Characteristics of Lead Isotope Evolution on a Continental Scale in the Earth

Abstract

Recent investigations of lead isotopes in oceanic sediments, beach and river sands, and oceanic volcanics are reviewed from one comprehensive point of view: relatively large amounts of uranium and small amounts of lead have been continuously transported from the interior of the earth to an outer protocontinental layer throughout geologic time; continental segments were formed at different times from this layer, but such activity was largely confined to the interval 3500 m.yrs ago to 2500 m.yrs ago; uranium is enriched in the surface rocks of the continents by magmatic activity, but such rocks are well-mixed with uranium depleted material throughout a depth of a few tens of km during orogeny, so that lead isotope effects resulting from uranium enrichment can only be seen during aging in the quiet period following orogeny; the uranium-poor source layers of continental basalts, on the other hand, are not well-mixed with the overlying uranium-rich layers because leads in young continental basalts are relatively non-radiogenic, showing the effects of aging in relatively isolated uranium poor chemical systems; oceanic basalts are moderately radiogenic and originate from moderately uranium-rich materials (the protocontinents).