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Talk Abstracts |
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Stony Brook VLab Research -- Progress and Plans
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Philip B. Allen |
We use theory to examine physical properties of iron-magnesium silicate rocks under
conditions found in the Earth's mantle. Our initial studies focus on the simplest such mineral, MgO, and make use of its measured properties under ambient conditions to test the theory intended for high P and T. The properties of particular interest to us are the magnetic state of Fe, and the heat conduction process, both by electromagnetic radiation and by lattice vibration.
A graduate student, Ryan Requist, uses electronic structure theory, in a local basis. He collaborates with Mark Pederson and Tunna Baruah at Naval Research Lab, who have a density functional cluster code. He also collaborates with Jim Muckerman at Brookhaven National Lab on quantum chemical (configuration interaction) calculations for the same clusters. An iron impurity is at the center of a cluster of MgO, embedded in the potential of the surrounding infinite matrix. The iron can be adjusted to various charge states. In charge state Q=+2, the d6 configuration has S=2 (high spin) at low pressures, and is a Jahn-Teller system. At high pressures, the spin collapses to S=0, which is not a Jahn-Teller system.
Another graduate student, Tao Sun, works on heat transport. He is exploiting the availability of good samples and of measured optical properties to study the conversion of photons into pairs of phonons. This process dominates in the infrared, and should be the limiting factor in the low temperature radiative thermal conductivity. At higher frequencies, more than two phonons are needed. Tao will estimate the strengths of these processes. He will also collaborate with Ryan Requist to study the processes of light absorption assisted by iron impurities. Tao is supervising two undergraduates. One of these is doing infrared transmission measurements on MgO, and the other is doing a classical molecular dynamics simulation of vibrational heat transport.
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Elasticity of Deep-Earth Materials at High P and T: Implication for Earths Lower Mantle
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Jay Bass1,2 |
Brillouin spectroscopy allows measurements of sound velocities and elasticity
on phases of geophysical interest at high pressures and temperatures.
This technique was used to measure the properties of numerous
important phases of Earths deep interior, such as Mg-perovskite,
aluminous perovskite, magnesiowustite, majorite garnets, and other.
Emphasis is now on measurements at elevated P-T conditions, and
measurements on dense polycrystals. Measurements to 60 GPa were made
using diamond anvil cells. High temperature is achieved by electrical
resistance and laser heating. Excellent results are obtained for
polycrystalline samples of dense oxides such as silicate spinels, and
(Mg,Al)(Si,Al)O3 –perovskites. A wide range of materials can now be
characterized. We have now interfaced a Brillouin spectrometer with
synchrotron radiation for the simultaneous determination of sound
velocities (by Brillouin) and density (by x-rays). Absolute pressures
can thus be measured allowing refinement of pressure scales for high
pressure experimentation.These and other results were used to infer
Earths average lower mantle composition and thermal structure by
comparing mineral properties at lower mantle P-T conditions to global
Earth models. A formal inversion procedure was used. Inversions of
density and bulk sound velocity do not provide robust compositional
and thermal models. Including shear properties in the inversions is
important to obtain unique solutions. We discuss the range of models
consistent with present lab results, and data needed to further
refine lower mantle models.
In collaboration with S.V. Sinogeikin1, Estelle
Mattern2, J.M. Jackson1, D. Lakshtanov1,
J. Matas2, J. Wang1, Y. Ricard2
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First Principles Study of the MgSiO3-FeSiO3-Al2O3 System at High-Pressure |
Razvan Caracas |
Using first-principles calculations we predict the effects of composition on perovskite-post-perovskite phase relation. The transition is predicted at 110 GPa for pure MgSiO3. The addition of Al2O3 slightly increases this transition pressure, and the addition of Fe2+ considerably reduces it; the FeSiO3 end-member term is stable in the post-perovskite modification with respect to perovskite at all pressures. We also determine the static equations of state, densities, elasticity and seismic wave velocities. At the transition Vp increases slightly, and Vs increases significantly, consistent with the seismic observations for D~T. The addition of both Fe2+ and Al2O3 decrease the seismic wave velocities.
In collaboration with Ronald E Cohen |
European Grid Middleware for VLAB |
Stefano Cozzini |
In this talk I will present the actual state of the European middleware for grid computing developed within the most important European projects. I am exploring ways to apply these technologies can be used in the Vlab computational GRID.
