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Abstract

One of the major goals of the Geosciences is to identify and comprehend material cycling in our planet in order to understand mantle geodynamic processes. These processes are fundamental to the formation of the continental crust and the atmosphere, mantle-crustatmosphere interactions, and planetary formation as a whole. With our systematic, integrated studies of He, Ne, Sr, Nd and Pb isotopes of globally distributed mid-ocean ridge basalts and ocean island basalts we have contributed significantly to refining existing models on the behaviour of He and Ne during melt formation and evolution as well as models of the geochemical structure of the entire mantle. We have e.g. shown that He and Ne are more compatible during melt formation than their mother isotopes 235U, 238U and 232Th. Therefore, it is no longer necessary to postulate the existence of a virtually non-degassed, isolated lower mantle reservoir. We have also shown that the upper mantle is chemically heterogeneous at different spatial and temporal scales, indicating that the mixing efficiency of the upper mantle is not as good as previously thought. In addition, based on He, Ne and Ar isotope and trace-element data from the Foundation hotspot we have shown that the associated near ridge seamounts erupt lavas with a trace-element signature characteristic of oceanic gabbro, which indicates the existence of recycled, virtually unchanged oceanic lithosphere in the plume source.