Electrical Conductivity of Ti-Bearing Hydrous Olivine Aggregates at High Temperature and High Pressure

摘要

We investigated the electrical conductivity of Ti-H-doped synthetic olivine aggregates at 4 GPa, 873-1273 K, and controlled oxygen fugacities. Under a given pressure and temperature, electrical conductivity depends on both hydrogen and Ti content, but these samples show different conductivity behavior from that observed in Ti-poor sample such as San Carlos olivine. We found that when Ti content is comparable to or larger than hydrogen content, Ti has notable effects on electrical conductivity, but the effects of Ti is different between the H-rich and the H-poor regimes. In the H-rich regime, electrical conductivity of olivine is weakly dependent on Ti content but has different sensitivity to water content than a Ti-poor olivine. In contrast, in the H-poor regime, electrical conductivity of Ti-rich olivine is substantially higher than the conductivity of Ti-poor olivine. As a consequence, the effect of hydrogen for the Ti-rich synthetic olivine on electrical conductivity is smaller than for the Ti-poor (natural) olivine for the modest H content expected in the asthenosphere, whereas in the H-poor lithosphere Ti will enhance the electrical conductivity substantially. Possible models to explain these observations are proposed including the interaction of Ti-related defects and H-related defects as well as the charge transfer caused by the hopping conduction due to Ti3+ double left right arrow Ti4+ under the H-poor conditions. We conclude that the addition of Ti to olivine affects the behavior of H-related defects, and therefore the applications of results from Ti-rich olivine samples to the Ti-poor real Earth need to be made with great care.