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Mantle plumes and mantle sources.

Basalts from many ocean islands define elongate arrays in Sr-Nd-Pb isotopic space; these likely reflect the dominance of binary mixing of mantle sources in intraplate volcanism. S. R. Hart et al. (1) observe that when these arrays are projected onto a ternary diagram bounded by mantle end-members DMM [depleted mid-ocean ridge basalt (MORB) mantle], HIMU (high U/Pb mantle), and EMI (enriched mantle 1), they are subparallel and point toward a composition on the DMM-HIMU join. Hart et al. suggest this composition is associated with a high ^3He/^4He ratio (>30 R_A) and therefore cannot be a ubiquitous upper-mantle mixture of DMM and HIMU (both of which have ^3He/^4He < 9 R_A). They instead argue in favor of a new isotopic component, resident in a deep mantle "Focus Zone" (FOZO), which is characterized by a high ^3He/^4He ratio or which acquires helium with a high ^3He/^4He signature from the core.