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3.1 h. Electrical conductivity of the orthopyroxene-ilmenite system and geophysical applications (Y.S. Xu and B.T. Poe, in collaboration with T.J. Shankland/Los Alamos and A.G. Duba/Livermore)

In the absence of direct sampling of the Earth's deep interior, compositional models have been developed that predict the mineral phases that should be stable at various depths. One such model is the "pyrolite model" in which pyroxene and its high pressure polymorphs are the next most abundant components in the mantle after olivine and its polymorphs wadsleyite and ringwoodite. If this model is correct, then these polymorphs of pyroxene will influence the bulk conductivity of the mantle, especially if their conductivities differ significantly from those of the polymorphs of olivine. Electrical conductivities of pyroxene and its high-pressure phases have not been studied as thoroughly as the olivine system. Most investigators determined the electrical conductivity of pyroxene at room pressure, while several investigators determined pyroxene conductivity at high pressures up to 3 GPa. Because assumptions and approximations are required in order to extrapolate results obtained at low pressures and temperatures to mantle conditions, here we report in-situ electrical conductivity measurements for the orthopyroxene - clinopyroxene - ilmenite + garnet system and their geophysical applications. The experimental method was described in the Annual Report of 1997. The starting material was San Carlos orthopyroxene Mg0.92Fe0.08SiO3 containing 2.89% Al2O3 by weight.

Figure 3.1-12 displays the logarithm of electrical conductivity vs. reciprocal temperature for orthopyroxene, clinopyroxene, and ilmenite + garnet; the activation enthalpies of orthopyroxene, clinopyroxene and ilmenite + garnet determined from the slopes of these curves are similar. Conductivity decreases by about 0.7 log units when orthopyroxene transforms to clinopyroxene and then increases about 0.7 log units when clinopyroxene transforms to ilmenite + garnet. Compared to perovskite data with the same starting material (see Annual Report 1998), the activation enthalpies of orthopyroxene, clinopyroxene and ilmenite + garnet are more than twice that of perovskite; Figure 3.1-12 shows that conductivity rises more than one order of magnitude when ilmenite + garnet transforms to perovskite at mantle conditions.

Fig. 3.1-12: Logarithm of electrical conductivity vs. reciprocal temperature for pyroxene and its high pressure phases. Abbreviations: opx is orthopyroxene, cpx is clinopyroxene, il is the ilmenite phase, gt is garnet, and pv is silicate perovskite.

Using the present results plus previous data (see Annual Report 1997 for olivine and its polymorphs; see Annual Report 1998 for perovskite), we have made calculations of electrical conductivity at physical conditions and mineralogical compositions appropriate for the Earth's mantle. The calculations are based on several spatial averaging schemes to combine recent laboratory measurements made under mantle-like conditions on the volumetrically dominant materials. We find that co-existing phases can affect the bulk conductivities when their conductivities are significantly different from those of the most abundant mineral in a region. A laboratory-based conductivity-depth profile Lab 1 was constructed using the effective medium theory average (Fig. 3.1-13a). The way to examine how the laboratory-based profile reflects the real mantle is to turn around the geophysical inverse process and compare apparent resistivities calculated from the laboratory-based profile with those from geophysical data. In order to obtain the apparent resistivity for the laboratory-based profile, we reverse the process of Patella's method (D. Patella, Geophysics, 41, 96-105, 1976.). Figure 3.1-13b shows the calculated apparent resistivity curves for layered models compared with that from field sites. The apparent resistivity curve BD calculated for the three-layer model BD (K. Bahr, and A. Duba, Earth Planet. Sci. Lett., submitted.), is shown in Figure 3.1-13a. The apparent resistivity curve Olsen99 of the field sites was obtained by averaging results from European observatories (N. Olsen, Geophys. J. Int., 138, 179-187, 1999). It is clear that the Lab 1 curve resembles that of the field sites except for a slight deviation at the intermediate periods (Fig. 3.1-13b). The BD curve deviates from Olsen99 at short periods. In order to obtain a better fit to Olsen99, we varied the layer conductivities slightly but kept their thickness constant. The resulting curve Lab 2 is closer to Olsen99 than Lab 1 and BD (Fig. 3.1-13b). The difference between Lab 1 and Lab 2 is well within uncertainties and is small except at the transition zone. The similar shapes for these results and the geophysically derived curves indicate that the laboratory-based conductivity profile is within the band of variation permitted by the geophysical models of the Earth's mantle.

Fig. 3.1-13:
a. Step-like laboratory-based conductivity-depth profile compared with geophysical models.
b. Comparison of apparent resistivity estimates for the laboratory-based layered mantle with those of field sites.

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