Extreme geochemical heterogeneity beneath the North Tonga Arc: Interaction of a subduction zone with intraplate seamount chains

We provide new geochemical and isotope data for lavas from six young submarine volcanoes from the northernmost oceanic Tonga island arc. These lavas have low concentrations of moderately incompatible trace elements, but are enriched in Nb and Ta compared to all other Tonga island arc lavas, and their Nd, Hf and Pb isotope compositions resemble those of rejuvenated lavas from the nearby Samoa hotspot. These unusual compositions indicate that the mantle source of North Tonga lavas was depleted by prior melting events, before re-enrichment by a partial melt enriched in highly incompatible elements with Nd, Hf and Pb isotope compositions similar to Samoan intraplate lavas. Samoa plume mantle is flowing southwards beneath the northern Tonga island arc and Lau Basin, in part guided by rifting of the Tonga plate lithosphere. A fluid component was later added from a mixture of subducted pelagic sediment and subducting Louisville Seamount Chain volcanic material. Our new data show that the Louisville component is only found in lavas from volcanoes occurring along ∼100 km of the arc front, indicating that the oceanic crust loses Pb by dehydration over a relatively small depth interval during subduction. The compositions of lavas from the northern Tonga island arc are therefore influenced by material from two hotspots; southwards inflow of mantle from the active Samoa plume beneath the arc within the past 6 Ma, and a subduction component from >80 Ma old subducted Louisville Seamount Chain basalts.