A revised estimate of the conductivity of iron alloy at high pressure and implications for the core energy balance

The theory of the electrical and thermal conductivities of iron alloy at high pressure is re-examined, with a downward revision of the estimated thermal conductivity of the core to 28-29 Wm(-1) K-1, almost uniform throughout. The consequent heat conduction at the top of the core, 3.8 terawatts, some of which can be carried back down by refrigerator action of compositional convection, is correspondingly smaller than other recent estimates. Using an estimate of the total (poloidal plus toroidal) field strength in the core derived from nutational coupling of the inner core, we take the present power requirement of the geomagnetic dynamo to be 0.3 terawatt. Allowing for the use of part of the compositional convective energy for refrigerator action to reverse some of the conducted heat, a core with no radiogenic heat could, on average, provide this much dynamo power for 3.8 billion years. If K-40 with a present output of 0.2 terawatt is introduced, then the inner core and the dynamo could both have existed for 4.5 billion years. These numbers are more consistent with recent observations of the solubility of K in Fe alloy at high pressure than the high K contents demanded by more conductive core models (c) 2007 Elsevier B.V. All rights reserved.