DI44A-01 INVITED
Synergy Between High-pressure Experiment and First-principle Computation in Study of Earth's Deep Interior
The physical properties of minerals, which constitute Earth's mantle and core, are important keys to understand the structure, composition, dynamics and evolution of the Earth's interior. Experimental and theoretical studies for physical properties of minerals were generally presented in separate papers often with a publication gap of a couple of years. In the last decade, theoretical methods based on the quantum mechanics have advanced to the point where they can provide reliable data at extreme conditions where high-pressure experiments can not achieve. Recently, our approach using both the high-pressure experiment and the first-principle computation was successful to discover new minerals and to predict physical properties of many materials at high pressures and temperatures. In the case of oxide studies, the discovery of new iron oxide (Fe2O3) in high-P experiments [1] inspired to discover the post-perovskite phase by ab initio calculations [2]. The identification of new aluminous oxide (Al2O3) by calculations [3] led the determination of the electrical conductivity at the base of the mantle [4]. In the case of carbonate studies, calculations solved the structure of the new phase which had not been identified by experiments [5] and helped to search new phases in experiments [6,7]. Ab initio calculations are also powerful tool to investigate iron alloys accompanied by the spin (or magnetic) transition. The synergy between our experiments and calculations also led to find new phases of iron sulfide in the Earth's core conditions [8]. The objectives of this talk are to highlight developments in our theoretical and experimental studies, to show the presentation of state of the art joint theoretical and experimental works of oxides, silicates, carbonates and iron alloys, and to consider challenges for the subject in the future. [1] Ono et al., J. Phys. Chem. Solid. 65, 1527+ (2004). [2] Oganov and Ono, Nature 430, 445+ (2004). [3] Oganov and Ono, PNAS 102, 10828+ (2005). [4] Ono et al., EPSL 246, 326+ (2006). [5] Oganov et al., EPSL 241, 96+ (2006). [6] Ono et al., Am. Mineral. 92, 1246+ (2007). [7] Oganov et al., EPSL 273, 38+ (2008). [8] Ono et al., EPSL 272, 481+ (2008).
DI44A-02
Redetermination of Post-Perovskite Structure of (Mg,Fe)SiO3
Since the post-perovskite (ppv) phase in MgSiO3 was discovered to be similar structure of CaIrO3 (Cmcm Z=4) under the conditions of near Earth's core-mantle boundary, many investigations have provided explanations for the presence of low seismic velocity. However, precise experimental structure analysis of ppv-(Mg1-xFex)SiO3 has never been reported because of the experimental difficulty. Fe and Mg cation distribution and ordering in ppv-(Mg,Fe)SiO3 due to cation properties including spins states are significant subject in lower mantle electronic and magnetic states. The present experiment aims the Rietveld profile fitting of ppv-(Mg0.6,Fe0.4)SiO3 by the precise powder diffraction measurements and the statistic process of the twenty observed. We found the possible structure models of ppv-(Mg,Fe)SiO3 by the Monte Carlo calculation. The most relable structure among those models in the Rietveld fitting of ppv-(Mg0.6Fe0.4)SiO3 is the structure of space group Pmma, in which Fe and Mg occupy two sites of M1 and M2: the site occupancies are (Fe0.25(3)Mg0.75(3)) in the larger M1 site and (Fe0.55(3)Mg0.45(3) ) in the larger and more distorted M2 site. Fe2+ in the M2 site is in the low-spin state, which reduces the ion radius. Consequently Fe2+ becomes smaller ion than Mg2+. These two sites are distributed alternatively zigzag mode in the direction of a. axis, About 70% of Fe atoms occupies at the M2 site. (Notice that Fe and Mg cations are randomly distributed in only one site in Cmcm.) The two-site model is consistence with the results of X-ray emission. Further more the first principles calculation also shows the two-site model is more energetically possible under high-pressure conditions. MgSiO3 may have the two-site structure of Pmma instead of Cmcm. If it is not the case, the Pmma structure is the new structure and the phase boundary in the solid solution (Mg1-xFex)SiO3 has to be determined. More data analyses based on precise powder diffraction measurements over a broader range of P-T conditions need to solve these problems.
