MR53A-1701
Elasticity and Sound Velocities of (Mg,Fe)SiO3 Orthopyroxenes Determined by Nuclear Resonant Inelastic X-ray Scattering
Orthopyroxene is one of the major mineral phases of the Earth's upper mantle. Comparison of the sound velocities and elastic properties of upper mantle silicate minerals with seismic observations is essential for constructing the next generation deep Earth models. Nuclear resonant inelastic X-ray scattering (NRIXS) is a high-resolution X-ray spectroscopic method that has only recently begun to be used for geophysical applications. The NRIXS technique provides access to the partial phonon density of states (PDOS) of the nuclear resonant isotope, in this case 57Fe. Sound velocities and other thermodynamic quantities of the material containing the resonant isotope can therefore be derived from the PDOS. NRIXS studies related to geophysical applications have primarily been conducted on high symmetry and/or iron-rich materials, but far fewer measurements have been conducted on low symmetry phases, such as orthorhombic enstatite. In this contribution, we present NRIXS measurements for three powdered samples of (Mg,57Fe)SiO3 orthoenstatite containing 20, 13, and 7 mole percent FeSiO3. The experiments were performed under ambient conditions at sector 3 ID-B of the Advanced Photon Source at Argonne National Laboratory. The monochromator was tuned from ±80 meV to ±100 meV (0.25 meV step size) around the nuclear transition energy of 14.4125 keV, with an energy resolution of 1 meV. The radiation emitted from the samples was observed with two avalanche photodiode detectors. One detector was placed close to the sample (~2mm away) to collect the inelastic scattered photons, and the other detector was placed downstream in the forward scattering direction, in order to obtain the resolution function independently. The quasi- harmonic model was used to extract the partial PDOS from the measured data. Using an improved empirical relation for the dispersion of the acoustic phonons at low-energy (long wavelength), the Debye sound velocity has been determined from the PDOS for each of the three compositions. We show that the sound velocities of orthoenstatite determined from NRIXS compare well with previous ultrasonic and Brillouin scattering studies on similar compositions.
MR53A-1702
Elasticity of MgO to 130 GPa: Implications for lower mantle mineralogy
Average composition and structure of the Earth's deep mantle can be approached by comparing observed seismic velocities to appropriate laboratory data collected for candidate minerals under relevant pressure and temperature conditions. It is widely believed that the Earth's lower mantle is primarily composed of silicate perovskite and ferropericlase. Precise knowledge of the elastic properties of MgSiO3 perovskite and MgO periclase, major end-members of constituent mineral phases of the lower mantle, under high-pressure condition is therefore crucial for constructing the accurate mineralogical model of the Earth's lower mantle. Recent technical advances in high-pressure Brillouin spectroscopic measurements using diamond anvil cell apparatus extended significantly the upper pressure limit for acoustic measurements. It is now possible to measure the aggregate shear wave velocity profiles of both MgSiO3 perovskite and post-perovskite phases up to a pressure of 172 GPa (Murakami et al., 2007a; Murakami et al., 2007b). Large number of studies under lower pressure condition have consistently yielded precise elastic constants of MgO at zero pressure. (e.g., Jackson and Niesler, 1982). Available experimental data for MgO from the direct sound wave velocity measurements under lower mantle pressure regime are however still limited to a maximum pressure of 55 GPa on single-crystal (Zha et al., 2000). Owing to the lack of acoustic data on MgO under whole pressure range of lower mantle, the pressure dependence of elastic velocities or moduli has thus far been poorly constrained. In particular, the pressure derivative of the shear modulus (G'0) reported in the previous experiments exhibited considerable variations ranging from 2.21 to 2.85, which prevents us from modeling the detailed lower mantle mineralogy. The large variation of G'0 values for MgO also makes it difficult to evaluate the effect of iron on G'0 of (Mg,Fe)O ferropericlase by comparing with the recent relatively low-pressure experimental results on (Mg,Fe)O. In order to resolve this issue, the aggregate shear wave velocities of MgO has been determined throughout Earth's lower mantle pressure regime approaching to 130 GPa using Brillouin spectroscopy in conjunction with synchrotron X-ray diffraction technique in a diamond anvil cell apparatus. High pressure Brillouin measurements were conducted using the newly-developed acoustic measurement system, recently installed in the SPring-8. Our new results have shown that the aggregate shear wave velocity and shear moduli at ambient pressure are highly consistent with earlier studies. However, the pressure derivative of the shear modulus (G'0) of MgO is distinctly lower than that of previous lower-pressure experiments. Compare with the previous results on (Mg,Fe)O ferropericlase, there is no clear correlation between iron content and G'0 at least for low iron concentration of ferropericlase under high-spin state of iron. Calculated shear wave velocity profile in lower mantle applied by the lower G'0 value of MgO can remarkably well reproduce the global seismological 1-D velocity profile model with a simple assumptions of an adiabatic geotherm and uniform bulk composition model. Combined with our recent results on the elasticity of MgSiO3 perovskite phase, we will discuss in detail the possible mineralogical and compositional model of the Earth's lower mantle.
MR53A-1703
Deformation experiments of polycrystalline sintered diamond materials in the DDIA with HIMEDIA pistons
Deformation experiments of polycrystalline sintered diamond materials were carried out at pressure of about 6 GPa and temperature of about 1273 K, within the stability field of diamond, using a combination of deformation-DIA (DDIA) and monochromatic X-rays. The experiments were performed at the GSECARS bending magnet beamline 13-BM-D at the Advanced Photon Source. Two deformation experiments were performed for two different types of sintered diamond materials. In the first and the second run, sintered diamond rods produced by Ringwood Superabrasives, Australia, and Diamond Innovations, USA, were employed as the samples, respectively. The first sample contains SiC catalyst, whereas, in the second sample, metal catalyst was removed. In both the runs, nano-polycrystalline sintered diamond named HIMEDIA was used as deformation pistons to deform the commercially available sintered diamonds under high pressure and temperature. The HIMEDIA was synthesized at 15 GPa and 2773 K and machined using laser- cutting device. In deformation runs, we used a cylindrical graphite furnace and generated temperature was estimated using a power-temperature relation determined in a separate run. The cell assembly was pressurized isotropically up to a load of 40 tons. At this fixed load, the sample was heated up to 1273K, and the deformation was started by moving the top and the bottom anvils toward the sample. During deformation, X-ray diffraction patterns and X-ray transmitted images were recorded one after the other. In both the runs, we observed ductile behavior of the sintered diamond rods up to about 18 percent axial strain in 150 minutes. In the diffraction patterns, lattice strains of diamond were also observed. Stress-strain curves of sintered diamond materials will be presented in the session.