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Improved Density Functionals for Water |
Erin E. Dahlke |
The accuracy of existing density functional methods for describing the noncovalent interaction energies in small water clusters is investigated by testing twenty-five density functionals against a data set of 28 water dimers and 8 water trimers whose structures are taken from the literature and from simulations. The most accurate functionals are found to be PW6B95 with a mean unsigned error of 0.13 kcal/mol and MPWB1K and B98 with mean unsigned errors of 0.15 kcal/mol; the best functional with no Hartree-Fock exchange is mPWLYP, which is a GGA with a mean unsigned error of 0.28 kcal/mol. In comparison, the most popular GGA functionals, PBE and BLYP, have mean unsigned errors of 0.52 and 1.03 kcal/mol respectively. Since GGAs are very cost efficient for both condensed-phase simulations and electronic structure calculations on large systems, we optimized four new GGAs for water. The best of these, PBE1W and MPWLYP1W, have mean unsigned errors of 0.12 and 0.17 kcal/mol respectively. These new functionals are well suited for use in condensed phase simulations of water and ice.
In collaboration with Don Truhlar
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Going beyond local density and gradient corrected XC functionals in Quantum-ESPRESSO |
Stefano de Gironcoli |
Density Functional Theory is, in principle, an exact theory of matter in its electronic ground state. The widespread success of local density and gradient corrected XC functionals in predicting material properties have changed the way computer simulations are used in order to understand material behavior at the atomistic level, and contributed in an essential way to the diffusion of first-principles methods among non-specialists.
These "traditional"functionals are however not without shortcomings and more accurate functionals are needed in a number of cases. Lack of complete self-interaction cancellation and inability to describe dispersion(van der Waals) interactions are among the main failures one must cope with
In order to overcome the first shortcoming, scientists in the physics and chemistry communities uses, beside Hartree-Fock, B3LYP, PBE0, and more recent and fancy functionals that include a certain percentage of HF. From a more fundamental DFT point of view the EXact-eXchange DFT scheme also requires HF matrix elements. These functionals are computationally very demanding to be applied in a periodic (plane-wave) formalism, but the progress in available computer power makes increasingly possible to afford for them.
Although this work is still in progress, I will report on the current effort going on at the DEMOCRITOS National Simulation Centre in Trieste in order to implement Hartee-Fock related density functionals in the Quantum-ESPRESSO package.
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Seismic Imaging of the Earth's Deep Interior |
Adam Dziewonski |
During the last quarter of the century the data sources and computer resources became sufficient to attempt reconstruction of three-dimensional anomalies of seismic wave-speeds in the Earth's interior, reaching all the way down to the center of the Earth.
The initial studies revealed for the first time very large-scale low-velocity structures at great depth: they reach to the core-mantle boundary and have lateral dimensions of up to 5,000 km and are 1,000-1,500 km tall. Their origin -- compositional or thermal -- is unknown, although the prevailing opinion is that composition is an mportant element in their origin. These two slow mega-structures (Great African Plume and Equatorial Pacific Plume Group) are surrounded by faster than average mantle in a pattern that is similar to the geometry of plate subduction over the last 200 million years. Another, planetary scale, result is that there is an abrupt, or very rapid, change in the power spectrum of the heterogeneities at the boundary between the upper and lower mantle (~ 650 km depth), indicating an impedance to flow between the two regions.
More detailed, regional, studies led to images that may be interpreted as penetration of the subducted slabs into the lower manle, thus indicating the whole mantle rather than layered convection. The difficulty with interpreting the seismic images is that they are snapshots of the Earth properties at the present time. Their interpretation requires extrapolation, which always carries some risk.
The 3-D models of seismic properties of the mantle require multi-disciplinary collaboration: with mineral physicists to understand how to interpret them in terms of temperature or compositional variations. With geodynamicists, who model flow in the mantle, to infer how these properties might have changed in time. With geochemists, who collect samples on the Earth's surface but are capable of determinig the time at which they have been formed, to establish how these variable properties affected the chemical evolution of our planet.
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A Framework for VLab |
Gordon Erlebacher |
In this talk I will present our implementation of WEBIS, which is an application of Naradabrokering, a middleware API based on the publish/subscribe paradigm. Our architecture permit arbitrary clients to submit tasks (visualization, computation, analysis) to one or several servers and retrieve the results, without knowledge of the available resources. This architecture is fault tolerant, scalable, and supports multiple protocols. Most importantly, it allows collaboration strategies to be incorporated in a natural way. We will discuss the possible usage patterns of our system based on web services, and its future
evolution towards a collaborative VLab.
