Iron may critically influence the physical properties and thermochemical structures of Earth’s lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and ...mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8–10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.
Seismic tomography reveals 2 extensive regions in the lowermost mantle, beneath Africa and the Pacific, that exhibit lower-than-average seismic wave speeds. These regions have been named the Large ...Low Shear Velocity Provinces (LLSVPs), and they have spatial scales on the order of 1000s and 100s of kilometers in width and height, respectively. Discovering their cause remains an important challenge in the deep Earth community because recognizing what they are has important, first-order implications toward understanding the nature of global mantle convection and therefore, heat and chemical transport and evolution through time. At about an order-of-magnitude smaller scale are Ultra Low Velocity Zones (ULVZs) that reside on the core-mantle boundary. ULVZs are typically up to 100s of kilometers laterally and only 10s of kilometers vertically. We don't know what LLSVPs and ULVZs are, and the primary question is whether they are thermal or compositional features, and/or both. In any case, there is every reason to suppose that they are linked by a dynamical relationship, and by better understanding one, we can discover more about the other. Here, we review the observations associated with LLSVPs and the various conceptual mantle models that the community is debating regarding their cause. ULVZs, as they may relate to the larger LLSVPs, are also reviewed, and dynamical linkages between the two are discussed. Better understanding both LLSVPs and ULVZs promises to provide critical insight into global scale mantle convection and therefore provide a foundation for understanding numerous other processes in the Earth's interior.
•A review of observations regarding Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle•A review of conceptual hypotheses regarding the cause of Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle•An overview of Ultra Low Velocity Zone (ULVZ) observations and ULVZs may be dynamically connected to LLSVPs.
The sound speed of seawater is not constant. It varies with season, day time, depth and range. This variation was not considered in marine seismic data processing and imaging before deep-water ...seismic acquisition became a routine activity. As a result, non-ignorable errors may be contained in the final migrated-image of deep-water seismic data. To eliminate such errors, we propose here a scheme for inverting the seawater velocity under the condition that the seabed has a complex topography. In this scheme, the seabed topography is represented by step grids constrained by measured water depth data, and the sound profile is assumed to be the one satisfying the Munk formula. Thus, only the parameters appearing in the Munk formula need to be inverted. This is quite different from the conventional inversion schemes that take the velocity as the inversion target. We use the conjugate gradient method to minimize the objective function given as the sum of squares of traveltime differences. Tests on synthetic common-shot data set show that our scheme presented here works as expected. Applications to real marine data set further validates the reasonability and feasibility of our scheme under complex seabed topography. To investigate the reliability of our inversion scheme, we migrate the real 2D data set using the Kirchhoff migration with the inverted Munk profile as the velocity model. The corresponding results show that the seawater velocity obtained by our inversion scheme improves the imaging quality of underwater structures.
The effects of partial melting on seismic velocity and attenuation have long been studied by focusing on the direct effects of melt, such as the poroelastic effect. The direct effects are generally ...very small for a very small melt fraction. Because geochemical studies have shown that the melt fraction during partial melting is very small (∼0.1%), it is difficult to explain upper-mantle low-velocity regions by the direct effects of melt. Recent experimental studies, by using a rock analog, have captured a significant enhancement of polycrystal anelasticity just before partial melting in the absence of melt. This newly recognized effect enables us to interpret seismological and geochemical observations consistently. The new anelasticity model significantly changes the interpretation of upper-mantle seismic structures. This review summarizes the recent progress in the understanding of polycrystal anelasticity, starting from a basic knowledge of linear anelasticity.
