New insights from thermohaline multiphase simulations into the mechanisms controlling vent fluid salinity following a diking event at fast-spreading ridges

Abstract

Hydrothermal fluids expelled at mid-ocean ridge vent sites show a large spatial and temporal variability in exit temperatures and chlorinities. At the East Pacific Rise (EPR), 9°50.3'N, time series data for a 25+ year period reveal a correlation between these variations and magmatic diking events. Heat input from dikes appears to cause phase separation within the rising fluids splitting them into low-salinity vapor and high-salinity brine phases. The intrusion of a new dike is therefore likely to result in a characteristic salinity signal, with early post-eruptive fluids showing vapor-influenced low salinity and later brine-influenced fluids showing high salinity values.We here use a 2-D multiphase hydrothermal flow model to relate these observations to processes and properties within the sub-seafloor such as permeability, porosity, background flow rates, and phase separation as well as segregation phenomena. We have grouped the time evolution of vent fluid salinity into four temporal stages and have identified how multiphase flow phenomena control vertical salt mass fluxes within each stage. Rock porosity and permeability as well as background temperature of the undisturbed hydrothermal system control the duration of the four stages and maximum venting salinity. Based on our results, we are able to reproduce the characteristics of time-series data from the EPR at 9°50.3'N and infer the likely ranges of rock properties and the hydrothermal conditions within the oceanic crust beneath.