Deep-Sea Hydrothermal Systems

Max temperatures: ~270 °C

Setting: Basalt-hosted, high-temperature hydrothermal field on the slow-spreading Mid-Atlantic Ridge north of the Azores. Moytirra is unusually located on a steep fault scarp forming the eastern wall of the axial valley, approximately 3.5 km from the axial volcanic ridge. The field includes active black smokers, sulfide–anhydrite chimneys and diffuse venting, illustrating strong interaction between tectonic faulting and an underlying magmatic heat source.

Max temperatures: up to ~63 °C

Setting: Low-temperature, basalt-hosted hydrothermal field located on the eastern flank of the Gigante Seamount complex in the Azores region. Luso was discovered in 2018 during deep-sea exploration using ROV Luso. Hydrothermal activity occurs through translucent, low-temperature fluids and distinctive ochre-coloured Fe–Si-rich hydrothermal structures. The precipitates are dominated by Fe oxyhydroxides and amorphous silica, with Fe-rich clay minerals locally present.

Max temperatures: ~285 °C

Setting: Shallow, basalt-hosted, high-temperature hydrothermal field located on an axial volcanic edifice of the Mid-Atlantic Ridge south of the Azores Triple Junction. The Menez Gwen segment is volcanically robust and lacks the pronounced central rift valley typical of many slow-spreading MAR segments. Hydrothermal activity includes focused high-temperature venting, lower-temperature diffuse flow and sulfide–sulfate mineralization. Vent-fluid chemistry is strongly influenced by phase separation under the relatively low-pressure conditions of this shallow MAR system.

Max temperatures: ~324–333 °C

Setting: Basalt-hosted, high-temperature hydrothermal field located near the centre of the Lucky Strike segment of the Mid-Atlantic Ridge, on a large axial volcanic edifice. The field hosts numerous high-temperature vent structures together with extensive lower-temperature diffuse flow. Hydrothermal-fluid compositions reflect strong fluid–rock interaction and phase separation within this relatively shallow MAR system.

Max temperatures: <10 °C; low-temperature diffuse/seep-like discharge

Setting: Ultramafic-hosted, low-temperature hydrothermal site associated with serpentinized mantle rocks near the Lucky Strike–North FAMOUS non-transform discontinuity. Hydrothermal activity is characterized by low-temperature diffuse or seep-like fluid discharge rather than high-temperature black-smoker venting. The site is associated with methane-rich hydrothermal signatures related to water–rock interaction and serpentinization.

Max measured seafloor temperatures: ~7–9 °C

Setting: Low-temperature hydrothermal field hosted by ultramafic and mafic rocks on the Saldanha Massif, within non-transform offset NTO5 between the FAMOUS and AMAR segments of the Mid-Atlantic Ridge. The massif is dominated by ultramafic and gabbroic rocks and records extensive serpentinization. Hydrothermal activity is predominantly diffuse, with fluids observed venting through small orifices in the sedimentary cover. Fluid–rock interaction and serpentinization play a major role in the evolution of this hydrothermal system.

Max temperatures: ~365 °C

Setting: High-temperature, ultramafic-hosted hydrothermal field located on the western flank of the Rainbow Massif, within a non-transform discontinuity of the Mid-Atlantic Ridge between the AMAR and South AMAR segments. The field is hosted predominantly by serpentinized ultramafic rocks and contains numerous vigorous black smokers. Hydrothermal fluids are acidic, exceptionally Fe- and metal-rich, and strongly enriched in H₂ and CH₄ compared with typical basalt-hosted MAR hydrothermal systems. Serpentinization is a major control on fluid chemistry, while the very high vent temperatures require an additional deep heat source.