Imaging the slow demise of slow-spread oceanic lithosphere and fluid cycling beneath the Lesser Antilles arc: first passive seismic images from the VoiLA project
IPGP - Îlot Cuvier
Séminaires Géosciences Marines
Imperial College London
The cycling of fluids in, through, and out of subduction zones has been crucial for Earth’s evolution, may drive volcanic eruptions and large earthquakes. Yet, the pathways of fluid and the budgets involved remain poorly understood. Most previous studies have focussed on circum-Pacific subduction zones, where the subducting oceanic lithosphere has formed at relatively fast spreading centres. In contrast, little work has been done on subduction of slow-spread oceanic lithosphere, such as in the Atlantic. The Lesser Antilles subduction zone is the Eastern Caribbean is only one of two subduction zones where Atlantic lithosphere is consumed and the subduction of fracture zones make this region an excellent natural laboratory to test models of water cycling and its impact on the composition of magmatic material erupted at the volcanic arc. In 2016, we deployed 34 ocean bottom seismometers across the Lesser Antilles arc, which recorded data for 14 months. We have detected and located ~500 local earthquakes in the subduction zone, which has allowed us to generate 3-D tomographic images of the subduction zone. In this talk, I will focus on the results of seismic attenuation imaging, which is sensitive to changes to fluid content and temperature. Therefore, interpreting these seismic attenuation images may help us delineate pathways of volatiles from the slab, through the mantle wedge, and up to the volcanic arc. Our results show that an attenuating mantle wedge (Qs~50) lies in the back-arc region - not directly beneath the arc as previously assumed - and requires significant mantle hydration given the expected cold temperature of the subduction zone. There is also a strong variation in mantle wedge seismic attenuation along the arc, which may be influenced by subducted fracture zones; this may result in a more hydrated mantle wedge. This along-arc variability in seismic attenuation is consistent with patterns of eruptive volumes and geochemical variability observed at the volcanic arc.