Oceanic Crust and Mantle

Abstract

The Nd isotopic data provide an important addition to petrogenetic modeling studies of igneous rocks because they provide previously unavailable information about the chemical composition of magma sources. Oceanic basalts present the possibility of applying the isotopic data for this purpose in a relatively straightforward way because there is little likelihood that the magmas are modified by contamination with old continental crust between the time they are generated in the mantle and when they are erupted at the surface. The Nd isotopic ratio (ε_Nd value) measured on a lava can therefore be safely assumed to represent the ratio in the mantle where the magma originated. Using an appropriate model the ε_Nd value can be used to infer some of the trace element characteristics of the mantle magma source, in particular the Sm/Nd ratio. This can, in turn, be compared to the Sm/Nd ratios of the lavas to evaluate petrogenetic models (Chap. 5). This approach requires assumptions, but is valuable because it allows the internal consistency of the models to be evaluated. The trace element partitioning models provide the basis for evaluating the relationship between the Sm/Nd ratio of a basalt magma and its source in the mantle. A sampling of data for oceanic basalts is shown in Fig. 8.1.