The Earth's mantle is divided into separate reservoirs. From basaltic lavas erupted today we know that the upper mantle is depleted in incompatible trace elements, whereas ocean island basalts (OIB) sample a deep, more enriched reservoir. The two incompatible elements potassium (K) and uranium (U) display a different ratio to each other in lavas from the upper and lower mantle reservoirs. This is surprising because they do not fractionate substantially during melting of the mantle and therefore continuous extraction of melts from the upper mantle over Earth's history cannot explain this disparity. Here a model is constructed in which U is insoluble during continental weathering in the anoxic conditions of the early Earth. This leads to recycling of oceanic crust with high K/U in the early Earth and low K/U after significant oxidative weathering commenced ca. 2200 Ma. This concept reproduces the observed K/U in both the upper and lower mantle reservoirs as well as the continental crust. The model is also shown to account for the thorium/uranium ratios and thereby Pb (lead) isotope compositions in these reservoirs. Successful model solutions imply that (1) chemical weathering fluxes to the oceans on the early Earth were significantly higher than present day, and (2) that the majority of the OIB reservoir is younger than 2200 Ma.