Characterizing source reservoirs of igneous rocks: A new perspective. Fractionation of radiogenic isotopes: A new tool for petrogenesis

The evaluation of the variably fractionated mantle derived rocks by the model under study shows that the mantle by and large is undepleted and isotopically homogeneous. The popular notion that the mantle is isotopically inhomogeneous is in fact just a perception resulting from the fractionation of radiogenic isotopes with respect to their non-radiogenic counterparts at the time of derivation of the magma from their source reservoirs. Hence, the true isotopic makeup of the mantle source reservoir can never be known. Therefore, predictions made by any model, including the one under study, regarding the isotopic makeup of the source reservoirs, do not reflect the actual chemical and isotopic state of the source reservoirs. These predictions basically reflect the degree of isotopic fractionation undergone by the igneous rocks at the time of extraction of the magma from their source reservoirs. This study further shows that the observed inverse correlation (mantle array) formed by the Sr and Nd isotopic ratios of modern mantle-derived oceanic basalts is not because of mixing of the end-members from the variably fractionated mantle reservoirs but instead caused by the coherent fractionation of radiogenic Sr and Nd isotopes with respect to their non-radiogenic counterparts during the magmatic processes.