Effects of an endothermic phase transition at 670 km depth in a spherical model of convection in the Earth's mantle
TL;DR: In this article, numerical modeling of mantle convection in a spherical shell with an endothermic phase change at 670 km depth reveals an inherently three-dimensional flow pattern, containing cylindrical plumes and linear sheets which behave differently in their ability to penetrate the phase change.
Abstract: Numerical modelling of mantle convection in a spherical shell with an endothermic phase change at 670 km depth reveals an inherently three-dimensional flow pattern, containing cylindrical plumes and linear sheets which behave differently in their ability to penetrate the phase change. The dynamics are dominated by accumulation of downwelling cold material above 670 km depth, resulting in frequent avalanches of upper-mantle material into the lower mantle. This process generates long-wavelength lateral heterogeneity, helping to resolve the contradiction between seismic tomographic observations and expectations from mantle convection simulations.
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Curtin University1, University of Western Australia2, Lund University3, University of Adelaide4, Geological Survey of Canada5, Carleton University6, University of Brasília7, University of Bergen8, University of New Mexico9, Macquarie University10, Stockholm University11, University of Copenhagen12, Russian Academy of Sciences13
TL;DR: A brief synthesis of the current state of knowledge on the formation and break-up of the early Neoproterozoic supercontinent Rodinia and the subsequent assembly of Gondwanaland is presented in this paper.
2,790 citations
TL;DR: In this article, the authors compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts Crustal thickness ranges from 20 to 40 km, and the lower crust is characterized by high (70-76 km s?1) compressional wave velocities.
Abstract: Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts Such provinces do not originate at “normal” seafloor spreading centers We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (70-76 km s?1) compressional wave velocities Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java’s rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features
1,367 citations
TL;DR: In this article, the problem of phase equilibrium is reduced to a linear optimization problem that is independent of the functional form used for the equations of state of individual phases of the aggregate.
Abstract: [1] Geodynamic models commonly assume equations of state as a function of pressure and temperature. This form is legitimate for homogenous materials, but it is impossible to formulate a general equation of state for a polyphase aggregate, e.g., a rock, as a function of pressure and temperature because these variables cannot distinguish all possible states of the aggregate. In consequence, the governing equations of a geodynamic model based on a pressure-temperature equation of state are singular at the conditions of low-order phase transformations. An equation of state as a function of specific entropy, specific volume, and chemical composition eliminates this difficulty and, additionally, leads to a robust formulation of the energy and mass conservation equations. In this formulation, energy and mass conservation furnish evolution equations for entropy and volume and the equation of state serves as an update rule for temperature and pressure. Although this formulation is straightforward, the computation of phase equilibria as a function of entropy and volume is challenging because the equations of state for individual phases are usually expressed as a function of temperature and pressure. This challenge can be met by an algorithm in which continuous equations of state are approximated by a series of discrete states: a representation that reduces the phase equilibrium problem to a linear optimization problem that is independent of the functional form used for the equations of state of individual phases. Because the efficiency of the optimization decays as an exponential function of the dimension of the function to be optimized, direct solution of the linearized optimization problem is impractical. Successive linear programming alleviates this difficulty. A pragmatic alternative to optimization as an explicit function of entropy and volume is to calculate phase relations over the range of pressure-temperature conditions of interest. Numerical interpolation can then be used to generate tables for any thermodynamic property as a function of any choice of independent variables. Regardless of the independent variables of the governing equations, a consistent definition of pressure, and the coupling of equilibrium kinetics to deformation, is only possible if the continuity equation accounts for dilational strain.
831 citations
Cites background from "Effects of an endothermic phase tra..."
...[2] For decades phase equilibria have been accounted for in geodynamic models by parameterizations that permit separation of chemical and nonchemical energetic effects [e.g., Richter, 1973; Christensen and Yuen, 1985; Connolly and Thompson, 1989; Tackley et al., 1993; Podladchikov et al., 1994]....
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...[2] For decades phase equilibria have beenaccounted for in geodynamic models by parameterizations that permit separation of chemical and nonchemical energetic effects [e.g., Richter, 1973; Christensen and Yuen, 1985; Connolly and Thompson, 1989; Tackley et al., 1993; Podladchikov et al., 1994]....
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Journal Article•
TL;DR: In this paper, two high-resolution models of the P-wave and S-wave seismic structure of the mantle were derived independently using different inversion techniques and different data sets, but they show excellent correlation for many large-scale as well as smaller scale structures throughout the lower mantle.
Abstract: Two new global high-resolution models of the P-wave and S-wave seismic structure of the mantle were derived independently using different inversion techniques and different data sets, but they show excellent correlation for many large-scale as well as smaller scale structures throughout the lower mantle. The two models show that high-velocity anomalies in the lower mantle are dominated by long linear features that can be associated with the sites of ancient subduction. The images suggest that most subduction-related mantle flow continues well into the lower mantle and that slabs may ultimately reach the core-mantle boundary. The models are available from anonymous ftp at maestro.geo.utexas.edu in directory pub/grand and at brolga.mit. edu in directory pub/GSAtoday.
746 citations
TL;DR: In this paper, the authors show that superplume events in the late Paleozoic and mid-Cretaceous may have been caused by minor slab avalanches as the 660-km discontinuity became more permeable to the passage of slabs with time.
