The electrical conductivity of upper-mantle rocks: water content in the upper mantle

Abstract

The electrical conductivity of upper-mantle rocks—dunite, pyroxenite, and lherzolite—was measured at ∼2–3 GPa and ∼1,273–1,573 K using impedance spectra within a frequency range of 0.1–106 Hz. The oxygen fugacity was controlled by a Mo–MoO2 solid buffer. The results indicate that the electrical conductivity of lherzolite and pyroxenite are approximately half and one order of magnitude higher than that of dunite, respectively. A preliminary model involving water and iron content effects on the electrical conductivity was derived and is summarized by the relation: $$ \sigma = 10^{{5.2\,\pm\,0.4}} \,C^{{0.67\,\pm\,0.07}}_{W} \,\exp{\left( { - \frac{{(183\,\pm\,9){\text{kJ}}/{\text{mol}}}} {{RT}}} \right)}\,\exp {\left( {X\,\frac{{(79\, \pm\,3){\text{kJ}}/{\text{mol}}}} {{RT}}} \right)}.$$ The results also indicate that pyroxenes dominate the bulk conductivity of upper mantle in hydrous conditions and suggest the maximum water content in oceanic upper mantle is as high as ∼0.09 wt%.