MR52A-01 INVITED
Jointly Constraining Upper Mantle Seismic, Thermal and Compositional Structures From Mineral Physics and Seismic Data
The upper mantle velocity structures and thermal and compositional models are important to the understanding of the dynamics and evolution of the mantle. Several phase transformations exist in the upper mantle, and they are sensitive to mantle composition, temperature and chemical interactions between the olivine- and pyroxene- normative components. With the accumulation of in-situ measurements of elastic properties and accurate determination of phase equilibria data, we can now explore various chemical interactions of various phase assemblages, and quantitatively calculate seismic velocity structures for various mantle temperature and compositions. Mantle compositional and thermal structures can thus be quantitatively constrained by jointly modeling mineral physics data and seismic observations. In this representation, we constrain fine seismic SH velocity structures near the 660-km discontinuity beneath South America and northeast Asia and P and SH velocity structures in the upper mantle beneath southern Africa, and explore thermal and compositional models appropriate for explaining the inferred seismic structures in the three regions on the basis of mineral physics data. Beneath South America and northeast Asia, SH velocity structures near the 660-km discontinuity are found to be different. Beneath South America, the velocity gradient above the 660-km discontinuity is larger than that of PREM, while the velocity jump across the discontinuity is the same as PREM. Beneath northeast Asia, the velocity gradient above the 660-km discontinuity is the same as that of PREM, while the velocity jump across the discontinuity is larger than PREM. Both regions are characterized by a large velocity gradient extending about 80 km deep below the 660-km discontinuity. These different velocity structures can be explained by different mantle temperature or composition, in particular, the aluminum content in mantle composition. The presence of garnet 80 km below the 660-km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3.4%. The different velocity gradients above the 660-km discontinuity between South America and northeast Asia can be explained by either a difference in mantle temperature of about 100° C(with that beneath South America being lower) or a difference in aluminum content of about 1% (with that beneath South America being lower) between the two regions. Beneath southern Africa, the SH and P wave data suggest that a low velocity zone is present with velocity reductions of at least -5% for S wave and -2% for P wave beneath a 150-210 km thick high-velocity lithospheric lid, and the P/S ratio is larger (1.88) in the transition zone than in the lithospheric lid (1.70). The inferred P wave velocity jump across the 660-km discontinuity is small (<4%), while the inferred SH wave velocity jump across the discontinuity is comparable to that in PREM. The low velocity zone can be explained by a high temperature gradient of 6 ° C/km or presence of partial melt. The different P/S velocity ratios between the lithospheric lid and the transition zone can be explained by a difference in aluminum content of the mantle composition, with values of 1% in the lithospheric lid and 4% in the transition zone, respectively. The inferred P and SH velocity jumps suggest a bulk sound velocity decrease across the 660-km discontinuity.
MR52A-02 INVITED
Constraints on Lateral Variations in Temperature and Composition in the Upper Mantle From Inversion of Long Period Seismic Waveforms.
Our goal is the construction of a model of lateral variations in temperature and composition in the upper mantle obtained directly from the inversion of a global collection of long period three component multimode seismic waveforms. We start from a reference 1D model corresponding to a known pyrolite composition and a 60My geotherm connected with a 1300oC mantle adiabat, which was obtained by Monte-Carlo inversion of a global dataset of body wave travel times and fundamental mode free oscillation frequencies. We first inverted for lateral variations in temperature alone, taking into account the non-linear effects of anelasticity, and including corrections for radial anisotropy. Temperature can explain a large part of the signal contained in the waveforms, however, under cratons, a compositional signature is present. Also, the average 1D profile obtained from such an inversion is not compatible with a dry pyrolite composition, but rather, suggests a gradual enrichment in a garnet rich component with depth. Starting from our 3D temperature model, we present the first results of waveform inversion, utilizing both phase and amplitude information, and including lateral variations in composition, parametrized in terms of basalt depletion.
MR52A-03 INVITED
Sound velocities of mantle and subducted slab lithologies: Constraints from combined in situ X-ray and ultrasonic measurements
We have developed techniques to precisely measure the sound velocities of high-pressure phases at pressures to 22 GPa and temperatures to 1800K using a combination of synchrotron in situ X-ray and ultrasonic measurements with a Kawai-type multianvil apparatus. Using these techniques, we measured the sound velocities of ringwoodite and majorite in a pyrolite composition , as well as those of MORB in the garnetite facies (majorite + minor stishovite). We found quite strong non-linear decreases of Vp and Vs with temperature, particularly for Vs in majorite, under these P, T conditions of the mantle transition region (MTR). Our results suggest that pyrolite yields seismic velocities consistent with typical seismological models for the MTR, except for the bottom part of this region, whereas piclogite or basalt compositions may lead to velocities substantially lower than those seismologically derived. In contrast, harzburgite would better fit to the seismological models in the lower half of the mantle transition region, provided that the geotherm in this region is of adiabatic, which may be due to accumulation of main bodies of stagnant slabs (depleted peridotite or harzburgite) near the 660 km discontinuity.
