Introduction

Magmas derived from mantle plumes provide a powerful method to recognize and map compositional domains in the mantle, which in turn constrain mantle processes. The most definitive tracers are isotopic ratios of daughter elements of radioactive nuclides or concentration ratios of incompatible elements. Incompatible elements are transferred almost entirely to magmas upon melting in the mantle. For moderate or large degrees of melting, both isotopic and incompatible element ratios transfer from a mantle source to a derivative magma, providing a geochemical “signature” of the source.

Incompatible element distributions for basalts derived from four different mantle sources are shown in Figure 5.1 normalized to primitive mantle composition. Elements are arranged from most incompatible (during lherzolite melting) on the left (Rb) to least incompatible on the right (Y). Because incompatible elements are largely transferred from source to liquid, the element distributions in each basalt should reflect the element distributions in the respective mantle sources. Ocean-ridge basalts (MORB) record a depleted mantle source in which the most incompatible elements are most depleted. This source, which appears to reside in the upper mantle, is one of the restite reservoirs formed as continental crust has been extracted from the mantle over time (e.g., Hofmann 1997). Notice that oceanic plateau basalts (Ontong Java) record a mantle source somewhat less depleted in highly incompatible elements than MORB and that many oceanic island (OIB) and island arc basalts are derived from sources enriched in these elements.