HYDRATION AND DEHYDRATION OF OCEANIC-CRUST CONTROLS PB EVOLUTION IN THE MANTLE

The position of the Pb isotopic compositions of mid-ocean ridge basalt s (MORE) to the right of the geochron has long puzzled geochemists. Ur anium is more incompatible than Pb during mantle melting, and the mant le source of MORB, being depleted in incompatible elements, should hav e low Pb isotope ratios and plot to the left of the geochron. This has been called the ''lead paradox''. MORE have a second peculiar charact eristic: Pb concentrations are drastically depleted relative to other elements of similar compatibility such as Ce or Nd. We suggest that bo th characteristics can be explained by preferential mobility of Pb dur ing hydrothermal alteration of the oceanic crust associated with sea-d oor spreading, and subsequent dehydration during subduction. A portion of this Pb migrates into the mantle wedge and is then added to the co ntinental crust via are magmas. Parts of the Pb-depleted recycled ocea nic crust are stored at some deep level in the mantle and eventually b ecome the source of ocean island basalts, the rest mixes into the mant le to become the MORE source. This model is evaluated quantitatively a nd special attention is given to the evolution of the Ce/Pb ratio in t he depleted mantle from the beginning of Earth history to the present.