Geochemical enrichment of lavas in the northern Lau Basin may reflect the influx of Samoanplume mantle into the region. We report major and trace element abundances and He-Sr-Nd-Hf-Pbisotopic measurements for 23 submarine volcanic glasses covering 10 locations in the northern Lau and North Fiji Basins, and for three samples from Wallis Island, which lies between Samoa and the Lau Basin. These data extend the western limit of geochemical observations in the Basins and improve the resolution of North-South variations in isotopic ratios. The Samoan hot spot track runs along the length of the northern trace of the Lau and North Fiji Basins. We find evidence for a Samoan-plume component in lavas as far West as South Pandora Ridge (SPR), North Fiji Basin. Isotopic signatures in SPR samples are similar to those found in Samoan Upolu shield lavas, but show a slight shift toward MORB-like compositions. We explain the origin of the enriched signatures by a model in which Samoan-plume material and ambient depleted mantle undergo decompression melting during upwelling after transiting from beneath the thick Pacific lithosphere to beneath the thin lithosphere in the northern Lau and North Fiji Basins. Other lavas found in the region with highly depleted isotopic signatures may represent isolated pockets of depleted mantle in the basins that evaded this enrichment process. We further find that mixing between the two components in our model, a variably degassed high-3 He/ 4He Samoan component and depleted MORB, can explain the diversity among geochemical data from the northern Lau Basin.

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Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as approximately 30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism. Results indicate that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid, yielding transport times commensurate with those indicated by uranium-thorium studies.

We report the first known occurrence of high‐Ca boninites within an active submarine island arc, at Volcano A within the Tonga Arc. Both the whole rock and a population of melt inclusions (in Fo86–92 olivines) from a dredged satellite cone have compositions classified as high‐Ca boninite. All samples from Volcano A, however, may be related to parental boninites, given the similarity in their rare earth element patterns and their coherency along a similar liquid line of descent. The primary high‐Ca boninite liquids were generated in the mantle wedge by high cumulative degrees of melting (>∼24%) at typical mantle wedge temperatures (<1300°C) driven by an influx of slab‐derived fluid (>4 wt % H2O in primary liquids). We propose a two‐stage model for generating primary boninite liquids at Volcano A: (1) melting of fertile peridotite within the Lau back‐arc basin, followed by (2) remelting of this residual peridotite with slab‐derived fluid beneath the Tonga Arc. The occurrence of high‐Ca boninites at Volcano A is related to the relative location and duration of back‐arc spreading. Here, the Eastern Lau Spreading Center has been processing mantle for ∼1 Ma, and corner flow circulation brings mantle from the back‐arc melting regime into the arc melting regime at a rate that is a significant fraction (>30%) of the convergence rate. On the basis of Si6.0 and Ti6.0 relationships, we argue that a significant portion of the central Tonga Arc near Volcano A, as well as several other arc volcanoes with active back‐arc basins, are also erupting basaltic andesites with boninite parentage.