Kurnosov et al. (2017) but differed significantly from that proposed in the recent study of Fu et al. (2019). Using the elastic stiffness coefficients of bridgmanite obtained in this study at different pressures, we calculate the pressure dependence of the adiabatic bulk modulus, KS0 = 257.1(6) GPa, K'S0 = 3.71(4), and of the shear modulus, G0 = 175.6(2) GPa, G'0 = 1.86(1). These elastic parameters are included in a self‐consistent thermodynamic model to calculate seismic wave velocities along a lower mantle adiabat for a primitive upper mantle bulk composition in the FeO‐CaO‐MgO‐SiO2 system, which is currently the most complex system for which sufficient data exist. This preliminary model provides a good match to the vS and vP of 1D seismic models, implying that the composition of the lower mantle may be closer to pyrolite, rather than being more bridgmanite rich. Key Points: We measured both compressional and shear wave velocities of MgSiO3 bridgmanite up to mid‐lower mantle pressure for the first timeThe effect of chemistry on the elastic moduli and elastic anisotropy of bridgmanite at lower mantle pressures is discussedWe compare the seismic velocities for a simplified primitive upper mantle composition with 1D seismic models of Earth's lower mantle [ABSTRACT FROM AUTHOR]

The mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle. Constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics. Seismic anisotropy, i.e., the directional dependence of seismic wave velocity, provides us with the most direct constraints on mantle deformation. Its detection below ~250 km depth is challenging, and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism. Here, we present a global model of azimuthal anisotropy for the top 1000. km of the mantle. We used a dataset composed of fundamental and higher mode Rayleigh wave phase velocity maps, which provides resolution of azimuthal anisotropy to much greater depths than in previous studies. Our model unravels the presence of significant anisotropy in the transition zone, challenging common views of mantle deformation mechanisms, and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur. The newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the MTZ and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material. While the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios, possible explanations for our findings include recrystallization during phase transformations, or changes in the slip system across the MTZ boundaries, which in turn could be explained by changes in the water content of mantle material, consistent with the idea that the transition zone acts as a water filter. © 2013.

The mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle. Constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics. Seismic anisotropy, i.e., the directional dependence of seismic wave velocity, provides us with the most direct constraints on mantle deformation. Its detection below ~250 km depth is challenging, and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism. Here, we present a global model of azimuthal anisotropy for the top 1000. km of the mantle. We used a dataset composed of fundamental and higher mode Rayleigh wave phase velocity maps, which provides resolution of azimuthal anisotropy to much greater depths than in previous studies. Our model unravels the presence of significant anisotropy in the transition zone, challenging common views of mantle deformation mechanisms, and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur. The newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the MTZ and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material. While the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios, possible explanations for our findings include recrystallization during phase transformations, or changes in the slip system across the MTZ boundaries, which in turn could be explained by changes in the water content of mantle material, consistent with the idea that the transition zone acts as a water filter. © 2013.

We performed deformation experiments on dry natural single crystals of magnetite and ilmenite to determine the rheological behavior of these oxide minerals as a function of temperature, orientation, and oxygen fugacity. Samples were deformed at temperatures of 825–1150 ∘ C to axial strains of up to 15–24% under approximately constant stress conditions up to 120 MPa in a dead-load-type creep rig at ambient pressure in a controlled gas atmosphere. Oxygen fugacity ranged from 10 - 9.4 to 10 - 4 atm. Ilmenite creep was insensitive to oxygen fugacity, while magnetite displayed a strong, non-monotonic oxygen fugacity dependence, with creep rates varying as f O 2 - 0.7 and f O 2 0.4 at more reducing and more oxidizing conditions, respectively. Dislocation creep rates of magnetite single crystals were weakly dependent on crystallographic orientation with stress exponents that varied between 2.8 and 4.3 (mean 3.5 ± 0.4). Magnetite compressed parallel to <100>, <110>, and <111> axes exhibited apparent activation energies of 315±5, 345±30, and 290±5 kJ/mol, respectively. We estimated f O 2 -independent magnetite activation energies of 715 ± 150, 725 ± 145, and 690 ± 150 kJ/mol for <100>, <110>, and <111> orientations, respectively, in the region of negative f O 2 -dependence. Ilmenite single crystals were compressed parallel, normal, and inclined to the c-axis. Stress exponents of 3.4, 4.3, and 3.9 indicate dislocation creep with activation energies of 420 ± 35, 345 ± 30, and 360 ± 40 kJ/mol, respectively, for these orientations. Mechanical anisotropy in ilmenite is notably higher than in magnetite, as expected from its lower crystal symmetry. Constitutive equations were formulated for ilmenite and magnetite creep. [ABSTRACT FROM AUTHOR]