741 citations
References
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TL;DR: In this paper, a large data set consisting of about 1000 normal mode periods, 500 summary travel time observations, 100 normal mode Q values, mass and moment of inertia have been inverted to obtain the radial distribution of elastic properties, Q values and density in the Earth's interior.
9,266 citations
Book•
05 May 1982
TL;DR: A comprehensive and quantitative study of the fundamental aspects of plate tectonics is presented in this paper, with an introduction to heat flow, elasticity and flexure, fluid mechanics, faulting, gravity, and flow in porous media.
Abstract: A comprehensive and quantitative study of the fundamental aspects of plate tectonics. Provides an introduction to heat flow, elasticity and flexure, fluid mechanics, faulting, gravity, and flow in porous media, with a wide range of geological applications. Contains detailed coverage of mantle convection and mantle rheology. Includes a wide variety of practical problems.
2,292 citations
TL;DR: In this article, the phase boundaries between ilmenite-perovskite in MgSiO3 and between Mg2SiO4 spinel and MgO periclase were determined using a uniaxial split-sphere apparatus.
Abstract: The high-pressure transformation in MgSiO3 and those in the spinel phases of compositions from Mg2SiO4 to (Mg0.5Fe0.5)2SiO4 in the Mg2SiO4-Fe2SiO4 system were investigated using a uniaxial split-sphere apparatus. The phase boundaries between ilmenite-perovskite in MgSiO3 and between Mg2SiO4 spinel and the assemblage of MgSiO3 perovskite and MgO periclase were determined to be P(GPa) = 26.8–0.0025T(°C) and P(GPa) = 27.6–0.0028T(°C), respectively, in the temperature range 1000–1600°C. The pseudobinary diagrams for the system Mg2SiO4-Fe2SiO4 were determined at temperatures of 1100°C and 1600°C. It was demonstrated that the magnesian spinel (with Fe/Mg + Fe < 0.22 at 1100°C and <0.26 at 1600°C) dissociates into perovskite and magnesiowustite within an extremely narrow pressure interval (<0.15 GPa at 1600°C). The dissociation pressure was found to be almost independent of iron content and to coincide to that at 670 km depth within experimental uncertainties. These experimental results indicate that the sharpness of the 670-km discontinuity may indeed be due to this dissociation in a peridotitic or pyrolitic mantle. The current status of our understanding of deep mantle mineralogy and chemistry is discussed based on recent high-pressure and high-temperature experiments.
954 citations
TL;DR: In this paper, the authors performed two-dimensional numerical calculations of convection in a domain containing a divariant phase change and found that the critical value of the negative Clapeyron slope, which must be surpassed in order to induce layered convection, decreases in magnitude with increasing Rayleigh number Ra in the range 104 ≤ Ra ≤ 2×106.
Abstract: We report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection. Two-dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the “extended Boussinesq approximation,” i.e., considering the effects of adiabatic gradient, latent heat, and frictional heating in the energy equation. We find that the critical value of the negative Clapeyron slope, which must be surpassed in order to induce layered convection, decreases in magnitude with increasing Rayleigh number Ra in the range 104 ≤ Ra ≤ 2×106. Near the critical Clapeyron slope, vacillations between double- and single-layer convection or strongly leaking double-layer convection are possible. The breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences. The value of the critical Clapeyron slope also seems little affected by the width of the transition zone or by its depth. A possible superplastic rheology within the transition zone would tend to favor layered convection. Scaling the model results to the 670-km discontinuity in the earth's mantle as a possible endothermic phase boundary, we estimate the critical Clapeyron slope to be in the range of −4 to −8 MPa/K (−40 to −80 bar/K). The possibility that the spinel → perovskite + periclase transition is within this range appears to be remote but certainly cannot be neglected.
628 citations
TL;DR: In this article, the authors discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks, which minimizes a problem with tomographic studies of regional scale.
Abstract: The P wave velocity structure beneath the Western Pacific is found from the International Seismological Center first arrival data. Special attention was paid to the deep structure beneath the Wadati-Benioff zone. We discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks. This way of discretization minimizes a problem with tomographic studies of regional scale: difficulty in making corrections for the effects outside the region of interest. Our solution is iterative with the alternate step of the relocation of earthquakes, using the whole mantle model of Inoue et al. (1990) as a starting model. A first-order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model. The essential result depends little on whether the reference spherical model is smooth or discontinuous near 400- and 670-km depths. We examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere. The resolution is depth dependent but in general good enough to see the slab configuration beneath the Southern Kurile-Japan-Izu-Bonin arcs and the Java arc. It is relatively poor beneath the Northern Kurile and Mariana arcs. The seismic image of subducting slab beneath the Southern Kurile to Bonin arcs bends to subhorizontal near the leading edge of the Wadati-Benioff zone and extends continentward over a distance of more than 1000 km. The subhorizontal portion of the slab connects a high-velocity blob to the bottom that reaches a depth of at least 800 km across the 670-km discontinuity under the Japan arc. Although the image of the Java slab directly penetrates the 670-km discontinuity, it then bends to a shallow dip with a considerable spread, reaching a depth of about 1200 km. These results suggest that descending slabs of lithosphere in the Western Pacific tend to be stagnant in the transition zone under a subtle control of the 670-km discontinuity. Although stagnant slab materials eventually descend into the lower mantle, they no longer maintain their original configuration below the 670-km discontinuity.
627 citations