MR31A-0136
2D Micro-XAS mapping in Diamond Anvil Cell: Application for Post-Spinel Transition
Energy Dispersive X-ray Absorption Spectroscopy (EDXAS) is a now a well-established method which has been applied to a broad range of applications. The advantages of an energy dispersive spectrometer, that features no movement of optics during acquisition leading to an enhanced stability of energy scale, spot size and position, combined with a micron sized spot and the option of fluorescence detection, has made it possible to address 2- dimensional mapping with micron resolution on heterogeneous samples, providing full XAS information on each pixel. It is worth noting that due to the absence of mechanical scanning of the monochromator, the spatial resolution is not affected by the energy scan and remains fixed to the dimensions of the probe. In addition, the energy scale is preserved. Moreover, the dwell time per pixel is short enough to make it practically possible to acquire 100 x 100 pixel images in a few hours. We tested 2D mapping in transmission mode to perform "in-situ" investigations in the diamond anvil cell. Maps of redox and speciation at extreme conditions of pressure and temperature yield information on possible phase transitions and/or chemical reactions that occur at P and T conditions in the Earth interiors. As test sample, we chose a major component of Earth's transition zone, ringwoodite [γ-(Mg,Fe)2SiO4]. Sample synthesized in large-volume press at 19 GPa and 1700 C from natural olivine (Mg0.88,Fe0.12)2SiO4 was polished, loaded into the DAC, compressed to desire pressure, and laser-heated. We aquired Fe K-edge XANES maps at different pressures, up to ~ 40 GPa, before and after laser heating, covering for each map an area of 200 x 200 m2 at 5 m resolution. We found that laser heating does not result in re-distribution of iron between heated and non-heated areas. Within precision of measurements there are no detectable changes in iron oxidation state upon decomposition of ringwoodite in to silicate perovskite and magnesowüstite. We also observe that iron preferably partitioning in to magnesowüstite.
MR31A-0137
Synthesis of Post-perovskite Phases from Almandine-Pyrope Garnets at High Pressure
The post-perovskite phase could be the dominant phase at the core-mantle boundary and may be responsible for the anomalous seismic discontinuities within and across the D" layer. However, the chemical complexity and extreme conditions near the core-mantle region may strongly affect the structure and stability field of post- perovskite phase. To understand the stability of post-perovskite phase across a wide compositional range, including Al2O3-rich compositions, we have synthesized and investigated the properties of post- perovskite along the pyrope-almandine, (Mg,Fe)3Al2Si3O12, join. Three natural samples, Py58Alm38Gr3 (Alm38), Py43Alm54Gr2 (Alm54), Py21Alm73Gr5 (Alm73), were used in this study. The samples were mixed with gold or platinum as laser absorber and internal pressure standards. NaCl or Ar was used as a pressure medium and thermal insulators. In-situ high-pressure laser-heating experiments were carried out at 13ID-D beamline of GSECARS, Advanced Photon Source. The incident x-ray beam was focused to 4-10 μm2. The high temperatures were achieved by a double-sided laser-heating technique. The two-dimensional images of x-ray diffraction patterns were collected with the exposure time in the range of 10-600 seconds. The Alm38 samples were compressed to ~145 GPa and heated at 1460-1830 K for less than an hour, both post-perovskite and perovskite phases were observed. However, when heating at temperature >1900K for longer times, the Al2O3 post-perovskite phase can also be observed, indicating possible disproportionation. The Alm54 samples were compressed to > 148 GPa and heated to 1780K and both post- perovskite and perovskite phases were observed. At ~155 GPa and 2080 K some new peaks were observed for this composition that may be the Al2O3 post-perovskite phase. The Alm73 samples were compressed at pressure > 150 GPa and heated at 2040-2570K, only the post-perovskite phase was observed. Our results suggest that post-perovskite phase can incorporate high iron and aluminum content but requires higher pressures and temperatures for synthesis.