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Data Management within the UK E-science |
Adil Hasan |
The e-science Data Management Group provides an improved framework for data management, curation and preservation for CCLRC and the UK e-Science Community by engaging in leading edge research into metadata, data access technologies, security architecture and various aspects of data curation. The group also applies and deploys proven tools in collaborative research projects and provides a range of services both for CCLRC and the UK Research Community. In this talk we describe the areas within data management and curation that the group concentrates on and present a number of applications and approaches deployed by the group in a range of e-science projects.
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Connecting the Dots: Synthesizing Experiments, Observations, and Processes in the Deep Earth
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John Hernlund |
Earth's deep interior exhibits a mystifying range of features which have yet to be explained within a unified framework analogous to plate tectonics at Earth's surface. Proceeding a bit further with this analogy, it might be might be said that we need a "plate tectonic" paradigm for the deep Earth which connects a large variety of seemingly disparate facts, providing a link between observations and process. To carry out such a task, it is first necessary to map observations into physically meaningful descriptions of the present state of the deep Earth. Thus mineral physics plays a central role in forging ahead, mostly because direct interaction with these environments is not possible. Additionally, mineral physics plays a critical role in helping to guide our knowledge of what kinds of processes may occur, through knowledge of rheology, chemical equilibria, equations of state, etc.. In this talk, I'll focus on some of the first-order features of the deep Earth and explore some of the ways in which they might be connected to first-order processes, with an emphasis upon how mineral physics can help distinguish between the various alternatives.
In collaboration with Paul Tackley.
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Hydrous Melting in the Mantle |
Marc M. Hirschmann |
The distribution of H2O in the mantle is both an expression and an influence on the dynamics of mantle convection, but the total H2O stored in Earth's mantle remains highly uncertain. Estimates vary by approximately an order of magnitude, from less than a quarter of the mass of H2O in the world's oceans to ~4 ocean masses. Water in the mantle is stored chiefly as a trace substituent in nominally anhydrous minerals (NAMs) and in some special circumstances, in dense hydrous magnesium silicates (DHMS). A key process governing the distribution of H2O in the mantle is hydrous melting, which occurs owing to variations in H2O storage capacity of NAMs as a function of temperature, pressure, and mantle mineralogy. Such melting may occur in a variety of settings in Earth's mantle, including in oceanic basalt sources and in deeper regions above and below the transition zone. The 50-200 ppm H2O in the upper mantle likely derives from a blend of sources that may include residues of hydrous partial melting, either in the deep mantle and/or beneath arcs or oceanic islands. Relative to the large storage capacity in the transition zone, low storage capacities above and below may lead to hydrous melting for material upwelling through 410 km or downwelling through 670 km.
The apparently very low storage capacity of the lower mantle (<20 ppm H2O) may force melting even if downwelling rocks have normal upper mantle H2O (50-200 ppm) concentrations. Very low storage capacity in the lower mantle, if verified experimentally, presents a challenge to the view that the H2O -rich sources ofoceanic island basalts reside in the lower mantle. Current research of hydrous melting in the mantle is proceeding along several fronts. Studies of geochemistry of erupted basalts place
considerable constraints
on H2O concentrations in different reservoirs in the mantle and on the behavior of H2O during melting. Experimental determinations of H2O storage capacities of NAMs at appropriate temperatures and pressures are fundamental inputs required to determine the locus and consequences of such melting and though available constraints have increased greatly in recent years, many uncertainties persist. Theoretical understanding of the thermodynamics of H2O substitution in relevant minerals and melts remains very limited, in large part because the solution properties of H2O in silicate melts are not known well at very high pressure. However, recent studies have allowed thermodynamic prediction of hydrous melting at conditions prevailing in basalt source regions (<150 km). The challenge is to extend such models to conditions where deeper melting may be
occurring.
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A Visualization Approach to Understanding Minerals Properties |
Bijaya B. Karki |
In recent years, large amounts of data related to the structural, electronic and mechanical properties of minerals (in their solid and liquid phases) are routinely produced by massively parallel computer simulations. Such simulations are based on approaches ranging from the empirical molecular dynamics (MD) to the sophisticated quantum mechanical density functional theory (DFT). The resulting massive data sets are often time-dependent and three-dimensional in the nature. Gaining insight into these data is, however, a non-trivial task. Recently, we have initiated to adopt a visualization approach to facilitate understanding of various data related to geophysically relevant minerals such as silicates and oxides. In this talk, I will present our current progress in this endeavor by considering some visualization case studies. These deal with the multivariate elasticity data, electronic density distributions and MD-produced atomic data.