DI44A-03
Waveform inversion for localized seismic structure and its application to D
In order to fully extract information on localized seismic structure from observed seismic data, we have developed a methodology for seismic waveform inversion. The calculation of synthetic seismograms and their partial derivatives are the key steps in such an inversion. We have developed accurate and efficient methods for calculating broadband synthetic seismograms for spherically symmetric transversely isotropic media for both shallow and deep events, which allows us to compute synthetics up to 2 Hz or higher frequencies (Kawai et al. 2006, GJI). Then, wWe formulate the inverse problem of waveform inversion for localized structure using the efficient algorithm of Geller and Hara (1993), computing partial derivatives for the 3-D anisotropic elastic parameters, including anelasticity, at particular points in space. Our method allows us to conduct both local and multi-scale global waveform inversion using pixel (or local shell) parameterization. We previouslyhave conducted waveform inversion for the vertical profile of the shear velocity in the lowermost mantle beneath Central America and the Arctic, beneath which the shear velocity is faster than the global average (Kawai et al., 2007ab, GRL). The obtained models suggest that the S-velocity increase in D'' may be localized in the zone from 100-200 km above the core-mantle boundary (CMB), while the S-velocity does not significantly deviate from PREM in the zone from 0-100 km above the CMB. In this studywork, we studied D'' beneath the Pacific, where the S-velocity is supposed thought to be slower than the global average on the basis of by many tomographic studies. models (e.g. Takeuchi 2007). We use the transverse component of broadband waveforms (for the period range, 8- 200 s). observed waveforms. We found 1-1.5% velocity decreases and increases in the zones from 400-500 km and from 300-400 km above the CMB, respectively. In addition, we found 0.5-1% velocity increases and decreases in the zones from 100-200 km and from 0-100 km above the CMB, respectively. This is interpreted using the experimental results for phase relations in pyrlolite and basalt (Ohta et al. 2008). The shallowerupper velocity decrease can be interpreted as due to the phase transitions in basalt from Mg-pv to Mg-ppv (Tsuchiya and Tsuchiya 2006) and from CaC2-type to α-PbO2type SiO2 (Karki et al. 1997), and the upper shallower velocity increase can be interpreted as due to the phase transition in pyrolite from pv to ppv (Tsuchiya et al. 2004).
DI44A-04 INVITED
Confessions of a seismologist: What we can or cannot say about phase, temperature, and chemistry of the lower mantle based on seismic tomography
Seismic tomography has revealed regions of extreme heterogeneity at the core-mantle boundary (CMB). Most notably, the broad, slow anomalies in the central Pacific and Africa and circum-Pacific fast anomalies. For years, the deep-Earth community has attempted to distinguish between the thermal and chemical contributions to the observed increases and decreases in seismic velocity. With the discovery of a possible post-perovskite phase transition in the lowermost mantle (Murakami et al., 2004, Oganov and Ono, 2004), it is now pertinent to evaluate the effect of post-perovskite on tomography models. Analysis of tomography can reveal a few things with some certainty. 1) The ratio of shear to compressional velocity (R) is very unstable and not a useful measure of chemical heterogeneity. A more robust measure is the bi-modal distribution of shear velocity, but not compressional velocity in the lowermost mantle. 2) While it could be possible for post-perovskite to cause an anti-correlation of bulk sound speed and shear velocity, the observed shear velocities near the CMB are far too slow to be due to the phase change alone and still require a change in chemistry in these regions. 3) If the fast anomalies in the lowermost mantle are assumed to be due to cold temperatures and the subsequent presence of post-perovskite, then we can place tight bounds on the temperatures near the CMB.
DI44A-05 INVITED
Dynamical Effects of the Post-Perovskite Transition in Thermal and Thermo-Chemical Mantle Convection
Several studies have focused on the post-perovskite (PPV) transition's possible dynamical effect, as well as the complex seismological structures that may arise through the interplay of variations in temperature, composition and the PPV phase transition. Here these issues are explored using numerical models of thermal and thermo-chemical convection in various geometries including a three-dimensional spherical shell. A zero-, single- or double- crossing of the PPV phase boundary is observed depending on the temperatures of the CMB and deep mantle; this evolves with time as the core and mantle cool. The PPV transition has a minor effect on the dynamics and mantle temperature, mildly destabilizing the lower boundary layer and slightly increasing mantle temperature, depending on its depth relative to the thermal boundary layer. If piles of dense subducted MORB accumulate above the CMB then there is an anticorrelation between regions with a thick PPV layer and hot dense piles, but with a composition-dependent PPV transition this can change. Lateral variations in the occurrence of PPV are likely the dominant contributor to long-wavelength lateral shear-wave velocity heterogeneity in the deepest mantle, depending on some uncertain scaling parameters. The different contributions to seismic heterogeneity have different spectral slopes: temperature is "red", composition is "white" and PPV is intermediate. When compositional effects on the stability of PPV are taken into account, a large variety of complex behavior can occur, generating structures such as discontinuities, gaps or holes, and multiple (i.e., more than 2) crossings.
