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To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds.

The composition of Archean volcanic crust can be characterized by a higher Mg/Si ratio than modern mid-ocean ridge basalt (MORB), because of the higher degree melting from the warmer mantle in the Archean. Although modern MORB may become less dense than the surrounding mantle beneath the mantle transition zone (MTZ), the Mg-rich composition of Archean volcanic crust may result in the different density, and therefore different sinking behavior near the MTZ. In order to understand the compositional effect of Archean volcanic crust on the sinking behaviors and the scale of mantle mixing in the Archean, we investigated the mineralogy and density of Archean volcanic crust near the MTZ (470–910 km-depth). We conducted experiments at 19–34 GPa and 1400–2400 K using the laser-heated diamond anvil cell (LHDAC) combined with in-situ X-ray diffraction (XRD). The in-situ XRD and the chemical analysis revealed that Archean volcanic crust forms garnet and ringwoodite (84 and 16 vol%, respectively), which gradually transforms to Brg and CaPv (82 and 18 vol%, respectively) at 23–25 GPa and 1800 K. Our in-situ XRD experiments allowed us to measure the volumes of stable phases and to estimate their densities at high pressure and temperature. The results suggest that Archean volcanic crust maintains greater density than the pyrolitic mantle in the Archean regardless of temperature at 20–34 GPa (570–850 km-depth), promoting further sinking into the deeper mantle in the Archean. We also considered the density of the subducting slab in the Archean. The density model showed that the subducting slab is still denser or at least equally dense as the surrounding pyrolitic mantle in the Archean.

Nitrogen (N) is extremely depleted in the bulk silicate Earth (BSE). However, whether the silicate magma ocean was as N‐poor as the present‐day BSE is unknown. We performed multi‐anvil experiments at 20 GPa and 1,673−2,073 K to determine the dihedral angle of Fe−Ni−N alloy melt in ringwoodite matrix to investigate whether percolation of Fe‐rich alloy melt in the solid mantle can explain N depletion in the BSE. The dihedral angles ranged from 112° to 137°, surpassing the wetting boundary. Our experiments suggest that N removal from the mantle by percolation of Fe‐rich alloy melt to the Earth's core is unlikely. Therefore, besides N loss to space during planetesimal and planetary differentiation, as well as its segregation into the Earth core, the stranded Fe‐rich metal in the deep mantle could be a hidden N reservoir, contributing to the anomalous depletion of N in the observable BSE. Plain Language Summary: Understanding how and when the present‐day inventory of nitrogen (N) in the bulk silicate Earth (BSE) was established is important to gain insights into Earth's habitability. A key question remains as to why N is strongly depleted in the BSE than carbon and hydrogen. Efficient segregation of N into the metallic core in the final stage of Earth's formation is postulated to be one of the primary causes behind this depletion. However, it is not clear whether the silicate magma ocean (MO) was as depleted in N as the present‐day BSE due to the uncertainties in the degree of metal‐silicate equilibration during the final stages of Earth's formation. Post‐MO crystallization, Fe−Ni alloy precipitated in the reduced mantle owing to the disproportionation of ferrous iron. We used high‐pressure experiments to examine whether this Fe−Ni alloy melt can trap the excess N and percolate through the solid mantle to the core. Our experiments show that the percolation of Fe−Ni−N melt is unlikely owing to its dihedral angle in silicate phases being larger than the wetting boundary. Instead, the Fe−Ni−N alloy stranded in the mantle can be a hidden N reservoir and the present‐day BSE may not be as N‐depleted as predicted. Key Points: Dihedral angles of Fe‐Ni‐N melt in ringwoodite matrix ranged from 112° to 137°, surpassing the wetting boundaryPercolation of Fe−Ni−N melt through the solid mantle cannot explain N depletion in the bulk silicate EarthFe−Ni−N alloy can be a hidden N reservoir in the mantle if excess N was present in the Earth's mantle post core‐mantle differentiation [ABSTRACT FROM AUTHOR]

We report Raman and infrared absorption spectroscopy along with X-ray diffraction for brucite-type beta-Cd(OH)2 to 28 GPa at 300 K. The OH-stretching modes soften with pressure and disappear at 21 GPa with their widths increasing rapidly above 5 GPa, consistent with a gradual disordering of the H sublattice at 5 20 GPa similar to that previously observed for Co(OH)2. Asymmetry in the peak shapes of the OH-stretching modes suggests the existence of diverse disordered sitesfor H atoms in Cd(OH)2 under pressure. Above 15 GPa, the A1g(T) lattice mode shows non-linear behavior and softens to 21 GPa, at which pressure significant changes are observed: new Raman modes appear, two Raman-active lattice modes and the OH-stretching modes of the low-pressure phase disappears, and the positions of some X-ray diffraction lines change abruptly with the appearance of weak new diffraction features. These observations suggest that amorphization of the H sublattice is accompanied by a crystalline-to-crystalline transition at 21 GPa in Cd(OH)2, which has not been previously observed in the brucite-type hydroxides. The Raman spectra of the high-pressure phase of Cd(OH)2 is similar to those of the high-pressure phase of single-crystal Ca(OH)2 of which structure has been tentatively assigned to the Sr(OH)2 type.