Dense hydrous magnesium silicate (DHMS) phase E is a potential water carrier in subducting slabs that can transport water to the Earth's deep mantle between the bottom of the upper mantle and the ...uppermost transition zone. Therefore, knowledge on the high pressure‒temperature (P‒T) full elastic moduli of phase E at relevant mantle conditions is important in deciphering the existence of DHMS phases and their influences on seismic profiles in the region; however, the high P‒T elasticity data of phase E still remains lacking. In this work, we determined the combined effect of P‒T on the single‐crystal elasticity of phase E up to 24 GPa and 900 K by in situ X‐ray diffraction and Brillouin scattering measurements in externally‐heated diamond anvil cells. The aggregate elastic moduli and compressional‐wave (VP) and shear‐wave (VS) velocities of phase E are then derived by analyzing the single‐crystal elasticity and density data using the third‐order finite‐strain equations. We found that phase E exhibits much lower bulk and shear moduli and acoustic velocities than the most abundant constituent minerals in the upper mantle and transition zone, such as olivine, clinopyroxene, garnet, and wadsleyite. The modeled results using the obtained elasticity results show that the existence of phase E in a hydrated pyrolite model can result in relatively lower Vp and Vs profiles and negative velocity anomalies in seismic observations. The existence of phase E with relatively lower velocity profiles could be a possible origin of the low‐velocity layers atop the 410‐km discontinuity in some cold and highly‐hydrated regions.
Plain Language Summary
Deep‐mantle water storage and circulation remains one of the most intriguing issues in geoscience. Serpentine and its high pressure‒temperature (P‒T) phases, namely high‐density magnesium silicates (phases A, superhydrous B, D, E, and H), are considered to be the dominant potential water carriers in subduction zones. Hence, their sound velocities and density at high P‒T conditions are of particular importance for interpreting seismic observations and understanding water circulation and geodynamic processes in subduction‐related environments. In this study, we report new experimental results on the high P‒T single‐crystal elasticity of phase E up to 24 GPa and 900 K obtained by in situ synchrotron X‐ray diffraction and Brillouin scattering measurements. The single‐crystal elasticity data of phase E are used to derive its aggregate sound velocities and build mantle velocity profiles for dry and hydrated pyrolite models. We found that the existence of phase E in a hydrated pyrolite model can result in relatively lower compressional‐wave and shear‐wave velocity profiles and negative velocity anomalies in seismic observations. This finding helps explain the origin of the low‐velocity layers atop the 410‐km discontinuity in some highly‐hydrated regions.
Key Points
Single‐crystal elasticity of the Mg‐endmember phase E has been measured at high pressure and temperature conditions
The aggregate sound velocities of phase E are much lower than those of the typical minerals in the upper mantle and mantle transition zone
The existence of phase E could contribute to the origin of the low‐velocity layers atop the 410 km discontinuity in some hydrated regions
Nitrogen Content in the Earth's Outer Core Bajgain, Suraj K.; Mookherjee, Mainak; Dasgupta, Rajdeep ...
Geophysical research letters,
16 January 2019, Letnik:
46, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Using first principles molecular dynamic simulations, we explore the effects of nitrogen (N) on the density and sound velocity of liquid iron and evaluate its potential as a light element in the ...Earth's outer core. Our results suggest that Fe‐N melt cannot simultaneously explain the density and seismic velocity of the Earth's outer core. Although ~2.0 wt.% N can explain the bulk sound velocity of the outer core, such N content only lowers the density of liquid Fe by ~3%. Matching both the velocity and density by the other light elements limits the N in the core to ≪2.0 wt.%. Our finding suggests that nitrogen is a minor to trace element in the Earth's core and is consistent with the geochemical mass balance with terrestrial abundance of N and alloy‐silicate partitioning data, which suggest that there cannot be significant N in the core.
Plain Language Summary
Physical properties of liquid iron cannot explain the seismological observations of density and sound wave velocities in the liquid outer core. In this study we explore the effect of light element nitrogen on the physical properties of liquid iron and provide an upper bound of the amount of nitrogen in the outer core if nitrogen were the sole light element.
