Just add water: backarc oceanic crust is different

Along backarc spreading systems, mantle melting involves a combination of pressure release melting, as at mid-ocean ridges, and hydrous flux melting, as beneath the volcanic arc. These end-member processes varyingly interact beneath the spreading center, with distance between the spreading center and arc volcanic front a key control on producing a range of crustal types. Yet the transition between these end-members can be abrupt. In the Lau basin, the Eastern Lau Spreading Center produces a bi-modal basin structure, with arc-proximal crust showing clear evidence of the role of water in enhancing melting beneath the spreading center and affecting final crustal composition. But as the ridge moves away from the arc, there is an abrupt transition to the formation of more normal oceanic crust – suggesting rapid changes in mantle water content or mantle flow. In contrast to more typical mid-ocean ridges, where spreading rate controls much of the character of new crustal formation, in backarc settings the influence of water can overprint the usual spreading-rate trends. This talk will present the latest discoveries from a series of geophysical studies that investigate the role of water in the Tonga-Lau backarc system, and the distinctive oceanic crust produced there.