MR53A-1704
Spin and structural transitions in AlFeO3 and FeAlO3 perovskite and post- perovskite
We use an advanced formalism within the density functional theory to investigate the perovskite and post- perovskite structures with AlFeO3 chemistry. We consider two ordered cases: one with Fe and then one with Al in octahedral coordination. For each case we investigate several spin configurations. We observe that up to 90 GPa the FeAlO3 perovskite structure, i.e. with Fe3+ in the inter-octahedral space, with antiferromagnetic configuration and large local magnetic moment is the most stable one. Beyond 90 GPa the post-perovskite structure of AlFeO3, i.e. with Fe3+ in octahedral coordination, with antiferromagnetic configuration and small local magnetic moment is the most stable one. The perovskite to post-perovskite phase transition at 90GPa triggers a partial collapse of the magnetic moment. The local magnetic moment vanishes beyond 150 GPa in post-perovskite.
MR53A-1705
Stress-Induced Proton Disorder in Hydrous Ringwoodite
The system (Fe,Mg)2SiO4 with its modifications olivine, wadsleyite and ringwoodite has been the subject of numerous studies in the past to determine the maximum solubility of hydrogen in their structures (e.g., Kudoh et al., 2000). Nowadays experimental work is focused on the effect of the water content on the physical, structural and thermodynamic properties. Camorro Perez et al. (2006) studied hydrous (1 and 1.7 wt % water) Mg-ringwoodite by in-situ high-pressure IR spectroscopy up to 30 GPa using Neon as pressure medium and reported a second-order phase transition at pressures of about 25 GPa with sudden disappearance of the prominent OH band centred at 3150 cm-1. To study this phenomenon in the whole system (Fe,Mg)2SiO4 we synthesized Fe- rich hydrated ringwoodite of different compositions ranging from xFe = 1.0 to xFe = 0.40. The samples were investigated by electron microprobe, secondary ion mass spectrometry (SIMS), conventional and synchrotron FTIR spectroscopy at ambient condition and to 30 GPa in situ in the diamond anvil cell (DAC). Water determination using SIMS in combination with IR spectroscopy yielded concentrations ranging from 0.6 wt % for the Fe- endmember to 2.5 wt % for the Mg-richest sample. Several DAC experiments were performed with ringwoodite of compositions xFe = 0.4, 0.5 and 0.6 in a Megabar diamond anvil cell. We performed the high pressure measurements in different pressure transmitting environments. In the experiments performed using CsI powder, or cryogenically loaded liquid Argon, between 10 and 12 GPa, the prominent OH-stretching suddenly disappears, and the lattice vibrations show discontinuities, independent of composition. In another set of experiments cryogenically loaded liquid Argon was annealed for one hour at 8.6 GPa to 120° C before further increasing the pressure to ensure more hydrostatic conditions (Wittlinger et al. 1997). In these experiments the OH stretching vibrations as well as lattice vibrations could be observed up to 30 GPa without any discontinuity and their pressure behaviour (dν/dP) can well be described by linear fits. Molecular vibrations are very sensitive to non-hydrostatic conditions and we interpret the disappearance of the OH-bands as a stress-induced proton disordering in hydrous ringwoodite due to the use of hard pressure transmiting media like CsI or argon without thermal annealing. Thus, our study cannot confirm the phase transition observed by Camorro Perez et al. (2006) in ringwoodite. But as they used Neon as pressure transmitting medium, which is known to become non-hydrostatic at pressure above 16 GPa (Bell and Mao, 1981) we argue that their observation of a sudden disappearance of the OH band may also be related to non-hydrostatic conditions. References Bell P.M. and Mao H.-K. (1981) Carnegie Inst. Wash Yrbk 80: 404-406. Camorro Perez E.M., Daniel I., Chervin J.-C., Dumas P., Bass J.D. and Inoue T. (2006) Phys. Chem. Minerals, 33, 502 510. Kudoh Y., Kuribayashi T., Mizohata H., Ohtani E., (2000) Phys. Chem. Mineral. 27, 474-479. Wittlinger J., Fischer R., Wener S., ScheiderJ., Schulz J. (1997) Acta Cryst B53, 745 749.
MR53A-1706
Effect of aluminium on the silica compressibility and transition to the CaCl2-form at high- pressure and high-temperature up to 100 GPa
The recycling of oceanic crust into the deep Earth offers a way of enriching the mantle in silica. In such a subduction context, the high-pressure phase of silica, stishovite, present in the basaltic and the sedimentary portions of the slab, has been shown to contain significant amounts of aluminium. The incorporation of this element and its possible coupling with oxygen vacancies (Si4+ = Al3+ + O) can influence significantly the density, compressibility and phase transitions of mantle phases. We investigated the effect of Al on stishovite properties up to ~100 GPa in a YAG or CO2-laser heated diamond anvil cell using in-situ X-ray diffraction at the ID30-ID27 beamline of ESRF. Using an Al-bearing silica glass as starting material, we performed more than 10 different experimental runs at various P and T conditions. Our first observation is that the transition from rutile to calcium chloride (CaCl2) phase of silica occurs at lower pressure, down to 23 GPa at 300 Kelvin, compared to 50 GPa in the pure SiO2 system. [1]. Also, we confirm that the stishovite room pressure volume is increased with increasing Al-content. Still, the volume difference of ~1% at 1 bar rapidly vanishes at higher pressures and the PV equation of state of Al-bearing stishovite at room pressure appears similar to that of Al-free stishovite at all pressures investigated to 100 GPa, within the experimental error. The CaCl2 distortion increases with pressure and decreases with temperature allowing to measure the clapeyron slope of the transition in-situ at high-pressure and high-temperature: dP/dT of 0.02 GPa/K. According to the phase diagram determined by in-situ X-ray diffraction, silica should be in the CaCl2 form in most of the transition and lower mantle. Also, aluminous silica, whether in the rutile or calcium chloride structure, is softer than pure silica. Our results show that the incorporation of ~2.3 weight % of alumina in silica decreases its bulk modulus of around 7-10% in case of the rutile form and of 3.5% in case of the CaCl2 form. References: [1] Andrault et al. (1998) Science, 23, 720-724.
