-
[show abstract]
[hide abstract]
ABSTRACT: The second critical endpoint in the basalt-H(2)O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.
Proceedings of the National Academy of Sciences 05/2011; 108(20):8177-82. · 9.68 Impact Factor
-
Earth and Planetary Sciences Letters 01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Coesite nanocrystals have been synthesized from periodic mesoporous organosilica (PMO) with (CH(2))(2) bridges heated at 300 °C for 150 min and 12 GPa. The crystals are not sintered, single crystalline, and have diameters of ca. 100-300 nm. Below 300 °C, an amorphous non-porous organosilica glass was obtained. Heating above 300 °C at 12 GPa results in the rapid crystal growth and micron size coesite crystals were formed.
Chemical Communications 12/2010; 46(46):8815-7. · 6.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the synthesis of optically transparent, mesoporous, monolithic diamond from periodic mesoporous carbon CMK-8 at a pressure of 21 GPa. The phase transformation is already complete at a mild synthesis temperature of 1,300 degrees C without the need of a catalyst. Surprisingly, the diamond is obtained as a mesoporous material despite the extreme pressure. X-ray diffraction, SEM, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, and Z-contrast experiments suggest that the mesoporous diamond is composed of interconnected diamond nanocrystals having diameters around 5-10 nm. The Brunauer Emmett Teller surface area was determined to be 33 m2 g(-1) according Kr sorption data. The mesostructure is diminished yet still detectable when the diamond is produced from CMK-8 at 1,600 degrees C and 21 GPa. The temperature dependence of the porosity indicates that the mesoporous diamond exists metastable and withstands transformation into a dense form at a significant rate due to its high kinetic inertness at the mild synthesis temperature. The findings point toward ultrahard porous materials with potential as mechanically highly stable membranes.
Proceedings of the National Academy of Sciences 08/2010; 107(31):13593-6. · 9.68 Impact Factor
-
Angewandte Chemie International Edition 06/2010; 49(25):4301-5. · 13.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the structural transformations of carbon filled SBA-16 mesoporous silica at 10 Gigapascal pressure and temperatures between 25 and 1800 °C in a multi-anvil apparatus. The silica begins to crystallize in the coesite structure at 400 °C under retention of the mesophase order. An irregular mesostructure of crystalline coesite is observed for samples treated at 800 °C. Formation of crystalline stishovite and graphite under mesoscale phase separation is observed at 1000 °C and 1300 °C. From a temperature of 1400 °C, the stishovite phase transforms into coesite and graphite transforms into diamond, respectively. The diamond phase adopts a micron-sized platelet-like morphology at 1400–1800 °C. The diamond crystallite size was determined to be 9–12 nm.
High Pressure Research 12/2009; 29(4):754-763. · 0.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Periodic mesoporous coesite was obtained by a modified nanocasting process from a periodic mesoporous silica SBA-16/C composite at a pressure of 12 GPa and 350 degrees C.
Journal of the American Chemical Society 07/2009; 131(28):9638-9. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic environment of the iron sites in post-perovskite (PPv) structured (57Fe,Mg)SiO 3 has been measured in-situ at 1.12 and 1.19 Mbar at room temperature using 57Fe synchrotron Mössbauer spectroscopy. Evaluation of the time spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The dominant site is consistent with an Fe3+ -like site in a high spin state. The second site is characterized by a small negative isomer shift with respect to a-iron and no quadrupole splitting, consistent with a metallic iron phase. Combined with SEM/EDS analyses of the quenched assemblage, our results are consistent with the presence of a metallic iron phase co-existing with a ferric-rich PPv. Such a reaction pathway may aid in our understanding of the chemical evolution of Earth's core-mantle-boundary region.
Geophysical Research Letters 05/2009; 36. · 3.79 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Faceted stishovite nanocrystals with sizes of 200-400 nm were synthesized at a pressure of 12 GPa and a temperature of 400 degrees C in a multianvil apparatus using mesoporous silica SBA-16 as the precursor.
Journal of the American Chemical Society 03/2009; 131(8):2764-5. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The structure of silicate glasses and the corresponding liquids at high pressure and their structure-property relations remain difficult questions in modern physical chemistry, geochemistry, and condensed matter physics. Here we report high- resolution solid-state O-17 3QMAS NMR spectra for mixed cation Ca-Na silicate glasses quenched from melts at high pressure up to 8 GPa. The spectra provide the experimental evidence for the varying pressure-dependence in two different types of nonbridging oxygen (NBO) environments (i.e., Na-O-Si and mixed (Ca,Na)-O-Si) in the single glass composition. The percentage of NBO drops significantly with increasing pressure and is a complex function of melt composition, including cation field strength of network modifying cations. A decrease in NBO fraction with pressure is negatively correlated with the element partitioning coefficient between crystals and liquids at high pressure.
The Journal of Physical Chemistry B 09/2008; 112(37):11756-61. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earth's interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earth's interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure-temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earth's interior.
Proceedings of the National Academy of Sciences 06/2007; 104(22):9182-6. · 9.68 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The boundary layer between the crystalline silicate lower mantle and the liquid iron core contains regions with ultralow seismic velocities. Such low compressional and shear wave velocities and high Poisson's ratio are also observed experimentally in post-perovskite silicate phase containing up to 40 mol% FeSiO3 endmember. The iron-rich post-perovskite silicate is stable at the pressure-temperature and chemical environment of the core-mantle boundary and can be formed by core-mantle reaction. Mantle dynamics may lead to further accumulation of this material into the ultralow-velocity patches that are observable by seismology.
