MR14A-01 INVITED 16:00h
Seismic discontinuities in the upper mantle
There are a number of seismological techniques which are able to see and to map discontinuities of physical parameters (density, velocity) in the upper mantle. Seismic tomography maps smooth parameter variations, not discontinuities. The best known technique are the receiver functions, which are rapidly developing since about ten years. Receiver functions are basically P-to-S or S-to-P converted waves at discontinuities. These are weak signals and they need summation of many seismic recordings and processing to become visible. These phases are much used to study the 410, 520 and 660 km discontinuities on a global scale, especially in subduction zones and near plumes. The topography of these discontinuities is thought to be a high resolution measure of in situ temperature. The crust-mantle boundary (Moho) is also well observed globally with receiver functions. Especially interesting is the subducted Moho and the possible effects of slab dehydration on the upper plate Moho. Reverberations within the crust often prevent usage of P receiver functions in the depth range between Moho and 410 discontinuity. These multiples however make it easy to determine absolute Moho depth and average crustal Vp/Vs on a much larger scale. S receiver functions have only very recently proven a good method to map the lithosphere-asthenosphere boundary (LAB). This boundary was so far rarely observed with high resolution body waves, mostly low resolution surface waves had been used. There are recent very detailed maps of the LAB near the Hawaii and Iceland plumes providing data for studies of plume-lithosphere interaction.
MR14A-02 INVITED 16:15h
Sound velocities of aluminous perovskite and ferropericlase at high-pressure: Implications for Earth's deep interior
Accurate determinations of the sound velocities of deep Earth material under the relevant pressures and temperatures are essential for understanding seismic observations in this region. Aluminum-bearing magnesium silicate perovskite and (Mg,Fe)O ferropericlase have been proposed to be the most abundant minerals in Earth's lower mantle, with aluminous perovskite occupying roughly half of Earth's volume. Despite its major role in the deep Earth, the sound velocities of aluminous perovskite at high-pressure have not been measured. Furthermore, some experimental and theoretical studies have indicated that the properties of magnesium silicate perovskite may be significantly altered by the presence of Al$^{3+}$, depending on how this ion is incorporated into the perovskite structure. This complexity adds an additional element of uncertainty to the properties of the actual aluminous perovskite in the lower mantle. Here we present the first high-pressure Brillouin scattering measurements on silicate perovskite. The experiments were performed on a polycrystalline aluminum-bearing MgSiO$_{3}$ perovskite sample (containing 5 wt.% Al$_{2}$O$_{3}$, as described in Jackson {\it et al.} 2004) in a diamond anvil cell, using methanol-ethanol-water as a pressure transmitting medium. The pressure-dependence of the aggregate compressional (V$_{P}$) and shear (V$_{S}$) wave velocities, as well as, the adiabatic bulk (K$_{0S}$) and shear ($\mu $) moduli were obtained. We combine these results with our recent high-pressure Brillouin data on single-crystal (Mg$_{0.94}$Fe$_{0.06}$)O ferropericlase and discuss the importance of shear properties for understanding Earth's lower mantle. Reference: Jackson, J.M., J. Zhang, and J.D. Bass (2004), {\it Geophys. Res. Lett.}, {\bf 31}, L10614, 2004GL019918R.
MR14A-03 16:30h
P-V-T equations of state of lower mantle minerals: Constraints on mantle composition models
Ferropericlase (Mg,Fe)O is likely a stable phase coexisting with silicate perovskite in the Earth's lower mantle. Determination of a reliable P-V-T equation-of-state of this phase is therefore crucial for developing compositional and mineralogical models of the Earth's interior. In this study, we report new compression data on ferropericlase up to 136 GPa, covering the entire pressure range of the lower mantle. The experiments were performed at the HPCAT 16-ID-B beamline (Advanced Photon Source), using monochromatic X-radiation and a CCD area detector. We used (Mg$_{0.6}$Fe$_{0.4}$)O as the starting material. The powdered sample was sandwiched between NaCl and a mixture of NaCl-Au in an externally heated high-temperature diamond anvil cell. The sample was annealed at each pressure increment by laser heating. High-quality diffraction data were collected up to 136 GPa. The same starting material was also studied up to 27 GPa and 2173 K in a multi-anvil apparatus by X-ray diffraction. A reliable P-V-T equation of state for (Mg$_{0.6}$Fe$_{0.4}$)O was developed by combining the two data sets. The new results, together with our recent P-V-T data for Al-bearing perovskite up to 105 GPa and 1000 K, provide solid density measurements for the two most important lower mantle minerals under simultaneous high pressure and temperature conditions. The new data are used to model the density profile of the lower mantle and provide tight constraints on its chemical composition.