MR53A-1707
Seismological and experimental constraints on metastable phase transformations and rheology of the Mariana slab
Recent seismological studies have observed a 5% low velocity wedge down to the depth of 630 km implying a wedge of metastable olivine and depressions of the 660 km discontinuity down to 690 km due to the delay of the post-spinel transformation inside the Mariana slab. Based on these seismic observations and newly calculated thermal structures of the slab in the present study, we suggest that the olivine-spinel and the post- spinel transformations occur at 630 km depth and 550°C, and 690 km depth and 700°C, respectively, in the central coldest part of the Mariana slab. Combining this information with recent experimental kinetic data, we constrain details of non-equilibrium transformations and viscosity of the Mariana slab. The observed depth of the metastable olivine inside the Mariana slab can be explained by growth- controlled olivine-spinel transformation under relatively dry condition such as the water content of about 100- 150 wt. ppm H2O. On the other hand, nucleation process controls the depth of the post-spinel transformation in the slab. Therefore the observed depressions of the 660 km discontinuity should be explained by combination of the overpressure needed for the nucleation and the negative Clapeyron slope for the transformation. The Clapeyron slope larger than -1.3 MPa/K is favored for the nucleation-controlled post- spinel transformation in the Mariana slab. Grain-size evolution and viscosity structures in the Mariana slab were estimated based on these metastable phase transformations. The slab weakening due to the grain-size reduction is limited in about 40 km widths below the metastable olivine wedge at 630-690 km depths, whereas it expands in more than 120 km widths at deeper than 690 km depth after the post-spinel transformation. This is because of the differences in mechanisms of the grain-size reduction between the olivine-spinel and the post-spinel transformations. These viscosity structures are consistent with behaviors of the Mariana slab observed by seismicity and seismic tomography that the Mariana slab vertically descends, and largely deforms and thickens at the top of the lower mantle.
MR53A-1708
Major Element Partitioning Between Lower Mantle Minerals
The Earth's lower mantle consists mainly of Al, Fe-bearing perovskite or post-perovskite (at depth >2500- 2700 km) with lesser amount of (Mg,Fe)O ferropericlase. The major element partitioning between these phases has strong effects on various physical properties, such as density, viscosity, electrical and thermal conductivities. Therefore, Fe-Mg distribution coefficients between perovskite and ferropericlase have been repeatedly reported in the literature. However, the distribution coefficient, especially in Al-bearing system, is poorly determined under high pressure. This is due to the difficulty of experiment as well as chemical heterogeneity in laser-heated DAC samples. Now we investigated the distribution coefficient in Al-bearing system up to deeper lower mantle condition with the laser heated diamond anvil cell (LHDAC) experiments. The starting material was gel with a pyrolitic composition. Sample was sandwiched by thermal insulation layer and metallic powder or sheet was used as laser absorber. High temperature condition was generated by YAG laser with double sided heating technique. X-ray diffraction measurements were conducted at Spring-8. Recovered sample was Ar ion-thinned using Ion Slicer, which can make much larger thin section for transmission electron microscopy (TEM) observation. The chemical compositions of recovered sample were acquired by field-emission-type electron microprobe analyzer (FE-EPMA) and energy dispersive X-ray spectroscopy (EDS) attached with TEM. Spatially wide examination using FE-EPMA demonstrats that the elements distribute heterogeneously in some samples. Relatively strong chemical heterogeneity was observed in the sample using powder laser absorber, compared to the sample using sheet absorber. The variation of local bulk composition significantly affected to the distribution coefficient. TEM-EDS analysis on each phases indicates that iron was enriched in ferropericlase compared to perovskite, and determined K*(Pv/Fp) = (XFe/XMg)Pv / (XFe/XMg)Fp was significantly lower than that in Al-free system. This is possibly due to the effect of Al-Fe3+ coupled substitution mechanism in perovskite as discussed in previous studies. Fe2+ and/or Fe3+ enrichment in perovskite possibly has considerable effect to physical properties of bulk lower mantle.
MR53A-1709
In situ Raman Study on the Phase Transition at 1500 K in MgSiO3 Enstatite
Quantifying the structural changes that occur in enstatite (MgSiO3) at high temperature (T) is essential to understand geophysical phenomena of the upper mantle. It has been known that enstatite undergoes a phase transition above 1000 K. However, the crystal structure of the high-T phase has been controversial for decades because the high-T phase is not quenchable, and changes in its diffraction pattern are subtle. Raman spectroscopy provides structural information and is complementary to diffraction methods in that it is sensitive to changes in bonding as well as symmetry. However, in situ high-T measurements have been difficult to obtain due to the intense thermal radiation from the sample at high T overwhelming the Raman signal. We measured Raman spectra of enstatite up to 1650 K at 1 bar using a nanosecond gated spectroscopy system which effectively reduces detection of the thermal radiation, and thus enhances the signal-to-background ratio of Raman spectra by 4 orders of magnitude. The starting material is natural orthoenstatite. We observed significant changes in intensities and frequencies of the Raman modes at 1500 K, suggesting that enstatite undergoes a phase transition. The fact that we did not observe the appearance or disappearance of major spectral features at the transition T indicates that the structural change at 1500 K may be displacive. Major low-frequency modes at 85 and 135 cm-1 show significant softening beginning from 1100 K, indicating reorganization of the structure around Mg atoms. A doublet at 680 cm-1, which is related to the bending vibration of the SiO4 tetrahedral chain (Si-O-Si), becomes a singlet at the transition temperature, consistent with configurational equivalence among the tetrahedral chains expected for protoenstatite. In addition, a doublet at 1025 cm-1, related to the Si-O stretching vibration, remains as a doublet even after the transition, indicating that the structure of the SiO4 tetrahedra may not change across the transition. Although major features of low-T enstatite reappear during cooling, we found some modes of the high-T phase survive even after quenching, indicating that enstatite may not be entirely reverted to orthoenstatite.
MR53A-1710
Equation of State of MgSiO3 Perovskite Based on A New MgO Pressure- Scale
It is generally accepted that (Mg,Fe)SiO3 perovskite is the most dominant phase in the Earth's lower mantle and the P-V-T equation of state are essentially important to constrain the chemical composition and temperature of the lower mantle. In order to determine the equation of state precisely in situ X-ray diffraction measurements of MgSiO3 perovskite were performed up to 62 GPa and 1500 K using a Kawai-type multianvil apparatus with sintered diamond anvils and synchrotron radiation. Mg2SiO4 forsterite was used as a starting material and MgSiO3 perovskite was synthesized during the heating period before the XRD measurements. MgO synthesized from the decomposition of the starting material was used for internal pressure standard. Pressures were evaluated with a new equation of state of MgO derived from pressure-scale-free experimental data (presented in the same session). Small amount of Au powder was mixed into the starting material as another possible pressure standard. Temperatures were measured using W97Re3-W75Re25 thermocouple and temperature fluctuations were less than 1 K for 1 hour even at 62 GPa and 1500 K. XRD profiles were acquired for 1 hour in the cooling cycle of each experimental run. Errors in the volume determination were about 0.5 per cent, and typical error in pressure calculation was 0.2 GPa. In this study, we provide internally consistent and precisely determined P-V- T data set of MgSiO3 perovskite, MgO, and Au to mid lower mantle conditions, which can be a primary reference in the study of the lower mantle.
