Fluid Geochemistry of the Capelinhos Vent Site. A Key to Understand the Lucky Strike Hydrothermal Vent Field (37°N, MAR).

Abstract:

The Lucky Strike hydrothermal field is situated at the mid-Atlantic ridge, south of the Azores, on top of a central volcano within the axial valley. The volcano is composed of a fossil lava lake surrounded by three volcanic cones. An Axial Magma Chamber (AMC) is reported 3.4km below the seafloor. The active venting sites are situated around the fossil lava lake and are directly linked to the heat supplied by the AMC. High temperature fluids from the Lucky Strike field were sampled in 2013, 2014 and 2015 in order to document the depth of the reaction zone, subsurface mixing, geographical control and magmatic degassing.

A new active site named Capelinhos was discovered approximately 1.5km eastward from the lava lake, during exploration by ROV Victor6000 - MoMARsat cruise, 2013. It is composed of 10m-high chimneys discharging black smoker-type fluid. Fluid temperatures were 328°C in 2013 and decreased to 318°C in 2014 and 2015. Capelinhos fluids are Cl-depleted by 55% compared to seawater indicating phase separation at depth. In comparison, the other sites range from 6% enrichment (2608/Y3 site) to 22% depletion (Eiffel tower site). Si geothermobarometry of Y3 site estimates quartz equilibration at P=300 bars and T=360-380°C, coherent with Fe/Mn geothermometer (T=370±10°C). For Capelinhos, Fe/Mn suggests 398°C (±10°C) which is close to the critical point of seawater (P=300 bars and T=407°C). Other geothermobarometer uses Si/Cl vapor-like fluid to constrain depth of the top of reaction zone and predicts significant bias due to mixing along the up-flow zone. Application gives P=~370 bars, T=~435°C at Capelinhos and P=~390 bars, T=~440°C at Eiffel tower. This is further sustained by end-member ⁸⁷Sr/⁸⁶Sr=0.7038, which indicates little interaction of Capelinhos vent fluids with seawater-derived fluid, compared to other vapor-like sites with ⁸⁷Sr/⁸⁶Sr=0.7043. Because of its external location, Capelinhos site isn’t influenced by the complex tectonic context of the lava lake.