MR31A-0138
Crystal Chemical Basis of the Perovskite to Post-Perovskite Transition
The phase transition from perovskite to post-perovskite can be understood in terms of the crystal-chemical principles that govern the compression of the perovskite structure. 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. When the B site is softer than the A site, as it is for perovskites with cations of formal charge +3 on both sites, the structure becomes less tilted with pressure and undergoes transitions to higher symmetries [1]. The post-perovskite transition will therefore not occur in such compounds. In +2:+4 perovskites such as MgSiO3, the structure becomes more tilted with pressure as a result of the A cation site being softer than the octahedral B site. As the tilt angles increase, the bond valence matching principle [2] that governs the structural evolution of perovskites requires an increased rate of compression of both cation polyhedra. At high compression there comes a point at which no further tilting is possible. This can be interpreted as the structural and elastic limit within the perovskite structure that drives the transition to the post-perovskite phase. The systematics of the bond valence matching principle also allow one to estimate the Clapeyron slope of the phase transition and that the coupled substitution of +3 cations (e.g. Fe + Al for Mg + Si) in MgSiO3 perovskite will delay the post-perovskite transition to higher pressures. [1] Angel, Zhao, Ross (2005) Phys. Rev. Lett., 95, Art. No. 025503. [2] Zhao, Ross, Angel (2004) Acta Cryst., 60, 263-271.
MR31A-0139
Thermodynamics of CaMgSi2O6-KAlSi2O6 clinopyroxene solid solution: Quantum mechanical and static lattice energy calculations
K-enriched clinopyroxenes are often found in mineral assemblages formed at high and ultrahigh pressures, such as inclusions in diamonds, eclogitic and peridotitic xenoliths in kimberlites and lamproites. Thermodynamic properties of the solid solution between diopside and potassium jadeite cannot be studied experimentally due to impossibility to synthesize crystals with more than 25% of KAlSi2O6. Here we show that the missing thermodynamic information can be obtained with the aid of computer simulations. A set of empirical interatomic potentials has been used for calculation of static lattice energies of 800 different structures in a 2x2x4 supercell of C2/c pyroxene with compositions intermediate between diopside (Di) and K-jadeite (KJd), as well as with different ordering states of the exchangeable K/Ca and Mg/Al cations. Excess static energies of these structures were cluster expanded in a basis set of 37 pair-interaction parameters. These parameters were used to constrain Monte Carlo simulations of temperature-dependent properties in the range of 273-2023 K and to calculate a T-X phase diagram for the solid solution. The simulations predicted the formation of stable intermediate compounds at 1/3, 5/12, 1/2, 7/12 and 2/3 of the KJd mole fraction. The compound at 1/2 with a space group P2/b is analogous to omphacite in the diopside - jadeite system. However, the cation distribution in this phase is inverted: K and Ca in K-omphacite occupy positions which would be Ca-rich and Na-rich, respectively, in Na-omphacite. The standard enthalpies and volumes of KJd and K-omphacite were estimated from first principles calculations, while the standard entropies, thermal expansion coefficients and bulk moduli were predicted on the basis of the force field lattice dynamics. The activity-composition relations in the disordered C2/c phase were approximated with respect to Di and KJd end-members with a Redlich-Kister polynomial. Using these results and thermodynamic data available in literature, the isopleths of KJd in CaMgSi2O6-KAlSi2O6 clinopyroxene in assemblages with sanidine + coesite, Si-wadeite + kyanite + coesite, Si-wadeite + kyanite + stishovite and hollandite + stishovite were computed. The predicted isopleths permit an estimation of the pressure or temperature of equilibration of K-clinopyroxenes in assemblages with sanidine and hollandite.
MR31A-0140
Single-crystal elastic properties of hydrous olivine
Brillouin scattering measurements were performed on hydrous olivine (Fo93) with 0.6wt% water at ambient conditions. The single-crystal elastic moduli were obtained from three pre-orientated samples that allowed velocities to be obtained in the (100), (010), (011) planes at ambient condition. From the single crystal moduli, the aggregate elastic moduli (VRH averages) are KS = 128.6(11) GPa and μ = 79.0(3) GPa for the adiabatic bulk modulus and shear modulus, respectively. The aggregate velocities are VP = 8.43(4) km/s and VS = 4.90(1) km/s for the compressional velocity and shear velocity, respectively. Comparing the results of this investigation with those of previous studies on anhydrous olivine with similar Fe contents, our results indicate the hydration has little effect on elastic properties of olivine at ambient conditions. Alternatively, our results may suggest that the previous studies underestimated the hydrogen content in the olivine samples used. It would appear that the velocity jump across the 410 km discontinuity in a wet mantle will be depend mostly on the water content of wadsleyite, since the elastic properties of this phase have been found to be far more sensitive to the H content of the sample. High pressure measurements are in progress. The high pressure derivatives of elastic moduli of hydrous olivine and hydrous wadsleyite are critical for constraining the water content hidden in the mantle transition zone.
