The thermal effect of fluid circulation in the subducting crust on slab melting in the Chile subduction zone

Fluids released from subducting slabs affect geochemical recycling and melt generation in the mantle wedge. The distribution of slab dehydration and the potential for slab melting are controlled by the composition/hydration of the slab entering a subduction zone and the pressure-temperature path that the slab follows. We examine the potential for along-strike changes in temperatures, fluid release, and slab melting for the subduction zone beneath the southern portion of the Southern Volcanic Zone (SVZ) in south central Chile. Because the age of the Nazca Plate entering the subduction zone decreases from ~14 Ma north of the Guafo Fracture Zone to ~6 Ma to the south, a southward warming of the subduction zone has been hypothesized. However, both north and south of Guafo Fracture Zone the geochemical signatures of southern SVZ arc lavas are similar, indicating 3-5 wt.% sediment melt and little to no contribution from melt of subducted basalt or aqueous fluids from subducted crust. We model temperatures in the system, use results of the thermal models and the thermodynamic calculation code Perple_X to estimate the pattern of dehydration-derived fluid release, and examine the potential locations for the onset of melting of the subducting slab. Surface heat flux observations in the region are most consistent with fluid circulation in the high permeability upper oceanic crust redistributing heat. This hydrothermal circulation preferentially cools the hottest parts of the system (i.e. those with the youngest subducting lithosphere). Models including the thermal effects of fluid circulation in the oceanic crust predict melting of the subducting sediment but not the basalt, consistent with the geochemical observations. In contrast, models that do not account for fluid circulation predict melting of both subducting sediment and basalt below the volcanic arc south of Guafo Fracture Zone. In our simulations with the effects of fluid circulation, the onset of sediment melting occurs under the volcanic arc, but dewatering of the subducting sediment and basalt is focused farther seaward (below the landward boundary of the stagnant mantle wedge corner). Thus, the sediment melt could enter the mantle wedge, contributing to the composition of the southern SVZ magmas, yet remain separate from the fluid derived from sediment dewatering which could migrate updip within the slab or into the wedge corner. Preferential hydrothermal cooling of the hottest segments of the system can help explain how there can be fairly uniform magma composition along the arc, despite large along-arc differences in the age of the subducting plate.