U44A-01
Determination of post-perovskite phase transition boundary in MgSiO3 up to 4400 K and implications for thermal structure at the base of the mantle
Thermal structure within the D" layer is of great significance because it determines the rheology and thus the dynamics in the D" and modulates the heat flow out of the outer core. A newly discovered post-perovskite phase transition can provide tighter constraint on the thermal structure within the D" based on the idea "double- crossing". In the thermal boundary layer, a steep geotherm is likely to intersect the post-perovskite phase transition boundary twice, which is supported by the presence of paired seismic discontinuities. Since this model strongly depends on the phase transition boundary, it should be determined more precisely. Here we conducted the phase equilibria experiments on MgSiO3 at a much wider range of PT conditions, 113-170 GPa and 2500-4400 K, in order to refine the post-perovskite phase transition boundary on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell. Both MgO and gold were used as internal pressure standards. Argon was loaded as a pressure medium and thermal insulation. Post-perovskite phase transition boundary was determined in both the forward and reverse directions. Our results show that the Clapeyron slope is larger than +11.5 MPa/K (Hirose et al., 2006 GRL) based on MgO scale and that resultant phase transition temperature at the CMB pressure is about 3300 K. This observation implies that even moderate temperature gradient in the D" could cause double-crossing.
U44A-02
Absence of P-wave Reflectivity Near the D" S-wave Velocity Discontinuity in the Lowermost Mantle beneath the Cocos Plate
An abrupt 1-2.5% S-wave velocity increase has been observed in the lowermost mantle beneath the Cocos plate in several studies. This is commonly attributed to the perovskite to post-perovskite phase transition. This phase transition is expected to have much weaker effects on P-wave velocities than on S-wave velocities and the depth range of the transition is expected to depend on Al and Fe content of the perovskite. We image lowermost mantle P-wave reflectivity beneath the Cocos plate using 1D stacking and 3D Kirchhoff migration. Our carefully processed data set comprises 8000 seismograms from deep South American earthquakes recorded by broadband and short-period seismic networks in western North America. Stacked P-wave source wavelets are deconvolved from the data for each event, allowing band-pass filtered signals to be combined for many events. Events are discarded if individual event-stacks do not show an impulsive PcP arrival with significant signal-to- noise ratio. Depth shifts are applied to each event to align PcP arrivals in the combined stacks. These shifts increase systematically from south to north. We observe a widespread weak P-wave reflector about 320 km above the CMB modeled well by a P-wave velocity (Vp) change of –0.2 to –0.4%, depending on the thickness of the velocity change. The depth of this relatively flat reflector is tightly constrained and is a few tens of km shallower than the local S-wave reflector, which may have regional topography of up to 100 km. We model a clear feature in the P-wave reflectivity with a change in dVp/dZ about 180 km above the CMB, accompanied by a sharp 0.2% increase in Vp. Different narrow band filters up to 2 Hz and forward modeling of double-array stacks show that this small velocity increase must occur over less than 10 km in depth. This also does not directly correspond to any significant feature in the S-wave velocity structure. The high signal-to-noise ratio of our locally binned data stacks allows us to preclude the existence of any other abrupt Vp change stronger than ±0.1 to ±0.4%. These upper bounds are influenced by the source-receiver geometry of the data and the thickness of the velocity change. A strong, positive velocity contrast, out-of-plane scatterer is observed in the migrations, and appears to originate shallower than 200 km above the CMB. Finally, we show that the PREM values for P-wave attenuation in the lowermost mantle are too low, and need to be at least three times greater locally to match our broadband observations.
