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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.

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

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

In-situ Raman measurements using a gated spectroscopy system revealed irreversible changes at 800-1000 [degrees]C in a natural red spinel (with 2 cation mol% Cr and 1 cation mol% Zn) and at 1100-1200 [degrees]C in a natural clear spinel (without Cr or Zn). Our observations of rapid broadening of a mode at 409 [cm.sup.-1] and the appearance of two weak modes at 210 and 520 [cm.sup.-1] at the transition temperature confirm the association of these features with cation disordering proposed by previous quench studies. Furthermore, we found that the frequencies of modes at 313 and 666 [cm.sup.-1] change at the transition temperature. The discontinuous frequency decrease of the mode at 313 [cm.sup.-1] and the increase in the frequency of the mode at 666 [cm.sup.-1] can be explained by the entrance of heavier Al atoms into the tetrahedral sites and the entrance of lighter Mg atoms into the octahedral sites, respectively. Our study demonstrates that in-situ Raman spectroscopy is a powerful tool for studying cation disordering in spinel-structured minerals at high temperature. Keywords: Spinel, Raman spectroscopy, order-disorder transition, high temperature

Raman spectra of [Fe.sub.2][O.sub.3] were measured to 62 GPa in a diamond anvil cell. All group theoretically predicted Raman-active phonon modes were detected to 54 GPa. In addition, some high-pressure spectra show an IR-active [E.sub.u], mode (-660 [cm.sup.-1]), possibly induced by surface defects or stress. This mode is related by a factor of two to a mode at 1320 [cm.sup.-1]. The assignment of the 1320 [cm.sub.-1] mode has been controversial (two-magnon scattering or two-phonon scattering), and our observation supports the phonon assignment. All Raman-active phonons show nonlinear pressure-induced shifts. The mode Gruneisen parameters and their logarithmic volume dependences for two low-frequency phonons, [A.sub.1g] and [E.sub.g], become negative and infinite, respectively, near 50 GPa as a result of the instability of the corundum structure at this pressure. Using the observed Raman-active phonons together with acoustic phonons previously measured by ultrasonics, and Kieffer's model, we calculate the phonon contribution to the thermodynamic parameters of hematite. Comparison with experimentally measured values allows an estimation of an upper bound to the magnon contribution to the heat capacity at ambient pressure. This increases continuously above the Morin temperature and reaches a maximum at the Neel temperature (~37%). The Raman spectra change dramatically at pressures greater than 54 GPa as indicated by the appearance of new peaks and a significant increase in background. Although direct structural analysis is not possible due to the low signal-to-background ratio and the lack of polarization information, we were able to examine the consistency of our Raman observation with the corundum-to-perovskite phase transformation using the results for an analog system: MgSi[O.sub.3] ilmenite (ordered corundum type) and perovskite. This analysis shows that observed new features in [Fe.sub.2][O.sub.3] Raman spectra may not be consistent with the GdFe[O.sub.3] perovskite structure.

Equation of state fits to experimental P,V,T data were examined by the inversion of synthetic data sets using the thermoelastic parameters of MgSiO(sub 3) perovskite. Our results show that by extending the pressure and temperature range to 130 GPa and 2500 K, the volume dependence of the Gruneisen parameter, q(mathematical expression not reproducible in ASCII), could be resolved to approximately 10% under the best circumstances. However, simulations also showed strong correlation between the bulk modulus, K(sub T0), and its pressure derivative, K'(sub T0), and q within the currently accepted uncertainty of elastic parameters for MgSiO(sub 3) perovskite. We considered the effect of random error based on the reported uncertainty for different measurement techniques. Even though the laser heated diamond-anvil cell (LHDAC) technique has significantly larger temperature uncertainty, the ability to extend the pressure and temperature ranges allows for improved resolution of higher order thermodynamic parameters. However, systematic error from temperature inhomogeneity in the LHDAC sample could result in overestimation of q. We also performed Birch-Murhanghan-Debye (BMD) equation of state (EOS) fits for currently available data sets. Consistent with the simulation results, combining recent LHDAC (Fiquet et al. 1998) and resistance heated diamond-anvil cell (RHDAC) (Saxena et al. 1999) with lower P-T measurements (Ross and Hazen 1989; Wang et al. 1994; Utsumi et al. 1995; Funamori et al. 1996) we obtained q = 2.0(3) and (gamma)(sub 0 = 1.42(4). The difference between q = 2.0(3) and the normally assumed value of q = 1 strongly affects calculated values for higher order thermoelastic parameters [e.g., (alpha), (mathematical expression not reproducible in ASCII) as well as first order parameters, such as density and bulk modulus at lower mantle conditions. However, possible systematic error sources need to be further investigated and measurements at higher P-T conditions promise to yield better constraints on the thermoelastic parameters of MgSiO(sub 3) perovskite.

The stability of synthetic cation-exchanged zeolites having the chabazite framework (CHA) and their cation-water interaction were studied using high-temperature reaction calorimetry. Four cations (K, Na, Li, and Ca) were exchanged into CHA. The enthalpies of formation were determined for all samples, and the partial molar enthalpy of hydration was measured by varying the water content of one Ca-CHA. The enthalpy of formation depends strongly on the exchanged cation, becoming more exothermic in the order Ca, Li, Na, K. The integral hydration enthalpy does not depend strongly on the nature of the cation, but becomes slightly less exothermic with increasing aluminum content. For the one Ca-CHA studied in detail, the average enthalpy of hydration is -34.6 +/- 1.2 kJ/mol relative to liquid water. A quadratic fit to the transposed temperature drop data gives (delta) h(sub H(sub 2)O) = -52.97 (+/- 4.74) + 2.94 (+/- 0.68) n (kJ/mol), where (delta) h(sub H(sub 2)O is the partial molar enthalpy of hydration and n is the number of water molecules per 24 oxygen formula.