Identifying fluid-fluxed versus decompression melting in the Tonga island arc – Lau back-arc region

Partial melting at destructive plate margins is usually linked to fluid addition from the subducting plate, but decompression melting and asymmetric “wet” and “dry” wings of the melting zone at back-arc spreading centres have also been invoked. Distinguishing between these models has proved difficult using conventional geochemical data. Here, we combine 230U–238Th disequilibria, H2O contents and Ba/Nb ratios on glasses to identify fluid-fluxed versus decompression melting across the Tonga Arc – Lau back-arc. From the arc front into the back-arc, (230U/238Th) disequilibria range from 0.55 to 1.14, H2O contents from 2 to 0.25 wt% and Ba/Nb ratios from 1000 to 5. The (230U/238Th) disequilibria and ratios of fluid mobile to immobile trace elements are amongst the most extreme reported from arcs and change to values typical of mid-ocean ridges. Our data provide evidence for the co-existence of both fluid-fluxed and decompression melting regimes and suggest that back-arcs situated close to the arc may be able to draw subduction-related material into the spreading axis. The systematic compositional change with increasing distance between arc and back-arc does not reflect changes in dehydration reaction in the subducting slab, but different proportions of slab material contributing to the back-arc spreading regime. A stepwise change at >100 km distance between arc and back-arc marks the separation between fluid-fluxed and decompression melting domains occurring over relatively short spatial distances. These are also associated with a transitional change in ridge morphology due to the appearance of an axial magma chamber at the southern East Lau Spreading Centre.