DI41A-1728
Systematic vertical changes of chemistry within lithosphere of shields?
Cratonic areas constitute large heterogeneities that extend down to at least 200km and that are identified by a specific chemistry. The differences and similarities between different cratons are however not reliably resolved. We compare four cratonic regions in Canada, South Africa, Finland and Australia using existing dispersion curves obtained by the analysis of seismic surface waves across particularly dense seismic arrays. By inverting these curves with an identical inversion method and parametrization we identify the reliably resolved differences, and similarities between the four study areas rather than obtaining a "best" model for each area. Alongside the inversion, we also compare the observed dispersion curves with predicted ones using models with either constant shear velocities or constant composition in the lithosphere. The three main conclusions of this study are that 1) models of constant composition within the lithosphere do not explain the dispersion curves, in fact the less physically-based models of constant shear velocity provide a better fit to the data for all of the areas; 2) a low velocity zone in the deep lithosphere or below the lithosphere is resolved beneath south Africa while the three other areas have no such low velocity zone; 3) in spite of age differences, the shear velocities in three areas are similar while the Yilgarn in Australia has significantly faster velocities.
DI41A-1729
The effects of compositional and rheological stratifications on the evolution of oceanic lithosphere from geophysical-petrological-dynamic modeling.
The oceanic lithosphere is not only a thermal, but also a chemical, and a mechanical boundary layer. Its thermodynamic properties, which in turn control its dynamic behaviour, depend ultimately on temperature, pressure, composition of the original source (i.e. upper mantle that has not experienced partial melting), and degree of melt depletion experienced at the mid-ocean ridge (MOR). Although the resulting compositional and rheological layering is expected to exert some control the evolution of the oceanic lithosphere and its associated geophysical observables (e.g. surface heat flow, bathymetry, seismic structure, etc), no systematic exploration with geophysical-petrological-dynamic models has been attempted so far. Here we use dynamic models coupled with melting and petrological models to explore 1) the influence of this layering on the development of small-scale convection under the oceans, 2) its role in determining the thickness of oceanic lithosphere, and 3) its feasibility as responsible for the deviations of seafloor and surface heat flow from predictions by conductive models in mature oceanic lithosphere. We show that the existence of small- scale convection is entirely compatible with experimental creep parameters and flow laws, and that the viscosity stratification due to melt extraction (i.e. H2O removal) is the main factor controlling the plate's thermal evolution, its asymptotic thickness, and the flattening of seafloor and surface heat flow at ages > 70 Ma. The effects of Al-rich phase transitions and compositional layering are relatively minor.
DI41A-1730
Is the lower mantle stagnant?
I calculate geotherms for the lower mantle (LM) from the solution to Fourier's equation for spherical shells with heating internally and below, by utilizing new models and measurements of thermal diffusivity (D) at temperature (T) and pressure (P), available constraints from mineral physics, various distributions of radiogenic elements, and integrating outward from the core-mantle boundary (CMB). A perovskite-rich LM can conductively carry heat from below any given radius under a wide range of conditions, mainly due to high thermal conductivity associated with lattice compression. The base model uses recent experimental measurements of eutectic melting in the Fe-S system (Chudinovskikh and Boehler, 2007, EPSL, p. 97) which indicate CMB temperatures of 3000 K, and assumes that 2 TW emanates from the core, consistent with power attributed to the geodynamo. For the continental crust, a low estimate of 8 TW is used, for a total Q=30 TW, indicated by spherical harmonic analysis of heat flux data (Hamza et al. 2008, IJES). I use recent experimental measurements of D of perovskite-family minerals at high temperature using laser flash analysis (Hofmeister, in press, PEPI). For D(T) of MgO, dD/dP and estimation of the effects of Fe content and the transition to post-perovskite, we use the damped harmonic oscillator model (Hofmeister et al, 2007, Treatise in Geophysics). High temperature values for heat capacity are used and density from PREM. A small radiative contribution for 1 mm grains (Hofmeister (2005, J. Geodynamics) is assumed. Base model calculations provide T =1700 K at 670 km, which is consistent with phase equilibria. Effects on the geotherm are: negligible for ppv being present, minor for the amount of radiative transfer or of internal heating in the LM, moderate for proportion of perovskite or changes in core heat or CMB temperatures. For example, a pure perovskite LM without radiative transfer is conductive having T from 2000 to 4000 K from 670 km to the CMB. Much higher Q still provides reasonable geotherms, given leeway in other parameters. For plausible values of input parameters, the lower mantle is stable, although very weak unicells (Nu = 1) cannot be ruled out. Convection is layered with all the "action" in the upper mantle.