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Selenelogical Tomography - Inferring the Composition of the Moon from the Apollo Lunar Seismic Data, Mass and Moment of Inertia
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Amir Khan |
The internal structure and composition of a planet or satellite are important constraints on theories for how such bodies formed and evolved. Of all geophysical methods used to study a planet's structure, seismology is uniquely suited to determine many of the parameters that are critically important to understand the dynamic behaviour of the planet. For this reason seismology has played a leading role in the study of the internal structure of the Earth. The only other solar system body from which we have seismic observations pertinent to its interior properties is the Moon, thus giving us an opportunity to examine planetary formation in general without being tied to the Earth. Issues that, in principle, can be adressed geophysically and which hold the potential of providing constraints on lunar formation and evolution, include the question of whether the Moon has a metallic iron core, the depth of differentiation needed to produce the plagioclase rich highland crust, its bulk composition and the question whether it bears any generic relationship to that of the Earth's mantle. From 1969 to 1972 the US Apollo program installed one short-lived and four long-lived seismometers on the Moon. The latter instruments were operated until 1977. The data collected by the Apollo seismic network provided the basis for a number of studies of lunar seismicity and internal structure. However, as seismology does not present an end in itself, it can only be used as an indirect means to infer internal state and composition of the Moon, by, for example, comparing laboratory measurements of seismic wave velocities made on returned samples and terrestrial analogues with those obtained from the inversions. Unlike the aforementioned studies where the objective was centered on obtaining a seismic velocity model, the main purpose of the present study is to attempt to infer the composition and mineralogy of the lunar mantle directly by inverting the Apollo lunar seismic arrival time data set, mass and moment of inertia.
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Water in Nominally Anhydrous Mantle Minerals |
David L. Kohlstedt |
Small amounts of water dramatically affect both the chemical and physical behavior of mantle minerals and rocks. As one example, at high pressures, the melting point of mantle rocks is several hundred degrees lower under hydrous than under anhydrous conditions. In another area, the diffusion of silicon in olivine crystals is over three orders of magnitude faster in a hydrous environment at a pressure of 2 GPa than in an anhydrous environment at a pressure of 0.1 MPa. The viscosities of mantle minerals and rocks are likewise dramatically lowered by the presence of trace concentrations of water. In nominally anhydrous minerals, water-derived protons influence physical properties through their role as point defects. Experimental determinations of the dependencies of proton solubility, cation diffusivity, and mineral viscosity on water fugacity provide constraints on the mechanisms of incorporation of protons in nominally anhydrous silicate minerals. The introduction of protons into the crystal structure of olivine, (Mg,Fe)2SiO4, results in the formation of defect associates involving protons with vacancies on the octahedrally coordinated (metal) and tetrahedrally coordinated (silicon) cation sublattices. These increases in vacancy concentrations lead directly to increases in cation diffusivities and decreases in mineral and rock viscosities.
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Structural experiments at high-pressure: present status and outlook
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Martin Kunz |
The exploration of materials at extreme conditions relies heavily on computational crystallography, which can probe atomic structure at conditions which are not accessible to current experimental techniques. While these predictions are of immense value to direct scientific research, it remains the ultimate goal of any scientific prediction to be experimentally tested. For this reason the development of new techniques, which push the frontier of high-pressure experiments to more extreme conditions is of central interest to the community. There are two main thrusts in pushing the limits, namely the quest for experiments at higher pressures and temperatures on the one hand and the attempt to extract more subtle structural details at moderate conditions. The high-pressure community on the West Coast is addressing this need with a variety of initiatives at the ALS and SSRL.
The newly commissioned high-pressure beamline (12.2.2) at the ALS currently allows for powder diffraction with externally and internally heated diamond anvil cells. End station 1 offers a focus spot of 150 x 90 m and is operating with a static CCD detector to enable powder diffraction experiments. This end-station is planned to be expanded into a monochromatic high-pressure single crystal diffraction station enabling the extraction of accurate structural data up to ~ 60 GPa. End station 2 sits at the focus spot of a pair of Kirkpatrick-Baez mirrors giving a focal spot of 10 x 10 m and is interfaced with a double-sided laser-heating system, enabling the combination of pressures in the Mbar range with temperatures around 3000 K.
To push the limit of pressures and temperatures even further, a project to interface laser-generated shock waves with the intense X-ray pulses of a free electron laser has been initiated. This would for the first time allow us to probe matter in a quantitative way at shock-wave conditions using diffraction methods. This project is planned to be installed at the LINAC Coherent Light Source (LCLS) in Stanford. In this talk, the status and prospect of these
various projects is given together with scientific examples.