The interplay between stress and chemical processes is a fundamental aspect of how rocks evolve, relevant for understanding fracturing due to metamorphic volume change, deformation by pressure solution and diffusion creep, and the effects of stress on mineral reactions in crust and mantle. There is no agreed microscale theory for how stress and chemistry interact, so here I review support from eight different types of the experiment for a relationship between stress and chemistry which is specific to individual interfaces: (chemical potential) = (Helmholtz free energy) + (normal stress at interface) × (molar volume). The experiments encompass temperatures from -100 to 1300 degrees C and pressures from 1 bar to 1.8 GPa. The equation applies to boundaries with fluid and to incoherent solid–solid boundaries. It is broadly in accord with experiments that describe the behaviours of free and stressed crystal faces next to solutions, that document flow laws for pressure solution and diffusion creep, that address polymorphic transformations under stress, and that investigate volume changes in solid-state reactions. The accord is not in all cases quantitative, but the equation is still used to assist the explanation. An implication is that the chemical potential varies depending on the interface, so there is no unique driving force for reaction in stressed systems. Instead, the overall evolution will be determined by combinations of reaction pathways and kinetic factors. The equation described here should be a foundation for grain-scale models, which are a prerequisite for predicting larger scale Earth behaviour when stress and chemical processes interact. It is relevant for all depths in the Earth from the uppermost crust (pressure solution in basin compaction, creep on faults), reactive fluid flow systems (serpentinisation), the deeper crust (orogenic metamorphism), the upper mantle (diffusion creep), the transition zone (phase changes in stressed subducting slabs) to the lower mantle and core mantle boundary (diffusion creep). [ABSTRACT FROM AUTHOR]

Abstract Elastic thermobarometry can be used to constrain the pressure and temperature conditions of mineral crystallization by exploiting the difference in the elastic evolution of a mineral inclusion and its host during cooling and decompression. In this work we examine the pressure‐temperature sensitivity of >5,000 untested inclusion‐host pairs. Hosts such as diamond and zircon are ideal host minerals because their low compressibility makes them rigid containment vessels. Highly compressible inclusions such as albite, graphite, and quartz serve as the most reliable barometers. We provide three case studies of inclusion‐host pairs from different geologic settings to demonstrate the advantages and challenges associated with these mineral pairs. Apatite inclusions in olivine from Yellowstone caldera mostly record negative residual pressures (tension) and suggest magmatic crystallization at ~0.4 GPa. Rutile inclusions in garnet from Verpeneset eclogites record near ambient conditions and do not recover reasonable metamorphic conditions of rutile entrapment. These results suggest that stiff inclusions may have a tensile strain limit, a possible limitation of elastic thermobarometry. Albite inclusions in epidote from a blueschist (Syros, Greece) record geologically reasonable entrapment pressures, but a large range of residual pressures that may be caused by the complex anisotropy of both phases. Our theoretical and applied results indicate that elastic thermobarometry has the potential to be used to understand petrologic processes in diverse geologic environments, including mantle, metamorphic, and magmatic settings but that each elastic thermobarometer requires careful evaluation.