DI41A-1731
Perisphere, Carbonatites, Delamination and Tiny Time Capsules
Combining mineral physics, thermodynamics, petrology and high-resolution seismology with mass balance
calculations (MBC), yields completely different solutions for the structure and evolution of the mantle than
canonical models (CM) based on assumptions augmented by intuitive interpretations of isotope data and
color cross-sections and the Boussinesq Approximation. Slab fluids, carbonatites, sulfides, upper crustal
debris, delaminated lower crust (low-velocity-anomalies, LVA) and permanently buoyant perisphere (high-
velocity, HVA; FOZO), collect in the shallow mantle. Eclogites (LVA) collect in the transition zone (TZ). Slab
eclogite is dense, and displaces uncontaminated undegassed MORB out. Carbonatites depress melting
points (LVA), scavenge volatiles from the mantle and, along with delaminated, refertilized and ultra-refractory
components, allow closure of MBC. The deep mantle is barren, dense, inaccessible and irrelevant, for
current fluxes. Marble-cake, and one- and two-layer models do not pass physical plausibility or ground-truth
tests. The mantle is heterogeneous–stratified and lumpy–"anomalies" in seismic velocity, topography and
magma chemistry form at ambient temperatures. Whole mantle convection assumes scaling rules and
approximations (Boussinesq) between density, temperatures and shear velocity that are implausible.
Carbonated silicates, eclogites, harzburgites, and TZ thicknesses, violate these rules. Shallow refractory
(HVA) lithologies produce little 4He, and serve as containers for preserving high 3He/4He signatures outside
"the convecting mantle" by buoyancy, strength, high melting point, low diffusivities and low temperatures.
Midplate magmas interact with shallow Commmon Components (C-), thus explaining concentrations and
variability. Metasomatism, magma-rock interactions and refractory containers eliminate the need for deep
sources and hidden reservoirs. The mantle retains about 23% of all of the 40Ar ever produced by β-
decay. MORB is 7% of the mantle and contains 20 % of all 4He and 40Ar ever produced. The TZ is a
plausible reservoir for MORB, noble gases and recycled crust. The amount of 3He in midplate magmas is
trivial and can be accounted for by shallow contamination. Of the 1500 moles/a of 3He lost from the mantle,
only 3 moles are contributed by High 3He/4He Hotspots [HiHeHo]; 0.3 moles are in the non-MORB-like
fraction. "High-3He" models are based on fluxes of 3He that represent 0.02 % of the total flux. 3He contents
of HiHeHo are extraordinarily low, 1-2 orders of magnitude less than MORB and 5 orders of magnitude less
than CM predict; these models are about as wrong as they can be. Metasomatism and low-level
contamination provide the distinctive midplate isotopes signatures, not deep undegassed reservoirs.
Carbonatitic melts are so enriched in U, Th, Nb, Ta and Hf that they cannot be ignored in MBC. In isotope
space they extend from EM1-HIMU to FOZO; some have high 3He/4He ratios and 3He. Carbonatites and
MORB are the helium-rich components, not OIB or lower mantle. Ultra-depleted, ultra-refractory and ultra-
buoyant peridotites can store noble gases for long periods of time. The reservoirs tapped for 3He by OIB
need contain only 10-5 of the number of atoms in the MORB reservoir. He, Ne and Os data have been used
to argue for primitive, lower mantle and core sources for alkalic and midplate volcanoes. Shallow metasomatic
processes and magma-wallrock interactions are the more traditional explanations. Isotope diversity is
achieved in-situ, without invoking oceanic crust or deep recycling. Ancient metasomatism and trapping of
helium in otherwise depleted lithologies results in high and diverse 3He/4He ratios except where MORB
overwhelms shallow metasomatic signatures.
http://www.mantleplumes.org/TomographyProblems.html
DI41A-1732
Adiabatic Mantle Upwelling
Mantle dynamics is commonly treated as an isentropic process (i.e. reversible adiabatic). The overall entropy production is usually considered to be small, and mainly localized within the thermal boundary layer (heat conduction), and around the upwelling and downwelling regions (viscous dissipation). Recently Ganguly has developed a thermodynamic formulation of adiabatic decompression and melting in the mantle that includes the effect of entropy production due to irreversible expansion during upwelling. This effect was also considered earlier by Waldbaum, Ramberg and Spera. We have combined the irreversible adiabatic formulation of upwelling with chemical equilibrium computation on the basis of minimization of Gibbs free energy using the database of Saxena. This approach yields a set of accurate adiabatic temperature profiles. The departure from the conventional isentropic gradient is defined by upwelling mantle materials of various density (or pressure) contrasts. Our calculations suggest that Earth's mantle experienced a faster cooling during the initial thermal evolution. However, when combined with appropriate viscosity model, the surface heat flow is such that the Urey ratio (ratio between the surface heat flow and internal heat production) is significantly lower than that in the previous models and in better agreement with geochemical estimate. We have also derived a new formulation for the heat transport that considers the temperature variations induced by an irreversible adiabatic volume change in a gravitational field in combination with a mantle flow model. The term describing the adiabatic contribution reduces to the well known isentropic term when the difference in the gradient between the flow pressure and lithostatic pressure become negligible, i.e. when upwelling mantle moves very slowly. The geodynamic model, which also includes a mineralogically dependent viscosity model, has been coupled with G minimization formulation and applied to study the evolution of an isochemical thermal plume. The preliminary result of the dynamic model shows significant departure from the adiabatic reversible gradient. The overall excess temperature created by the irreversible adiabatic expansion during upwelling is particularly localized in the upper mantle, which, in turn, enhances the dynamics of this portion of the mantle, thereby facilitating local chemical homogenization. The plume heat flux variation with depth is much less than previously estimated, implying that the current prediction of the radiogenic internal heating rate is probably overestimated. Our results shows also interesting common features with the superadiabatic temperature change in the lower mantle retrieved from seismological Earth models (variation of Bullen parameter with depth). The early evolution of a thermal plume obtained with the correct adiabatic formulation better explains the anomalously high temperature required for the formation of komatiites in the lower portion of the upper mantle and transition zone.