In collaboration with Simon M. Clark, Wendel A. Caldwell, and Raymond Jeanloz,
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Materials properties and their influence on mantle and plate dynamics
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Carolina Lithgow-Berteloni |
The dynamics of the solid Earth are greatly influenced by material properties and their dependence on pressure, temperature and composition. From density and thermal conductivity to viscosity. I examine the properties relevant to convection in the Earth's mantle and the rheology of plates and how variations in the pressure and temperature range of the Earth's mantle might affect the nature of convection and the evolution of surface properties. I will also examine the advances in generating plates self-consistently from the mantle and the choices of rheological properties appropriate for that generation. I will end with a view of future challenges in geodynamics that will greatly benefit from interactions between mineral physicists and fluid dynamicists.
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Minerals Deformation at Deep Earth Conditions from Atomistic Simulations
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Caetano Rodrigues Miranda |
Very little information is available on the microscopic deformation processes in Earth materials at high temperature and pressure conditions. A crutial point is the characterisation of the dislocation primary slip systems. This information is essential to modelling and analysis of crystal preferred orientations. Atomistic simulations have recently proved to be a reliable source of information about elasticity, slip systems and dislocation properties in a variety of systems ranging from the metals to covalent, and to ionic solids. Simulations are particularly useful at extreme conditions, where laboratory experiments are still a challenge. In this work, we have calculated the so called Generalized Stacking Fault surfaces (GSF) for MgO and Iron (work in progress) at high temperature and pressure conditions by using atomistic simulations. The GSF calculations are based on the determination of the energy cost (with respect to the ideal crystal) of shearing along a given direction of two halves of the crystal cut along the glide plane. Using the GSF in the framework of Peierls-Nabarro model, the properties of dislocations motion, dominant slip system and associated stress has been obtained for MgO and hopefully for Iron at Earth's interior conditions.
In collaboration with Sandro Scandolo
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Extra-solar Planets - Needs for Material Properties at Very High Pressure
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Richard O"Connell |
The discovery of massive extra-solar planets has revealed a class of planets with internal states that may differ from solar system planets. Massive terrestrial planets (super-Earths) have internal pressures much higher than the Earth. Models of the internal state of such planets require equations of state of terrestrial materials at pressures several times higher than those in the Earth. Understanding the thermal state and evolution will require better knowledge of defect properties and rheology at such high pressures.
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Overview of the NaradaBrokering Substrate
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Shrideep Pallickara |
The NaradaBrokering project at the Community Grids Lab at Indiana University is an open source project that researches issues pertaining to distributed middleware systems. The NaradaBrokering substrate is based on the publish/subscribe paradigm and facilitates the development of loosely-coupled, distributed, robust applications. This talk presents an overview of the substrate's capabilities and the services that can be leveraged by clients. The talk will also describe the more recent support, within the substrate, for specifications in the Grid/Web Services domain.
URL:http://www.naradabrokering.org/
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Grid Portals for VLAB |
Marlon Pierce |
We review the initial versions of the VLAB Portal for the Quantum Espresso codes. The initial portal provides simple remote file and job execution capability, but we are built upon standards that will enable us to expand the portal's functionality in collaboration with third party developers. The portal itself is built using the standard JSR 168, which allows us to plug in third party components and reuse software between portal projects. This should facilitate distributed development and simplify collaborations with VLAB partners. The portal's remote functionality is based on the Globus toolkit, which provides foundations for secure remote command execution and file management. We hope that the initial implementation descriptions will lead to requirements and input from the VLAB community.
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Fe-liquid segregation in Deforming Metal-Silicate Systems: Coupling Core Forming Compositions with Transport Phenomena
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Tracy Rushmer |
The segregation and macroscopic transport phenomena leading ultimately to the formation of metallic cores in planetary silicate mantles is a fundamental yet poorly understood process. Here we report the results of a series of deformation experiments on a sample of partially molten Kernouve H6 chondrite (T = 900 - 1050oC) aimed at determining the siderophile concentrations and associated partition coefficients in both Fe-S-Ni-O quench and Fe-Ni metal as a function of degree of melting, and to provide insight into the melt segregation mechanism(s). The geochemical results show the S content in the segregated Fe-rich liquid metal decreases with increasing degree of melting. As the S content of the liquid metal also strongly affects the partitioning of highly siderophile elements between solid and liquid metal, an increase in porosity (Fe liquid melt fraction) from 5 to 30% lowersDsm/lm for HSE by several orders of magnitude. The relationship between melt fraction and porosity is used to compare the migration rate of liquid metal driven by buoyancy pressure gradients with a new theoretical model of melt segregation in a deforming porous medium that takes into account the coupling between volume strain(dilatancy) and shear stress. For buoyancy driven porous flow, highest transport velocities occur at highest porosities, implying the fastest flow velocities will carry Fe-rich liquid metal with low sulfur contents, preferentially enriched in incompatible HSEs. Predicted characteristic timescales of liquid metal transport due to buoyancy effects (diapirism and porous flow) for a c. 100 km-sized planetesimals are contrasted with shear-induced segregation velocities set up in response to external perturbations via impacts, an important process during the final stages of planetary accretion. A novel feature of our analysis is that liquid metal segregated previously into a planetary core by buoyancy instabilities (e.g. porous flow or a raining mechanism), might be drawn locally back into the silicate lower mantle by pressure gradients linked to surface impacts providing a physical mechanism for return flow of siderophile elements across the CMB.
