measurements (7). During x-ray spectra acquisition, temperature variations were monitored An in situ angle-dispersive x-ray diffraction study was undertaken of iron in a laserand kept within 50 to 100 K. Pressures were heated, diamond-anvil cell up to 2375 kelvin and between 30 and 100 gigapascals in measured at room temperature before and af-AI , O, -and Si0,-pressure media. The resolution and reliability of diffraction peak inter laser heating, using ruby fluorescence and tensities allow quantitative assessment of a structural model. The results confirm that iron A1,03 or SiO, compression curves (8). undergoes a phase transformation at high pressures and temperatures. The space groupAngle-dispersive x-ray diffraction meais Pbcm for an orthorhombic lattice, and the atomic topology is close to that of E surements were performed on the dedicated hexagonal close-packed iron.high-pressure beamline ID30 at ESRE. A n x-ray beam of wavelength 0.4245 (1) A was selected from an undulator using a channelcut Si(ll1) monochromator. The mono-Iron is the dominant component of liable in situ diffraction experiments dur-chromatic beam was focused using two sin-Earth's core, so information on its behav-ing laser heating in the DAC.

Abstract. Samples prepared from synthetic single crystals of olivine have electrical conductivity that differs significantly from samples prepared, under similar conditions, from San Carlos olivine. In particular, the oxygen fugacity with which the sample was treated before being loaded into the laser-heated diamond anvil cell (LHDAC) has a large effect on the electrical conductivity of magnesiowiistite/perovskite assemblages synthesized from San Carlos olivine and no significant effect on those synthesized from the synthetic olivines. This effect is likely due to the presence of nickel in San Carlos olivine. However, because the main effect of nickel in olivine is to diminish the extent of the redox stability field of olivine, this points to a possible effect of the internal oxygen fugacity of the LHDAC on the assemblages produced during transformation. Electron microscopy examination of samples transformed to magnesiowiistite/perovskite assemblages in the LHDAC under identical conditions detected substantially more metallic iron in assemblages produced from San Carlos olivine. Analysis of the experimental procedure employed for transformation of silicates in the LHDAC indicates that oxygen from air trapped in the porous powder during pressurization can provide a condition of constant oxygen fugacity in the material. In the temperature gradient that exists during laser heating, a condition of constant oxygen fugacity can lead to very complicated phase assemblages, varying from highly oxidized to highly reduced, at least for materials containing transition metals. IntroductionThe electrical conductivity of Earth's mantle serves to filter variations in the magnetic field generated by the dynamo, which results from convective motions in the metallic core.Thus observations of changes in the magnetic field at the surface must take account of mantle electrical conductivity in order to use these variations to understand the mechanism responsible for dynamo generation. Further, angular momentum transfer by electromagnetic coupling from the core to the mantle also depends on electrical conductivity of the mantle. The upper mantle of the Earth is predominantly olivine and pyroxene, and it is reasonable to assume that this is the composition of the lower mantle [Ringwood, 1979]; however, at depths of about 670 km, both pyroxene and olivine undergo phase transformations to perovskite, with the additional phase magnesiowfistite being formed due to the stoichiometry of olivine [Liu, 1975].The laser-heated diamond anvil cell (LHDAC) is widely used to prepare phase assemblages of materials at pressures of Earth's mantle and core. Typically, ground-up samples are sealed from interaction with the external atmosphere by the diamonds and the gasketing material. In our synthesis experiments the material is then heated to central temperatures up to about 3000 K with a 25-txm-diameter laser spot. In this paper we report the results of an investigation of the electrical con- Boland and Duba [1986] showed that the presence of this amo...

We report recent experimental results on the electrical conductivity of Earth's mantle materials synthesised and measured in a laser heated diamond anvil cell. This results show an effect of oxygen partial pressure on the conductivity and on the chemistry of the high pressure phases synthesised in the DAC. Previous treatments under controlled fO2 conditions have a significant effect for natural olivines, attributed to the presence of a small amount of nickel. No effect was observed for pure synthetic olivines. The gas trapped in the sample during the closure of the cell has an influence on the produced material. Our results on the electrical conductivity of perovskite and magnesiowustite are in good agreement with recent geomagnetic data. [electrical conductivity, oxygen fugacity, perovskite, magnesiowustite].