DI41A-1733
Thermo-chemical interpretation of one-dimensional seismic reference models for the lower mantle
Average seismic structure of the lower mantle is extremely tightly constrained by global travel times. Very smoothly varying velocity-depth gradients between depths of 800 and 2500 km are commonly taken as an indication of a homogeneous lower mantle composition. However, with increasing depth, lower-mantle gradients are less and less well matched by those expected for a thermally well-mixed adiabatic-subadiabatic mantle of a homogeneous pyrolitic composition. We map out thermal and chemical sensitivity of seismic wave velocities as a function of lower mantle pressure and temperature, and the uncertainties in the velocities and derivatives due to uncertainties in the mineral physics parameters and EoS. Taking these uncertainties into account, we cannot fully preclude an adiabatic pyrolitic average lower mantle structure. However, seismic constraints are better matched by a mantle that is (1) superadiabatic plus has an increasingly fast composition with depth, e.g. Si-rich, potentially from an increasing proportion of basaltic components with depth, or (2) becomes increasingly iron-rich (primitive?) with depth, with temperatures similar to those of an adiabat compatible with upper mantle temperature constraints. Additional seismic and/or dynamic constraints, as well as reduction of the mineral physics uncertainties may help to distinguish between the possibilities.
DI41A-1734
Waveform Complexity Caused by D" Structure along Slab-edges
A lower mantle S wave triplication (Scd) has been recognized for many years and appears to be explained by the recently discovered perovskite (PV) to post-perovskite (PPV) phase change. Seismic observations of Scd display (1) rapid changes in strength and timing relative to S and ScS and (2) early arrivals beneath fast lower mantle regions. While the latter feature can be explained by a Clapeyron slope (γ) of 6 MPa/K and a velocity jump of 1.5% when corrected by tomographic predictions, it does not explain the first feature. We expand on this mapping approach by attempting a new parameterization that requires a sample of D" near the ScS bounce point (δVS) where the phase height (hph) and velocity jump (β) are functions of (δVS). These parameters are determined by modeling dense record sections collected from USArray and PASSCAL data where Grand's tomographic model is the most detailed in D" structure beneath Central America. The new phase boundary model suggests sharp gradient around the high phase boundary plateau beneath Central America. This sharp feature agrees with the geodynamic model for transient slab in the lower mantle with post-perovskite phase transition. The halo-like structure could be related to plume activities along the slab. Moreover, the edges of the slabs delimitated by both P and S waves display very rapid changes in phase boundary heights producing Scd and ScS multipathing, which can explain their unstable nature. By applying our new multi-path detector analysis on numerous USArray data, we obtain a patch (200 km) with no phase boundaries which appears to be a plume or ULVZ.
DI41A-1735
Earth's CMB topography and mantle convection
Better understanding topography on Earth's core-mantle boundary (CMB) may provide important constraints on mantle dynamics, specifically the style of mantle convection, and on lower mantle heterogeneity. For example, the origin of large, lowermost mantle low shear wave velocity provinces beneath the central Pacific and Africa is not well constrained, but are likely related to both mantle dynamics and CMB topography. Two competing hypotheses for these anomalies are: thermal upwellings (e.g., plume clusters) or large intrinsically dense piles of primitive mantle material (e.g., thermochemical piles). Here we discuss the results from our current 3D investigation of CMB topography in two styles of mantle convection: 1) an isochemical mantle with plume clusters, and 2) a thermochemical mantle with large, intrinsically dense piles. In this study, we numerically investigate 3D spherical models of mantle convection and calculate maps of topography (CMB and surface, with self-gravitation included) and geoid (CMB and surface). Maps of CMB topography and geoid (CMB and surface) are produced, and compared to observed CMB topography (e.g., Morelli and Dziewonski, 1987; Boschi and Dziewonski, 2000; Sze and van der Hilst, 2003) and surface geoid (e.g., Earth Geopotential Model, 1996). Our predicted surface geoid maps provide a key image of how CMB topography, for any given model, will affect the geoid. The results of this work emphasize the importance in using a suite of observables (in this case, topography and geoid maps for CMB and surface) to constrain whole mantle dynamics and lower mantle structure.
DI41A-1736
The Heat Flow out of the Core and its Temporal Fluctuations
The heat flux at the surface of the mantle displays very large lateral variations on a
long wavelength and the same is expected to be true for the heat flux across the core
mantle boundary (CMB). Time variations of mantle dynamics, as evidenced by the Wilson cycle
with a period of about 400 Myr, reshuffle these large scale lateral variations and should
lead to time fluctuations of heat flow that could be as high as 30% of the total
(Labrosse & Jaupart, 2007).
