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

Abstract

I present here a model, based on the Rb–Sr systematics, for characterizing the chemical and isotopic makeup of the source reservoirs of igneous rocks. The model is primarily based on the correlation between the Rb/Sr, and [(Rb/Sr)/(87Rb/86Sr)] ratios of rocks. However, this model does not require the direct input of the initial Sr isotopic ratio. The model under discussion in fact measures the degree of fractionation undergone by the source reservoir either with respect to its model 87Rb/86Sr ratio or to its present day model 87Sr/86Sr ratio. The latter method would produce results very similar to the one involving the use of initial 87Sr/86Sr ratio. Both the model isotopic ratios required for this are obtained from the various relations comprising the model itself under study.The proposed model is not dependent on any extraterrestrial data, and is internally consistent with respect to the various parameters used in defining this model. It has been shown that the parameters used are interdependent, mutually impose constraints on each other, and collectively produce outcomes which are commensurate with the expected results. Furthermore, the model was tested on the actual data of rocks taken from the literature, and the predictions made by the model in every test case came out completely consistent with their published results, which substantiates not only the validity of this model, but also the authenticity and aptness of the various parameters used in defining this model.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.