MR53A-1711
Elastic Properties and Seismic Velocities of Amphibole
Water plays a major role in the evolution of the Earth; it drives arc magmatism, changes the composition of mantle melts, and is linked to subduction zone seismicity. Water released during dehydration of subducted slab constituents may alter the initially anhydrous overlying mantle. A full understanding of water in subduction zones requires quantification of the amount of water in the mantle, its location, and its flow rates. Seismology provides the highest resolution information but requires calibration of the dependence of mineral elastic properties on water content. Recent disagreement in the amount of water in the mantle wedge is associated with differing choices for serpentine elasticity. The second most abundant upper mantle mineral group, the pyroxenes, can be hydrated to form amphiboles. Available single crystal elasticity data for amphiboles remain extraordinarily weak. Thus, the prediction of seismic velocities as a function of water content in hydrated assemblages containing amphiboles is problematic. We report new one bar single crystal properties for amphiboles using Impulsive Stimulated Light Scattering. The similarity and differences between the current data and earlier work will be discussed.
MR53A-1712
Structure and density of perovskite from subducted oceanic crust in the lower mantle.
We have synthesised large single-crystals of Fe/Al-rich magnesium silicate perovskite from a MORB tholeitic
bulk composition at 25 GPa and 1800°C. Microprobe analysis and Mössbauer spectroscopy are
consistent with the formula
Mg0.59Fe2+0.08Fe3+0.34Al0.34Si0.65O3. The data are
consistent with a coupled substitution mechanism with Al substituting Si at the octahedral site and ferric Fe
substituting Mg at the A site. A small amount of Fe2+ is also present at the A site.
Single-crystal structural refinements confirm that the introduction of Fe/Al into the magnesium perovskite
structure gives rise to an increase of the orthorhombic distortion due to an increase of the octahedral tilting.
Moreover, the presence of a significant amount of Al into the B site gives rise to an increase of the
octahedral distortion.
The compressibility of a single-crystal from this sample was studied at ambient temperature by means of X-
ray diffraction using both ethanol/methanol and He as pressure transmitting media. Results show a decrease
in bulk modulus but a significantly increase of K' with respect to pure MgSiO3 and peridotitic
perovskite. The change in compressibility is mainly due to softening of the c axis. The increase of K'
may explain why larger bulk moduli values have been obtained from in-situ DAC studies of Fe/Al-bearing
perovskite for which K' has been always fixed to the value of 4.
Complete sets of intensity data have been collected at several pressures in order to constrain the effect of
Fe/Al substitution on the increase in distortion of the perovskite structure with pressure.
Results have implications for density and anisotropy of subducted oceanic crust.
MR53A-1713
In-situ Elasticity Measurements on Fayalite to 8.7 GPa and 1073 K
While the elastic properties of the Mg-rich olivine have been studied using many different techniques, studies on the Fe-rich olivine are relatively scarce, especially at simultaneous high pressure and high temperature. These properties are essential to a complete understanding the structure and dynamics, the oxidation state, and deformation properties of the upper mantle. In this study, elastic wave velocities measurements on polycrystalline fayalite (Fe2SiO4) were carried out using simultaneous X-ray diffraction, X-ray radiography, and ultrasonic interferometry to 8.7 GPa and 1073 K. The experiment was conducted in a DIA-type cubic anvil apparatus (SAM85) installed at the superwiggler beamline X17B2 in Brookhaven National Laboratory. Travel time, sample length, and X-ray diffraction patterns (sample volume) were collected along multiple heating/cooling cycles, from which compressional and shear wave velocities, elastic bulk and shear moduli and their pressure and temperature derivatives were derived. These results are compared with the elasticity data of the (Mg,Fe)2SiO4 olivine solid solution from the literature and the effect of Fe/(Fe+Mg) ratio on elasticity of olivine is discussed.
MR53A-1714
Effects of Hydration on Thermal Expansion of Forsterite, Wadsleyite, and Ringwoodite
Crystals of hydrous forsterite, wadsleyite and ringwoodite with 8900, 16,600, and 25,000 ppm by weight H2O, respectively, were synthesized at Bayerisches Geoinstitut in Bayreuth, Germany. X-ray single- crystal diffraction was used to measure the thermal expansion coefficients of forsterite, wadsleyite, ringwoodite and their hydrous forms at ambient pressure, from temperatures as low as 133K to temperatures as high as 919K. Second order polynomial fitting was applied to thermal expansion of each crystal to derive the expansion coefficients in the form of α=a0+a1*T. The single crystal of anhydrous wadsleyite persisted up to 859K, and the single crystal of anhydrous ringwoodite broke up at 911K. Hydrous forsterite was observed to dehydrate at 919K, whereas the dehydration of hydrous wadsleyite started at 655K. From room temperature to high temperatures in this study, the mean thermal volume expansion coefficients of forsterite, wadsleyite and ringwoodite are 36.4(5)*10-6 K-1, 30.8(9)*10-6 K- 1 and 30.6(9)* 10-6 K-1 respectively for the anhydrous forms, and 38.1(9)* 10-6 K-1, 35.8(14)* 10-6 K-1 and 34.8(8)* 10-6 K-1 respectively for the hydrous forms. Hence, forsterite, wadsleyite, and ringwoodite all have larger thermal expansion coefficients in the hydrous forms compared with the anhydrous forms.
MR53A-1715
Crystal structural features of the M-site vacancies in Fe-free and Fe-bearing hydrous forsterites
The result of the structural refinement of Fe-free hydrous forsterite (Kudoh et al, J.M.P.S. 2006) revealed that the cation vacancies at the octahedral M sites predominantly occur at the M1 site. In contrast to the case of Fe-free hydrous forsterite, the result of the structural refinement of synthetic Fe-bearing hydrous foraterite (Kudoh et al, J.M.P.S. 2007) revealed that the cation vacancies at the octahedral M sites predominantly occur at the M2 site. In the case of Fe-free hydrous forsterite, the size of M1O6 octahedron is smaller than the ideal size to fit the edge lengths shared with rigid SiO4 tetrahedron. The size of M2O6 octahedron is close to the ideal size. The structural strain from the ideal size is larger in M1 site than in M2 site. Two H atoms may replace the Mg atom predominantly at the M1 site to reduce the structural strain, giving the vacancy at the M1 site. In the case of Fe-bearing hydrous forsterite, the Fe atoms occupy both M1 and M2 sites, leading the combinations of Mg-Mg, Fe-Fe, Mg-Fe and Fe-Mg for the occupation of M1-M2 sites. Among these, the combination of Fe in M1 site and Mg in M2 site may have maximum structural strain from the ideal size. For the reduction of the structural strain, two H atoms may replace the Mg atom predominantly at the M2 site, giving the vacancy at the M2 site.
MR53A-1716
Phase equilibrium and structural properties in the Ca-Al-Si-O system up to the P,T conditions of the Earth's lowermost transition zone.