U44A-03 INVITED
Global Predictions of the Post-Perovskite Phase Thickness
A lower mantle S-wave triplication with a Scd branch occurring between S and ScS appears to be explained by a recently discovered Perovskite (PV) to Post-Perovskite (PPV) phase-change. Recent mineral physics experiments indicate a positive Clapeyron slope (γ) between 5 to 13 MPa/K with a small S-velocity jump and an even smaller P-velocity jump cross the phase boundary. Earlier, Sidorin et al. (1999) used a mapping of δVs from Grand's tomographic model to estimate ΔT (temperature) and the differential times (Scd-S) to determine a global reference height (hph = 200 km) and a γ = 6 MPa/K assuming a jump in shear velocity β = 1.5%. Here we use the δVs to estimate hph and β locally by matching synthetics to observations over the S-Scd-ScS window to set up a mapping parameterization. The timing separation between (Scd-S) fixes the phase onset (PV to PPV) and (ScS-Scd) fixes the phase return (PPV to PV). We find that the velocity jump is twice as strong beneath slow regions as fast regions requiring distinct reference heights indicative of changing chemistry. Moreover, the edges of the supposed buckled slabs delineated by both P and S-waves in tomographic display very rapid changes in phase boundary heights producing complex Scd waveforms. These features can explain the scatter in (Scd-S) data sets and the unstable nature of this phase with easy detection to no detection commonly observed. We generate global models for various values of γ's, displaying the temperature at the phase boundary and PPV thickness. The temperature variation for γ = 3 Mpa/K is over 1000 K which appears extreme based on effects produced by sinking slab rates. Values of 6 <γ< 12 MPa/K appears to be easiest to reconcile with seismic data, although the PPV thickness is poorly sampled beneath the Superplumes.
U44A-04 INVITED
Inverse scattering of ScS and SKKS waves: high resolution, large scale imaging of Earth's core-mantle boundary region
With 3-D inverse scattering of broad band waves we have begun to explore the deep mantle on an unprecedented spatial scale. We combine concepts from inverse scattering (generalized Radon transform, or GRT) and statistics into an approach toward imaging structure near Earth's core-mantle boundary with large amounts of broad-band, three-component ScS and SKKS seismograms acquired by global seismograph networks. Extracting structural information requires few restrictive a priori assumptions about the structures of interest, which makes it complementary to forward modeling approaches. With a generalized Radon transform (GRT) we map broad-band seismogram windows -- comprising the main arrivals and its coda and precursors -- into images that reveal multiple, piece-wise continuous (and statistically significant) interfaces in the lowermost mantle. Tomographic wavespeed perturbations and the variation in depth to a widespread interface 150-300 km above the CMB are consistent with a post-perovskite (ppv) transformation. Stratification below it may results from multiple phase boundary crossings, and a locally observed wavespeed drop above the CMB may mark the base of a ppv-rich lens. We use thermodynamic properties of these phase transitions to estimate temperatures just above the core-mantle boundary (CMB). We infer a temperature at the CMB of 3,950±200 K. Beneath Central America, a site of deep subduction, the deep mantle is relatively cold (ΔT=700±100 K) and core heat flux high (qcmb=80-160 mWm-2, for thermal conductivity κ=5-10 Wm-1K-1). Away from it, the heat flux reduces to 35-70 mWm-2. GRT imaging with ScS can reveal D" structure beneath selected geographical regions (e.g., central and north America, north Pacific, east Asia), but sampling of underside SKKS reflections may allow hemisphere scale D" imaging.
U44A-05
A Bound on Heat Flow Below a Double Crossing of the Perovskite-Postperovskite Phase Transition
A double crossing of the perovskite-postperovskite phase transition in (Mg,Fe)SiO3 has been proposed to explain seismic reflections from structures near the base of the mantle. Estimates of temperature inferred from the phase transitions can be extrapolated to the core-mantle boundary (CMB) using a simple model for the thermal boundary layer. However, a complication arises when flow is driven by the (negative) buoyancy of the postperovskite phase. Latent heat release and advective transport increase the temperature gradient and heat flow below the region of postperovskite. A minimum bound on the temperature gradient at the CMB is obtained by noting that the temperature gradient at the base of the postperovskite region must be at least as steep as the transition temperature with depth. Both the temperature and heat flow at the CMB depend on the vertical velocity at the phase transition. A representative velocity of 1 mm/yr gives a local heat flux of 160 m W m-2. Even higher heat flow is predicted as the velocity increases. Such high heat flow is unlikely, so the heat flow bound can be used as a constraint on both the viscosity of the lower mantle and the geometry of the postperovskite layer.
