COOLING OF THE EARTH, UREY RATIOS, AND THE PROBLEM OF POTASSIUM IN THE CORE

The compatibility of mantle Urey ratios U(m) calculated from thermal e volution models with those calculated from geochemical models is consi dered. The initial heat generation rate in the mantle used for the the rmal evolution models was derived from two geochemical models of the p resent day primitive mantle with concentrations of U of 20 ppb (2.7 ti mes chondritic) and 26 ppb (3.5 times chondritic), respectively. The d epletion of the mantle through continental crust production was allowe d for; the effect of including the continental crust on U(m) is a redu ction of its value by about 0.1. To balance the observed average value of the surface heat flow q(s) it was assumed that the core may contai n K up to cosmochemically constrained quantities. For 26 ppb U in the present primitive mantle no core heat sources are required. The presen t value of U(m) for this model is 0.57 and the core heat flow is about 12% of q(s). With 20 ppb U in the present primitive mantle, heat prod uction equivalent to one to two silicate earth budgets of K is require d in the core to balance q(s) and U(m) is between 0.35 and 0.4. The co re heat flow is then between 25% and 33% of q(s).