MR14A-04 16:45h
Thermoelastic Parameters of Aluminate Perovskites from Single Crystal Vibrational Spectroscopy at Ambient and High Pressures
Perovskites form a major part of the lower mantle. Raman spectra of perovskites are very weak and all modes for any one orthorhombic perovskite have never been observed. The distribution of modes is important for vibrational modelling of thermoelastic parameters. Effect of varying chemistry and pressure on the spectrum is examined. Room-temperature polarized single-crystal Raman spectra have been measured for both GdAlO$_3$ and YAlO$_3$, which crystallize in the orthorhombic (Pbnm) perovskite structure. Of the 24 possible modes in 4 symmetries, 20 and 17 modes for gadolinium and yttrium aluminates, respectively, were observed. The spectra appear remarkably similar to those of other perovskites regardless of chemistry. Correlation of peaks to structural factors revealed that for the observed modes, lattice parameter plays a dominant role in determining the Raman frequencies while the mass of the A cation plays a minor role. The mass of the B cation plays no obvious role in the Raman spectrum. Comparisons were made to single crystal spectra of 19 other orthorhombic perovskites. Due to their nearly cubic nature, the modes are weak and have not yet all been observed for any one perovskite. Good approximations of the missing modes are given based on the comparisons and symmetry considerations. Raman spectra of yttrium aluminate perovskite have been collected to 24 GPa. These data are used to estimate the effect of pressure on thermoelastic parameters such as heat capacity, entropy, and thermal expansion.
MR14A-05 17:00h
Spin Transition in Earth's Lower Mantle: (Mg$_{x}$Fe$_{1 - x}$)O Magnesiow\"{u}stite at High Pressures
New experiments confirm W. S. Fyfe's prediction of 45 years ago that Fe$^{2+}$ should undergo a pressure-induced electronic transition deep in the Earth's mantle, from high-spin (paramagnetic) to low-spin (diamagnetic) states. We used $^{57}$Fe M\"{o}ssbauer ($\gamma$-ray absorption) spectroscopy and powder x-ray diffraction with gasketed diamond-anvil cells to characterize the (Mg, Fe)O solid-solution at elevated pressures, because high pressure - temperature experiments on peridotitic (upper mantle-like) bulk compositions indicate that Mg$_{0.80}$Fe$_{0.20}$O magnesiow\"{u}stite is likely the second-most abundant mineral of the Earth's lower mantle. Prior work shows that the Fe$_{0.94}$O w\"{u}stite endmember loses its magnetic moment above $\sim$ 70 to 140 GPa (onset to completion) at 300 K. Because the magnetic-ordering (N\'{e}el) temperature of w\"{u}stite exceeds 300 K above 20 GPa, and increases with increasing pressure as expected, the observed collapse of magnetism offers direct evidence that the iron ion has transformed to the diamagnetic (low-spin) state at deep-mantle pressures. Similar experiments conducted on Mg$_{0.20}$Fe$_{0.80}$O, Mg$_{0.50}$Fe$_{0.50}$O and Mg$_{0.80}$Fe$_{0.20}$O compositions show that the high- to low-spin transition begins at lower pressure with increasing Mg content: 80 ($\pm10$), 60 ($\pm10$) and 40 ($\pm10$) GPa, respectively, are required for inducing significant transformation at 300 K (the corresponding value for w\"{u}stite is 90 $\pm10$ GPa). The variation in transition pressure correlates with the composition-dependence of the (Mg, Fe)O lattice parameter, such that the spin change occurs for all compositions when the metal-oxygen bond length is reduced to 197 ($\pm2$) pm. In the high-spin state, all the magnesiow\"{u}stites we studied were found to be antiferromagntically ordered at low temperatures (zero-pressure N\'{e}el temperatures of 190 K, 140 K, 80 K and 25 K, respectively, for Mg/(Mg + Fe) ratios of 0, 0.20, 0.50 and 0.80), and it is the appearance of a non-magnetic site under pressure that signals the onset of the spin transition. Contrary to prior expectations, there is no evidence of any significant change in volume or bulk modulus across the Fe-spin transition in magnesiow\"{u}stite. Therefore, this important bonding change may cause little if any seismological anomaly, and its occurrence deep in the Earth's mantle can only be inferred through the combination of laboratory experiment and geophysical observation.