Science 05/2006; 312(5773):564-5. · 31.20 Impact Factor
-
Wendy L Mao,
Yue Meng,
Guoyin Shen,
Vitali B Prakapenka,
Andrew J Campbell,
Dion L Heinz,
Jinfu Shu,
Razvan Caracas,
Ronald E Cohen, Yingwei Fei,
Russell J Hemley,
Ho-kwang Mao
[show abstract]
[hide abstract]
ABSTRACT: High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the core-mantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.
Proceedings of the National Academy of Sciences 08/2005; 102(28):9751-3. · 9.68 Impact Factor
-
Science 06/2005; 308(5725):1120-1. · 31.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Recent missions have provided important insights into the martian interior, even though all of them aimed to elucidate the
planet's surface. In their Perspective,
Fei and Bertka
argue that the available evidence points to a liquid Martian core and a solid iron-rich silicate mantle. However, the size
of the core is poorly constrained. Because better knowledge of the properties of the martian interior will shed light on the
evolution not only of Mars but also of other terrestrial planets, the authors call for a mission that focuses on the martian
interior.
Science 05/2005; 308(5725):1120-1121. · 31.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report spectroscopic evidence for the pressure-induced structural changes in B2O3 glass quenched from melts at pressures up to 6 GPa using solid-state NMR. While all borons are tri-coordinated at 1 atm, the fraction of tetra-coordinated boron increases with pressure, being about 5% and 27% in the B2O3 glass quenched from melts at 2 and 6 GPa, respectively. The fraction of boroxol ring species increases with pressure up to 2 GPa and apparently decreases with further compression up to 6 GPa. Two densification mechanisms are proposed to explain the variation of boron species with pressure.
Physical Review Letters 05/2005; 94(16):165507. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute 57Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the "three-doublet" model, which assumes two Fe2+ -like sites and one Fe3+ -like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe3+/ΣFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe3+ remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high- spin to a low-spin state.
American Mineralogist 03/2005; 90:199--205. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic spin state of iron in lower mantle perovskite is one of the fundamental parameters that governs the physics and chemistry of the most voluminous and massive shell in the Earth. We present experimental evidence for spin-pairing transition in aluminum-bearing silicate perovskite (Mg,Fe)(Si,Al)O(3) under the lower mantle pressures. Our results demonstrate that as pressure increases, iron in perovskite transforms gradually from the initial high-spin state toward the final low-spin state. At 100 GPa, both aluminum-free and aluminum-bearing samples exhibit a mixed spin state. The residual magnetic moment in the aluminum-bearing perovskite is significantly higher than that in its aluminum-free counterpart. The observed spin evolution with pressure can be explained by the presence of multiple iron species and the occurrence of partial spin-paring transitions in the perovskite. Pressure-induced spin-pairing transitions in the perovskite would have important bearing on the magnetic, thermoelastic, and transport properties of the lower mantle, and on the distribution of iron in the Earth's interior.
Proceedings of the National Academy of Sciences 10/2004; 101(39):14027-30. · 9.68 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic spin state of iron in lower mantle perovskite is one of the fundamental parameters that governs the physics and chemistry of the most voluminous and massive shell in the Earth. We present experimental evidence for spin-pairing transition in aluminum-bearing silicate perovskite (Mg,Fe)(Si,Al)O3 under the lower mantle pressures. Our results demonstrate that as pressure increases, iron in perovskite transforms gradually from the initial high-spin state toward the final low-spin state. At 100 GPa, both aluminum-free and aluminum-bearing samples exhibit a mixed spin state. The residual magnetic moment in the aluminum-bearing perovskite is significantly higher than that in its aluminum-free counterpart. The observed spin evolution with pressure can be explained by the presence of multiple iron species and the occurrence of partial spin-paring transitions in the perovskite. Pressure induced spin-pairing transitions in the perovskite would have important bearing on the magnetic, thermoelastic, and transport properties of the lower mantle, and on the distribution of iron in the Earth’s interior.
Proceedings of the National Academy of Sciences 09/2004; 101:14027–14030. · 9.68 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Magnetic, elastic, thermodynamic, and vibrational properties of the most iron-rich sulfide, Fe3S, known to date have been studied with synchrotron Mossbauer spectroscopy (SMS) and nuclear resonant inelastic X-ray scattering (NRIXS) up to 57 GPa at room temperature. The magnetic hyperfine fields derived from the time spectra of the synchrotron Mossbauer spectroscopy show that the low-pressure magnetic phase displays two magnetic hyperfine field sites and that a magnetic collapse occurs at 21 GPa. The magnetic to non-magnetic transition significantly affects the elastic, thermodynamic, and vibrational properties of Fe3S. The magnetic collapse of Fe3S may also affect the phase relations in the iron–sulfur system, changing the solubility of sulfur in iron under higher pressures. Determination of the physical properties of the non-magnetic Fe3S phase is important for the interpretation of the amount and properties of sulfur present in the planetary cores. Sound velocities of Fe3S obtained from the measured partial phonon density of states (PDOS) for 57Fe incorporated in the alloy show that Fe3S has higher compressional and shear wave velocity than those of hcp-Fe and hcp-Fe0.92Ni0.08 alloy under high pressures, making sulfur a potential light element in the Earth's core based on geophysical arguments. The VP and VS of the non-magnetic Fe3S follow a Birch's law trend whereas the slopes decrease in the magnetic phase, indicating that the decrease of the magnetic moment significantly affects the sound velocities. If the Martian core is in the solid state containing 14.2 wt.% sulfur, it is likely that the non-magnetic Fe3 phase is a dominant component and that our measured sound velocities of Fe3S can be used to construct the corresponding velocity profile of the Martian core. It is also conceivable that Fe3P and Fe3C undergo similar magnetic phase transitions under high pressures.
Earth and Planetary Science Letters 07/2004; 226. · 4.18 Impact Factor