DI44A-06 INVITED
Structural Feature and Shear-Velocity Structure of the"Pacific Anomaly"
Seismic tomography has revealed two broad, seismically slow anomalies in the lower mantle, with one beneath the Pacific Ocean and the other beneath Africa. Here we term them the "Pacific Anomaly" and the "African Anomaly". Detailed mapping of the geometric and velocity features of the anomalies is crucial to further understanding of the origin, formation and dynamical evolution of these anomalies. In this study, we constrain structural feature and shear velocity structure of the Pacific Anomaly on the basis of forward travel time and waveform modeling of seismic data sampling a great arc across the anomaly from eastern Asia to southern South America. Our collected dataset consists of direct S, Sdiff, ScS, SKS and SKKS phases recorded in the Global Seismographic Network, the China National Digital Seismographic Network, the F-net in Japan and several temporary seismic arrays, from earthquakes occurring in the Solomon Islands, the Tonga-Fiji Islands, and the southern East Pacific Rise. After corrected for the effects of earthquake mislocation and the seismic heterogeneities outside the Pacific Anomaly, seismic observations suggest that the Pacific Anomaly along the great arc consists of at least two separated portions with a 740-km wide gap between them. The western portion of the anomaly is about 1050 km wide, extends at least 730 km above the core-mantle boundary (CMB) and exhibits a trapezoidal shape with lateral dimension increasing slightly with depth. The eastern portion of the anomaly has an 1800-km wide base and reaches at least 340 km above the CMB beneath the mid-Pacific. The waveforms and travel times for the seismic data sampling the western portion of the anomaly can be best explained by a negative shear velocity gradient from -3.0% at the top (730 km above the CMB) to -3.5% at 100 km above the CMB and an average shear velocity reduction of - 5% in the bottom 100 km of the mantle. The seismic data sampling the eastern portion of the anomaly can be explained by a uniform velocity reduction of -3%. Waveform modeling further suggests a 100-km thick low-velocity layer with a shear velocity reduction of -10% located at the edge of the western portion of the anomaly. Combining the latest results from others, we present a general picture of structural and velocity structures of the Pacific Anomaly. The structural and velocity features suggest that the Pacific Anomaly represents a cluster of metastable thermo-chemical piles.
DI44A-07 INVITED
Implications for Long-Term Mantle History of the Restricted Distribution of Large Igneous Province (LIP) Plume Sources at the Core-Mantle Boundary (CMB)
We have found, by rotation of LIPs of the past 300 My to their eruption sites in a paleomagnetic reference frame corrected for true polar wander, that those sites concentrate vertically above the margins at the CMB of the two Large Low Shear Wave Velocity Provinces(LLSVPs) of the deep mantle (Torsvik et al. 2006). This surprising discovery of narrow (< 200 wide) Plume Generation Zones stable for at least 300 My on the CMB at the LLSVP margins is consistent with the idea that the LLSVPs are compositionally (and probably also thermally) distinct dense bodies (each making up ca. 1 percent of mantle mass) rather than thermally buoyant "superplumes". The "centers of mass" of the two LLSVPs are antipodally disposed close to the equator, an intriguing possible further indication of long-term stability because the positively elevated part of the residual geoid, which matches the LLSVPs and therefore also appears also to have been stable for at least 300 My finds an analog in the aeroid of Mars of which the elevated regions are themselves antipodal on the equator. Because some volcanoes of Mars perhaps > 3.8 My in age are concentrated on the rims of the elevated aeroid it is worth considering the implications of the possible isolation of the LLSVPs from the rest of the mantle through most of Earth history. If the 2 percent of mantle mass that makes the LLSVPs has escaped being involved in making ocean floor it will be more Fe rich and denser than the average mantle. If it has also escaped being involved in making continent it will be richer in U,Th and K and hotter. It will have distinctive noble gas concentrations and could be the source (by diffusion) of the Earth's current 3He flux (Burke et al. 2008). If a velocity change attributable to a perovskite/post-perovskite transition can be mapped consistently both within and outside the LLSVPs it will help in testing the idea that the interiors of LLSVPs are hotter than the rest of the deep mantle.
DI44A-08 INVITED
The African and Pacific Superplumes and Thermochemical Piles and Their Relationship to Supercontinent Pangea
The long-wavelength structure for the present-day Earth's mantle is characterized by circum-Pacific subduction and the antipodal African and Pacific superplumes. The African and Pacific superplumes are anchored on two major thermochemical piles that extend from the core-mantle boundary (CMB) to possibly >500 km above CMB. These two superplumes are where most of large igneous provinces (LIPs) and plume-related volcanism are originated in the last 250 Ma. The thermochemical piles may provide distinct geochemical signatures observed in oceanic island basalts, although it remains controversial whether the piles consist of primordial mantle materials or recycled crust and lithosphere. Geodynamic modeling has demonstrated that the main structural features of the mantle including the circum-Pacific subduction, African and Pacific superplumes, and the thermochemical piles, are closely related to mantle convection associated with plate motion history for the last 120 Ma. However, outstanding questions remain. When did the African and Pacific superplumes and thermochemical piles start to take the current forms? How stable and stationary have they been in the mantle? How are they related to the observations of tectonics and volcanism priori to 120 Ma ago? Our recent studies on long-wavelength mantle convection and supercontinent cycles suggest that the African and Pacific superplumes and thermochemical piles are dynamic features and that they may move laterally in response to mantle flow associated with surface plate motion, such as past subduction and convergence between Laurentia and Gondwana. In particular, our studies suggest that the African superplume and pile did not form until Laurentia and Gondwana collided to form Pangea, while the Pacific anomaly may have been there for a longer time. Our results also suggest that, after lengthy convergence between Laurentia and Gondwana that pushed away the pile materials away from the African hemisphere, later subduction surrounding Pangea may not bring enough chemically dense mantle materials to form the African pile, if the pile consists of the primordial mantle, thus suggesting an origin of the recycled crust and lithosphere for the pile. While focusing on the African anomaly, we will also discuss potential ways to constrain the evolution of the Pacific superplume and pile.