Electrical conductivities of pyrope-almandine garnets up to 19 GPa and 17008C

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 reß ect 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, speciÞ cally Fe-Mg
substitution, on the electrical conductivity proÞ le 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.

Amthauer, G., Annersten, H., and Hafner, S.S. (1976) The Mössbauer spectrum of
57Fe in silicate garnets. Zeitscrift für Kristallographie, 143, 14 55.
Bahr, K. and Duba, A. (2000) Is the asthenosphere electrically anisotropic? Earth
and Planetary Science Letters, 178, 87 95.
Banks, R.J. (1969) Geomagnetic variations and the electrical conductivity of
the upper mantle. Geophysical Journal of the Royal Astronomical Society,
17, 457.
Darken, L.S. and Gurry, R.W. (1945) The system iron-oxygen. I. The wüstite
Þ eld and related equilibria. Journal of the American Chemical Society, 67,
1398 1412.
Duba, A., Boland, J.N., and Ringwood, A.E. (1973) The electrical conductivity of
pyroxene. Journal of Geology, 81, 727 735.
Duffy, T.S. and Anderson, D.L. (1989) Seismic velocities in mantle minerals and
the mineralogy of the upper mantle. Journal of Geophysical Research, 94,
1895 1912.
Ganguly, J., Cheng, W., and Chakraborty, S. (1998) Cation diffusion in aluminosilicate
garnets: experimental determination in pyrope-almandine diffusion
couples. Contributions to Mineralogy and Petrology, 131, 171 180.
Hinze, E., Will, G., and Cemic, L. (1981) Electrical conductivity measurements
on synthetic olivines and on olivine, enstatite, and diopside from Dreiser
Weiher, Eifel (Germany) under deÞ ned thermodynamic activities as a function
of temperature and pressure. Physics of the Earth and Planetary Interiors,
25, 245 254.
Hirsch, L.M. (1991) The Fe-FeO buffer at lower mantle pressures and temperatures.
Geophysical Research Letters, 18, 1309 1312.
Hirsch, L.M., Shankland, T.J., and Duba, A.G. (1993) Electrical conduction and
polaron mobility in Fe-bearing olivine. Geophysical Journal International,
114, 36 44.
Ita, J. and Stixrude, L. (1992) Petrology, elasticity, and composition of the mantle
transition zone. Journal of Geophysical Research, 97, 6849 6866.
Ito, E. and Katsura, T. (1989) A temperature proÞ le in the mantle transition zone.
Geophysical Research Letters, 16, 425 428.
Karato, S. (1990) The role of hydrogen in the electrical conductivity of the upper
mantle. Nature 347, 272 273.
Katsura, T., Sato, K., and Ito, E. (1998) Electrical conductivity of silicate perovskite
at lower-mantle conditions. Nature, 395, 493 495.
Kohlstedt, D.L., Keppler, H., and Rubie, D.C. (1996) Solubility of water in the α,
β, and γ phases of (Mg,Fe)2SiO4. Contributions to Mineralogy and Petrology,
123, 345 357.
Lacam, A. (1983) Pressure and composition dependence of the electrical conductivity
of iron-rich synthetic olivines to 200 kbar. Physics and Chemistry of
Minerals, 9, 127 132.
Li, B.S., Liebermann, R.C., and Weidner, D.J. (2001) P-V-V-p-V-s-T measurements
on wadsleyite to 7 GPa and 873 K: Implications for the 410 km seismic discontinuity.
Journal of Geophysical Research, 106(B12), 30579 30591.
Li, X. and Jeanloz, R. (1990) Laboratory studies of the electrical conductivity of
silicate perovskites at high temperatures and pressures. Journal of Geophysical
Research, 95, 5067 5078.
Lu, R. and Keppler, H. (1997) Water solubility in pyrope to 100 kbar. Contributions
to Mineralogy and Petrology, 129, 35 42.
McCammon, C.A. (2004) Mössbauer spectroscopy: Applications. In A. Beran and
E. Libowitsky, Eds., Spectroscopic Methods in Mineralogy, 6, p. 369 398.
Eötvös University Press, Budapest.
McCammon, C. (2006) Mantle oxidation state and oxygen fugacity: Constraints
on mantle chemistry, structure and dynamics. In R.D. van der Hilst, J. Bass, J.
Matas, and J. Trampert, Eds., Structure, Composition and Evolution of Earth s
Mantle, in press. American Geophysical Union, Washington D.C.
Olsen, N. (1999) Long-period (30 days 1 year) electromagnetic sounding and
the electrical conductivity of the lower mantle beneath Europe. Geophysical
Journal International, 138, 179 187.
Poe, B.T. and Xu, Y. (1999) In-situ complex impedance spectroscopy of mantle
minerals measured at 20 GPa and 1400 °C. Phase Transitions, 68, 453 466.
Poe, B.T., Romano, C., Nestola, F., and Rubie, D.C. (2005) Electrical conductivity
of hydrous single crystal San Carlos Olivine, Eos Trans. AGU, 86(52), Fall
Meeting Supplement, Abstract MR41A-0895.
Roberts, J.J. and Tyburczy, J.A. (1991) Frequency-dependent electrical properties
of polycrystalline olivine compacts. Journal of Geophysical Research,
96, 16205 16222.
(1993) Impedance spectroscopy of single and polycrystalline olivine:
evidence for grain boundary transport. Physics and Chemistry of Minerals,
20, 19 26.
Schober, M. (1971) The electrical conductivity of some samples of natural olivine
at high temperatures and pressures. Zeitschrift für Geophysik, 37, 283 292.
Schock, R.N., Duba, A.G., and Shankland, T.J. (1989) Electrical conduction in
olivine. Journal of Geophysical Research, 94, 5829 5839.
Shankland, T.J., Peyronneau, J., and Poirier, J.-P. (1993) Electrical conductivity
of the Earth s lower mantle. Nature, 344, 453 455.
Seifert, K.F., Will, G., and Voigt, R. (1982) Electrical conductivity measurements
on synthetic pyroxenes MgSiO3-FeSiO3 at high pressures and temperatures
under deÞ ned thermodynamic conditions. In W. Schreyer, Ed., High-pressure
Researches in Geoscience, p.419 432. Schweizerbart sche, Stuttgart.
Wang, D., Xu, Y., and Karato, S. (2005) Effect of water on electrical conductivity
in olivine, Eos Trans. AGU, 86(52), Fall Meeting Supplement, Abstract
MR41A-0894.
Will, G., Cemic, L., Hinze, E., Seifert, K.F., and Voigt, R. (1979) Electrical conductivity
measurements on olivines and pyroxenes under deÞ ned thermodynamic
activities as a function of temperature and pressure. Physics and Chemistry
of Minerals, 4, 189 197.
Xu, Y., McCammon, C., and Poe, B.T. (1998a) The effect of alumina on the electrical
conductivity of silicate perovskite. Science, 282, 922 924.
Xu, Y., Poe, B.T., Shankland, T.J., and Rubie, D.C. (1998b) Electrical conductivity
of minerals of the mantle transition zone. Science, 280, 1415 1418.
Xu, Y., Shankland, T.J., and Poe, B.T. (2000a) Laboratory-based electrical
conductivity in the Earth s mantle. Journal of Geophysical Research, 105,
27865 27875.
Xu, Y., Shankland, T.J., and Duba, A.G. (2000b) Pressure effect on electrical
conductivity of mantle olivine. Physics of the Earth and Planetary Interiors,
118, 149 161.