Ca- and Al-bearing silicates have been extensively investigated for their importance into the mantle processes. In the transition zone, majorite garnet and Ca-perovskite are reported to be the major hosts of Ca as well as deposits of Al [1]. At higher P, Ca gets completely exsolved from the majorite to the Ca-pv. It is observed that Al plays a significant role in the Ca-Si-O system by inducing new phase equilibriums [2,3,4], and influencing the physical properties of the Ca-pv [5]. It is then suggested that phase transitions and chemical reactions involving Ca- and Al-bearing silicates could conciliate mineralogical results and seismological observations in the region of the transition zone [3,6]. We studied the phase equilibrium at HP-HT in the grossular and anorthite systems using a Kawai-type multi- anvil apparatus up to 25 GPa and 2000 K. Grossular garnet is reported to decompose into a mix of corundum and Ca-pv [3] or to transform into a perovskite of grossular composition [7]. Our study shows that grossular mainly decomposes into a mix of Al-Ca-pv and CAS phase in the same P range and higher T. Anorthite is also found to mainly decompose into a mix of Al-Ca-pv and CAS phase. Despite some Al-content in the Ca-pv, the CAS phase is the major host of Al, which suggests according to previous studies [3,5], a lower solubility of Al into the perovskite structure at high T. The Al-content of the Ca-pv as well as the presence of another Al-rich Ca-bearing silicate in zone of variable temperatures and composition as subducting slabs are discussed for their possible implication into the transition zone processes. [1] T. Irifune, 1994. Nature 370, 131-133 [2] L. Gautron et al., 1996. Phys. Earth Planet. Int. 94, 71-81 [3] N. Takafuji et al., 2002. Phys. Chem. Mineral 29, 532-537 [4] S. Zhai and E. Ito, 2008. Phys. Earth Planet. Int. In press [5] T. Kurashina et al., 2004. Phys. Earth Planet. Int. 145, 67-7 [6] L. Stixrude et al., 1996. Am. Mineral. 81, 1293-1296 [7] H. Yusa et al., 1995. Phys. Earth Planet. Int. 25, 25-31
MR53A-1717
Intercalibration of FTIR and SIMS for Hydrogen Measurements in Garnets
The importance of understanding the hydrogen incorporated in nominally anhydrous minerals (NAMs) that influences their physical and chemical properties significantly is well accepted, however, quantitatively experimental constraints between hydrogen concentration and physical and chemical properties are only meaningful if hydrogen concentration can be accurately determined. Fourier Transform Infrared Spectroscopy (FTIR) may be the most powerful technique since it provides information about both bulk concentration and local bonding environments, while Secondary Ion Mass Spec-trometry (SIMS) has held promise as an ideal method for hydrogen analyses for its advantages of high spatial resolution and insensitivity to crystal orientation. Great efforts have been made to establish experimental standards for these two techniques since they have no rigorous self-calibration. We here present new Fourier Transform Infrared Spectroscopy (FTIR) and Secondary Ion Mass Spectrometry (SIMS) analyses of 1H in 11 natural garnets (grossular, spessartite and pyrope). This set of garnets extends to a wider range of H2O contents (~4 to 6790 wt. ppm H2O) than previous studies and can be more reliable if more garnets can be investigated. We routinely achieve a less than 5 ppm by weight H2O with high vacuum quality, the use of a Cs+ primary beam, and carefully prepared samples using a resin-free mounting technique (described in Aubaud C. et al., 2007. Intercalibration of FTIR and SIMS for hydrogen measurements in glasses and nominally anhydrous minerals. American Mineralogist, 92, 811-828). Although some scatter is observed, the straight line with a slope of 0.0722 defined by all garnets in a plot of (H)FTIR vs. (H+/Si+)SIMS* (SiO2)EMP suggests small matrix effects, while some previous efforts demonstrated the existence of such matrix effects. Discrepancies between FTIR and SIMS measurements can be partially distributed to the impurities, which have different hydrogen concentration as the host crystals, because much larger volume is sampled by FTIR than SIMS.
MR53A-1718
A New High-Pressure Phase With Mg2Al2O5 Composition
As MgAl2O4 is one of major components of both spinel phase and calcium ferrite phase in the Earthfs mantle, many researchers have studied its high-pressure phase relations. However, the high- pressure high-temperature phase relation has been unclear at temperature higher than 2000 °C. By performing high-pressure experiments, we found that MgAl2O4 spinel decomposes to Mg2Al2O5 and Al2O3 at 20-25 GPa and temperature higher than 2000 °C. This is the first report of a compound with the composition of Mg2Al2O5. The powder X-ray diffraction pattern can not be explained by crystal structures of already known high-pressure phases like brownmillerite-type Ca2Al2O5. In this study, we tried to construct a crystal structure model of the new Mg2Al2O5 phase. The obtained structure model was used for structure refinement by the Rietveld analysis. The Mg2Al2O5 phase for a powder XRD measurement was synthesized using a Kawai-type multi-anvil high-pressure apparatus at Gakushuin University. The starting material of a mixture of MgO:Al2O3 = 2:1 (in molar ratio) was hold at 23 GPa and 2500 °C for 10 min. After quenching, the sample was recoverd to ambient pressure. The sintered polycrystalline sample was crushed by a die and ground into powder. The powder X-ray diffraction profile was measured using Rigaku RINT2500V (Cr Kα, 45 kV, 250 mA) with the step size of 0.02°. The Rietveld analysis was made using RIETAN-2000 program. The XRD profile showed that symmetry of the Mg2Al2O5 phase is orthorhombic. The extinction rules of the powder XRD peaks suggested the space group of Pba2 or Pbam. Since it was found that the XRD pattern is close to that of (Mg, Fe2+)2(Fe3+, Al)(BO3)O2 ludwigite, a structure model of the Mg2Al2O5 phase was constructed based on it. It should be noted that planner three coordinated sites in ludwigite in which boron places were moved by 0.5 in atomic coordinate along c-axis, because interatomic distances between cation and oxygen in the planner three coordinated sites are too short for Mg2+ and Al3+. In this structure model, (Mg, Al)O6 octahedra connected by edge-sharing and corner-sharing form triangular tunnels along c-axis. Cation sites occupied by only Mg are placed in the tunnels. The sites have a coordination environment of trigonal prism- type MgO6. By the Rietveld analysis, lattice parameters were determined as a = 9.3710(6) Å, b = 12.1952(6) Å, c = 2.7916(2) Å. The calculated density of 3.801(1) g/cm3 is consistent with the high-pressure phase relations. R factor by adopting the space group of Pbam is smaller than that by Pba2. Therefore, it is suggested that the space group of the Mg2Al2O5 phase is Pbam
MR53A-1719
High pressure behavior of phase-X
A recent study by Konzett and Fei (2000) has revealed that K-rich amphibole breaks down to produce a hydrous K-rich silicate phase with P63cm symmetry. The structure of this phase, which has been named phase-X, consists of octahedral MgO sheets held together by dislicate Si2O7 groups, with K seated in the large interstitial spaces between layers. While it has been shown that phase-X can serve as a host of potassium and water in the mantle, very little is known about its structural, thermodynamic and elastic behaviour at elevated pressure. We are undertaking an in-depth analysis of the crystal chemistry of phase-X, relating structure to the stability and thermodynamic properties using First Principles simulations based on Density Functional Theory. We find the equation of state for anhydrous phase-X to be well represented by a third order Birch-Murnaghan finite strain expansion, with KO = 132 GPa, K' = 4.40 and VO = 284.58 Å3, where K is the bulk modulus, prime indicates pressure derivatives, O refers to zero pressure. In addition to investigating the possible locations within the structure available to protons (i.e. water), we calculate the full elastic tensor for this phase at high pressure, from which the seismic velocities and their dependence on water content are derived. References Konzett, J and Fei, Y (2000) Transport and storage of potassium in the Earth's upper mantle and transition Zone: an experimental study to 23 GPa in simplified and natural bulk compositions. Journal of Petrology, 41, 583-603.