Droplets of an iron-bearing calcia magnesia aluminosilicate (Fe-CMAS) melt were reacted under distinctly reducing conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] - 2.4 x [10.sup.-13] and 6.4 x [10.sup.-15] atm) at high temperature (~1400[degrees]C) and ambient pressure. The low [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] environment was maintained by a flowing gas mixture of CO and C[O.sub.2], with a high content of CO. Molten metallic iron alloyed with silicon and carbon formed on the surface of the melt; no metal was observed in sample interiors. A color change from brown to pale blue confined to the outer layer of the melt indicated that essentially complete reduction of [Fe.sup.3+] to [Fe.sup.2+] had occurred in this region. Analysis of the reaction kinetics, particularly in comparison to melts of similar polymerization but free of CaO, reveals that the concentration of electron holes has decreased to such an extent that ionic transport in the melt is significantly slowed and the diffusion of CO as a neutral species becomes dominant and rate-limiting: molecular CO, initially incorporated into the melt as a physically dissolved species, subsequently reacts to form chemically dissolved (bonded into the melt structure) C[O.sup.2-.sub.3] anions, consuming electron holes in the process. The chemical diffusion coefficient for CO in the reduced melt at 1400[degrees]C is estimated as [D.sub.CO] [approximately equal to] 4 x [10.sup.-4] [cm.sup.2]/s, consistent with that of other, similarly sized molecular species (e.g., [H.sub.2] and [H.sub.2]O) for similarly polymerized melts, as reported by other investigators. Upon quenching, the droplet acts as a closed system. Internal redox couples see the reduction of the carbonate so as to form bubbles of CO, the composition of which are confirmed with Raman spectroscopy. The open-system reduction and closed-system quenching dynamics are analyzed following an Ellingham-diagram approach. Keywords: Redox, silicate melts, carbon, diffusion, reactions, kinetics DOI: 10.2138/am.2010.3375

The thicknesses and Clapeyron slopes of mantle phase boundaries strongly influence the seismic detectability of the boundaries and convection in the mantle. The unusually large positive Clapeyron slope found for the boundary between perovskite (Pv) and post-perovskite (pPv) (1-3) (the 'pPv boundary') would destabilize high-temperature anomalies in the lowermost mantle (4), in disagreement with the seismic observations (5). Here we report the thickness of the pPv boundary in ([Mg.sub.0.91][Fe.sup.2+.sub.0.09])Si[O.sub.3] and ([Mg.sub.0.9][Fe.sup.3+.sub.0.1])([Al.sub.0.1][Si.sub.0.9])[O.sub.3] as determined in a laser-heated diamond-anvil cell under in situ high-pressure (up to 145 GPa), high-temperature (up to 3,000 K) conditions. The measured Clapeyron slope is consistent with the D' discontinuity (6). In both systems, however, the pPv boundary thickness increases to 400-600 ± 100 km, which is substantially greater than the thickness of the D' discontinuity (< 30 km) (7). Although the [Fe.sup.2+] buffering effect of ferropericlase (8-10) could decrease the pPv boundary thickness, the boundary may remain thick in a pyrolitic composition because of the effects of Al and the rapid temperature increase in the D' layer. The pPv boundary would be particularly thick in regions with an elevated Al content and/or a low Mg/Si ratio, reducing the effects of the large positive Clapeyron slope on the buoyancy of thermal anomalies and stabilizing compositional heterogeneities in the lowermost mantle. If the pPv transition is the source of the D' discontinuity, regions with sharp discontinuities may require distinct compositions, such as a higher Mg/Si ratio or a lower Al content., The lowest 200-400 km of the mantle (the D' layer) is known to have seismic properties distinct from those of the overlying mantle, including a laterally varying discontinuity at the [...]

