ELECTRICAL-CONDUCTIVITY OF OLIVINE, WADSLEYITE, AND RINGWOODITE UNDERUPPER-MANTLE CONDITIONS

Geophysical models show that electrical conductivity in Earth's mantle rises about two orders of magnitude through the transition zone in th e depth range 410 to 660 kilometers. Impedance measurements obtained o n Mg1.8Fe0.2SiO4 olivine, wadsleyite, and ringwoodite at up to 20 giga pascals and 1400 degrees C show that the electrical conductivities of wadsleyite and ringwoodite are similar and are almost two orders of ma gnitude higher than that of olivine. A conductivity-depth profile to 6 60 kilometers, based on these laboratory data, shows a conductivity in crease of almost two orders of magnitude across the 410-kilometer disc ontinuity; such a profile favors a two-layer model for the upper mantl e. Activation enthalpies of 1.2 to 1.7 electron volts permit appreciab le lateral variations of conductivity with lateral temperature variati ons.