MR53A-1720
Tetrahedral Occupancy of Ferric Iron in (Mg,Fe)O at High Pressures
(Mg,Fe)O, the second most abundant phase in the lower mantle, plays important roles in the dynamics of the deep Earth. The presence of ferric iron (Fe3+) in (Mg,Fe)O can have significant effects on its transport properties such as atomic diffusivity, electrical conductivity, and viscosity, since the distribution of Fe3+ affects the equilibrium concentration of point defects. Fe3+ is generally assumed to substitute into octahedrally-coordinated cation sites in (Mg,Fe)O. Our previous thermodynamic analysis, however, suggested that Fe3+ occupies tetrahedrally-coordinated interstitial sites at high pressures. Two different sites of Fe3+ occupancy are predicted by different pressure dependence of concentration of Fe3+. Therefore, more detailed studies on the atomistic mechanisms of Fe3+ dissolution are necessary to provide us with some insight into the transport properties of the lower mantle. In this study, we have conducted Mössbauer spectroscopy of (Mg,Fe)O single crystals synthesized at 1200-2000 °C and 5-15 GPa under different oxygen fugacity using a multianvil apparatus. The Mössbauer spectra were recorded at room temperature in the transmission mode. Each obtained Mössbauer spectrum consists of a dominant quadrupole doublet of ferrous iron (Fe2+) superimposed with a minor peak of ferric iron (Fe3+). The site occupation and concentration of Fe3+ are estimated based on the position of the isomer shift and the absorption intensity. These data suggest that 1) Fe3+ occupies octahedral sites at low pressures and temperatures while it occupies tetrahedral sites at high pressures and temperatures, 2) Fe3+ solubility decreases with temperature in the regime of octahedral occupancy, while it slightly increases in the regime of tetrahedral occupancy. Based on these results, we infer that Fe3+ in (Mg,Fe)O predominantly occupies tetrahedral sites in the lower mantle.
MR53A-1721
Phase transitions and density changes in pyrolite up to 50 GPa using multianvil apparatus with sintered diamond anvils
Pyrolite has been thought a representative model of the Earthfs mantle composition. Therefore, detailed studies of the phase relations and mineral physics properties in pyrolite over a wide pressure and temperature condition are required in order to understand the mineralogy and dynamics of the entire mantle. In this study, we have conducted in situ X-ray diffraction experiments to clarify the phase relations and chemical compositions of the individual phases, and associated density changes in pyrolite under the lower mantle conditions up to 50 GPa by using a combination of a multianvil apparatus and synchrotron radiation. In order to generate these high pressures, sintered-diamond cubes with truncated edge length of 1.5 mm were used as second stage anvils. The produced pressure was evaluated by the Au pressure scale proposed by Tsuchiya (2003). Only three phases, MgSiO3-rich perovskite, CaSiO3-rich perovskite and (Mg, Fe)O ferropericlase, were observed from 28 to 47 GPa and at 1873 to 2073 K on the basis of both in situ X-ray observations and electron microprobe analyses on the recovered samples, which are consistent with some earlier studies on pyrolite or peridotite compositions using laser-heated diamond anvil cell. The density change in pyrolite along a typical geotherm down to a depth of ~ 1200 km was evaluated by combining the observed unit-cell volume and chemical composition data on the coexisting individual high-pressure phases, which agrees quite well with the density profile in PREM, suggesting that pyrolite is a good compositional model for this region of the lower mantle.
MR53A-1722
Equation of state of nanocrystalline forsterite
Grain size and grain shape are important parameters for physical properties of minerals as well as for geophysical processes like deformation. Even though the occurrences of nanominerals in the crust and the mantle are limited their presence might have an important influence. For instance, mechanical grinding of rocks associated to earthquake can generate nanoparticles and their presence is thought to control earthquake instability. In the context of subducting slab, experiments show presence of nanoparticle of ringwoodite which plays a important role in deep earthquakes. However, the influence of nano grain size on elastic properties of minerals is poorly studied. This work presents a study of equation of state of nano-forsterite. Nanocrystalline forsterite has been synthesized using a sol-gel method. A particle size of about 50 nm has been obtained. In-situ hydrostatic high pressure and high temperature experiments have been performed at X17B2 (NSLS) using the DIA-type multianvil press. Two samples of macron size and nano size particles of forsterite have been studied simultaneously up to 10 GPa and 1300°C for comparison. The bulk modulus of both samples and its derivative with pressure and temperature will presented and the consequences for geological processes will be discussed.
MR53A-1723
Characterizing of X-ray Diffraction on Beamline 12.2.2 Advanced Light Source
The lattice parameter of NIST standard reference material LaB6 has been determined by x-ray diffraction on beamline 12.2.2 at Advance Light Source. Ten separate measurements of the lattice parameter at each energy point and sample-to-detector distance were made and the beam energy ranges from 15 to 30KEv. The delta(2theta) and FWHM are highly consist over the whole diffraction angle. The lattice parameter of LaB6 was refined with GSAS and high accurate of 4.1560(6) was obtained.
