Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle

It has long been known that the measured kappa (Th-232/U-238, Or kappa) Of mid-ocean ridge basalts (MORB) and, by inference, the upper mantle, is much lower than the time-integrated kappa recorded by Pb isotope ratios (kappa( Pb)). We examine models that can reconcile this kappa conundrum by in situ decay of U and Th in the upper mantle. Monte Carlo simulations reveal a res tricted range of permissible 'in situ' paths of MORE mantle evolution. Thes e solutions require a roughly constant Th-232/U-238 ratio during early Eart h history, followed by a period of steadily decreasing Th-232/U-238 from th e end of the Archean to the present. These model criteria make good geologi cal sense in terms of post-Archean recycling of crustal uranium back into t he mantle, Preferential recycling of uranium, relative to thorium, can resu lt from the high aqueous mobility of uranium in the oxidising environment a t the Earth's surface. Soluble uranium is transported from continents to th e altered oceanic crust and ultimately, by subduction, back into the mantle . In contrast, insoluble thorium remains in the weathered continental resid ue. This process is only likely to have become important after the marked i ncrease in atmospheric oxygen fugacity at similar to 2.2 Ga which led to a change in the predominant surface oxidation state of uranium. The uranium f luxes required in this Post-Archean Uranium Recycling (PURE) model are comp atible with estimates derived from present-day fluxes of 'excess' continent al uranium returned to the mantle by subduction, integrated over some 2 Ga. Further modelling of Earth evolution in the context of differentiation int o crust, depleted mantle and recycled plume reservoirs demonstrates the via bility of this scenario in explaining modem-day lead isotopic signatures of both MORE and 'HIMU' ocean island basalts. We emphasise that resolution of the kappa conundrum does not require a steady state upper mantle with its lead isotope ratios buffered by entrainment of material from another, deepe r reservoir. (C) 1999 Elsevier Science B.V. All rights reserved.