The heat flow transported by mantle convection controls the energy available to drive the
geodynamo. The present value of the heat flow out of the core is poorly constrained but a
value lower than what is conducted along an isentrope at the top of the core has been
proposed. Even if this is not true at the present time, the large amplitude fluctuations
that are expected make it plausible in the past. When an inner core is growing,
compositional convection may be able to overcome a stable thermal stratification. On the
other hand, before the onset of inner core crystallisation, periods of sub-isentropic heat
flow would lead to dynamo extinction periods, for which no evidence exists for the last 3
Gyr at least. The buffering effect of a dense basal magma ocean on the heat flow
fluctuations is found to be too small to solve that problem and, moreover, its high
content in heat producing radioactive isotopes make the heat flow from the core lower than
that to the solid mantle. Therefore, the existence of such fluctuations restricts the
domain of acceptable values of the long-term super-isentropic heat flow out of the core to
the higher end of the previously discussed range.
http://perso.ens-
lyon.fr/stephane.labrosse
DI41A-1737
Mantle Convection Models with Temperature- and Depth-dependent Thermal Expansivity
We investigated the effects of temperature- and depth-dependent thermal expansivity in 2D cylindrical shell and 3D plane layer mantle convection models. Most previous mantle convection models have employed either a constant coefficient of thermal expansion, α, or a depth-dependent α = α (z), where z is the vertical coordinate antiparallel to gravity. We consider α to have the form α (z,T) = αz(z) αT(T) where αz(z) increases with height z, (or decreases with depth) and αT(T) increases with temperature, T. We find that the depth dependence and temperature dependence of α each have a significant, but opposite, effect on the mean surface heat flux (or Nusselt number) and the mean surface velocity of the convecting system. For α = αz(z), a decrease of α with depth by a factor of four across the mantle causes a decrease of surface heat flux and a decrease in mean surface velocity, relative to the constant-α case. However, when the temperature dependence of α is also included, αT(T) effectively compensates for the effects of αz(z) such that the predicted decrease in heat flow and surface velocity are either eliminated or, in some cases, become increases. Consequently, previous studies that include only the effects of depth dependence of α may underestimate surface heat flow and surface velocities by a significant amount.
DI41A-1738
Vertically Discontinuous Seismic Signatures From Continuous Thermochemical Plumes
To interpret seismic signatures associated with mantle upwellings, we must understand the distribution of thermochemical heterogeneities within mantle plumes. Thermochemical heterogeneities are expected to arise within plumes by the incorporation of subducted lithosphere (Eclogite and Harzburgite) that has reached the plume source region (thermal boundary layers in the mantle). We analyze laboratory experiments in conjunction with seismic velocity models to predict the seismic signature of thermochemical plumes. Laboratory experiments are fully three-dimensional and use glucose syrup (Rayleigh number: 106) to model the mantle and a two-layer subducted lithosphere, where composition (viscosity and density) is controlled by water content. Experiments show heterogeneous upwellings with variations in both temperature and composition that are more complex than predicted in previous plume models. Spatial distributions for temperature and composition in representative, repeatable types of thermochemical upwellings are tracked through time, scaled to mantle values and used to calculate predicted seismic velocities. Apparent seismic velocity signals are estimated for patterns in thermochemical heterogeneity with length scales ranging from 1 to 300 km and excess temperatures from 50 to 300°C. Results show that if plumes are purely thermal they can be identified in the usual way, by slow velocities. However, if plumes are a mixture of compositions, as predicted by laboratory models, their velocity structure is more complex. An Ecolgite lens within a plume at ~300km depth with an excess temperature of 250°C can have the same velocity as regular mantle with no excess temperature. A Harzburgite lobe of a plume head (up to half of the plume volume) at 300km depth with an excess temperature of 225°C can have the same Vs as regular mantle with no excess temperature, but can only mask up to 55°C in Vp. Spatial variations in temperature control velocity structure above 300km, which appears slow for all compositions in a hot plume. Complex seismic signatures are predicted for mantle upwellings forming from source regions containing oceanic lithosphere, including strong variations in velocity with depth. Strong thermal signatures above 300km may be masked by compositional effects below 300km. Results predict that a vertically continuous upwelling will give a spatially discontinuous seismic result.
DI41A-1739
The Influence of the Perovskite-post Perovskite Phase Transition and the Spin Transition in Iron on Layered Mantle Convection
The interaction between the endothermic phase transition(s) at the base of the transition zone and both subducting lithosphere and uprising plumes originating from the deep mantle remains a key issue in geodynamics. The rates of both heat and mass transfer between the upper and the lower mantle are controlled by this interaction. Factors that govern its impact include not only the Clapeyron slope of the transition but also the contrast in physical properties across the interface, notably viscosity. Mass transfer across the level of the endothermic transition at 660 km depth is inhibited by a viscosity increase. Evidence for such inhibition derives from several lines of argument. For example, seismic tomographic images of downgoing slabs trapped in the transition region above 660 km depth are common. We will describe a sequence of new simulations of convective mixing which include the most accurate representations of the temperature and pressure dependence of thermal conductivity, thermal expansively and viscosity as well as the presence of the near surface exothermic and endothermic traditional phase transitions that bracket the transition zone as well as the Perovskite-post Perovskite (Pv-pPv) deep mantle transition that appears to define the D" layer. In these new models we investigate the influence Pv-pPv transition and the spin transition in iron on mantle dynamics, with a focus upon the permeability to mass transfer of the 660 phase transition. This endothermic transition operating in conjunction with a modest but abrupt viscosity increase at 660 km depth results in an episodic but significant decrease in the radial mass flux. The frequency of mantle avalanches decreases as the viscosity contrast increases and the mass flux in each avalanche becomes more diffuse and less intense.
