Fluid migration, deep dehydration, and melt generation in the Lesser Antilles subduction zone

Volatiles play a pivotal role in subduction zones dynamics, associated geological hazards and mineralization, yet their pathways remain partially understood. The Lesser Antilles subduction zone can yield insights to volatile recycling as a global end-member, where old oceanic lithosphere formed by slow spreading slowly subducts. Here we use seismograms from local earthquakes recorded by a temporary deployment of ocean-bottom seismometers in the fore- and back-arc during the VoiLA (Volatile Recycling in the Lesser Antilles) experiment to characterize the 3-D properties of the slab, back-arc and mantle wedge in the north-central Lesser Antilles subduction zone. Along the top of the slab, defined by the underlying Wadati-Benioff seismicity, we find low P-wave velocity extending to 130–150 km depth, deeper than expected for magmatic oceanic crust. The deep low velocities together with high Vp/Vs at 60–80 km and 120–150 km depth are consistent with a significantly tectonised and serpentinised slab top, as expected for lithosphere formed by slow spreading. The most prominent high Vp/Vs anomalies in the slab correlates with two projected fracture zones and the obliquely subducting boundary between Proto-Caribbean and Equatorial Atlantic lithosphere, indicating these structures enhance hydration of the oceanic lithosphere and subsequent dehydration when subducted. Deep dehydration of slab mantle serpentinite is evidenced by high Vp/Vs anomalies in the back-arc offshore Guadeloupe and Dominica. Right above the slab, the asthenospheric mantle wedge is imaged beneath the back-arc as high Vp/Vs and moderate Vp feature, indicative for fluids rising from the slab through the overlaying cold boundary layer. The fluids might be dragged down with the subducting slab before rising upwards to induce melting further to the west. The variation in seismic properties along the subducting slab and in the back-arc mantle wedge shows that the changes in hydration of the incoming plate govern the dehydration processes at depth. The highest Vp/Vs anomaly in the back-arc west of Dominica at depth greater than 120 km, together with the anomaly at 60–80 km depth on the slab east of the island, appear to track the source and path of excess volatiles that may explain the relatively high magmatic output observed on the north-central islands of the Lesser Antilles arc.