Volcanic and hydrothermal processes in submarine calderas: The Kulo Lasi example (SW Pacific)
2018
Abstract The study area is located at the transition between the northern end of the Tonga Trench and the North Fiji
fracture zone, where tectonic movements are reputed to be the fastest in the world. To the southeast of Futuna Island, a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a NE-SW oriented volcanic graben and the N-S oriented Alofi ridge. In 2010, the active
volcanoKulo Lasi, which represents the most recent volcanic episode in the Futuna area, was discovered in the center of this extensive volcanic zone. Kulo Lasi is a 20 km diameter
shield volcanothat rises 400 m above the seafloor. It is composed of basaltic to
trachy-andesitic lava with no obvious geochemical affinity with the Tonga subduction that occurs 500 km to the east. The central
calderais 5 km in diameter and 300 m deep and is located at a water depth of 1500 m. Diving operations with the submersible Nautile and high-resolution AUV mapping, have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the
caldera. Four tectono-volcanic stages can be distinguished at Kulo Lasi
caldera. In stage 1, the
shield volcanois built. Annular reverse faults develop at the summit and control circulation of water/rock-dominated hydrothermal fluids and high-temperature alteration of rocks along the nascent normal faults. Mixing of hydrothermal fluids with seawater is favored along normal superficial faults, leading to the formation of low-temperature Fe/Mn mineralization at the summit of the
volcano. During stage 2, the
calderacollapse, gradually revealing outcrops of the altered and mineralized zones formed during Stage 1. As the
magma chambercools and collapses, less heat is available. As a result, medium to low-temperature ( 2 , materialized by the presence of native sulfur depositions on the surface of the most recent lava flows. During stage 4 (present day), black smoker sulfide
chimneys, controlled by the deep reverse faults, form at the surface of the most recent lava flows at the outer part of the
calderafloor. The fluids emitted result from the mixing of a deep fluid of the water/rock reaction type and of shallow seawater
superheatedin contact with the hot dykes. Hydrogen is generated during reaction of seawater with the hot dikes and H 2 is enriched in the mixed fluids. Active (43 °C) siliceous
chimneysat the base of the
calderawalls and low-temperature vents (6 °C) perched on the
calderawalls suggests a decrease in the venting temperature at a distance from the floor of the
caldera. In
chimneys, the
sphalerite/
pyrrhotite/isocubanite/barite
paragenesis, probably linked to the high H 2 concentrations in the fluids, indicate unusual reducing conditions for a back-arc. The
sulfide mineralizationalso has unusual trace element concentrations. The concentration in Pb and Ba appears characteristic of back-arc environments while the enrichment in Co, Se, and Sn is more common to mineralization associated with basalt in mature back-arcs and associated with
ultramafic rockson
mid ocean ridges.
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