DI41A-1740
Sensitivity to initial CMB temperature of structure and evolution of the Earth's mantle and core based on 3-D spherical models of thermo-chemical mantle convection
The early thermal state of the Earth, just after core-mantle differentiation, is expected to be extremely high temperature, especially the temperature of the core, hence core-mantle boundary (CMB). To understand the evolution of the coupled core-mantle system, it is therefore important to test the effect of extremely high initial CMB temperature, which our previous studies did not do [e.g., Nakagawa and Tackley, 2005]. Here a range of 4500 K to 6000 K for the initial CMB temperature is used to investigate the thermal history of the Earth's mantle and core using 3-D spherical mantle convection models with yielding-induced place tectonics and a parameterized core heat balance, focusing particularly on the time-scale for formation of compositional anomalies near the CMB and the parameter combinations that lead to the correct present day inner core boundary (ICB) temperature. Preliminary results are as follows: 1. The final CMB temperature and heat flux are likely to converge a certain range regardless of initial CMB temperature, 2. The time scale for formation of compositional anomalies in the CMB region is likely to be longer when the initial CMB temperature is extremely high because then plume activity is more vigorous and entrains compositionally-anomalous material. In this presentation, we will also investigate the plausibility of the Basal Magma Ocean hypothesis, so far investigated using only simple models [Labrosse et al., 2007], in more realistic mantle convection models with core-cooling and realistic mineral physics.
DI41A-1741
Transient Slabs and Plumes in the Lower Mantle in Compressible Models With the Post-Perovskite Phase Transition
Compressible convection modeling provides a more realistic framework with which to study convection in the Earth's mantle as the models incorporate processes that are not captured by the commonly applied Boussinesq approximations such as adiabatic and latent heating, viscous dissipation and density stratification. Global scale steady state compressible convection models reveal much about the nature of convection in the Earth's mantle. Few studies, however, investigate the detailed effects of compressibility on the evolution of specific structures in the lower mantle such as slabs and plumes. Previous research suggests that an incompressible thermal slab interacting with a phase transition can explain the observed D" triplication and the temperature structure of the lower mantle may result in a double-crossing of the post- perovskite phase boundary. We develop a suite of half-annulus incompressible and compressible models to explore the transient interaction of a thermal slab in the lower mantle with the post-perovskite phase transition. An initial condition is prescribed such that a thermal slab is emplaced from the base of the lithosphere to approximately 600 km above the core-mantle boundary. The descent of the slab through the phase boundary and its interaction with the lower thermal boundary layer is expected to produce strong lateral gradients in seismic velocity anomaly that greatly influence travel-times. We generate synthetic seismograms using ray tracing and then compare to global waveform observations. While finding that the post-perovskite phase transition promotes more vigorous plume activity the models suggest that this effect is not of fundamental importance to the dynamics of the system. Our results suggest that incompressible slabs are passive structures at the core-mantle such that diffusive processes dominate. By comparison, compressible slabs more actively participate in the overall flow structure and develop sharper edges in the temperature field. A detailed study is now underway to understand these differences by considering the additional heating terms that are introduced into the energy equation for compressible flow.
DI41A-1742
Why post-perovskite should have a low viscosity and its dynamical consequences .
In the past 4 years the discovery of the post-perovskite (PPV) phase of ( Mg, Fe)SiO3 [1,2] has revolutionized the geosciences. Previous work on PPV has focussed mainly on aspects of equation of state, such as density, elastic constants, Clapeyron slopes and temperature intercepts. These studies have brought a consistent picture of the thermal and seismic structure of the lower mantle. However, the viscosity of PPV holds the key to any sort of predictability concerning the instabilities generated in the D" layer. However, viscosities of solids are extremely difficult to measure experimentally or to predict theoretically, due to a disparity of time and length scales and extremely small deformation rates. We propose an alternative approach, based on a combination of evidence from mineral physics, geomagnetism and geodynamics, to conclude that PPV viscosity is about 2-3 orders of magnitude less than that for perovskite and this has important dynamical consequences. One can show that the viscosity based on diffusion is proportional to the inverse of the electrical conductivity based on ionic processes of oxygen anions. Based on analogy with Al2O3, for which direct measurements of the conductivity under pressure were available since 1996 [3], Oganov and Ono proposed in 2005 that PPV should have a higher (by ~2 orders of magnitude) ionic electrical conductivity than perovskite [4,5]. This was confirmed by recent experiments of Ohta on (Mg,Fe)SiO3 [6] and suggests that there should be a concomitant decrease of the PPV viscosity. Geoid modelling work with laterally varying viscosity also reveals that the the long wavelength regions of the D" associated with descending slabs with PPV to have a viscosity around 100 to 1000 times smaller than the viscosity associated with regions having a lower seismic velocity anomalies. By means of a two-dimensional finite-element model, we show that the presence of a very low viscosity PPV lens above the CMB exerts a strong influence on the heat flux from the core, inducing sharper heat flow peaks than models without the PPV transition. The time dependence of the heat flow is also more chaotic with PPV transition. This finding may have important implications for core-mantle coupling and the character of the Earth's magnetic field. [1] Murakami M., Hirose K., Kawamura K., Sata N., Ohishi Y. (2004). Science 304, 855-858. [2] Oganov A.R. & Ono S. (2004). Nature 430, 445-448. [3] Weir S.T., Mitchell A.C., Nellis W.J. (1996). J. Appl. Phys. 80, 1522-1525. [4] Oganov A.R., Ono S. (2005). Proc. Natl. Acad. Sci. 102, 10828-10831. [5] Ono S., Oganov A.R., Koyama T., Shimizu H. (2006). Earth Planet. Sci. Lett. 246, 326-335. [6] Ohta K, Onoda S, Hirose K, Sinmyo R., Shimizu K., Sata N., Ohishi Y., Yasuhara A. (2008). Science 320, 89-91.