MR53A-1724
Pyrometry in the Multianvil Press: New approach for temperature measurement in large volume press experiments
Temperature measurement in large volume press experiments has been based on thermocouple emf, which has well known problems: unknown pressure dependence of emf [e.g., 1], chemical reaction between thermocouple and other materials, deformation related texture development in the thermocouple wires [2], and so on. Thus, different techniques to measure temperatures in large volume press experiments other than thermocouples are required to measure accurate temperatures under high pressures. Here we report a new development using pyrometry in the multianvil press, where temperatures are derived on the basis of spectral radiometry. Several high pressure runs were conducted using the 1000 ton press with a DIA module installed at 13 ID-D GSECARS beamline at Advanced Photon Source (APS) [3]. The cubic pressure medium, 14 mm edge length, was made of soft-fired pyrophyllite with a graphite furnace. A moissanite (SiC) single crystal was built inside the pressure medium as a window for the thermal emission signal to go through. An MgO disk with 1.0 mm thickness was inserted in a gap between the top of the SiC crystal and thermocouple hot junction. The bottom of the window crystal was in direct contact with the tip of the anvil, which had a 1.5 mm diameter hole drilled all the way through the anvil axis. An optical fiber was inserted in this hole and the open end of fiber was in contact with the SiC crystal. Thermal spectral radiance from the inner cell assembly was obtained via the fiber and recorded by an Ocean Optics HP2000 spectrometer. The system response of spectrometer was calibrated by a tungsten ribbon ramp (OL550S, Optronic Laboratories, Inc.) with standard of spectral radiance. The cell assembly was compressed up to target value of 15 tons and then temperature was increased up to 1573 K. Radiation spectra were mainly obtained above 873 K and typical integration time was 1 ms or 10 ms. Data collection was done in the process of increase and decrease of temperature. In one in-situ X-ray experiment, pressures near the thermocouple were derived from the equations of state for gold [4] or MgO [5], and the pressure values varied up to approximately 1 GPa at various temperatures. Two different calculations were made to infer temperature based on pyrometry. One was made based on Planck radiation function and the other was made by the J-function, which was defined based on the law of Wien [6]. Calculated temperatures above about 1000 K were generally consistent with those obtained from the thermocouple emf. However, below about 1000 K, temperatures based on pyrometry in the process of increase of temperature were way off from those obtained by thermocouple emf. Recent results and future improvements will be discussed. Reference [1] I. C. Getting and G. C. Kennedy J. Appl. Phys., 41, 4552-4562, 1970. [2] J. Li, C. Hadidiacos, H.-K. Mao, Y. Fei and R. J. Hemley High Pressure Research, 23, 389-401, 2003. [3] Y. Wang, M. Rivers, S. Sutton, N. Nishiyama, T. Uchida, T. Sanehira Phys. Earth Planet. Inter. in press. [4] S.-H. Shim, T. S. Duffy and K. Takemura Earth Planet. Sci. Lett., 203, 729-739, 2002. [5] M. Matsui, S. C. Parker and M. Leslie Am. Mineral., 85, 312-316, 2000. [6] T. Yagi and J. Susaki, in High-Pressure Research: Application to Earth and Planetary Sciences, edited by Y. Syono and M. H. Manghnani, pp.51-54, TERRAPUB, Tokyo / AGU, Washington D.C., 1992.
MR53A-1725
High-pressure single-crystal X-ray diffraction study on Super Fluorous Phase B up to 7.4 GPa
Super hydrous phase B (SUP-B), Mg10Si3O14(OH)4, is known as one of dense hydrous magnesium silicate minerals (DHMS). Also, super fluorous phase B (sup-B-F), Mg10Si3O14F4, was firstly synthesized by Hazen et al. (1997). In the crystal structure of SUP-B-F, F replaces OH in sup-B structure. Therefore, differences of behavior between SUP-B-F and SUP-B under high-pressure conditions directory reflect the effect of hydrogen bonding. In this study, to clarify the behavior of SUP-B-F under high-pressure condition, SUP-B-F was synthesized at 21 GPa and 1400°C conditions using the Kawai type multi anvil apparatus installed in Gakushu-in University. Using a synthesized single-crystal (0.04 × 0.04 × 0.03 mm3), high-pressure single-crystal X-ray diffraction measurements were conducted up to 7.4 GPa with synchrotron radiation. Lattice constants were determined at several pressure points up to 7.4 GPa. The isothermal bulk modulus of SUP-B-F, calculated from the Birch-Murnaghan Equation of State, was 154(2) GPa with a pressure derivative K'=4 (assuming). This value is significantly larger than that of super hydrous phase B (142.8(5) GPa with Kf=5.7 by Crichton et al., 1999). The X-ray reflection intensity data set was collected at 5.0 GPa using the four-circle diffractometer installed at the beam line BL-10A in Photon Factory, Japan. 198 reflections were used for structure refinement (the number of measured reflections is 786). After Lorentz- polarization correction and background reduction were applied for all intensity data, the crystallographic equivalent reflections (Orthorhombic, Pnnm) were averaged. Structure refinement results were R=5.0% and wR2=13.1%, respectively.
MR53A-1726
Polarizability of SiO Bonds in Olivine as a Function of Temperature: a Raman Study
Olivine undergoes no phase transition between 300 K and 850 K. Hence, Raman band intensities and band positions should vary monotonously upon heating. We find that, upon heating to 670 K, Raman bands in the 700-1000 cm1 range decrease monotonously in intensity and positions. The effect is reversible. Above 670 K, however, the Raman bands increase again in intensity. Upon cooling from 850 K to 300 K there is at first a hysteresis and then full recovery. A prior study of the dielectric polarization of olivine at the limit of 0 Hz indicates a similar 2-step T-dependent behavior, which correlates well with the 2-step break-up of peroxy defects in the olivine structure. The break-up of the peroxy bonds leads to an extended delocalization of the wavefunction associated with the O state. Although the concentration of peroxy defects is probably only in the 100-500 ppm range, their effect is felt across the entire olivine structure. As the wavefunction associated with the hole state spreads out, all oxygens loose some of their electron density and their polarizability increases. This effect can explain the anomalous T-dependent behavior of the Raman bands.
MR53A-1727
Structure of interatomic Fe-B compounds at high pressures and temperatures
The interatomic Fe-B compounds consisting of relatively large transition-metal atoms and smaller metalloids have many useful properties, such as wear resistance, corrosion resistance, good electric and thermal conduction, ferromagnetism and a potential for incompressible superhard material. Due to these the magnetic and other properties of such compounds have been very actively studied over the three decades [1-5]. The interatomic Fe-B compounds constitute an ideal testing ground, because it allows one to investigate a change in magnetic properties with different B and Fe concentrations and they could be a model of iron diluted compositions of the Earth outer core [6]. There are only a few studies on Fe-B compounds at high pressure and most of them are focused on the amorphous state of the compounds [7, 8]. A detailed study of the Fe-B compounds at high pressure was not found in literature. We synthesized and studied the stable FeB, Fe2B, and unstable Fe3B at high pressures and high temperatures. The experiment included a synthesis the materials, chemical analysis, structural analysis and determination of the equation of states. This study consequently important and represent a significant challenge for a thorough understanding of the properties of these interstitial compounds.
