A boundary layer model for submarine hydrothermal heat flows at on‐axis and near‐axis regions

Abstract

Hydrothermal discharge from oceanic crust makes up a significant portion of Earth's overall heat flux. In this paper we investigate two aspects of oceanic hydrothermal heat flux that are not well understood: (1) the relative partitioning of heat flux between high‐temperature and low‐temperature flows at oceanic spreading ridges and (2) hydrothermal behavior of the near‐axis region, where seismic data suggest that a zone of partial melt extends quasi‐vertically into the lower crust at the East Pacific Rise. We apply steady state boundary layer theory to each system by assuming circulation occurs near a hot isothermal wall that laterally transfers heat to and induces convection within an adjacent fluid‐filled medium. We vary the permeability of the medium over a range of 10−14 to 10−10 m2. For model 1 we assume a wall temperature of 150°C, corresponding to a relatively impermeable anhydrite wall within layer 2A extrusives, whereas for model 2 we assume a wall temperature of 800°C that extends to the base of the crust. The boundary layer results for model 1 suggest that diffuse flow makes up 50% or less of the heat output at the ridge axis, which is at the low end of some observations. Should more accurate and complete heat flux data be obtained and indicate that a high fraction of ridge axis heat loss results from diffuse flow, another mechanism in addition to the boundary layer model proposed here may be required. Boundary layer results for near‐axis circulation, model 2, indicate that temperature distributions are in reasonable agreement with those inferred from seismic tomographic studies of the oceanic crust and with other numerical models. For the near‐axis model, results also show that heat transfer in the hydrothermal boundary layer is greater than the input from steady state generation of the oceanic crust by seafloor spreading. This difference in heat input suggests that the width of the partial melt zone may be decreasing with time. Thus, the application of the boundary layer theory to complex hydrothermal processes near oceanic spreading centers provides important insights into key system processes.