Seismic Constraints on Water and Melt Pathways and Fluxes Through Island Arc System
Abstract:Seismological studies using land and ocean bottom seismographs can image velocity anomalies resulting from the presence of melt, fluids, and hydrated minerals in arc and backarc systems. The largest uncertainty in subduction zone water inputs derives from the lack of constraints on hydration of the incoming oceanic mantle. Results from several subduction zones show reduced mantle seismic velocities associated with extensional faulting as the incoming plate bends, suggesting up to 30% uppermost mantle serpentinization associated with water flow through faults. Extensional earthquakes occur to depths of about 15 km below the Moho at most subduction zones, suggesting the water storage capacity of the subducting uppermost mantle may be much larger than previously thought.
Island arc systems with active backarc spreading centers offer an opportunity to image mantle wedge processes and compare with geochemical and petrological outputs. In Mariana, mantle seismic anomalies associated with arc and backarc melting are separated by a high velocity, low attenuation region at shallow depths (< 80 km), implying distinct arc and backarc melting regions, with the anomalies coalescing at greater depths. The maximum anomaly in the backarc is shallower (~ 30 km) than in the arc (~ 60 km), consistent with final equilibrium depths estimated from basalt thermobarometry. In the Lau basin the slow anomaly beneath the spreading center is displaced westward with greater depths, suggesting that partial melting occurs along an upwelling limb of mantle flow originating west of the backarc. The observed Lau backarc anomalies are roughly inversely proportional to inferred mantle water content, suggesting that water reduces the melt porosity. Water may increase the efficiency of melt transport and reduce porosity by lowering the melt viscosity, increasing grain size through faster grain growth, or by causing a different topology of melt within the mantle rock. A lower melt porosity for aqueous melts is also consistent with the smaller amplitude seismic anomaly seen for the water-rich volcanic arc melting regions compared to the backarc melt production zone.