Key Points
Nitrogen decreases density and enhances bulk sound velocity of liquid iron
We provide an upper bound of nitrogen in the liquid outer core
The amount of the light elements required to explain density and velocity correlates with the atomic number of the light elements
Single‐crystal elasticity of both α‐ and β‐orthopyroxene was determined up to 20 GPa and 300 K by Brillouin scattering. Using the derived full elastic moduli (Cij), we investigated the contribution ...of the metastable pyroxene to the seismically observed 3%–5% low‐velocity anomalies along the subducting slab in the top transition zone. Our modeled results show that a harzburgite wedge with a 1000‐K colder geotherm and metastable α‐orthopyroxene and olivine displays compressional (VP) and shear‐wave (VS) velocities 3.0%–3.6(6)% and 2.0%–2.8(6)% lower than the surrounding mantle at 410–460 km depth, respectively. At deeper depth up to 520 km, VP and VS of this metastable wedge with β‐orthopyroxene and olivine are 3.6%–4.4(6)% and 2.8%–4.3(6)% lower than the pyrolitic mantle, respectively. The presence of both metastable orthopyroxene and olivine instead of metastable olivine alone helps better explain the origin of the low‐velocity anomalies within the subduction slab in the top transition zone.
Plain Language Summary
Subducting slab plays a significant role in transportation the surface material to the Earth's deep interior. It is normally imaged as a high‐velocity body compared to the surrounding mantle. However, seismic studies detected the existence of 3%–5% low compressional‐wave velocity anomalies accompanied with strong seismic shear‐wave anisotropies within the subducting slab at the top transition zone in various locations of the Earth which cannot be explained by the presence of metastable olivine alone. Besides olivine, orthopyroxene could also remain metastable in the coldest harzburgite layer of the slab. Here we report experimental results on the single‐crystal elasticity of both α‐ and β‐orthopyroxene up to 20 GPa and 300 K. These experimental results allow us to provide a comprehensive evaluation on the velocity profiles of the coldest harzburgite layer of the slab. We found that the coldest harzburgite layer with 22–37 vol.% orthopyroxene and 62–78 vol.% olivine has the VP and VS 3.0%–4.4(6)% and 2.0%–4.5(6)% lower than those of the pyrolitic mantle in the mantle transition zone, respectively. The observed 3%–5% low‐velocity anomalies within the slab in the top transition zone should be explained by the metastable orthopyroxene and olivine instead of metastable olivine alone.
Key Points
Single‐crystal elasticity of α‐ and β‐orthopyroxene was determined to 20 GPa and shows anomalous change across the phase transition
The obtained results were used to model the velocity profiles of the coldest harzburgite layer of the sinking subduction slab
We find that the low‐velocity anomalies within the slab at the top transition zone should be caused by metastable orthopyroxene and olivine
The crystal orientation fabric (COF) in ice cores provides detailed information, such as grain size and distribution and the orientation of the crystals in relation to the large-scale glacier flow. ...These data are relevant for a profound understanding of the dynamics and deformation history of glaciers and ice sheets. The intrinsic, mechanical anisotropy of the ice crystals causes an anisotropy of the polycrystalline ice of glaciers and affects the velocity of acoustic waves propagating through the ice. Here, we employ such acoustic waves to obtain the seismic anisotropy of ice core samples and compare the results with calculated acoustic velocities derived from COF analyses. These samples originate from an ice core from Rhonegletscher (Rhone Glacier), a temperate glacier in the Swiss Alps. Point-contact transducers transmit ultrasonic P waves with a dominant frequency of 1 MHz into the ice core samples and measure variations in the travel times of these waves for a set of azimuthal angles. In addition, the elasticity tensor is obtained from laboratory-measured COF, and we calculate the associated seismic velocities. We compare these COF-derived velocity profiles with the measured ultrasonic profiles. Especially in the presence of large ice grains, these two methods show significantly different velocities since the ultrasonic measurements examine a limited volume of the ice core, whereas the COF-derived velocities are integrated over larger parts of the core. This discrepancy between the ultrasonic and COF-derived profiles decreases with an increasing number of grains that are available within the sampling volume, and both methods provide consistent results in the presence of a similar amount of grains. We also explore the limitations of ultrasonic measurements and provide suggestions for improving their results. These ultrasonic measurements could be employed continuously along the ice cores. They are suitable to support the COF analyses by bridging the gaps between discrete measurements since these ultrasonic measurements can be acquired within minutes and do not require an extensive preparation of ice samples when using point-contact transducers.
