Electrical conductivity of Ti-bearing hydrous olivine aggregates at high temperature and high pressure

The role of hydrogen to enhance many defect-related properties such as high-pressure experiments from the electrical conductivity and plastic deformation in olivine is well established. A large number of previous available investigations are mainly concentrated on the effects of hydrogen on high-pressure physical properties of olivine, and the influence of impurities other than hydrogen have not been studied in any detail. However a series of papers suggested the importance of some charged impurities such as Ti on hydrogen dissolution. We investigated the electrical conductivity of Ti (titanium) -H (hydrogen) doped synthetic olivine aggregates at 4 GPa, 873−1273 K and controlled oxygen fugacities using three different solid oxygen buffers (e.g. Re−ReO₂, Ni−NiO and Mo−MoO₂). Under a given pressure and temperature, electrical conductivity depends both on hydrogen and Ti content, but these samples show different conductivity behavior from that observed in Ti-poor sample (San Carlos olivine; ~20−30 ppm wt TiO₂). We found that when Ti content is comparable to or larger than hydrogen content, Ti has strong 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 approximately independent of Ti content, and 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 in 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. 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 small polaron hopping conduction. 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 a great care.