One of the most exciting advances in Earth science in the last several decades has been our increased understanding of the structure and composition of the mantle. Seismic tomography and isotope geochemistry have been major players in those advances. The isotopic studies used basalts from ocean basins to minimize the problems of possible crustal contamination. Data from mid-ocean ridge basalts (MORBs) and oceanic island basalts (OIBs) revealed a relatively detailed picture of the isotope geochemistry of the sub-oceanic mantle [e.g. Hofmann, 1997] that led to the recognition of four principal magma components that define end-member compositions. These are DMM, HIMU, EMI, and EM2 (see Table 1 for further details). All of these components except DMM have been attributed to subduction of different materials, such as oceanic and continental crust and lithosphere, down into the mantle. Several studies have indicated that the mantle is isotopically heterogeneous with heterogeneities that were probably established at least several billions of years ago.
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