MR14A-06 17:15h
In situ X-ray diffraction and X-ray emission study of magnesiowustite in Earth's lower mantle conditions: implications to the geophysics and geochemistry of the lower mantle
The thermodynamic behavior of iron in mantle host phases plays an important role in understanding geochemical modeling, geodynamic simulation, and seismic wave observations of the Earth's deep interior. In particular, electronic spin transitions in the iron-bearing lower mantle phases, magnesiowustite and silicate perovskite, have important geophysical and geochemical consequences such as density change, iron partitioning, change of radiative thermal conductivity, and compositional layering in the lower mantle. Recent X-ray emission spectroscopic studies at high pressures and room temperature have found such high-spin to low-spin transitions of iron in magnesiowüstite and silicate perovskite (Badro et al., 2003, 2004; Li et al., 2004), but there remains uncertainty about associated volume change. Here we use in situ X-ray diffraction and X-ray emission spectroscopic techniques to measure the density/volume change related to the electronic transition in magnesiowüstite (Mg,Fe)O with various compositions at lower mantle conditions. Our X-ray emission spectroscopic results show that the high-spin to low-spin transition of iron in magnesiowüstite occurs gradually over a wide range of pressure. We also studied ferropericlase with both techniques, and found a similar, gradual density increase over a wide range of pressure due to the electronic collapse. We will also address the temperature and compositional effect on the spin transition of iron in magnesiowüstite. These results suggest that the electronic spin transition of iron in magnesiowüstite does not contribute significantly to the geochemical and geophysical signatures in the lower mantle.
MR14A-07 17:30h
Elasticity of Ferropericlase at High Pressure and Temperature: Implications in Earth's Interior
(Mg,Fe)O is a major phase in the lower mantle of the Earth. To date, elasticity studies on (Mg,Fe)O were conducted along high-pressure regime. Static compression studies have explored the behaviors at higher P-T spaces, but provided only indirect determination of the isothermal bulk modulus and its pressure derivatives, and no information on the shear properties. By combining synchrotron X-radiation and ultrasonic velocity measurements, we are able to conduct simultaneous measurements of sound velocities using ultrasonic interferometry and unit cell parameters using X-ray diffraction, and sample length using X-radiographic imaging, at high P and T conditions (Vp-Vs-Volume-P-T). Elasticity of ferropericlase (Mg,Fe)O with 17 mole percent of Fe were measured at high pressures and temperatures. Our high temperature results for (Mg,Fe)O showed that the intrinsic contribution to the temperature derivative is weak for the adiabatic bulk moduli (Ks) and zero for the shear moduli (G). This observation is different from the "general" trend; the intrinsic temperature effect on the shear modulus tends to be greater than the bulk modulus. In this meeting, we will present and discuss the results of the elasticity of (Mg,Fe)O at high pressures and high temperatures, and the implications in the lower mantle.
MR14A-08 17:45h
The equation of state of the lower mantle for a pyrolite composition: Implications from the PVT data, PREM and a linear shock-wave Us-Up asumption
For a direct comparison with PREM, the density for a pyrolite lower mantle has been calculated using various PVT data sets available in literature. Care was taken to account the effects of iron, aluminum and other minor elements on the unit cell volumes of individual phases. It was found that the density calculated from the high pressure and temperature data of Funamori et al. (1994) could match PREM at 700km depth and 1900K within an uncertainty less than .5%, if the pressure were re-defined in terms of the MgO scale proposed by Speziale et al. (2001) and the gold scale by Shim et al. (2002). The mutual-consistency among laboratory data, the pyrolite model and PREM indicates that the lower mantle is likely of a pyrolite composition, and that the two new equations for MgO and gold should be used as an appropriate choice for defining the pressure scale. The one proposed for gold by Anderson et al. (1989), on the other hand, gives a pressure that is about 1.5GPa lower at 28 GPa and 1900K. The difference represents a correction on $\gamma$, the Gruneisen parameter that defines the thermal pressure. It may provide an explanation for discrepancy observed between the 660 km seismic discontinuity and the ringwoodite to perovskite and MgO phase boundary. An equation of state is proposed for the entire region of the lower mantle, in which PREM defines the principal reference adiabat, and $\gamma$, the Gruneisen parameter, defines both the temperature gradient and the thermal pressure with respect to the adiabat of PREM. For such an equation, $\gamma$ as a function of density (or pressure) was determined under two constraints. The first one is provided by the bulk sound velocity estimated along the room temperature isotherm from the pressure-density data for a pyrolite composition. The second one is a hypothetical Hugoniut constructed under a linear Us-Up relationship observed in shock wave experiments. At the lower mantle conditions, $\gamma$ was found close to be a constant, with a value higher than that estimated in previous studies. As a consequence, the temperature rise and the thermal expansion as a function of pressure along the adiabat defined by PREM should also be higher. The Anderson-Gruneisen parameter, $\delta_{s}$, has also been estimated. At the lower part of the lower mantle, it was found to be less than 1, which may explain the negative correlation between the lateral variations of the bulk sound and shear wave velocities observed in tomography studies.