During periods of volcanic activity, hydrothermal fluid chemistry changes drastically, becoming unusually dilute due to enhanced degrees of phase separation. Despite decreases in nearly all other metals, these dilute fluids maintain surprisingly high dissolved Fe concentrations. This is demonstrated by a 17 yr time series from 9°50′N on the East Pacific Rise, where two eruption cycles are separated by a decade of steady-state chemical and physical conditions. We report experimental data confirming a sharp increase in Fe solubility in low-salinity and low-density vapors that constitutes a reversal in behavior exhibited in near-critical vapors characteristic of the steady-state condition. In accordance with field observations during the eruptions, a fundamental divergence between the otherwise similar behaviors of Fe and Mn also results. This helps explain how Fe fluxes are maintained during magmatic events, which may have important implications for the succession and temporal evolution of vent-related fauna. Calibrated geochemical proxies for subseafloor reaction conditions (pressure-temperature) now allow us to elucidate hydrothermal processes from steady state through eruptive and recovery stages at the 9°50′N system.

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