Sr-Nd and Ar-He isotopic relationships: comparison, geotectonic implications and approximation by model computing

Sr-Nd and Ar-He isotope relationships are used as the principal data for a five-reservoir model of the Earth evolution and, correspondingly, contemporary results obtained for Sm-Nd, Rb-Sr, K-Ar and U-He systematics are briefly discussed. The U-He system seems to be the only one which enables one to make a choice between evolution and mixing models. He escape accompanied by isotope fractionation provides an extremely low content of both isotopes, especially the light one, 3He, in the atmosphere, which is one of the end-member terrestrial reservoirs. So the mixing of He from three end-member reservoirs (that is, the depleted mantle, the crust and the atmosphere) cannot be responsible for high 3He/4He ratios in helium which are observed in some rocks and fluids of hot-spot areas and are interpreted as undepleted mantle emanations. A good agreement between the observed Sr-Nd and Ar-He data and the calculated results supports the model described in the paper. Several preliminary results inferred from this mode, viz. some peculiarities of Ar and Sr evolution in fractionated reservoirs, calculated K/Rb ratios in the Earth, etc., are discussed. (orig.)