Deep-Sea Observatory EMSO-Azores (Lucky Strike, 37º17’N MAR): Impact of Fluid Circulation Pathway on Chemical Hydrothermal Fluxes

Abstract:

The operational EMSO-Azores deep-sea observatory provides a unique opportunity combining yearly hydrothermal fluid sampling along with continuous monitoring of physical parameters (temperature, seismicity, pressure,…) to better constrain hydrothermal fluid flow dynamics both in time and space at the Lucky Strike vent field. Repeated sampling at this field since its discovery in the mid 90’s shows persistent inter-site differences the end-member hot fluid compositions, revealing 4 groups of sites that possibly reflect various processes, such as phase separation, conductive cooling, tidal pumping, or magmatic activity. Three groups are at the main field, and a fourth one (Capelinhos) is 1,5 km to the E. We also identify a period of high CO₂ content of hydrothermal fluids (up to 120 mmol/l), clearly showing replenishment of the magmatic chamber in 2008 and 2010.

The Si-Cl geothermobarometer of the most vapor-dominated fluid (270 mmol/l, Capelinhos) gives a pressure of 370 bars and a temperature of ˜435ºC in the reaction zone. If hydrothermal cells operate in a cold hydrostatic pressure gradient, the transition between the reaction and upflow zones is located at about 2400 mbsf, with a magma lens at ˜3400 mbsf. Si-based geothermobarometry and empirical Fe/Mn geothermometry argue for P and T conditions close to seawater critical point at 300 bars (i.e. ˜1300mbsf) for Capelinhos. In contrast, the 3 other groups exhibit temperatures much lower than 380ºC, as well as reduced end-member Fe and Mn concentrations compared to those of Capelinhos. These physico-chemical features reflect the effects of conductive cooling along the upflow zone. Conductive cooling can vary spatially owing to variable substratum permeability across the hydrothermal field, a requisite to induce along-axis oriented hydrothermal cell.

The end-member chemical features of the 4 groups suggest that local variations in tectonism and volcanism exert a strong control on heat, mass and chemical fluxes.