The control of alkalies and uranium in seawater by ocean crust alteration

Alteration of the oceanic crust during hydrothermal circulation of seawater produces fluxes of K, Rb and Cs between these reservoirs which are significant compared to the river input of these elements. The ocean crust U flux, on the other hand, is probably not significant. The upper crust, altered at low temperature, is a sink for all of these elements (as shown by direct analysis of upper crustal materials). The lower crust is a source for K, Rb, and Cs, based on the observation that high-temperature fluids exiting the crust as “host springs” are enhanced over seawater in K, Rb and Cs concentration. While the sign of the hot spring fluxes may be correct, the absolute magnitudes cannot be, as the calculated yearly hot spring flux of Rb and Cs significantly exceeds the total Rb and Cs inventory of newly formed unaltered crust. By modelling the crust as a melt/cumulate combination, we show that the crust as a whole is a sink for K, Rb, Cs, and probably U, with yearly fluxes of1.1 × 10¹³, 2.6 ×10¹⁰, 6.0 × 10⁸ and 1.0 × 10⁹g, respectively (com to yearly river dissolved fluxes of7.4 × 10¹³, 3.5 × 10¹⁰, 6.4 × 10⁸and1.0 × 10¹⁰g, respectively). The alteratio oceanic crust appears capable of quantitatively balancing the river inputs of Rb and Cs. For K, an additional sink comprising～ 85% of the river input is necessary. Because this missing K sink cannot be arbitrarily manipulated without destroying the Rb and Cs balances, a sink with K/Rb higher than the continental crust is required, and may possible be found in the sediments of the continental shelves.