In collaboration with Nick Petford, Munir Humayun, and Andrew J. Campbell
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Thermal Conductivity from Atomistic Simulations: Some Preliminary Findings, Many Challenges Ahead |
Sandro Scandolo |
The thermal conductivity of minerals is a crucial parameter to understand and model the dynamics of the Earth's interior. I will review recent attempts to calculate the thermal conductivity of solids using atomistic simulations, trying to emphasize virtues and weaknesses of the various approaches, and illustrating strategies for the grand challenge of calculating thermal conductivities from first principles.
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Phase Transitions and Physical Properties of Silicates at High Pressure and High Temperature |
Sang-Heon Dan Shim |
High-pressure and high-temperature measurements have shown that major elements in the lower mantle (O, Si, Mg, Fe, Ca) are predominantly hosted in materials with perovskite structures, (Mg,Fe)SiO3 and CaSiO3. Therefore, it is essential to study their physical properties and phase transitions in order to understand the structure and dynamics of the lower mantle. Although pressure and temperature conditions of the deep mantle can be achieved using the laser-heated diamond-anvil cell, it is still experimentally challenging to measure many of the key properties at extreme conditions. Several recent studies have demonstrated the way in which combining experiments and first-principles calculations can be a very powerful way to study materials at extreme conditions. The stability of (Mg,Fe)SiO3 perovskite, the dominant lower-mantle mineral, at the deep mantle has been questioned by some earlier measurements. Our recent synchrotron X-ray diffraction measurements in the laser-heated diamond cell have confirmed the stability of both Mg-silicate and Ca-silicate perovskites to 2600-km and 2400-km depth conditions, respectively, which is consistent with recent first principles calculations. We also confirmed that Mg-silicate perovskite undergoes a major transformation at the bottom of the mantle, i.e., the post-perovskite transition. First-principles studies of this material have demonstrated that the physical properties of the post-perovskite phase may explain important seismic observations of the lowermost mantle. Earlier studies reported a cubic structure for CaSiO3 perovskite while first principles calculations proposed a non-cubic unit cell at the ground state. We found that CaSiO3 perovskite has a tetragonal unit cell at lower-mantle pressures. Physical properties of these silicate perovskites are important to understand seismic studies of the lower mantle. Earlier low-pressure measurements have reported that iron up to 20 % in Mg-silicate perovskite does not affect the bulk modulus. However, our recent measurements at lower-mantle pressures have indicated that the bulk modulus does change with iron content. We constrained the bulk modulus of (Mg0.9Fe0.1)SiO3 perovskite with well-studied gold pressure scale and found that the bulk modulus is lower than what has been reported for the lower mantle. In order to understand the effect of ferric iron in lower-mantle minerals, we studied (Ca0.75Fe+30.25)(Si0.75Fe+30.25)O3. We found that Ca-silicate perovskite can take at least 25 % ferric iron in both the dodecahedral and the octahedral sites. It appears
that ferric iron induces the onset of phase transitions in Ca-silicate perovskite: cubic → tetragonal → orthorhombic at mantle pressures. The volume of Ca-silicate perovskite decreases
slightly with ferric iron substitution, which will significantly increase the density of Ca-silicate perovskite.