Elastic thermobarometry can be used to constrain the pressure and temperature conditions of mineral crystallization by exploiting the difference in the elastic evolution of a mineral inclusion and its host during cooling and decompression. In this work we examine the pressure‐temperature sensitivity of >5,000 untested inclusion‐host pairs. Hosts such as diamond and zircon are ideal host minerals because their low compressibility makes them rigid containment vessels. Highly compressible inclusions such as albite, graphite, and quartz serve as the most reliable barometers. We provide three case studies of inclusion‐host pairs from different geologic settings to demonstrate the advantages and challenges associated with these mineral pairs. Apatite inclusions in olivine from Yellowstone caldera mostly record negative residual pressures (tension) and suggest magmatic crystallization at ~0.4 GPa. Rutile inclusions in garnet from Verpeneset eclogites record near ambient conditions and do not recover reasonable metamorphic conditions of rutile entrapment. These results suggest that stiff inclusions may have a tensile strain limit, a possible limitation of elastic thermobarometry. Albite inclusions in epidote from a blueschist (Syros, Greece) record geologically reasonable entrapment pressures, but a large range of residual pressures that may be caused by the complex anisotropy of both phases. Our theoretical and applied results indicate that elastic thermobarometry has the potential to be used to understand petrologic processes in diverse geologic environments, including mantle, metamorphic, and magmatic settings but that each elastic thermobarometer requires careful evaluation. Plain Language Summary: Determining the pressures and temperatures at which rocks forms is crucial to understanding processes that occur on Earth. The pressures and temperatures at which rocks form can give insights into processes such as the following: At what depths (pressures) do magmas form? How deep (pressure) do rocks go where continents or ocean plates collide? What are the conditions under which diamonds form? The geologic community has for many years developed methods to constrain the pressure and temperatures at which rocks form. Recent developments have tried to take advantage of the difference in the mechanical and physical properties of two minerals—one being trapped (inclusion) inside of another (host). The trapped mineral can retain some pressure at the Earth's surface, and we can try to estimate the initial conditions at which the inclusion was trapped by the host mineral. Here, we present new potential inclusion and host pairs that can be used to constrain these initial pressure and temperature conditions and discuss potential limitations associated with these mineral pairs. Some of these inclusion‐host pairs may provide the potential to constrain the formation conditions of rocks that previously did not have suitable barometers or thermometers. Key Points: Elastic thermobarometry has the potential to constrain pressures and temperatures of rocks from mantle, metamorphic, and magmatic settingsStiff inclusions that should preserve significant tension at ambient pressures may not be usable as elastic thermobarometersFeldspar inclusions are promising because of their high compressibility but are complicated by complex anisotropy [ABSTRACT FROM AUTHOR]

This paper describes riesite, a new high-pressure polymorph of TiO2 from the Ries impact structure, Germany. Riesite occurs in shock-induced melt veins within xenoliths of bedrock in suevite. It is structurally closely related to srilankite from which it differs by having two distinct cation sites rather than one and through its monoclinic symmetry. It is indicative that riesite forms only upon release from the shock state upon back transformation from akaogiite. [ABSTRACT FROM AUTHOR]

This article is dedicated to the occurrence, relevance, and structure of minerals whose formation involves high pressure. This includes minerals that occur in the interior of the Earth as well as minerals that are found in shock-metamorphized meteorites and terrestrial impactites. I discuss the chemical and physical reasons that render the definition of high-pressure minerals meaningful, in distinction from minerals that occur under surface-near conditions on Earth or at high temperatures in space or on Earth. Pressure-induced structural transformations in rock-forming minerals define the basic divisions of Earth's mantle in the upper mantle, transition zone, and lower mantle. Moreover, the solubility of minor chemical components in these minerals and the occurrence of accessory phases are influential in mixing and segregating chemical elements in Earth as an evolving planet. Brief descriptions of the currently known high-pressure minerals are presented. Over the past 10 years more high-pressure minerals have been discovered than during the previous 50 years, based on the list of minerals accepted by the IMA. The previously unexpected richness in distinct high-pressure mineral species allows for assessment of differentiation processes in the deep Earth. [ABSTRACT FROM AUTHOR]

CaSiO3 polymorphs are abundant in only unique geological settings on the Earth’s surface and are the major Ca-bearing phases at deep mantle condition. An accurate and comprehensive study of their density and structural evolution with pressure and temperature is still lacking. Therefore, in this study we report the elastic behavior and structural evolution of wollastonite and CaSiO3-walstromite with pressure. Both minerals are characterized by first order phase transitions to denser structures. The deformations that lead to these transformations allow a volume increase ofthe bigger polyhedra, which might ease cation substitution in the structural sites of these phases. Furthermore, their geometrical features are clear analogies with those predicted and observed for tetrahedrally-structured ultra-high-pressure carbonates, which are unfortunately unquenchable. Indeed, wollastonite and CaSiO3-walstromite have a close resemblance to ultra-high-pressure chain- and ring-carbonates. This suggests a rich polymorphism also for tetrahedral carbonates, which might increase the compositional range of these phases, including continuous solid solutions involving cations with different size (Ca vs. Mg in particular) and important minor or trace elements incorporation.