•NSFnets involve the VP and VV formulations of the Navier-Stokes equations.•NSFnets can directly simulate and sustain turbulence at Reτ∼1,000.•A study on the weights in the loss function to enhance ...accuracy is performed.•Transfer learning of NSFnets can reduce computational cost and enhance accuracy.•NSFnets can solve the ill-posed or inverse problems better than CFD solvers.
In the last 50 years there has been a tremendous progress in solving numerically the Navier-Stokes equations using finite differences, finite elements, spectral, and even meshless methods. Yet, in many real cases, we still cannot incorporate seamlessly (multi-fidelity) data into existing algorithms, and for industrial-complexity applications the mesh generation is time consuming and still an art. Moreover, solving ill-posed problems (e.g., lacking boundary conditions) or inverse problems is often prohibitively expensive and requires different formulations and new computer codes. Here, we employ physics-informed neural networks (PINNs), encoding the governing equations directly into the deep neural network via automatic differentiation, to overcome some of the aforementioned limitations for simulating incompressible laminar and turbulent flows. We develop the Navier-Stokes flow nets (NSFnets) by considering two different mathematical formulations of the Navier-Stokes equations: the velocity-pressure (VP) formulation and the vorticity-velocity (VV) formulation. Since this is a new approach, we first select some standard benchmark problems to assess the accuracy, convergence rate, computational cost and flexibility of NSFnets; analytical solutions and direct numerical simulation (DNS) databases provide proper initial and boundary conditions for the NSFnet simulations. The spatial and temporal coordinates are the inputs of the NSFnets, while the instantaneous velocity and pressure fields are the outputs for the VP-NSFnet, and the instantaneous velocity and vorticity fields are the outputs for the VV-NSFnet. This is unsupervised learning and, hence, no labeled data are required beyond boundary and initial conditions and the fluid properties. The residuals of the VP or VV governing equations, together with the initial and boundary conditions, are embedded into the loss function of the NSFnets. No data is provided for the pressure to the VP-NSFnet, which is a hidden state and is obtained via the incompressibility constraint without extra computational cost. Unlike the traditional numerical methods, NSFnets inherit the properties of neural networks (NNs), hence the total error is composed of the approximation, the optimization, and the generalization errors. Here, we empirically attempt to quantify these errors by varying the sampling (“residual”) points, the iterative solvers, and the size of the NN architecture. For the laminar flow solutions, we show that both the VP and the VV formulations are comparable in accuracy but their best performance corresponds to different NN architectures. The initial convergence rate is fast but the error eventually saturates to a plateau due to the dominance of the optimization error. For the turbulent channel flow, we show that NSFnets can sustain turbulence at Reτ∼1,000, but due to expensive training we only consider part of the channel domain and enforce velocity boundary conditions on the subdomain boundaries provided by the DNS data base. We also perform a systematic study on the weights used in the loss function for balancing the data and physics components, and investigate a new way of computing the weights dynamically to accelerate training and enhance accuracy. In the last part, we demonstrate how NSFnets should be used in practice, namely for ill-posed problems with incomplete or noisy boundary conditions as well as for inverse problems. We obtain reasonably accurate solutions for such cases as well without the need to change the NSFnets and at the same computational cost as in the forward well-posed problems. We also present a simple example of transfer learning that will aid in accelerating the training of NSFnets for different parameter settings.
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