MR31A-0141
Cation vacancies and possible hydrogen atom positions in Fe-bearing hydrous forsterite
The crystal structure of a single crystal of synthetic Fe-bearing hydrous forsterite (Mg1.85Fe0.14Si0.99H0.06O4) synthesized at 13.5 GPa and 1400 °C has been investigated using X-ray diffractometry. The data show a site occupancy of 96(3)% Mg + 4(1)% Fe + 0(4)% at the M1 site and a site occupancy of 89(3)% Mg + 8(1)% Fe + 3(4)% at the M2 site, indicating that the cation vacancies at the octahedral M sites predominantly occur at the M2 site. The occupancy at the T site was 99.0% Si (fixed) + 0(1)% Fe + 1(1)% , indicating a small number of vacancies with no detectable Fe occupation. A comparison of the calculated bond distances and reported stretching frequencies of the polarized IR spectra shows that the most probable H atom locations are near to the edges of the MO6 octahedra for both M-site and T- site vacancies.
MR31A-0142
Elastic Properties of SiO2 Stishovite to 50GPa
Stishovite is a major phase in the crustal portion of subducted slabs in the bottom of the upper amntle and in the lower mantle. We have measured the acoustic velocities, densities, and Raman frequencies of pure SiO2 stishovite at pressures up to 50 GPa. The acoustic velocities data were used to calculate single-crystal and aggregate elastic moduli for stishovite. For single-crystal elastic moduli we obtained: C11 = 462(3) GPa, C33 = 769(5) GPa, C44 = 258(2) GPa, C66 = 299(3) GPa, C12 = 201(2) GPa, C13 = 190(2) GPa. Aggregate elastic moduli were fit at values: KS0 = 313(3) GPa, K0' = 4.41(20); G0 = 226(2) GPa. Our data are in excellent agreement with previous ambient and low-pressure measurements performed with Brillouin scattering (Weidner et al., 1982) and with first principles calculations ( Karki et al., 1997), The shear modulus varies strongly as a function of pressure. Our high-pressure measurements on variation of elastic properties with increasing pressure are in excellent agreement with the Landau theory calculations (Carpenter et al., 2000) which predict the high-pressure elastic properties in vicinity of second-order phase transitions. There is also reasonagle agreement with than the predicted high-pressure behavior fromfirst-principles methods.
MR31A-0143
Mineral physics of the perovskite to post-perovskite transition in CaIrO3 - seismological implications for the lower mantle
In the system CaIrO3 the perovskite (pv, Pbnm) to post-perovskite (ppv, Cmcm) phase transition is accessible at 1-5 GPa and 1450-1610 C. Both of the CaIrO3-phases are quenchable to ambient conditions, making the system suitable for studies of crystallographic, crystal chemical and mineral physical properties that may provide important guidelines to the behaviour of MgSiO3-based compositions of the lower mantle. Combined results from experimental phase relations, experimental pV-EoS [1], thermal expansivity and density functional theory (DFT) modeling [2] provide insights into the seismologic and geodynamic features of the pv-ppv-transition in the lowermost mantle. Experimental phase relations in the 1-5 GPa range and DFT modeling give Clapeyron slopes for the pv-ppv-transition of 24.5 and 19 MPa/K, respectively, which is about twice the published dp/dT-slopes of the MgSiO3-system. The compressibility of the CaIrO3-crystal structures derived from pv-EoS experiments and DFT-modelling are very similar. In ppv the b-axis compressibility is about twice that of the other two axes. In pv the a- and c-axis compressibilities exceed that of the b-axis by factors of 3 and 2, respectively. The bulk modulus change of the pv to ppv transition is negative, whereas the DFT-derived shear moduli of the two phases give a positive delta-G of the same transition. These results are consistent with DFT-modelling of the MgSiO3-system and with seismological data [3,4]. The apparent suitability of the CaIrO3-component as a low-pressure analogue for the lower mantle MgSiO3-component in terms of crystallographic and mineral physics properties has encouraged an investigation of the incorporation of trivalent cations. By studying the incorporation of a small selection of ions with variable radii in CaIrO3-based pv and ppv, we are searching for systematic relations between various trivalent cation proportions, substitution mechanisms and elasticity parameters. [1] T Boffa-Ballaran, RG Tronnes, D Frost, Equations of state of CaIrO3 perovskite and post-perovskite phases. Am Mineral 92, Oct. 2007. [2] S Stolen, RG Tronnes, The perovskite to post-perovskite transition in CaIrO3: Clapeyron slope and changes in bulk and shear moduli by density functional theory. Phys Earth Planet Int 164, 50-62, 2007. [3] J Wookey, S Stackhouse, J-M Kendall, J Brodholt, GD Price, Efficacy of the post-perovskite phase as an explanation for the lowermost-mantle seismic properties. Nature 438, 1004-1007, 2005. [4] T Lay, J Hernlund, EJ Garnero, MS Thorne, A post-perovskite lens and D" heat flux beneath the central Pacific. Science 314, 1272-1276, 2006.