Karl Przibram is one of the pioneers of early solid state physics in the field of the interdependence of coloration effects and luminescence in solids (crystals, minerals) induced by radiation. In 1921 Przibram discovered the effect of radio-photoluminescence, the light-stimulated phosphorescence in activated crystals induced by gamma rays. In 1926 Przibram was the first to use the term, Farbzentrum (color center, F-center), and in 1923 he advanced the view of atomic centers as carriers of coloration. Being a pupil of Ludwig Boltzmann and Franz S. Exner, he dedicated his early work to condensation and conductivity phenomena in gases and Brownian motion. Under the influence of Stefan Meyer, he began his lifelong interest in mineralogy, setting up his own research group at the Vienna Radium Institute, which pioneered investigations on thermoluminescence and gave a first description of glow curves. Being of Jewish descent, Przibram had to leave Austria after the Nazis took power; he found shelter in Belgium and returned to Austria in 1946 as professor for experimental physics at the University of Vienna. This paper is a first attempt to give an overview of the cultural and scientific background of Przibram's life and science in context of the cultural and political developments from 1900 to 1950 in Austria. [ABSTRACT FROM AUTHOR]

Previous seismological studies have focused on the large‐scale heterogeneity of the D″ layer beneath East Asia. Here, we use high‐frequency PcP precursors recorded at dense networks in China and Japan to constrain the small‐scale heterogeneity in this region. By forward modeling of the stacks of recorded PcP precursory signal, we find that the small‐scale heterogeneity in the D″ layer beneath East Asia is characterized by a P wave velocity perturbation of 0.1‐0.2% and a dominant length scale of 4‐20 km. Although the resolved length scale is similar to the dimension of the subducted oceanic crust, the weak velocity perturbation is difficult to explain by basaltic rock in the lowermost mantle. Postperovskite transition of the supposed slab might also not be viable for generating weak heterogeneity; thus, new explanations are needed from geodynamics and mineral physics to account for the different strength of the large‐ and small‐scale heterogeneity. Plain Language Summary: Most global tomography models show high‐velocity anomalies near the core‐mantle boundary beneath East Asia, which is part of the circum‐Pacific fast anomalies in the lowermost mantle. Previous seismological, mineral physical, and geodynamical studies have proposed two hypotheses to interpret the origin of high‐velocity anomalies: the accumulation of old slabs and the perovskite to postperovskite phase transition. In this work, by modeling PcP precursors in recordings, we observe weak small‐scale heterogeneity, which cannot be explained by these two hypotheses; thus, new mechanisms are needed to account for the different strength of heterogeneity between large scale and small scale. Key Points: The D″ layer beneath East Asia features of prominent fast velocity anomaly at large scale, but characters at small scale is not well knownVery weak small‐scale heterogeneity of the D″ layer beneath East Asia is resolved from recorded PcP precursorsThe transition of the slab to pPv might be one of the interpretations of weak small‐scale scattering [ABSTRACT FROM AUTHOR]

The core concerns of plate tectonics theory are the dynamics of subducting plates, which can be studied by integrating multidisciplinary fields such as seismology, mineral physics, rock geochemistry, geological formation studies, sedimentology, and numerical simulations. By establishing a theoretical model and solving it with numerical methods, one can replicate the dynamic effects of a subducting plate, quantifying its evolution and the surface response. Simulations can also explain the observations and experimental results of other disciplines. Therefore, numerical models are among the most important tools for studying the dynamics of subducting plates. This paper provides a review on recent advances in the numerical modeling of subducting plate dynamics. It covers various aspects, namely, the origin of plate tectonics, the initiation process and thermal structure of subducting slab, and the main subduction slab dynamics in the upper mantle, mantle transition zone, and lower mantle. The results of numerical models are based on the theoretical equations of mass, momentum, and energy conservation. To better understand the dynamic progress of subducting plates, the simulation results must be verified in comparisons with the results from natural observations by geology, geophysics and geochemistry. With the substantial increase in computing power and continuous improvement of simulation methods, numerical models will become a more accurate and efficient means of studying the frontier issues of Earth sciences, including subducting plate dynamics. [ABSTRACT FROM AUTHOR]

The late Taro Takahashi earned a particularly well-deserved reputation for his research at Lamont Geological Observatory on carbon dioxide and its transfer between the atmosphere and the oceans. However, his accomplishments in Mineral Physics, the field embracing the high-pressure–high-temperature properties of materials, has received less attention in spite of his major contributions to this emerging field focused on the interiors of Earth and other planets. In 1963, I was thrilled when he was offered a faculty position in the Geology Department at the University of Rochester, where I had recently joined the faculty. Taro and I worked together for the next 10 years with our talented students exploring the blossoming field just becoming known as Mineral Physics, the name introduced by Orson Anderson and Ed Schreiber, who were also engaged in measuring physical properties at high pressures and temperatures. While their specialty was ultrasonic velocities in minerals subjected to high pressures and temperatures, ours was the determination of crystal structures, compressibilities, and densities of such minerals as iron, its alloys, and silicate minerals, especially those synthesized at high-pressure, such as silicates with the spinel structure. These were materials expected to be found in the Earth’s interior and could therefore provide background for the interpretation of geophysical observations.

