Brent T. Poe

Laboratory‐based electrical conductivity in the Earth's mantle

Recent laboratory measurements of electrical conductivity of mantle minerals are used in forward calculations for mantle conditions of temperature and pressure. The electrical conductivity of the Earths mantle is influenced by many factors, which include temperature, pressure, the coexistence of multiple mineral phases, and oxygen fugacity. In order to treat these factors and to estimate the resulting uncertainties, we have used a variety of spatial averaging schemes for mixtures of the mantle minerals and have incorporated effects of oxygen fugacity. In addition, to better calculate lower mantle conductivities, we report new measurements for electrical conductivity of magnesiowustite (Mg0.89Fe0.11)O. Because the effective medium theory averages lie between the Hashin-Shtrikman bounds for the whole mantle, a laboratory-based conductivity-depth profile was constructed using this averaging scheme. Comparison of apparent resistivities calculated from the laboratory-based conductivity profile with those from field geophysical models shows that the two approaches agree well.

Vibrational spectroscopy of silicate liquids and glasses

The application of vibrational spectroscopy to the study of silicate liquids and glasses is described, and new Raman data for K2Si2O5 and K2Si4O9 compositions are presented. The timescale of the vibrational spectroscopic experiments relative to relaxation timescales in the melts is discussed. Silicate systems are usually described as “liquid” or “glassy” based on experimental measurements of viscosity or heat capacity. These have a much longer characteristic measurement timescale than the vibrational spectroscopic experiments. Because of the long structural relaxation times for silicate frameworks over the normal laboratory temperature range, silicate “glasses” and “liquids” always show the same, unrelaxed response to the vibrational spectroscopic experiment. This is one reason for the observed close similarity between “glass” and “melt” spectra. Vibrational spectroscopy can readily be used to investigate structural changes which occur within supercooled silicate liquids due to structural relaxation on the laboratory timescale, above the glass transition temperature, Tg. The vibrational spectroscopies are complementary to other spectroscopic methods, including nuclear magnetic resonance, for this type of study. Our new Raman spectroscopic results on K-disilicate and -tetrasilicate glasses and liquids show effects due to structural relaxation above Tg. The spectra for K2Si2O5 show evidence for an increase in the concentration of Q2 silicate species with increasing temperature. We have determined the enthalpy change for the 2Q3  Q2 + Q4 speciation reaction in K2Si2O5 to be ∼ 20 kJ mol−1, of the same order of magnitude as those obtained for liquids near Na2Si2O5 composition by previous workers. For K2Si4O9 glass, the Raman data show evidence for a different type of structural relaxation. The intensity of a peak near 590 cm−1 increases with increasing temperature above Tg, which is interpreted as an increase in the proportion of three-membered siloxane rings in the liquid. The enthalpy change for formation of these three-membered rings is also 20 kJ mol−1, consistent with the results of a previous study on SiO2 glass.

Al Coordination Changes in High-Pressure Aluminosilicate Liquids

Understanding the effect of pressure on aluminosilicate glass and liquid structure is critical to understanding magma flow at depth. Aluminum coordination has been predicted by mineral phase analysis and molecular dynamic calculations to change with increasing pressure. Nuclear magnetic resonance studies of glasses quenched from high pressure provide clear evidence for an increase in the average coordination of Al with pressure.

Al and Si coordination in SiO21bAl2O3 glasses and liquids: A study by NMR and IR spectroscopy and MD simulations

Abstract Structural properties of a series of SiO 2 1bAl 2 O 3 glasses with up to 59 mole% Al 2 O 3 were investigated by magic angle spinning NMR and infrared absorption spectroscopies. The NMR results showed that Al is present in IV- and VI-coordinated sites. The relative proportions of these species depends on composition and quench rate. The 27 Al NMR results have been used to “calibrate” the IR absorption spectra, in that features in the IR spectra have been assigned to Al1bO stretching vibrations of different AlO x polyhedral species. This will be useful for future in situ studies of these glasses and melts at high pressures or high temperature, when NMR experiments are difficult to perform, or do not give information on structural sites. For comparison with these results, molecular dynamics simulations of SiO 2 1bAl 2 O 3 liquids were carried out for several compositions along the join. The pair distribution functions were analyzed to give information on the distribution of Al and Si coordination species in the liquids. At high silica content, both Si and Al predominantly occupy IV-coordinated sites, though the presence of some III-coordinated Al is indicated. With increasing Al 2 O 3 content, the proportion of Al V and Al VI species is increased, and the average Al coordination number for Al 2 O 3 liquid approaches 5.0. Concomitant increases in Si V coordination sites are observed, and the average Si coordination increases to ∼ 4.7 near pure Al 2 O 3 .

Spinel‐Si3N4: Multi‐Anvil Press Synthesis and Structural Refinement

The third known polymorph of silicon nitride, which is cubic and was only recently discovered, has been prepared from two further, different precursors—Si2N2(NH) and a-Si3N4—in a high-pressure, high-temperature synthesis using multi-anvil presses. The synthesis and characterization of the products is described, which included a structural determination by Rietveld refinement of powder X-ray diffraction data. Spinel-type c-Si3N4 is significantly harder than the α and β phases and may possibly find applications as an ultrahard material.

