The role of iron and manganese in elucidating the temperature of subseafloor hydrothermal reactions: insights from experimental and field data

Abstract

Field/experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal fluids reflects fluid-mineral reaction at temperatures higher than typically measured at the seafloor. In order to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T(°C) = 331.24 + 112.41*log[Fe/Mn] has been calibrated between 350 °C and 450 °C. The apparent Fe-Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. This methodology is used to interpret new data from 13°N on the East Pacific Rise (EPR) and the Lucky Strike Seamount on the Mid-Atlantic Ridge where high-temperature fluids both enriched and depleted in chloride, relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, phase separation is also a dominant process affecting the chemistry of hydrothermal fluids; and vapor-liquid partition coefficients for accessory metals (Na-normalized as NaCl dictates phase equilibria) have therefore been experimentally derived for temperature between 360 and 460 °C, where those > Na represent increasing affinity for the liquid phase. These data reveal the relationship Cu(I) < Na < Fe(II) < Zn < Ni(II) ≤ Mg ≤ Mn(II) < Co(II) < Ca < Sr < Ba and a dependence on the cationic radii. The depth below seafloor of the magmatic heat source at Lucky Strike is greater than that of faster spreading systems such as EPR 9-13°N, which host higher temperature vents with greater transition metal concentrations. The combined experimental/field data suggest the resulting increased residence time in the up-flow zone allows re-equilibration at temperatures lower than those resulting in phase separation. Fluid chemistry measured in the immediate aftermath of eruptions at EPR 9-10°N conversely shows large deviations from Fe/Mn equilibrium consistent with partitioning due to phase separation. The eruptive data also exhibit that P-T conditions were sufficiently extreme that Fe is behaving in a volatile manner observed in the experiments and in some instances subseafloor halite saturation may have occurred.