DI41A-1743
Perovskite and Post Perovskite Phase Relation in the MgSiO3-Al2O3 System
It has been believed that a few mol% Al2O3 would dissolve into (Mg,Fe)SiO3 in the Earth's lower mantle. Existence of aluminum in MgSiO3 is thought to change the volumes, elasticity and stability relations of perovskite (Pv) and post perovskite (pPV). The phase diagram of Al-bearing MgSiO3 reported both experimentally and theoretically shows that Al drastically increases the pPV transition pressures with significant Pv+pPV co-existence regions. The large two phase loops are however irreconcilable to the transition seismically detectable as the D" discontinuity. Here we investigated finite temperature thermodynamic properties in the MgSiO3-Al2O3 system by means of the density functional first principles method with multiple configuration sampling. Based on the results, we will propose revised high-P,T phase equilibria in this system. Through the calculations, interesting structural properties of aluminous Pv and Rh2O3(II) solutions were also discovered. Research supported by the Ehime Univ Project Fund and in part by JSPS.
DI41A-1744
Stability and Elasticity of High Iron and Aluminum Post-Perovskite Phases and Their Implications for the D" Layer
To evaluate the iron and aluminum effects on the post-perovskite phase at deep mantle conditions, it is important to study the potential mantle silicates containing both iron and aluminum. In this study, three different compositions of natural garnet along pyrope-almandine join, Pyr21Alm73Gr5, Pyr43Alm54Gr2, Pyr58Alm38Gr3, were used as starting materials to investigate the stability and elasticity of high iron- and aluminum-bearing post-perovskite phase at deep mantle conditions. In situ high-pressure and high- temperature experiments were conducted at beamline 13-ID-D of GSECARS, Advanced Photon Source. A monochromatic beam with a wavelength of 0.3044 Å and a MAR CCD detector were used for X-ray diffraction data collections. Samples were loaded in the symmetrical diamond-anvil cells and heated by the double-sided laser heating system. Our results showed that the post-perovskite phase can be successfully synthesized from three different compositions at pressure greater than 160 GPa and temperature higher than 1600 K. This indicates that the post-perovskite phase can simultaneously accommodate high aluminum and high iron contents. However, Al2O3-post-perovskite phase can also be observed from some runs for Pyr43Alm54Gr2 and Pyr58Alm38Gr3, showing that there is actually a limit for incorporating the aluminum into the post-perovskite phase but not for iron. In addition, we also found that the volume of post- perovskite phases can also be affected by the incorporated amount of iron. Our pressure-volume results showed that high-iron post-perovskite phases have larger volumes and the iron effect is greater at pressure above 120 GPa.
DI41A-1745
First principles study on elastic, thermodynamic and vibrational properties of MgGeO3 post-perovskite
Now the relationship between the seismic velocity heterogeneity observed in the deep mantle and the
properties of post-perovskite phase of MgSiO3, possibly the most abundant mineral component in the
Earthfs lower mantle, is being discussed extensively. Since the MgSiO3 has quite high transition
pressure, study of low-pressure analogs is also important. Although the high-P,T phase relation of
MgGeO3 is predicted to be quite similar to those of MgSiO3 including the Clapeyron slope, it is still
not well understood how similar the thermodynamic and elastic properties of analogs to those of
MgSiO3. We show the physical properties of MgGeO3 perovskite and post-perovskite calculated
based on the density functional first principles methods, and compare them to those reported for MgSiO3.