MR31A-0144
Results From in Situ High P-T Melting and Phase Equilibria Experiments on the Allende Meteorite
Because chondritic materials are thought to be the building blocks of terrestrial planets and planetesimals, crystallization of chondritic and peridotitic material can be used to simulate accretion and differentiation of a bulk planet. The objective of this study is to measure the liquidus phases and temperatures for a number of planetary mantle analog materials at P>20 GPa. Experiments were conducted in the Large Volume Press at the Advanced Photon Source, Argonne National Laboratory. Phases were identified using energy-dispersive X-ray diffraction (EDXRD) with a fixed diffraction angle (2θ) of ~6° and data collection times of 60 sec. Heating runs up to 2200°C were performed at 400, 600, and 700 tons, sampling a pressure range from 18-32 GPa. A 3mm TEL beamline modified Fei-type assembly was used in experiments: a Re furnace with lanthanum chromite insulating sleeve, alumina or MgO end caps, graphite capsule packed with powdered starting materials, and X-ray windows of a slit in the Re furnace and alumina or graphite plugs in the lanthanum chromite. A pressed pellet of MgO powder doped with diamond powder was used as a pressure standard and packed between the capsule and thermocouple. The majorite liquidus temperature occurs near 2050°C, comparable to previous results (Agee et al. 1995; Asahara et al., 2004). The majorite-Mg-perovskite cotectic occurs at 22 GPa, as opposed to around 25 GPa. However, given the limited number of experiments and uncertainties introduced from previously not applying (1) a P-T relationship from relaxation of the assembly during heating, and (2) a thermal gradient from temperatures measured at the thermocouple across the capsule, significant differences in P and T are not surprising. It is possible to apply a P-T correction based on (1) and (2) above, and identify a majorite-ferropericlase cotectic for previous studies at around 22 GPa. This is particularly true for Agee et al. (1995), where calibrations were performed at 1200°C. Pressure at 2000°C can be 3 GPa lower than that at 1200°C (Leinenweber et al., 2006). Although Mg-perovskite was identified as the liquidus phase above 22 GPa, the ferropericlase-out line is very steep, and may become the liquidus phase above 25 GPa. This shallow liquidus is more consistent with a deeper, hotter magma ocean model.
MR31A-0145
Structure and Properties of Amorphous MgSiO3 in Earth's Mantle: A View from Synchrotron Inelastic X-ray Scattering
The structure of amorphous MgSiO3 at high pressure is essential for understanding magmatic processes in the Earth's interior and chemical differentiation of the Earth in the Hadean magma ocean. The pressure- induced structural changes in the Mg-silicate melts play an important role in the macroscopic thermodynamic and transport properties at high pressure. In spite of the importance and implications for global geophysical processes in the Earth's interior, the high-pressure structure of MgSiO3 glasses and melts has not been well-understood, contrary to its crystalline analogues, due to the inherent structural disorder and the lack of suitable experimental probes at high pressures. We have recently shown that synchrotron in-situ inelastic x-ray scattering at high pressure provides detailed pressure-induced electronic bonding changes in amorphous oxides (Lee SK et al. Nature Materials 2005 69, p3695; Lin et al. Phys. Review B. 2007 75, 012201; Lee SK et al. Phys. Rev. Lett. 2007, 98, 105502), providing improved understanding of atomistic origins of melt properties at high pressure (Lee SK, Geochim. Cosmochim. Acta. 2005, 69, p3695). Here, we report the inelastic x-ray scattering spectra for diverse binary and ternary oxide glasses including MgiSiO3 at pressure up to 40 GPa and revealed the previously unknown structural details of their pressure-induced electronic bonding changes. Direct in-situ measurements provide evidence for a gradual coordination transformation of framework cations as well as anions with multiple densification mechanisms and show that the onset of an electronic bonding transition in MgiSiO3 glass occurs between 12 to 20 GPa. While it is well established that the lattice structure of mantle minerals exhibits control on element partitioning behavior, our modeling based a strucutral input from x-ray Raman scattering indicates that the oxygen-specific bonding changes in MgiSiO3 melt with pressures also exhibits a profound control on elemental partition coefficients between silicate melts and crystal in Earth's mantle.