MR52A-04 INVITED
Pressure Induced Slip-System Transition in Olivine: Laboratory Experiments and Geophysical Implications
Olivine deforms by dislocation creep in the shallow upper mantle which results in seismic-velocity anisotropies allowing characterization of convective flows in this region. Seismic anisotropy is interpreted from lattice preferred orientations produced experimentally in olivine, which depends on the dominant dislocation slip systems. At pressure P<3 GPa, mantle temperature (T) and in dry conditions, olivine [100] dislocation slip dominates the less active [001] slip. This tends to align crystal fast velocity [100] axis with the principal shear direction. Yet recent deformation experiments carried out at P> 3 GPa (Couvy et al., 2004, EJM, 16, 877; Raterron et al., 2007, Am. Miner., 92, 1436), as well as a theoretical study based on first-principle calculations (Durinck et al., 2005, PCM, 32, 646), show that olivine [001] slip may be dominant in the (P,T) range of the deep upper mantle. This would promote a shear-parallel slow-velocity [001] axis which may explain the seismic-velocity attenuation with depth observed in this region (Mainprice et al., 2005, Nature, 433, 731). The [100]-slip/[001]- slip transition has previously been attributed to the presence of water (e.g., Jung et al., 2006, Tectonophysics, 421, 1). A pressure effect with similar consequences would have strong implications on our present understanding of mantle flow. In order to determine the effect of P on both [100] and [001] slip activities, deformation experiments were carried out on dry oriented forsterite crystals at P>6 GPa and T=1400°C, using the Deformation-DIA apparatus at the X17B2 beamline of the NSLS (Upton, NY). Constant applied stress σ <300 MPa and specimen strain rates were monitored in situ using time-resolved x-ray diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of run products reveals that dislocation creep was responsible for sample deformation. The collected data were then compared with those previously obtained by Darot and Gueguen (1981, JGR, 86, 6219) on identical forsterite crystals deformed in comparable T and σ conditions, but at room P. A slip transition with increasing P, from dominant [100]-slip to dominant [001]-slip, will be documented. The extrapolation of crystals rheological laws to upper-mantle conditions, which shows that [001]-slip activity should be dominant in deep upper mantle, as well as the corresponding implication on upper mantle viscosity will also be presented.
MR52A-05
Single-crystal elasticity of hydrous wadsleyite to 12 GPa
Wadsleyite, β-Mg2SiO4, is expected to be the dominant mineral in the Earth's transition zone from 410 to 520 km depth. This mineral has the greatest water storage capacity among the olivine polymorphs (e.g. Smyth et al., 1987; Kohlstedt et al., 1996) and could contain up to 0.9 wt% H2O under transition zone conditions (Demouchy et al., 2005). Previously, we reported that the elasticity of wadsleyite decreases strongly with increasing water content at ambient conditions (Mao et al., 2007). Pressure derivatives of bulk and shear moduli are needed to extrapolate elastic moduli to high-pressure conditions. Static compression studies suggest that the presence of hydroxyl might increase the pressure derivative of the bulk modulus for wadsleyite (Smyth et al., 2005; Holl et al., 2007). Here, we conducted high-pressure Brillouin measurements to 12 GPa to determine the single-crystal elasticity of wadsleyite with 0.84 wt% H2O. Three platelets were used in the Brillouin measurements. For each platelet, we collected spectra at 100 steps in a total of 19 directions over a range of 180 degrees. With the exception of C55, the elastic constants, Cij, of hydrous wadsleyite follow the similar trends as anhydrous wadsleyite with increasing pressure (Zha et al., 1997). Pressure derivatives of the bulk and shear moduli of wadsleyite with 0.84 wt% H2O are 4.2(1) and 1.4(1) respectively. These values are not significantly different from the corresponding values of anhydrous wadsleyite (e.g. Zha et al., 1997). Thus, the presences of 0.84 wt% H2O has no detectable effect on the pressure derivatives of the bulk and shear moduli. We calculate the effect of water on the compressional and shear wave velocities for wadsleyite at 410 km (~13.8 GPa, along a 1400°C adiabat). If earth's transition zone is water saturated (0.9 wt% H2O), this could lead to a 3.0% reduction in compressional wave velocities and a 3.3% reduction in shear wave velocities.