U44A-06
Using a New Multi-Discipline Approach to Predict Seismic Tomography from Geodynamical Models of Mantle Convection
Understanding the first-order dynamical and compositional structure of Earth's mantle is a fundamental goal in solid-earth geophysics. Rudimentary knowledge of mantle structure relies on observations from geochemistry and seismic tomography and on inferences from theoretical convection models. Seismic tomography has shown the lower mantle to be dominated by a long-wavelength lateral velocity structure, characterized by broad low shear-wave velocity anomalies beneath Africa and the Central Pacific. Mantle convection models which describe the lower mantle as homogeneous explain the anomalies as clusters of small thermal plumes in an isochemical mantle while heterogeneous mantle models place the anomalies into the context of thermochemical piles characterized by an anomalously dense chemical component. Direct comparisons of temperature fields from theoretical calculations to the observed global shear-wave seismic tomography models suggest that models of thermochemical convection better recreate the lower mantle velocity structure however significant differences in the resolution of seismic and geodynamic models complicate such direct comparison. The parameterization of tomographic models is an order of magnitude larger than that of the numerical models, and the former are inherently spatially heterogeneous. Consequently, tomographic models distort heterogeneities, resulting in a blurred image of mantle structure. It is therefore plausible that the anomalies are poorly-imaged clusters of thermal plumes and not dense piles as consensus suggests, reopening the possibility of isochemical convection as being the mode of heat and mass transport which characterizes Earth's mantle. Using a new multi- discipline technique based on first-principle mineral physics, we perform calculations to predict seismic tomography from theoretical geodynamical models. We investigate whether seismic observations can be used as a constraint to distinguish between numerical models of plume clusters in an isochemical mantle and thermochemical piles in a chemically heterogeneous mantle as possible causes of the seismically observed lower mantle velocity structure. We find that our geodynamically predicted tomographic images differ significantly from the original numerical temperature fields, suggesting that the difference in resolution between seismic models and theoretical models is an important parameter to take into account when investigating lower mantle structure.
U44A-07 INVITED
The Basal Magma Ocean
The presence of partial melt at the base of Earth's mantle today implies a more extensive basal melt layer in its hotter past, stabilized by its enrichment in Fe and small or inverted molar volume difference relative to solids at high pressure. The low viscosity of such a basal magma ocean formed early in Earth's history ensures it was subject to vigorous convection and nearly isentropic conditions, and it will have undergone slow fractional crystallization from the top down at a rate determined by secular cooling and its phase diagram, likely enriching it in Fe-bearing components and thus further increasing its density. The basal magma ocean also became further enriched in other incompatible elements as it crystallized, and its evolution can be constrained by 1) the systematic differences in \varepsilon142Nd of Earth samples compared to chondrites 2) the initial thickness of about 1000 km to provide for the hidden reservoir of heat producing elements, 3) coupled mantle and core thermal evolution models, and 4) the present day ULVZ thickness of order several km. Plausible thermal evolutions yield an approximate exponential decrease in thickness with time, and all of the estimates consistently converge on e-folding time scales for decay of around 1 Gyr. The remaining 4-6 TW of heat produced in this layer at the present time decreases the amount of core cooling required of the high temperature gradients in D" implied by post-perovskite-related seismic discontinuities, and core thermal evolutions predict a decreased cooling rate in the past such that a geodynamo may not have been viable in Earth's early history.
U44A-08
Deep mantle dynamics in 3D spherical geometry incorporating a realistic phase diagram calculated by free energy minimization
Deep mantle dynamics and the resulting thermo-chemical-phase structures are here studied using thermo- chemical mantle convection simulations in a 3D spherical shell that incorporate composition-dependent phase diagrams calculated by free energy minimization. This improves on our previous studies, which used simple depth-dependent thermodynamic properties and calculated seismic anomalies based on linearized derivatives around a pyrolitic mean composition. Realistic mineral assemblages of mantle rocks have several high pressure and temperature phases, which vary substantially as composition changes from MORB-like to harzburgitic. Linearized treatments probably do not adequately capture the variation of physical properties with composition and temperature. In order to get closer to a realistic mineralogy, we here calculate composition- dependent mineral assemblages and their physical properties using the code PERPLEX, which minimizes free energy for a given combination of oxides as a function of temperature and pressure, and use the resulting properties in a 3-D spherical numerical model of thermo-chemical mantle convection, with three-dimensionally- varying physical properties [Nakagawa et al., 2007 in Goldschmidt conference]. Preliminary results are that while thermo-chemical structures are not greatly different from in the previous treatment, the spectral profiles of seismic anomalies seem to match seismic tomographic models more closely. Here we extend these results to focus on seismic signatures of the deep mantle including the post-perovskite phase transition. There is still uncertainty in the thermodynamic properties of the post-perovskite phase; hence the phase relationship of post-perovskite and its composition-dependence is treated as ‘adjustable' within mineral physics uncertainties. The thermal- chemical-phase structures in our latest numerical simulation models are compared to the latest seismologically- observed structures.