From the magma chamber to microhabitats: dynamics of diffuse hydrothermal circulations at mid-ocean ridges

Zoom link: https://u-paris.zoom.us/j/84823718402?pwd=YlBtWGd1RDYrVzk0QmVtSkpxandMdz09 Meeting ID: 848 2371 8402 Password: 708450 The 1km² Lucky Strike hydrothermal field is one of the largest found at mid-ocean ridges. In this field, a submarine observatory has been established to investigate both the links between geological, physical and chemical processes and with the hosted fauna at hydrothermal sites for different spatial and temporal scales. The hydrothermal activity is visible at the seafloor through chimney edifices called “black smokers” but also through diffuse fluids of lower temperature that are scattered on larger surfaces. Very little is known about the different mechanisms of formation and the dynamics of circulation of diffuse fluids in the subseafloor despite their important role for the hydrothermal fauna. We studied diffuse fluids during a 3 years monitoring at 2 hydrothermal vent sites from the Lucky Strike hydrothermal field (Tour Eiffel and White Castle). We documented the time and space variability of diffuse venting temperature and chemistry, the effect of tidal loading and currents, fluid formation by mixing, cooling or heating of end-member hydrothermal fluids and seawater associated with minerals precipitation/dissolution. Based on these results, we developed a conceptual geological model for the dynamics of vent site scale hydrothermal circulations at the Lucky Strike field. This model confirms the role of the thin volcaniclastic formation deposited on the brecciated basalt substratum of both sites to control the dynamic of circulation and of venting of diffuse fluids. We propose that fluids that actually flow out of the basalts underneath are relatively hot (> 80°C) and end-member rich (> 9%) fluids, coming out of a network of meter-spaced permeable cracks in the basalt substratum, up to 25 m from the black smokers at both vent sites. These fluids would generate a diffuse heat flow estimated as about 7.5 MW at White Castle, and 28.9 MW at Tour Eiffel. We use these observations to constrain a 2D thermo-hydraulic model of hot fluid circulation in a porous media, in order to predict the geometry of fluid circulation in the shallow subseafloor and the formation of diffuse fluids close to black smokers, as a response to changes in key physical parameters such as the width of the upflow zone, and the velocity of hydrothermal fluids at the base of the system. We also explore the effect of sulfate precipitation from entrained seawater. To test these models, we use observations on the dimensions of the venting domains at the seafloor, and the temperature of hydrothermal effluents. Our results show that low permeability for the porous media or high velocity for the upflowing hydrothermal fluids are necessary in order to have high-temperature fluids >300°C vents. To be able to have close lateral diffuse fluids >20°C, anhydrite precipitation to change local permeability and fluid flow is necessary.