Lavas from the islands of Tafahi and Niuatoputapu, at the northern end of the active Tonga-Kermadec arc in the southwest Pacific, were erupted at a convergent plate margin, yet they can be shown to contain a contribution from two different mantle plumes. High concentrations of Nb relative to other high field strength elements in these lavas, compared to other Tonga lavas, reflect an ocean island basalt component in the mantle wedge derived from the nearby Samoa mantle plume. Pb isotope compositions indicate that most of the Pb in these lavas is derived from the oceanic crust of the plume-generated Louisville Seamount Chain, which is being subducted beneath the Tonga arc. These two plume components were thus introduced into the arc lavas in very different ways and provide insight into upper-mantle dynamics and magma-generation processes occurring in an active arc–back-arc system.

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