Differentiation and Evolution of the Mantle [and Discussion]

Evidence for isotopic heterogeneity (particularly with respect to Nd and Sr isotopes) in sub-oceanic and sub-continental mantle is reviewed. The ranges of isotopic heterogeneity in both sub-ocean and sub-continental mantle, as deduced from basalts and ultramafic nodules, are comparable. The isotope data are compared with the results obtained from simple two-reservoir models with time-dependent transport between the reservoirs. The deduced isotopic characteristics of the upper mantle are readily reproduced if only 50% of the mantle is involved in the production of the continental crust.