Excess 3He in mantle-derived rocks relative to atmospheric abundance shows that degassing of Earth is a continuing process. The importance of chemical changes induced by fluids from the deep mantle on the oceanic and continental lithosphere has been recognized increasingly in recent years. I propose that the parent-daughter element fractionation caused by such fluids dominates the Sr, Nd, and Pb isotope evolution of the mantle in the second half of Earth's history. A simple two-layered mantle model based on such a premise seems to be consistent with the present knowledge of the Sr, Nd, Pb, and He isotopic characteristics of mantle-derived rocks. The model predicts two populations of intraplate oceanic-island volcanism: “primary” fluid-induced volcanism and “secondary” volcanism caused by remelting of ancient metasomatized domains. The model implies a new interpretation of the Nd-Sr isotopic covariations in oceanic basalts.

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