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Hydrogen incorporation into orthoenstatite (Pbca), low-clinoenstatite (P[2.sub.1]/c), and high-pressure clinoenstatite (C2/c) has been investigated using polarized and unpolarized infrared spectroscopy. Using shifts in OH stretching frequencies between the spectra and data from different crystal models, we test various models for hydrogen incorporation. The only significant differences between orthoenstatite and low-clinoenstatite spectra relate to anisotropy of the higher wavenumber bands, which implies a change in orientation of longer OH dipoles between the two structures. High-pressure clinoenstatite reverts to low-clinoestatite during depressurization, but subtle differences are noted between IR spectra of samples synthesized in the high-pressure clinoenstatite and low-clinoenstatite stability fields. Differences probably relate to the splitting of oxygen sites into two sets of non-equivalent sites during transformation of high-pressure clinoenstatite. The most realistic models for hydrogen incorporation into all three polymorphs involve association of hydrogen with the underbonded O2a and O2b sites. However, changes in OH dipole orientation between the different polymorphs and the effects of phase transitions on water solubility in the system MgSi[O.sub.3] mean the effects and implications of hydrogen incorporation into the three polymorphs may differ considerably. Keywords: Enstatite, transition, hydrogen, water, mantle

A new technique for performing long duration (up to 300 hours) high-pressure annealing experiments under water-saturated conditions has been developed. This technique has been used to investigate water-solubility and speciation in natural, gem-quality chromian diopside. Capsule design for the technique is a variant of the double-capsule technique, and relies on the use of a semi-permeable Pt membrane, which permits free hydrogen diffusion into samples, but protects samples from reacting with buffer mixtures. The investigation of a natural single crystal of chromian diopside revealed a very unusual annealing behavior: water contents increase sharply after a short annealing period and then decrease slowly to some metastable equilibrium value. The main process that takes place during the annealing experiments is hydrogen diffusion coupled with [Fe.sup.3+] reduction. This essentially reverses the main mechanism for hydrogen loss from mantle samples during exhumation, and the technique therefore provides sample-specific information on original water contents. Absorption bands at 3646 and 3434 [cm.sup.-1] in IR spectra from annealed samples suggest two main mechanisms for hydrogen incorporation in the diopside sample: (1) incorporation of hydrogen onto the O2 site, with vibration of the OH dipole in the direction of a nearby O3 site (along the edge of an M2 site), and (2) incorporation of hydrogen onto the O2 site with vibration of the OH dipole toward a nearby O1 site (along a shared M1-M2 edge) or O2 site (along the edge of an MI site). The ratio of peak heights between the absorption bands at 3646 and 3434 [cm.sup.-1] is independent of water fugacity but dependent on oxygen fugacity, and appears to provide a measure of the redox state 'frozen' into the sample. This ratio could be used to determine whether pyroxenes from upper-mantle xenoliths had experienced concurrent hydrogen-loss and oxidation during exhumation.

Conjointure Arthurienne: actes de la 'Classe d'excellence' de la Chaire Francqui, 1998. Ed. by JULIETTE DOR. Louvain-La-Neuve: Institut d'Etudes medievales de l'Universite Catholique de Louvain. 2000. vi + 125 pp. [...]

The high-pressure stability of antigorite in the systems MSH and MASH was investigated using two structurally and chemically well-constrained natural samples. Careful sample selection and characterization ensured that the only significant difference between the samples was Al content, one sample being essentially A1 free, and the other containing 3.06(2) wt% [Al.sub.2][O.sub.3]. In the system MSH, the reaction antigorite = forsterite + clinoenstatite + water was bracketed, under water-saturated conditions, between 630 and 650[degrees]C at 1.6 GPa, between 620 and 660 [degrees]C at 2.5 GPa, between 620 and 660[degrees]C at 3.9 GPa, and between 4.5 and 5.0 GPa at 520[degrees]C. In the system MASH, the reaction antigorite = forsterite + clinoenstatite + chlorite + water was bracketed, under water-saturated conditions, between 660 and 700[degrees]C at 2.0 GPa, between 660 and 680[degrees]C at 2.9 GPa, and between 5.0 and 5.5 GPa at 600[degrees]C. At pressures above 5.8 GPa, intersection of this reaction with the reaction chlorite + clinoenstatite = pyrope + forsterite + water leads to an additional reaction whereby the Al component of the antigorite is transferred to pyrope upon antigorite breakdown. The addition of a few weight percent [Al.sub.2][O.sub.3] into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures, possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets. This effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure. In addition, antigorite breakdown in the system MASH transfers a significant volume of water to chlorite-bearing assemblages, thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system MSH.

We report the thermal Equation of State (EoS) of the non-magnetic Fe3C phase based on in situ X-ray diffraction (XRD) experiments to 117 GPa and 2100 K. High-pressure and temperature unit-cell volume measurements of Fe3C were conducted in a laser-heated diamond-anvil cell. Our pressurevolume-temperature (P-V-T) data together with existing data were fit to the Vinet equation of state with the Mie-Grüneisen-Debye thermal pressure model, yielding V0 = 151.6(12) ų, K0 = 232(24) GPa, K'0 = 5.09(46), ?0 = 2.3(3), and q = 3.4 (9) with ?0 = 407 K (fixed). The high-T data were also fit to the thermal pressure model with a constant aKT term, PTh = aKT(?T), and there is no observable pressure or temperature dependence, which implies minor contributions from the anharmonic and electronic terms. Using the established EoS for Fe3C, we made thermodynamic calculations on the P-T locations of the breakdown reaction of Fe3C into Fe7C3 and Fe. The reaction is located at 87 GPa and 300 K and 251 GPa and 3000 K. An invariant point occurs where Fe, Fe3C, Fe7C3, and liquid are stable, which places constraints on the liquidus temperature of the outer core, namely inner core crystallization temperature, as the inner core would be comprised by the liquidus phase. Two possible P-T locations for the invariant point were predicted from existing experimental data and the reaction calculated in this study. The two models result in different liquidus "phases" at the outer core-inner core boundary pressure: Fe3C at 5300 K and Fe7C3 at 3700 K. The Fe7C3 inner core can account for the density, as observed by seismology, while the Fe3C inner core cannot. The relevance of the system Fe-C to Earth's core can be resolved by constructing a thermodynamic model for melting relations under core conditions as the two models predict very different liquidus temperatures. [ABSTRACT FROM AUTHOR]