In collaboration with B. Scott Lundin and Javier Santillan
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Simulating Fluid Phase Equilibria of Water from First Principles |
J. Ilja Siepmann |
Efficient Monte Carlo algorithms are combined with the Quickstep routines of CP2K to develop a program that allows for direct Gibbs ensemble Monte Carlo simulations of fluid phase equilibria using a first-principles description of the physical system. Configurational-bias Monte Carlo techniques and pre-biasing using an inexpensive approximate potential are employed to increase the sampling efficiency and to reduce the frequency of expensive ab initio energy evaluations. A water representation based on the Becke-Lee-Yang-Parr exchange and correlation functionals together with norm-conserving Goedecker-Teter-Hutter pseudopotentials, and a triple-zeta valence basis yields a saturated liquid density of 900 kg/m3 at 323 K, and normal boiling and critical temperatures of 350 and 550 K, respectively. An analysis of the structural and electronic properties of the saturated liquid phase shows an increase of the asymmetry of the local hydrogen-bonded structure despite the persistence of a four-fold coordination, and decreases of the molecular dipole moment and of the spread of the lowest unoccupied molecular orbital with increasing temperature.
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Grain Size Evolution, Rheology and Mantle Dynamics |
Slava Solomatov |
Grain size strongly affects the rheology of rocks and plays a big role in mantle dynamics and evolution. Therefore, understanding the grain size is an important problem in geophysics. The grain size is controlled by several processes including grain growth, Ostwald ripening, phase transformations, and dynamic recrystallization. Although these processes have been relatively well studied and constrained for various materials, the mantle is unique at least in two important aspects. First, the mantle is extremely "dirty". It consists of several mineral phases with lots of impurities. Second, the time scale is "geological" - millions and billions of years. This means that theory and numerical simulations are critical in bridging the gap between the laboratory and geological time scales. In this report we will discuss numerical simulations of coarsening in multi-phase systems, using Monte Carlo Potts models, and the implications of our results for the Earth and other terrestrial planets.
In collaboration with V. S. Solomatov and R. El-Khozondar
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Relevance Of Structure, Thermodynamic And Transport Properties Of Amorphous Geomaterials (Glasses, Metastable And Equilibrium Liquids) To The Evolution Of The Terrestrial Planets
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F. J. Spera |
The liquid state, long a frontier area in materials research and atomic scale modeling, is especially pertinent to current problems in the geochemical and geodynamical evolution of the Earth and other terrestrial planets in the solar system and beyond. The accretion and very earliest history of the Earth is informed from radiometric tracers, specifically the isotopic systems W-Hf and Sm-Nd and by dynamical modeling (shock wave physics and impact modeling). Further progress demands better understanding of thermodynamic and transport properties of molten silicates in the system SiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O, K2O, H2O, and TiO2. The isotopic record shows that Earth accretion was accomplished within ~30 Myr of condensation of the first solids (CAI inclusions) within the solar nebula at to=4570 3 Ma. This implies, based on the dynamics of collisional runaway accretion and later era of giant (lunar-forming) impact, that Earth accreted fast and hot--probably entirely or significantly molten. The isotopic record of Sm-Nd suggests that progressive crystallization of the Earth"s early "magma ocean&qout; may have concentrated (and sequestered over geologic time) certain elements (including radiogenic ones) near the bottom of a relatively FeO-rich silicate magma ocean. Phases precipitating at the base of the silicate magma ocean at ~ 135 GPa and T~ 5000 K may include post-perovskite (pPv), perovskite (Pv), magnesiowustite, Ca-rich Pv and perhaps other unknown phases. Fractional crystallization of the early magma ocean sets initial conditions on composition and structure relevant to subsequent earth history. An understanding of the properties of multicomponent silicate liquids at high pressure and temperature is a critical need. Although a robust thermodynamic model of multicomponent silicate liquid exists for P ~3 GPa (Ghiorso, 2004), extrapolation to pressures > 10 GPa requires an EOS tailored to accommodate the large isothermal expansivity and isobaric compressibility of silicate liquids and configurational changes in the short-range melt structure with pressure and temperature. It has been found that explicit separation of the volumetric consequences of isostructural compression and expansion (vibrational effects) from volumetric effects rooted in changing melt structure (configurational effects i.e., changing cation-oxygen coordination environments), improves extrapolations of the EOS to high pressure. This separation is an essential ingredient for obtaining a thermodynamic (Gibbs energy) and transport description of multicomponent natural silicate liquids at high temperature and pressure. Molecular Dynamics simulations prove critical to calibration of liquid EOS"s because coordination statistics provide the necessary link between melt structure and melt density, information difficult to obtain otherwise. The MD algorithm uses the Particle-Ewald Mesh (PEM) formalism for computation of long-range forces and determination of properties including the internal energy, Helmholtz free energy, isochoric heat capacity, tracer diffusivity, melt shear viscosity, thermal conductivity, etc. We are performing 104-105 particle MD experiments on melts in the system CMASF using Coulomb-Born-van der Waals effective two-body potentials to develop a first-order thermodynamic model of liquids to CMB temperatures and pressures. Typical results are presented for molten MgSiO3 using simple potentials. Incorporation of H2O into the model necessitates modified two-body (Rahman-Stillinger-Lemberg) and three-body (Stillinger-Weber) terms. A second-generation model based on first-principles modeling awaits the future.