MR52A-06
Water at the Bottom of Upper Mantle: A Key to Reconcile Mineral Physics Composition Model with Seismic Wave Velocity Jump at 410-km Discontinuity
Laboratory experiments demonstrate that water can be transported from Earth's surface to its deep interior (e.g. Ohtani 2005). Presence of water significantly influences crytal structure (e.g. Smyth 2006), elasticity (e.g. Jacobsen 2006) and phase stabilities (e.g. Chen et al. 2002, Smyth and Frost 2002) of minerals in Earth's mantle. We report the experimental result of water effect on equation of state of San Carlos olivine measured using synchrotron x-rays in a diamond anvil cell at pressures up to 11 GPa. The bulk modulus of San Carlos olivine is reduced by 5% when the sample contains saturated water (4 wt%) with respect to anhydrous sample. The experiments are conducted at the National Synchrotron Light Source (NSLS); a mixture of methanol and ethanol (4:1 by volume) is used as pressure medium. Based the experimental result and previously published data, we re-estimate the seismic velocity at 410-km discontinuity (1400°C adiabat) based on pyrolite composition model. An Earth model program developed by Weidner and Wang (2000) that calculates phase fractions from experimental data on partitioning coefficients of Ca, Fe, and Al between the olivine- and pyroxene- normative components at any given pressure and temperature condition is used. The results indicate that the long-standing discrepancy in magnitude of seismic velocitie jump at 410km depth between mineral physics model and seismic observations can be reasonably negligible with presence of water. This result supports that water is likely present at the bottom of the Earth's upper mantle.
MR52A-07
Polyhedral Control of Tilt Transitions in Perovskites
Advances in single-crystal structure determinations have shown that the structural evolution of ABO3 perovskites with increasing pressure is determined by the relative compressibility of the octahedral "B" cation site and the extra-framework "A" cation site. With increasing pressure, the octahedral tilts change and the octahedral site compresses in such a way that the increase in bond valence sums at the A and B sites is equal [1]. The extent of tilting of a given perovskite in a given space group is limited. With increasing temperature phase transitions occur to different tilt systems with smaller tilts and higher symmetry. We have previously shown that the bond-valence matching principle leads to the prediction that the Clapeyron slopes of phase transitions between two perovskite tilt systems [2]. The P-T slopes of phase boundaries are negative for perovskites with +3 cations in both sites, and positive for perovskites such as MgSiO3 and CaSiO3 with +2 cations on the A site and +4 cations in the octahedral B site. Examination of structural data for a wide variety of perovskites now shows that phase transitions from one tilt system to another occur when the structural tolerance factor tobs (derived from bond lengths) reaches a critical value. Experimental data show that the critical value corresponds to the point at which the global instability indices of the two phases become equal. Each space-group change has its own critical value of tobs, but otherwise appears to be invariant with chemistry or pressure or temperature. In combination with the bond-valence matching principle, the critical values can therefore be used to calculate phase diagrams for perovskites. [1] Zhao, Ross, Angel (2004) Acta Cryst., 60, 263-271. [2] Angel, Zhao, Ross (2005) Physical Review Letters, 95, 025503
MR52A-08
Transformational Weakening During the Perovskite-Post Perovskite Phase Transition in CaIrO3
The D" layer plays a fundamental role in core - mantle interactions. The recent discovery of a CaIrO3 structured phase of MgSiO3, the so called post-perovskite (ppv) phase, significantly altered our understanding of the seismic and dynamic properties of the D". Here we report results of a study of the rheology during transformation from pv to pvv. The experiments were performed using the D-DIA on the X17B2 beamline at NSLS. A sample of presintered perovskite structured CaIrO3 was deformed, along with a MgO stress calibrant, during heating at 3 GPa and up to 1000°C in a 8mm D-DIA assembly. The stress was measured by X-ray diffraction from the MgO sample and a proxy for the phase proportions was the relative strengths of unique perovskite and post-perovskite diffraction peaks from the CaIrO3 sample. The strain was measured by X-radiography, from the position of Platinum foils placed above, between and below the two samples. From this data the viscosity of the CaIrO3 sample can be calculated as well as the progress of the reaction. During the transformation to post-perovskite we observed a two fold weakening of the pv and ppv mixture; followed by an increase in the strength of the mixture to greater than that of that starting perovskite-structured material. If MgSiO3 behaves in the same way during pv - ppv transformation it will cause a viscous decoupling between the lower mantle and the D". This in turn has major implications for the topography that can be supported at the top of the D" layer.