MR31A-0146
Density and Fe-Mg partitioning changes in pyrolite to 50 GPa
Pyrolite is 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, Fe-Mg partitioning and density changes in pyrolite under the lower mantle conditions by using a combination of a multianvil apparatus and synchrotron radiation. In order to generate higher pressures equivalent to the lower mantle conditions, sintered-diamond cubes with truncated edge length of 1.5 mm were used as second stage anvils. MgSiO3-rich perovskite (MgPv), CaSiO3-rich perovskite (CaPv) and (Mg, Fe)O magnesiowustite (Mw) were observed from 28 to 47 GPa and at 1873 to 2073 K, which are based on the Tsuchiyafs pressure scale of gold. This mineral assemblage is consistent with the previous studies, and the calculated bulk densities of pyrolite are close to those in PREM. We also observed the Fe-Mg partition coefficient between MgPv and Mw, KD=(Fe/Mg)Pv/(Fe/Mg)Mw, significantly decrease from 0.8 to 0.5 with increasing pressure in this above range.
MR31A-0147
Vibrational And Elastic Properties Of Delta-AlOOH and Phase D
It has been believed that water is transported into the deep mantle by hydrous minerals in subducting cold slabs. Existence of water in deep Earth minerals is known to affect their viscosity, melting temperature, and elastic properties. We have found using first principles techniques that both of δ-AlOOH, and phase D which are relevant to the carrier of water into the lower mantle, change to have the symmetric hydrogen bond at lower mantle pressures. Here we report the effects of hydrogen bond symmetrization on the vibrational properties of δ-AlOOH and phase D. We found that as increasing hydrogen bond strength by compression, the OH stretching frequencies decrease until the hydrogen bond changes to be symmetric. After the symmetrization of the hydrogen bond, the corresponding OH stretching frequencies, in turn, gradually increase with increasing pressure. These softening and subsequent hardening of the OH vibrational modes, which have also been reported in high pressure molecular solid, ice VII and VIII phases at ~60 GPa, occurs in hydrous mineral, δ-AlOOH and phase D, at much lower pressures of ~30 GPa and ~40 GPa, respectively. These vibrational properties can be good indications for experimental detection of the hydrogen bond symmetrization. We also report the elastic properties of δ-AlOOH and phase D and discuss seismic signatures expected to occur associated with the hydrogen bond symmetrization. Research supported in part by JSPS.
MR31A-0148
Micro-texture and Structure of High-pressure Quenched Graphite and Related Carbon Materials
There have been extensive studies in room-temperature compression of graphite and related carbon materials such as nanotubes and fullerene. Some reports claimed that the transformation of carbon hybridized state from sp2 to sp3 takes place under high pressure at room temperature, and the hardness of the quench products may be comparable to that of cubic diamond. Here, we investigated the micro-texture and structure involved in such high-pressure quenched carbon materials using high-resolution electron microscopy. High- pressure experiments were conducted on a variety of carbon materials including graphite (synthetic, highly- oriented sheet), single/multi-walled carbon nanotubes, amorphous carbons in a diamond anvil cell (DAC, with 250 μm culet non-beveled anvils) at room temperature. Pelletized sample was loaded into a 70 μm hall, drilled in a preindented Re gasket, without a pressure medium. The sample was compressed up to 70 ~ 90 GPa at room temperature, kept at the highest pressure at least overnight, and then decompressed. The pressure dependence of graphite E2g( G) Raman band at ~1580cm-1 was measured on compression and decompression. A1g( D) band, so called defect band at ~1350 cm-1, was also collected for the recovered products. The quenched materials were examined by high-resolution (HR) field emission (FE-) SEM and (HR)TEM. A focused ion beam (FIB) was employed to fabricate thin cross-sections of the samples. The most notable change in texture upon compression was observed in multi-walled carbon nanotube (MWNT); the elongated tubes were fragmented into short rods (ca. 100 - 300 nm in length and 80 - 100 nm in width, almost two times wider than that of the original MWNT). TEM observations showed that the short rod- shaped particles consist of piles of graphene shells (stacked walls of MWNT, characterized by (002) lattice fringes) which were significantly bent and fragmented. Some of those rod-shaped particles showed lattice fringes with an interlayer spacing of ca. 2 Å at the core regions, which may be derived from a high-pressure phase of carbon. More detailed textural observations and structural analysis based on electron diffraction are works in progress.