Conductivity structure of oceanic upper mantle beneath the Pacific plate

Summary. Ocean bottom magnetotelluric responses from three ages on the Pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age. The conductivity achieves its maximum value (- 0.1 S m-') at depths near 70, 120 and 180 km for lithospheric ages of 1, 30 and 72 Myr respectively. The existence of this conductive structure has firmly been established by constructing a number of different acceptable models, and also by obtaining unique averages over depth of the conductivity. An explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the hgh conductivity. Beneath JDF ( t = 1 Myr) the lower bound lies in the range 0.04-0.12; beneath CAL (t = 30 Myr) it lies in the range 0.02-0.10; and beneath NCP (t = 72 Myr) it lies in the range 0.008-0.02. The distributions of partial melt show that the major portion of melt beneath JDF lies between 40 and 90km and that melting has ceased by depths of about 170 km at all three sites. Geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value, and then (possibly) decreases with depth. The depth at which the maximum temperature is reached is about 125 km for CAL and about 175 km for NCP, and the maximum temperatures are estimated to be 1400-1500°C. Lastly, as an independent check upon the partial melt explanation, the location and magnitude of the seismic low-velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites