Subseafloor phase equilibria in high-temperature hydrothermal fluids of the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17'N)

As part of an integrated study conducted at the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17'N) in 2008, gas-tight sampling devices were used to collect high-temperature (∼300°C) hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression. Compared with previous observations from 1993 to 1997, the most substantial changes in vent fluid compositions are dramatically increased CO ₂ concentrations (∼5×, up to 133mmol/L) and the observation of vent fluids enriched in dissolved chloride relative to seawater. Combined with an increase in δ13CCO2 values by ∼4‰ in 2008, the elevated CO ₂ indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event. The additional supporting fluid chemistry is, however, similar to that of the previous sampling intervals, necessitating a reassessment of the subseafloor controls on vent fluid chemistry at Lucky Strike in the context of recently obtained geophysical data that provides the depth/extent of a steady-state magma chamber. Two-phase behavior is indicated by the chloride variability in the vent fluids; and comparison with experimental data for the associated chloride-dependent partitioning of minor/trace elements suggests the possibility of a similar source fluid for all the vent structures, while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end-members. A recently calibrated Fe/Mn geothermometer indicates minimum subseafloor equilibration temperatures of 350-385°C. However, constraints imposed by dissolved Si/Cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430-475°C and pressures near the top of the axial magma chamber (∼410-480bars), where magmatic CO ₂ becomes entrained in the circulating fluids. The distance between the magma chamber and the seafloor at Lucky Strike is substantially greater than at most faster spreading ridges; and we propose the resulting increased residence time in the up-flow zone leads to the re-equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction. Agreement between experimental data, thermodynamic model calculations, and dissolved concentrations of Fe, Cu, Zn, H ₂, and H ₂S in the Lucky Strike fluids reinforce the hypothesis of pH-redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical Fe/Mn geothermometer. In-situ pH measurements of the high-temperature fluids exiting the seafloor are also consistent with the model calculations.