The self-diffusion of silicon and oxygen in diopside (CaMgSi2O6) liquid up to 15 GPa

The self-diffusivities of silicon and oxygen in diopside (CaMgSi2O6) liquid have been measured at pressures and temperatures up to 15 GPa and 2300°C, using a 1200-tonne multianvil apparatus. Diffusion couples were prepared using finely ground, diopside glass, half of which was enriched in tracer isotopes 18O (5%) and 30Si (12%). Results indicate that silicon and oxygen self-diffusivities are coincident (within an accuracy range of 10% RSD, 1σ) up to 13 GPa and show an initial decrease with pressure up to 11 GPa (oxygen self-diffusivities are 9.8×10−10 m2/s at 3 GPa and 3.9×10−10 m2/s at 11 GPa) after which there is an increase with pressure up to 15 GPa (9.1×10−10 m2/s) at 2000°C. The activation energy of self-diffusion of both silicon and oxygen was calculated to be 267 kJ mol−1, with no observable pressure effect up to 3 GPa. Self-diffusivity is inversely proportional to viscosity, the relationship between these properties being well approximated by the Eyring equation using the diameter of an oxygen anion as the translation distance. The calculated viscosities are in good agreement with previous direct viscosity measurements at lower pressure although the previously found high positive activation volumes were not reproduced.

The Viscosities of dry and hydrous XAlSi3O8 (X=Li, Na, K, Ca0.5, Mg0.5) melts

The low-temperature viscosities of dry and hydrous X (X=Li, Na, K, Ca0.5, Mg0.5)AlSi3O8 melts have been investigated. The samples were hydrated via piston cylinder synthesis, and the water contents were subsequently determined by Karl-Fischer titration (KFT) and IR spectroscopy. Both the anhydrous and hydrous viscosities were measured using the micropenetration technique in the range of viscosities between 108.5 to 1011.9 Pa s, at 1 atm pressure and in the temperature ranges of 745–990°C and 400–790°C for the dry and wet melts, respectively. The range of water content varied for all of the samples from 0.70 to 3.13 wt.% H2O. The viscosities of dry melts vary, at fixed temperature, as a complex function of the identity of the cation in the order Li<Na<Ca≤Mg<K. This trend is interpreted as due to the combined effects of cation field strength and (Si, Al) distribution in these melts. With the introduction of water into these melts, the viscosity decreases for all of the compositions investigated. As water is further dissolved, the array of anhydrous viscosities converges into two distinct curves, for alkali-bearing and alkaline-earth-bearing aluminosilicate liquids, respectively. In contrast to the insensitivity of viscosity to alkali cation identity for hydrous melts, the alkali/aluminium ratio remains a sensitive control on viscosity. Thus, the viscosities of a slightly peralkaline albite glass (Naexc) are lower than all of the others, both for the dry and the hydrous systems. We suggest that, in the case of alkaline-earth-bearing melts, an aluminium pair must be closely related to a doubly charged cation, to maintain electrostatic neutrality. The increase in the size of smallest rearranging species, which participates in the viscous flow process, as well as clustering of silica-rich and alumina-rich domains on an “intermediate-range” scale, may be the factors resulting in the higher viscosities of Ca- and Mg-bearing compared to alkali-bearing liquids.

Electrical conductivities of pyrope-almandine garnets up to 19 GPa and 1700 °C

Abstract Electrical conductivities of polycrystalline garnets ranging in chemical composition from almandine (Fe3Al2Si3O12) to pyrope (Mg3Al2Si3O12) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe2+-Fe3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profile of the mantle, using a recently developed laboratory- derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py100 to Py85Alm15. Although only a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle.

In-situ high-temperature Raman spectroscopic studies of aluminosilicate liquids

We have measured in-situ Raman spectra of aluminosilicate glasses and liquids with albite (NaAlSi3 O8) and anorthite (CaAl2Si2O8) compositions at high temperatures, through their glass transition range up to 1700 and 2000 K, respectively. For these experiments, we have used a wire-loop heating device coupled with micro-Raman spectroscopy, in order to achieve effective spatial filtering of the extraneous thermal radiation. A major concern in this work is the development of methodology for reliably extracting the first and second order contributions to the Raman scattering spectra of aluminosilicate glasses and liquids from the high temperature experimental data, and analyzing these in terms of vibrational (anharmonic) and configurational changes. The changes in the first order Raman spectra with temperature are subtle. The principal low frequency band remains nearly constant with increasing temperature, indicating little change in the T-O-T angle, and that the angle bending vibration is quite harmonic. This is in contrast to vitreous SiO2, studied previously. Above Tg, intensity changes in the 560–590 cm−1 regions of both sets of spectra indicate configurational changes in the supercooled liquids, associated with formation of additional Al-O-Al linkages, or 3-membered (Al, Si)-containing rings. Additional intensity at 800 cm−1 reflects also some rearrangement of the Si-O-Al network.

The effect of fictive temperature on Al coordination in high-pressure (10 GPa) sodium aluminosilicate glasses

Abstract Typical liquidus temperatures can be over 1000°C greater than the glass transition temperatures for high-pressure aluminosilicate melts so the effect of temperature must be determined if glass data is to be used to approximate the structural speciation present in geologic melts. This study has investigated the effect of fictive temperature (Tf, taken as the temperature where the melt structure is the same as that of the glass) on the percentage of [5]Al and [6]Al species in two high-pressure (10 GPa) Na-aluminosilicate glasses (Na3AlSi7O17 and NaAlSi3O8) where one glass of each composition was quenched from the high-pressure melt while the other was annealed near the glass transition temperature. The 27Al MAS NMR spectra of the Na3AlSi7O17 samples show that the higher Tf (quenched) glass contains more high-coordinated Al than the lower Tf (annealed, 475°C) glass. However, the 27Al spectra of the NaAlSi3O8 samples show the opposite temperature dependency, which in addition to the lack of NBO in this glass, may suggest differing mechanisms for the generation of high-coordinated Al