Results indicate MgGeO3 is a much better analog to MgSiO3 compared to CaIrO3 in terms of
these physical properties but not fully comparable to MgSiO3.
http://www.sci.ehime-u.ac.jp/~usui/
DI41A-1746
Structural Transitions and Strengths of Mg-Silicate Glasses to 80 GPa
Early Earth's mantle may be molten to its deep interior. Therefore, physical processes in the magma ocean determine the initial conditions for the structure of the mantle. Seismologic studies have shown that the present-day mantle has melts atop the transition zone and the core-mantle boundary. Therefore, the properties of silicate melts are important for understanding the evolution of the Earth's mantle. We measured Raman scattering of silicate glasses, frozen forms of melts, with mantle related compositions, MgSiO3, Mg2SiO4, and CaSiO3, in the diamond-anvil cell up to 80 GPa. In MgSiO3 glass, Raman spectra show a structural transition associated with increases in the coordination number at 19-38 GPa and another transition likely related to changes in the Si-O polyhedral connectivity at 65-70 GPa. However, in CaSiO3 and Mg2SiO4 glasses, the former transition occurs at higher pressures (P) by 5-10 GPa and the latter transition does not occur to our maximum P, indicating that a less polymerized Si-O network increases the transition P. The compositional sensitivity of the transition P in silicate melts would result in dense MgSiO3 melt in a high-P structure and less dense Mg2SiO4 and CaSiO3 melts in low-P structures at mid-mantle depths. This density contrast will make Si-rich melts negatively buoyant but Si-poor melts postively buoyant, resulting in the Si-enriched deeper mantle. Therefore, the early magma ocean may be compositionally stratified and therefore contribute to the formation of "hidden geochemical reservoir". Our findings provide a physical mechanism for the early differentiation suggested from geochemical observations of short-lived 142Nd (Boyet and Carlson, 2005). We also measured the yield strength of MgSiO3, Mg2SiO4, and SiO2 glasses to 55 GPa in the diamond cell through the pressure gradients in the samples. The change in thickness with P was monitored visually by measuring the change in focal point from both sides of the sample. Yield strength does not appear to be signficantly different within uncertainties among glasses of different compositions. The measured strength of SiO2 glass is lower than previously reported, but we still observed the same trend from room pressure to 50 GPa (Meade and Jeanloz, 1988).
DI41A-1747
Melting experiments on pyrolite and MORB materials under deep lower mantle conditions
Deep mantle melting has profound implications for crystallization from magma ocean and partial melting at the base of the mantle in the primordial and present Earth, respectively. Partial melting causes strong differentiation in chemistry and density by segregation of partial melt. The melting phase relations and element partitioning are important to understand this differentiation. Melting experiments on mantle materials were made only up to 35 GPa by multi anvil press (i.e. Ito et al., 2004). Laser-heated diamond anvil cell (LHDAC) should be required for melting experiments at higher pressure conditions, but it has never been made because of technical difficulties of chemical analysis of recovered sample and generation of ultrahigh P-T conditions. Here, we investigated the phase relation of pyrolite and MORB materials using LHDAC up to 70 GPa. Starting materials were gels with chemical compositions of KLB-1 peridotite and MORB. The sample disk was coated with a film of Re as a laser absorber. Laser heating was made for single side by a Nd:YLF laser. The recovered sample from LHDAC was processed to thin film for field emission-type micro probe using Ion Slicer. The result obtained by map analysis indicated phase segregation along the expected temperature gradient for all melting samples. The phase with non-stoichiometric chemical composition was found at the laser absorber end, which should be hottest portion in the sample chamber. This is most likely quenched-liquid. Chemical composition of a phase adjacent to quenched-liquid is that of crystalline phase, which is likely the liquidus phase followed down by some crystalline phases. Such a texture most likely represents the melting phase relation and is very similar to that obtained in melting experiments in multi anvil apparatus. In melting experiments in pyrolitic system at 70 GPa, we confirmed that Mg-perovskite (Mg-pv) remains as a first liquidus phase, while Ca-perovskite (Ca-pv) become a second liquidus phase. Melting phase relations in MORB at 70 GPa showed that crystallization sequence is Mg-pv, Ca-pv, and stishovite (st) decreasing with temperature contrary to that at 27 GPa which is Ca-pv, st, Alminous phase, and Mg-pv (Hirose and Fei, 2002). Our results suggest that melting phase relations change with pressure even under deep lower mantle conditions.
DI41A-1748
Implications of Laser-Driven Shock Experiments for the Core-Mantle Boundary
New techniques using laser-driven shock waves reveal the pressure – temperature conditions at which mantle rocks (silicates and oxides) become metallic fluids, hence core-like constituents. Experiments reaching peak pressures and temperatures of about 700 GPa (7 Mbar) and 3 x 104 K (3 eV) allow interpolation with lower-pressure measurements to reveal the properties of MgO, MgSiO3 and SiO2 at conditions of deep planetary interiors. We document shock-induced melting of MgO (14,970 ± 300 K at 470 ± 10 GPa) and MgSiO3 (7740 ± 450 K at 275 ± 10 GPa), and confirm previous findings for SiO2 (melting at 5620 ± 450 K at 116 ± 10 GPa), by monitoring temperature as a function of time for each sample as it is compressed by a strong but decaying shock wave. The melts show evidence of reflectivity > 20% at visible wavelengths, implying metallization ('ionization') of these oxides through the combined effects of pressure and temperature. These results indicate a blurring of the distinction between silicate and metallic constituents at the core–mantle boundary, particularly at the high temperatures immediately following a late-stage giant (Moon-forming) impact.