MR53A-1728
Iron oxidation state in (Mg,Fe)O: Calibration of the flank method on synthetic samples and application to natural inclusions in Lower Mantle diamonds
Ferropericlase (Mg,Fe)O and (Mg,Fe)(Si,Al)O3 perovskite are believed to form the bulk of the Lower Mantle (LM). The interval of redox conditions in the LM is still debatable. It was shown that the Fe3+ concentration in perovskite is insensitive to oxygen fugacity, therefore we have turned our attention to (Mg,Fe)O. Our work involves calibrating the so-called "flank method" for synthetic (Mg,Fe)O, and applying the results to determine Fe3+/Fetot in ferropericlase inclusions from LM diamonds, as a direct tool for investigating LM redox conditions during diamond formation. Experiments were performed in a multi anvil apparatus to obtain ferropericlase crystals with high quality surfaces, and Fe3+/Fetot in them was determined by Moessbauer spectroscopy. Samples were subsequently analyzed using the EMP. The flank method sensu strictu consists of measuring two pre-defined energy positions FeLAlfa and FeLBeta, whose ratio is sensitive to Fe2+/Fe3+. Positive correlations of LBeta/LAlfa as a function of Fetot (wt percent) and Fe2+ (wt percent) were observed for (Mg,Fe)O similar to those reported in the literature for garnets. We applied a least-squares regression model to fit the three variables Fetot, LBeta/LAlfa and Fe2+, and chose the simplest equation that fit the data. Our calibration for (Mg,Fe)O with xFe between 2 and 36.6 wt percent shows an agreement of 1 sigma with Fe3+/Fetot determined by Moessbauer spectroscopy, but with the additional advantage of a spatial resolution on the order of 10 microns (compared to no smaller than 100 microns for Moessbauer). We applied the flank method calibration to 6 ferropericlase inclusions from ultra deep diamonds (4 from Mato Grosso and 2 from Machado River, Brazil). Results show that LBeta/LAlfa ratios are consistent with the trend observed for synthetic (Mg,Fe)O, with no obvious evidence for magnesioferrite exsolution. The obtained Fe3+/Fetot values between 8 and 13 percent, and their correlations with Na, Cr and Al show that the inclusions were formed at oxygen fugacity conditions ranging from very reduced to quite oxidised. The patterns follows previous data for ferropericlase diamond inclusions from Kankan (Guinea) and define more the range of oxygen fugacity existing during diamond formation in the LM.
MR53A-1729
Phase diagram determination up to 823K in minerals using Diamond Anvil Cell
Abstract: The ruby fluorescence technique is widely used for pressure measurement inside the diamond anvil cell. However, at higher temperatures estimation of pressure becomes complex due to the broadening of the fluorescence peaks. There are other fluorescence markers upon which temperature has a lower effect. For example Sm: SrB2O7. Here, we present a high pressure - high temperature calibration of Samarium doped SrB2O7. In order to minimize the error in determining the pressure, Strontium Borate along with Ruby were loaded in diamond anvil cell under hydrostatic conditions and fluorescence measurements were carried out upto temperatures of 823K under pressure. An equation routine is obtained to fit the peaks at various temperatures upto 823K under pressure. Using this data for pressure determination, phase diagram of Lawsonite was studied and the results are discussed.
MR53A-1730
In Situ Thermal Diffusivity Measurements Of MgSiO3 Perovskite At Lower Mantle Pressures
We report preliminary data on the thermal conductivity of (Mg0.9Fe0.1)SiO3 silicate perovskite performed at 125 GPa and 2000 - 4000 K using a transient heating technique system (Beck et al. Appl. Phys. Lett. 91 181914, 2007) in conjunction with a diamond anvil cell. In situ x-ray diffraction tests of the sample show that the original Enstatite sample (Mg0.9Fe0.1)SiO3 laser heated during 20 minutes at 125 GPa to 2500 K has compositionally changed into two separate assemblages of perovskite (vast-majority) and post-perovskite. The thermal conductivity has been measured to be 12.5 (W/m*K) and thermal diffusivity has been calculated (using extrapolated to high pressure values of density and heat capacity) to be 12.2(±0.7) * 10-6 (m2/s). The thermal conductivity obtained in this experiment is in the range of previously theorized data. (Osako et al. Geophys. Res. Lett. ISSN 0094-8276, 1991) and shows that this experimental method and results seem promising. We acknowledge the support of NSF-EAR- 0721449, DOE/BES, DOE/NNSA (CDAC), and the W.M. Keck Foundation. R.K. acknowledges support by the NSF Research Experience for Undergraduates (REU) Program at the Carnegie Institution of Washington. We thank Yue Meng (HPCAT) for help at the laser heating x-ray diffraction facility at HPCAT (APS).
MR53A-1731
Laser Generation and Detection of Longitudinal and Shear Acoustic Waves in a Diamond Anvil Cell
Understanding of the elastic behavior of minerals under high pressure is a crucial factor for developing a model of the Earth's structure because most information about the Earth's interior comes mainly from seismological data. Several methods have been developed including Brillouin scattering, ultrasonic interferometry, and impulsive stimulated light scattering (ISLS). In view of the merit and the limitation involved in each technique, we will present an innovative approach, using laser ultrasonic in diamond-anvil cell (LU- DAC) for non-transparent materials. We will demonstrate that, using LU-DAC, it is possible to obtain directly both longitudinal and shear velocities of a non-transparent under high pressures. Preliminary data obtained on an iron foil to 23 GPa will be presented and compared with those reported earlier. Potential for extending such measurements to higher pressures and high temperatures for non-transparent materials such as amorphous and liquids will also be discussed.
MR53A-1732
High Pressure Equation of State Studies Using Ethanol-Methanol And Argon As Pressure Medium
Experimental high pressure studies are extremely important to planetary science, material science and to the development of condensed matter theory. With experimental difficulties in creating the extreme pressure temperature conditions appropriate to planetary interiors, the approach used is to obtain the thermodynamic data on materials of interest by extrapolating the condensed matter theory which has been benchmarked with the outcome of high pressure experiments to the available high pressures. However the high pressure data used to match the theory heavily depends on the use of pressure media; like ethanol-methanol, silicon oil, argon and helium. Unfortunately still there exist controversy in the literature even with the use of helium as pressure medium as illustrated by the unsettled debate on Zn and Os among different groups. We have measured the equation of state of intermetallic compound AuIn2 and Cd0.8Hg0.2 alloy using ethanol-methanol and argon to the pressure of 20 GPa to confirm the appearance of anomalies in the data due to occurrence of subtle electronic phase transitions. However these anomalies can also be attributed to oriented lattice strains and local non-hydrostatic conditions. We have tried to remove these at room temperature by stabilizing the sample in argon medium in the diamond anvil cell with proper annealing as indicated by the uniformity of the pressure across the sample by ruby fluorescence measurements. We will present the data revealing the electronic transition in AuIn2 at 2.7 GPa and in Cd0.8Hg0.2 near 9 and 18 GPa.