This paper describes riesite, a new high-pressure polymorph of TiO2 from the Ries impact structure, Germany. Riesite occurs in shock-induced melt veins within xenoliths of bedrock in suevite. It is structurally closely related to srilankite from which it differs by having two distinct cation sites rather than one and through its monoclinic symmetry. It is indicative that riesite forms only upon release from the shock state upon back transformation from akaogiite.

Graphical abstract Highlights • Geophysical and mineral physics data indicate presence of the wet transition zone. • The mantle transition zone contains continental crustal components. • Fluids or volatile-rich magmas may be generated at the top of the lower mantle. • Hydrous δ-H solid solution reduces Al 2 O 3 contents in the lower mantle bridgmanite. • The iron-water reaction creates pyrite-FeOOH at the core-mantle boundary. Abstract The roles of water in the mantle transition zone, lower mantle, and the core-mantle boundary are investigated. The evidence for a wet mantle transition zone has been suggested based on hydrous mineral inclusions in diamond. Seismic wave velocity and electrical conductivity profiles together with mineral physics data are consistent with existence of stagnant slabs in a wet mantle transition zone. The transition zone may contain continental crustal components in these stagnant slabs. Dense hydrous magmas may exist at the base of the upper mantle. Fluids or volatile-rich magmas may also exist at the top of the lower mantle due to the large contrast in water contents between the mineral assemblages in the mantle transition zone and the lower mantle, and the crossing of the convective descent of the cold hydrated materials. Dense magmas are not likely to be formed at the top of the lower mantle and hydrous magmas generated in this region move upwards and metasomatize the overlying mantle transition zone. Water can be transported deeper into the lower mantle by gravitational collapse of the stagnant slabs, which supply water into the lower mantle, including the core-mantle boundary. Hydrous δ-H solid solution may be the most important hydrous phase in lower mantle, and existence of this phase reduces the aluminum content in coexisting bridgmanite and post-perovskite, and thus modifies the physical properties of the lower mantle. Hydrous δ-H solid solution can accumulate at the base of the lower mantle. The iron-water reaction at the core-mantle boundary can also create pyrite-type FeOOH which can be a potential candidate material for the ultralow velocity zone (ULVZ). [ABSTRACT FROM AUTHOR]

Half‐space cooling and plate models of varying complexity have been proposed to account for changes in basement depth and heat flow as a function of lithospheric age in the oceanic realm. Here, we revisit this well‐known problem by exploiting a revised and augmented database of 2,028 measurements of depth to oceanic basement, corrected for sedimentary loading and variable crustal thickness, and 3,597 corrected heat flow measurements. Joint inverse modeling of both databases shows that the half‐space cooling model yields a mid‐oceanic axial temperature that is >100°C hotter than permitted by petrologic constraints. It also fails to produce the observed flattening at old ages. Then, we investigate a suite of increasingly complex plate models and conclude that the optimal model requires incorporation of experimentally determined temperature‐ and pressure‐dependent conductivity, expansivity, and specific heat capacity, as well as a low‐conductivity crustal layer. This revised model has a mantle potential temperature of 1300 ± 50°C, which honors independent geochemical constraints and has an initial ridge depth of 2.6 ± 0.3 km with a plate thickness of 135 ± 30 km. It predicts that the maximum depth of intraplate earthquakes is bounded by the 700°C isothermal contour, consistent with laboratory creep experiments on olivine aggregates. Estimates of the lithosphere‐asthenosphere boundary derived from studies of azimuthal anisotropy coincide with the 1175 ± 50°C isotherm. The model can be used to isolate residual depth and gravity anomalies generated by flexural and sub‐plate convective processes. Key Points: New global inventories of oceanic basement depths and heat flow are assembledAn objective assessment of thermal models with increasingly complex parameterizations is carried outOptimal thermal structure for oceanic lithosphere is identified, and its implications examined [ABSTRACT FROM AUTHOR]

