Poe, Brent T., Claudia Romano, and James A. Tyburczy
Arizona State University
jim.tyburczy@asu.edu
Recent laboratory work has demonstrated that the electrical conductivity profile of the mantle can be reasonably approximated from the measured conductivities of the major mineral constituents such as olivine, wadsleyite, ringwoodite and silicate perovskite. Where the approximation shows lesser agreement with recent geophysical electrical conductivity models, particularly at transition zone depths, consideration of the effects of minor constituents that might affect the electrical properties of minerals is required. For the transition zone, only the conductivities of nominally anhydrous minerals have previously been measured, yet both wadsleyite and ringwoodite, which compose a substantial proportion of the transition zone, can incorporate up to several weight percent water into their structures. Knowing the effect of dissolved water on the electrical conductivities of these minerals may thus provide some constraints on the amount of water in the transition zone. We have obtained preliminary results for the electrical conductivity of hydrous wadsleyite at P,T conditions of the Earth's upper mantle. The sample material was synthesized at 12 GPa and 1100°C in a welded Pt capsule using a multianvil apparatus. The recovered material was cut into 0.5 mm thick disks for in-situ measurement by complex electrical impedance spectroscopy also at 12 GPa in a multianvil apparatus. Pre-and post-run analyses by secondary ion mass spectrometry (SIMS) indicate that the samples lost some water during the high P,T conductivity runs. Post run analyses were 0.40 wt % and 0.50 wt % H2O, respectively for two conductivity runs performed. We find that the water-bearing wadsleyite is approximately 1/2 to 1 orders of magnitude more conductive than the nominally dry wadsleyite (Xu et al., 2000) over the temperature range 600 to 1050°C and that conductivity increases with water content. The activation energies are all similar with values of about 1.0 eV. Furthermore, analysis of nominally dry wadsleyite prepared in the same manner as that of the previous workers yields a water content of 0.16 wt % H2O, suggesting that the previous 'nominally dry' results contained a small amount of water. Based on these laboratory results, Earth conductivity profiles in the transition zone (420-670 km depth) can be matched by those of dry wadsleyite, that is, no water is required to exist in the region.