A geochemical study of off-axis seamount lavas at the Valu Fa Ridge: Constraints on magma genesis and slab contributions in the southern Tonga subduction zone

New geochemical data are presented for fresh lavas from off-axis seamounts east and west of the Valu Fa Ridge and from seamounts south of the propagating rift tip of the Valu Fa Ridge. Most seamount lavas are basaltic and thus more primitive than the mostly andesitic to dacitic lavas from the adjacent Valu Fa Ridge. Slight differences in fluid-mobile element ratios and in Sr isotope ratios suggest small-scale heterogeneities of the slab component in this region with an apparently higher sedimentary input into the off-axis seamount lavas than into the island arc magmas. Off-axis volcanoes near back-arc basin spreading centres thus resemble off-axis seamounts adjacent to mid-oceanic ridges which also show more extreme compositions than the well-homogenized mid-ocean ridge basalts. The geochemical variations support a diapiric ascent of the hydrated or partially molten mantle leading to separate magma supply systems for the different volcanic structures. The mantle wedge between the Valu Fa Ridge and the island arc appears to be uniformly depleted in fluid-immobile incompatible elements compared to average depleted mid-ocean ridge basalts whereas lavas from seamounts west of the Valu Fa Ridge show less depletion in agreement with a depletion event of the whole mantle wedge in the backarc melting zone. Seamount lavas south of the Valu Fa Ridge propagating rift resemble the Valu Fa lavas indicating an increased magma production caused by the influx of water into the mantle.