Seismic multi-pathing effects, attributed to a contrast in seismic attenuation between the back-arc mantle wedge and subducted crust, are detected in central Honshu, northeastern Japan. We observe an initial broadened P-wave which is followed by a delayed higher frequency P-wave signal. Their discrepant frequencies are best explained by attenuation effects: delayed P-wave signals travel in the low-attenuation oceanic crust and therefore contain more high frequency components. The time separation between the initial broadened P-waves and the delayed P-wave signals are affected by the seismic velocity in the subducted oceanic crust. We observe systematic variation in the delay times of the later waves indicating an increase in seismic velocity in the oceanic crust (relative to the mantle wedge) at ∼130–150 km depth. High-frequency seismic simulations incorporating mineral-physics derived models show that a 4% Vp increase due to the blueschist decomposition and a 9% Vp increase associated with the (lawsonite, talc)–eclogite transition replicate the observed delay time variation. The blueschist breakdown may occur at a depth of ∼100 km and the (lawsonite, talc)–eclogite transition might be linked with the reduced seismicity level at depths greater than 150 km. Distinct from traditional guided waves, the multi-pathing effects in this study are mainly controlled by attenuation contrast and therefore may not require the oceanic crust to have low velocity and any special decoupling mechanism. The multi-pathing effects offer us another important tool to image subducted oceanic crust below back-arc mantle wedges, especially where guided waves are not observable. In this study, we demonstrate the value of observing and simulating high frequency seismic waves (>20 Hz) in advancing our understanding of subduction zones. [ABSTRACT FROM AUTHOR]

Abstract: Mapping the spatial distribution of water in the mantle transition zone (MTZ, 410‐ to 660‐km depth) may be approached by combining thermodynamic and experimental mineral physics data with regional studies of seismic velocity and seismic discontinuity structure. HyMaTZ (Hydrous Mantle Transition Zone) is a Python program with graphical user interface, which calculates and displays seismic velocities for different scenarios of hydration in the MTZ for comparison to global or regional seismic‐velocity models. The influence of water is applied through a regression to experimental data on how H2O influences the thermoelastic properties of (Mg,Fe)2SiO4 polymorphs: olivine, wadsleyite, and ringwoodite. Adiabatic temperature profiles are internally consistent with dry phase proportion models; however, modeling hydration in HyMaTZ affects only velocities and not phase proportions or discontinuity structure. For wadsleyite, adding 1.65 wt% H2O or increasing the iron content by 7 mol% leads to roughly equivalent reductions in VS as raising the temperature by 160 K with a pyrolite model in the upper part of the MTZ. The eastern U.S. low‐velocity anomaly, which has been interpreted as the result of dehydration of the Farallon slab in the top of the lower mantle, is consistent with hydration of wadsleyite to about 20% of its water storage capacity in the upper MTZ. Velocity gradients with depth in absolute shear velocity models are steeper in all seismic models than all mineralogical models, suggesting that the seismic velocity gradients should be lowered or varied with depth and/or an alternative compositional model is required. [ABSTRACT FROM AUTHOR]

Lower mantle tomography models consistently feature an increase in the ratio of shear-wave velocity ( V S ) to compressional-wave velocity ( V P ) variations and a negative correlation between shear-wave and bulk-sound velocity ( V C ) variations. These seismic characteristics, also observed in the recent SP12RTS model, have been interpreted to be indicative of large-scale chemical variations. Other explanations, such as the lower mantle post-perovskite (pPv) phase, which would not require chemical heterogeneity, have been explored less. Constraining the origin of these seismic features is important, as geodynamic simulations predict a fundamentally different style of mantle convection under both scenarios. Here, we investigate to what extent the presence of pPv explains the observed high V S / V P ratios and negative V S – V C correlation globally. We compare the statistical properties of SP12RTS with the statistics of synthetic tomography models, derived from both thermal and thermochemical models of 3-D global mantle convection. We convert the temperature fields of these models into seismic velocity structures using mineral physics lookup tables with and without pPv. We account for the limited tomographic resolution of SP12RTS using its resolution operator for both V S and V P structures. This allows for direct comparisons of the resulting velocity ratios and correlations. Although the tomographic filtering significantly affects the synthetic tomography images, we demonstrate that the effect of pPv remains evident in the ratios and correlations of seismic velocities. We find that lateral variations in the presence of pPv have a dominant influence on the V S / V P ratio and V S – V C correlation, which are thus unsuitable measures to constrain the presence of large-scale chemical variations in the lowermost mantle. To explain the decrease in the V S / V P ratio of SP12RTS close to the CMB, our results favour a pPv-bearing CMB region, which has implications for the stability field of pPv in the Earth's mantle. [ABSTRACT FROM AUTHOR]

