A model for the formation of transient event plumes above mid‐ocean ridge hydrothermal systems

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Abstract
Event plumes (or megaplumes) are ephemeral bodies of anomalously warm water which form in the water column above mid‐ocean ridges as the result of the catastrophic release of large volumes of hydrothermal fluid. During the formation of the largest event plumes, heat fluxes increase to at least 100 times those of a typical high‐temperature vent field. Recent observations reveal a strong association between event plumes and diking‐eruptive events. It has previously been suggested that event plumes are the result of increased fluxes throughout hydrothermal systems following a large increase in the permeability of a critical region. In this paper, an alternative model is explored whereby the volume of fluid expelled is not balanced by increased down‐flow but rather by the expansion of fluids near the base of the system. For most geologically plausible patterns of hydrothermal circulation, the fracturing and faulting associated with a diking event will result in a much larger decrease in the flow resistance of the upflow zone than the down‐flow zone. To balance fluxes, the pressure must decrease throughout the system. There is considerable evidence that the fluids near the base of mid‐ocean ridge hydrothermal convection cells lie near the two‐phase boundary for seawater and thus are very compressible. Simple calculations show that fluid expansion in a depressurized reaction zone can match the observed formation times and sizes of large event plumes only if the permeability of the upflow zone increases to very high values (∼10−9–10−10 m2) and if the plume is underlain by a sizable reaction zone containing at least 0.01–0.2 km3 of fluid at temperatures near the two‐phase curve. Fluid expansion in a smaller reaction zone is a plausible mechanism to incorporate a component of mature hydrothermal fluid into event plumes whose heat content comes primarily from cooling lava flows. It may also be responsible for smaller event plumes or temporal fluctuations in chronic plumes which are not linked to volcanic eruptions but which follow technically or magmatically induced increases in permeability.