Explosive emplacement of pyroclastic rocks containig abraded gabbro xenoliths at 3600 m water depth at Charles Darwin Volcanic Field, Cape Verde

Abstract:

Mildly explosive volcanic eruptions on the deep seafloor have been demonstrated in several cases. In contrast, we present here evidence for highly explosive activity producing diatreme-like ejecta on the surrounding seafloor at the Charles Darwin Volcanic Field (CDVF) located at about 3600 m water depth on the lower southwestern slope of the Cape Verdean Island of Santo Antão. We examined the 1 km diameter Kolá volcanic crater using photogrammetric reconstructions derived from ROV-based imaging followed by 3D quantification using a novel remote sensing workflow. The measured and calculated parameters of physical volcanology derived from the 3D model allow us to get a handle on explosive volcanic processes on the deep seafloor. Kolá crater comprises a complicated lithologic succession of highly fragmented deposits, including spheroidal juvenile lapilli that may contain quenched carbonatite melts, and were likely formed by spray granulation. The deposits comprise numerous well-rounded clasts of MORB-type gabbroic country rocks with diameters up to 20 cm, probably entrained and abraded by fluidization within the vent, that were laterally transported for hundreds of meters through water. In spite of the great depth, the Kolá crater features dense but highly fragmented volcanic deposits with an unexpected combination of large clast sizes and wide clast dispersal. This suggests an energetic eruptive environment which may have similarities with that seen in maar-type eruptions on land. The occurrence of carbonatite in the juvenile clasts provide evidence for a CO₂-rich magmatic fluid involved in the explosive events.