We investigate the role of composition on the viscosity of the lower crust through a joint inversion of seismic P-wave ( V p ) and S-wave ( V s ) velocities. We determine the efficacy of using seismic velocity to constrain viscosity, extending previous research demonstrating robust relationships between seismic velocity and crustal composition, as well as crustal composition and viscosity. First, we calculate equilibrium mineral assemblages and seismic velocities for a global compilation of crustal rocks at relevant pressures and temperatures. Second, we use a rheological mixing model that incorporates single-phase flow laws for major crust-forming minerals to calculate aggregate viscosity from predicted mineral assemblages. We find a robust correlation between crustal viscosity and V p together with V s in the α -quartz regime. Using seismic data, geodetic surface strain rates, and heat flow measurements from Southern California, our method predicts that lower crustal viscosity varies regionally by four orders of magnitude, and lower crustal stress varies by three orders of magnitude at 25 km depth. At least half of the total variability in stress can be attributed to composition, implying that regional lithology has a significant effect on lower crustal geodynamics. Finally, we use our method to predict the depth of the brittle–ductile transition and compare this to regional variations of the seismic–aseismic transition. The variations in the seismic–aseismic transition are not explained by the variations in our model rheology inferred from the geophysical observations. Thus, we conclude that fabric development, in conjunction with compositional variations (i.e., quartz and mica content), is required to explain the regional changes in the seismic–aseismic transition. [ABSTRACT FROM AUTHOR]

We analyze teleseismic P-to-S conversions for high-resolution imaging of the mantle transition zone beneath the Colorado Rocky Mountains using data from a dense PASSCAL seismic broadband deployment. A total of 6,021 P-to-S converted receiver functions are constructed using a multi-channel minimum-phase deconvolution method and migrated using the common converted point technique with the 3-D teleseismic P- and S-wave tomography models of Schmandt and Humphreys (2010) . The image finds that the average depths of the 410-km discontinuity (the 410) and 660-km discontinuity (the 660) at 408 ± 1.9 km and 649 ± 1.6 km respectively. The peak-to-peak topography of both discontinuities is 33 km and 27 km respectively. Additionally, prominent negative polarity phases are imaged both above and below the 410. To quantify the mean properties of the low-velocity layers about 410 km, we utilize double gradient layer models parameterization to fit the mean receiver function waveform. This waveform fitting is accomplished as a grid-search using anelastic synthetic seismograms. The best-fitting model reveals that the olivine–wadsleyite phase transformation width is 21 km, which is significantly larger than anhydrous mineral physics prediction (4–10 km) ( Smyth and Frost, 2002 ). The findings of a wide olivine–wadsleyite phase transformation and the negative polarity phases above and below the 410, suggest that the mantle, at least in the 350–450 km depth range, is significantly hydrated. Furthermore, a conspicuous negative polarity phase below the 660 is imaged in high velocity region, we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling. Our interpretation of these results, in tandem with the tomographic image of a Farallon slab segment at 800 km beneath the region ( Schmandt and Humphreys, 2010 ), is that hydrous and upwelling mantle contributes to the high-standing Colorado Rocky Mountains. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we discuss the occurrence of liebermannite (IMA 2013‐128), KAlSi3O8, a new, shock‐generated, high‐pressure tetragonal hollandite‐type structure silicate mineral, in the Zagami basaltic shergottite meteorite. Liebermannite crystallizes in space group I4/m with Z = 2, cell dimensions of a = 9.15 ± 0.14 (1σ) Å, c = 2.74 ± 0.13 Å, and a cell volume of 229 ± 19 Å3 (for the type material), as revealed by synchrotron diffraction. In Zagami, liebermannite likely formed via solid‐state transformation of primary igneous K‐feldspar during an impact event that achieved pressures of ~20 GPa or more. The mineral name is in honor of Robert C. Liebermann, a high‐pressure mineral physicist at Stony Brook University, New York, USA. [ABSTRACT FROM AUTHOR]