DI41A-1749
Ab-initio Predictions of Potassium Partitioning Between Lower Mantle Phases
The distribution of radioactive isotopes in the Earth's deep interior is of fundamental interest to geophysics, geochemistry and geodynamics as this could potentially influence the distribution of radiogenic heat production and hence the thermal evolution and dynamics of the Earth. 40K is the most abundant long- lived radiogenic isotope in the Earth and as such, where potassium resides in the Earth's interior is of interest. Isolated deep Earth reservoirs of potassium in the core or in Mg-silicate post-perovskite (Mg-ppv) have been previously shown to be unlikely [e.g., Lee et al., 2008]. The lower mantle comprises approximately two-thirds of the Earth's volume, hence the distribution of potassium between the lower mantle phases: iron-bearing Mg silicate perovskite (Mg, Fe)SiO3 (Mg-pv), magnesiowüstite (Mg, Fe)O (mw) and a calcium silicate perovskite CaSiO3 (Ca-pv), is useful to understanding radiogenic heat distribution. The large A-site of Ca-pv provides a potential environment for heat-producing elements [e.g., Kato et al., 1988]. Here we address the partitioning of potassium between iron-bearing Mg-pv and Ca-pv by means density- functional theory-based ab-initio electronic structure computations using a planewave code (VASP). Based on the energetics we compute the equilibrium constant for K distribution between Mg-pv and Ca-pv at lower- mantle conditions.
DI41A-1750
Elasticity of Hollandite
KAlSi3O8 (K-hollandite) is a high-pressure polymorph of potassium feldspar and has been believed to be the most abundant phase in the continental crustal material that has been subducted to pressure and temperature conditions relevant to the mantle transition zone. They are possible reservoirs for large ion lithophile elements such as K, Na, Pb, Sr and are potential candidates to generate EMII geochemical signatures. The stability of K-hollandite has been experimentally studied. Although there are geochemical signatures for crustal components, how well could we detect such pockets geophysically? Our study aims to address this issue by determining elastic property of hollandite. We are undertaking an indepth analysis of crystal chemistry of hollandite relating structure-property and stability of these phases using first-principle electronic structure simulations. We have explored the elastic properties of K- and Na- hollandite over a wide range of pressures. Results of compression for the K - hollandite phase is well represented by a third order Birch-Murnaghan finite strain expression with KO = 207 GPa, K'= 4.47 and VO= 233.12 Å 3. The zero pressure volume is 1.8 % smaller whereas bulk modulus is 13 % larger than experimental values. This is expected since the comparison is not direct, our results are static (0 K) as opposed to experimental data at 300 K. Corresponding values for Na and Ca- hollandite phase are KO= 196.83 GPa, K'= 3.54, VO= 225.85 Å 3 and KO= 197.92 GPa, K'= 3.31, VO= 232 Å 3. The bluk modulus of Na and Ca endmember are similar owing to similarity of the cation size, where as K being a larger cation renders a much larger volume and stiffer structure. Hollandite has a tetragonal symmetry (I4/m) at ambient conditions and undergoes phase transition at 22 GPa to a lower symmetry monoclinic phase (I2/m). For K-hollandite we are exploring this phase transitions and a finite strain fit for the high pressure phase shows KO= 192 GPa, K'= 4.28 and VO= 234.37 Å 3. At 0 K, the phase transition, based on crossover of enthalpy of tetragonal and monoclinic structure occurs at ~36 GPa as opposed to ~20 GPa at ambient (300 K). We will determine full elastic tensor for high symmetry (I4/m) hollandite and develop velocity models based on crustal components to compare with local seismic models and infer about the chemistry in the interior of the earth.
DI41A-1751
Thermoelasticity and phase relations of Fe2SiO4 composition at P and T conditions of the mantle
The mantle is comprised primarily of magnesium-iron silicates and oxides. The goal of this research is to examine how iron affects the phase stability, thermoelasticity, and density of mantle minerals in order to help explain seismological observations, and to constrain geochemical and geodynamic models of the mantle. Our experimental goal was to examine the Fe2SiO4 phase assemblage as a function of pressures and temperatures corresponding to the transition zone to the deep mantle. Several series of X-ray diffraction experiments in conjunction with laser heating in diamond anvil cells were performed at GSECARS at the Advanced Photon Source. Starting materials were either a polycrystalline natural fayalite, or a single crystal synthetic fayalite. In all cases, NaCl was used both as a pressure calibrant and a thermal insulator. We collected diffraction patterns of the assemblages up to pressures of 63 GPa and temperatures of 2500 K to determine phase stability and densities with respect to the NaCl pressure standard. We observe the alpha fayalite/ringwoodite transition and the ringwoodite/wustite plus stishovite transition in agreement with previous studies. For each of these phases, we determine density and a thermal expansion parameter at mantle-relevant pressures and temperatures. We find a thermal expansion for iron-ringwoodite of 1.3 (0.1) E-5 K-1 and a thermal expansion for the B1 phase of wustite of 2.1 (0.4) E-5 K-1. The new thermoelastic parameters are used to help refine models of iron's effect on the elastic behavior of the transition zone and mantle phases. Above 45 GPa and at high temperatures, we observe new diffraction peaks coincident with the disappearance of diffraction peaks arising from SiO2 and FeO. These results are used to redraw the FeO SiO2 phase diagram at the high P, T mid-mantle conditions.