Significantly reduced detection of thermal radiation in gated spectroscopy allowed us to measure the Raman scattering of natural enstatite up to 1550 K at 1 bar. The intrinsic anharmonicity, [a.sub.i] = [([delta]ln[v.sub.i])/[([delta]T).sub.v], of the Raman-active modes in orthoenstatite (OEn) was obtained from temperature (7) shifts of vibrational frequencies measured in this study combined with previous high-pressure (P) Raman scattering data. Although the [a.sub.i] values of the lattice modes of OEn are similar to those for forsterite (Fo), the Si-O stretching modes have significantly lower intrinsic anharmonicity in OEn than in Fo, suggesting that the connectivity of the Si[O.sub.4] tetrahedra plays an important role in mode anharmonicity. At the phase transition at 1500 K, a doublet related to the stretching vibration of bridging O atoms in the Si[O3 chains becomes a singlet, and a doublet related to the stretching vibration of non-bridging O atoms remains as a doublet, consistent with the expected spectral change for a phase transition from OEn to protoenstatite. Two intense, low-frequency modes of OEn show a strong nonlinear decrease in frequencies with heating that cannot be explained solely by thermal expansion. This may indicate the reorganization of the structure around Mg atoms and unkinking of the Si[O.sub.3] chains at temperatures well below the phase transition. Keywords: Raman spectroscopy, gated spectroscopy, orthoenstatite, protoenstatite, phase transition, high temperature DO1: 10.2138/am.2009.3210

We have measured Raman spectra of the postperovskite (PPv) phase in CaIr[O.sub.3] up to 30 GPa to constrain the Gruneisen parameter ([gamma]). We identified a total of 4 strong modes between 200 and 650 [cm.sup.-1], which is in contrast with the Raman spectra of [Mn.sub.2][O.sub.3] and MgGe[O.sub.3]-PPv where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPv CaIr[O.sub.3], which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIr[O.sub.3]. The spectroscopic Gruneisen parameter, [[gamma].sub.sp.0] = 1.66~1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Gruneisen parameter, [[gamma].sub.th,0] = 1.75 [+ or -] 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIr[O.sub.3]-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that [[gamma].sub.sp] constrained by Raman measurements provides a reasonable estimate on the [[gamma].sub.th] of the PPv phase in CaIr[O.sub.3]. Keywords: Postperovskite, CaIr[O.sub.3], Gruneisen parameter, Raman sepctroscopy, thermal equation of state

Using the CaIr[O.sub.3]-type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGe[O.sub.3] post-perovskite (pPv) phase were determined based on Rietveld refinements at 78-109 GPa and first-principles calculations based on density functional theory. The reproducibility of structural parameters obtained for different samples, consistency with theoretical calculations, and good agreement with expected bond lengths based on structurally similar materials all provide evidence for both validity of CaIr[O.sub.3]-type structure model for the pPv phase in MgGe[O.sub.3] exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGe[O.sub.3] pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the Ge[O.sub.6] octahedron is 1.9x larger than that for the Mg[O.sub.8] hendecahedron. Examination of neighboring O-O distances shows that the O-O distance aligned along the a direction is one of the longest and that aligned along c is one of the shortest, and these may be related to the lower compressibility along e compared with a. Comparison of structural features of MgGe[O.sub.3] pPv with those for MgSi[O.sub.3], NaMg[F.sub.3], and CaIr[O.sub.3] pPv show that MgSi[O.sub.3] pPv has more similarity with NaMg[F.sub.3] and MgGe[O.sub.3] pPv than with CaIr[O.sub.3] pPv in such parameters as degree of octahedral distortion, implying that both NaMg[F.sub.3] and MgGe[O.sub.3] pPv are better analogs to MgSi[O.sub.3] pPv than CaIr[O.sub.3] pPv. Keywords: Post-perovskite, MgGe[O.sub.3], Rietveld refinement, high-pressure experiment, first-principles calculation, laser-heated diamond anvil cell, density functional theory, polycrystalline X-ray diffraction

Intriguing seismic observations have been made for the bottom 400 km of Earth's mantle (the D' region) over the past few decades, yet the origin of these seismic structures has not been well understood. Recent theoretical calculations have predicted many unusual changes in physical properties across the postperovskite transition, perovskite (Pv) [right arrow] postperovskite (PPv), that may provide explanations for the seismic observations. Here, we report measurements of the crystal structure of ([Mg.sub.0.91][Fe.0.09])Si[O.sub.3]-PPv under quasi-hydrostatic conditions up to the pressure (P)-temperature (7) conditions expected for the core-mantle boundary (CMB). The measured crystal structure is in excellent agreement with the first-principles calculations. We found that bulk sound speed ([V.sub.[PHI]) decreases by 2.4 [+ or -] 1.4% across the PPv transition. Combined with the predicted shear-wave velocity ([V.sub.s]) increase, our measurements indicate that lateral variations in mineralogy between Pv and PPv may result in the anticorrelation between the [V.sub.[PHI] and Vs anomalies at the D' region. Also, density increases by 1.6 [+ or -] 0.4% and Gruneisen parameter decreases by 21 [+ or -] 15% across the PPv transition, which will dynamically stabilize the PPv lenses observed in recent seismic studies. equation of state | mantle | phase transition | bulk sound speed | Gruneisen parameter