For the past half century, I have been fortunate in maintaining collaborations with Czech scientists in the Czech Republic (formerly Czechoslovakia) from the Geofyzikálníústav-GFU (Institute of Geophysics) of the Československá Akademie Věd-ČSAV (Czechoslovak Academy of Sciences). These collaborations have included my exchange visits to Prague (Praha) and convening international workshops in 1976, 1986 and 1996 in castles used by the ČSAV as well as visits by Czech colleagues to Stony Brook University. This tribute is in memory of my dear friend and colleague Vladislav Babuška. [ABSTRACT FROM AUTHOR]

From 1964 to the early 1970s, Orson Anderson led a research program at the Lamont Geological Observatory in the newly-emerging field of “mineral physics”. In collaboration with colleagues Edward Schreiber and Naohiro Soga, Orson exploited the techniques of physical acoustics to study the behavior of the sound velocities of minerals at elevated pressures and temperatures. This research program also included semi-empirical studies of the relationships between the bulk modulus and the molar volume of solids, the use of lattice dynamics to calculate the elastic moduli of cubic structures as a function of pressure to predict instabilities, and theoretical investigations of the Lagrangian and Eulerian formulations of finite strain equations of state.

Evidence from seismology, mineral physics, and core dynamics suggests a layer with an overall stable stratification in the Earth’s outer core, possibly thermal in origin, extending below the core-mantle boundary (CMB) for several hundred kilometers. Yet vigorous deep mantle convection with locally elevated heat flux implies locally unstable thermal stratification below the CMB, consistent with interpretations of non-dipole geomagnetic field behavior that favor upwelling flows in places below the CMB. To resolve this apparent inconsistency, we investigate the structure of convection and magnetic fields in the core using numerical dynamos with laterally heterogeneous boundary heat flux. Strongly heterogeneous boundary heat flux generates localized convection beneath the CMB that coexists with an overall stable stratification there. Our partially stratified dynamos are distinguished by their time average magnetic field structures. Without stratification or with stratification confined to a thin layer, the octupole component is small and the CMB magnetic field structure includes polar intensity minima. With more extensive stratification, the octupole component is large and the magnetic field structure includes intense patches or high intensity lobes in the polar regions. Comparisons with the time-averaged geomagnetic field are generally favorable for partial stratification in a thin (<400 km) layer but unfavorable for stratification in a thick (∼1000 km) layer beneath the CMB. [ABSTRACT FROM AUTHOR]

Computations have helped elucidate the dynamics of Earth's mantle for several decades already. The numerical methods that underlie these simulations have greatly evolved within this time span, and today include dynamically changing and adaptively refined meshes, sophisticated and efficient solvers, and parallelization to large clusters of computers. At the same time, many of the methods - discussed in detail in a previous paper in this series - were developed and tested primarily using model problems that lack many of the complexities that are common to the realistic models our community wants to solve today. With several years of experience solving complex and realistic models, we here revisit some of the algorithm designs of the earlier paper and discuss the incorporation of more complex physics. In particular, we re-consider time stepping and mesh refinement algorithms, evaluate approaches to incorporate compressibility, and discuss dealing with strongly varying material coefficients, latent heat, and how to track chemical compositions and heterogeneities. Taken together and implemented in a high-performance, massively parallel code, the techniques discussed in this paper then allow for high resolution, 3-D, compressible, global mantle convection simulations with phase transitions, strongly temperature dependent viscosity and realistic material properties based on mineral physics data. [ABSTRACT FROM AUTHOR]

Unravelling the thermal structure of the outermost shell of our planet is key for understanding its evolution. We obtain temperatures from interpretation of global shear-velocity (VS) models. Long-wavelength thermal structure is well determined by seismic models and only slightly affected by compositional effects and uncertainties in mineral-physics properties. Absolute temperatures and gradients with depth, however, are not well constrained. Adding constraints from petrology, heat-flow observations and thermal evolution of oceanic lithosphere helps to better estimate absolute temperatures in the top part of the lithosphere. We produce global thermal models of the lithosphere at different spatial resolution, up to spherical-harmonics degree 24, and provide estimated standard deviations. All relevant physical properties, with the exception of thermal conductivity, are based on a self-consistent thermodynamical modelling approach. Our global thermal models also include density and compressional-wave velocities (VP) as obtained either assuming no lateral variations in composition or a simple reference 3-D compositional structure, which takes into account a chemically depleted continental lithosphere. The global thermal models should serve as the basis to move at a smaller spatial scale, where additional thermo-chemical variations required by geophysical observations can be included. [ABSTRACT FROM AUTHOR]