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Silicate liquids in Earth's Deep Interior |
Lars Stixrude |
Magma generation and transport are the primary mechanisms of differentiation of Earth's interior. Melting may have extended to much greater depths in the geologic past, producing magmas with compositions very different from those being erupted today. A thin zone of partial melt has been proposed at the base of mantle on the basis of seismic observations. Quantum molecular dynamic simulations are a powerful tool for exploring the structure and behavior of silicate liquids at deep mantle conditions, where little is yet known of there properties. We have found that the structure of Mg-metasilicate liquid changes markedly over the mantle pressure regime, that the volume contrast between liquid and coexisting solid decreases nearly 5-fold over this regime, and that the congruent solidus is 5400 K at the core-mantle boundary.
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PGeodynamical modeling of Earth"s Thermo-Chemical Evolution: the Importance of Mineral Physics |
Paul J. Tackley |
Over the years, many conceptual models have been proposed to reconcile geochemical observations with geophysical constraints arising from seismology and from dynamical convection modeling. These conceptual models can now be tested by incorporating melting-induced differentiation and tracking of major- and trace-element chemistry into numerical convection calculations that simulate the thermo-chemical evolution of an Earth-like planet. Using this approach, synthetic geochemical data can be generated and compared directly with observations to further constrain the range of allowable mantle models. It is, however, found that the results of such models are highly dependent on physical properties of mantle materials, many of which are quite uncertain. Particularly important properties in the presented results are the relative densities of different mineral assemblages (e.g., representing MORB, pyrolite or harzburgite) as a function of pressure, partition coefficients for trace elements upon melting (particularly noble gases), and the exact details of mantle phase transitions, including Clapeyron slopes, the relative depth of the perovskite transition in the olivine and pyroxene systems, the presence or not of ilmenite, and the details of the post-perovskite transition. Some other physical properties likely to be important are the dependence of thermal conductivity on temperature and pressure(including spin state), and the dependence of mantle viscosity on volatile content and grain size.
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High-to-Low Spin Transition in Minerals and its Effect on Elasticity
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Taku Tsuchiya |
High resolution X-ray spectroscopy has recently demonstrated that the major lower mantle (LM) minerals undergo a high-to-low spin transition at LM pressures (23-135 GPa). Previous failures of standard DFT and "LDA+U" approaches to describe this phenomenon have hindered its investigation and consequences of fundamental importance to geophysics. Here, using the rotational invariant first principles formulation of LDA+U with optimized effective U, we report the first successful study of this transition in low solute concentration (Mg(1-x)Fex)O, magnesiumwustite, which is believed to be the second most abundant phase of Earth's LM.
In collaboration with Renata M. Wentzcovitch and Stefano de Gironcoli
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TBA |
Koichiro Umemoto |
Abstract needed |
Post-Perovskite at D" Conditions: Insights on the Lowermost Mantle
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Renata M. Wentzcovitch |
The thermoelastic properties of the newly found post-perovskite polymorph of MgSiO3, more stable than the Pbnm-perovskite phase at conditions close to those expected in Earth's D" region, has been investigated by first-principles and contrasted with those of the perovskite phase. We predict the major seismic trends such as velocity discontinuities, ratios of velocities and density anomalies, and anisotropy in aggregates with preferred orientation that should occur in the presence of this phase change. Consequences of this model mineralogy for the D" region will be discussed.
In collaboration with Taku Tsuchiya, Jun Tsuchiya, and Koichiro Umemoto
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The Role of Transport Properties in Lower Mantle Convection
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David A. Yuen |
The dynamics of plumes and slabs are governed by both the viscosity and thermal conductivity in the lwoer mantle. Much attention has been paid to the role played by viscosity stratification. Save for the work of Hofmeister, not much effort has been devoted on thermal conductivity. Yet we know that the temperature equation is the time-dependent partial differential equation governing mantle convection, while the momentum equation follows the temperature equation at each time step, as a slave variable. Therefore nonlinearties in the transport properties of the temperature or master equation can exert strong effects on mantle convection. We will point out the role of radiative heat-transfer in controlling plume dynamics and also in causing thermal assimilation of the subducting slabs. We will also discuss the role played by viscosity hill in the lower mantle on plume dynamics, especially the formation of superplumes with large radius.
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