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Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
An important end-member of hydrothermal circulation at mid-ocean ridges is the low-temperature diffuse flow found at all known black smoker vent fields. Diffuse flow is the result of the subsurface mixing of cold seawater with hot black smoker fluid. The conditions that allow this mixing to take place are poorly understood. To gain further understanding of diffuse flow a simple pipe model has been used to explore the relationship between the sub-seafloor permeability structure and the temperature of the exiting fluid. Calculations with this simple model show a wide and continuously varying exit temperature from black smoker temperatures to diffuse flow temperatures as the permeability structure is changed. The results show that for low temperatures to be observed at the surface, the upper part of the circulation for that part of the discharge has to be relatively permeable, and that the deeper sections of the upflow must be much more impermeable. The model suggests that the early stages of evolution of a hydrothermal system are characterized by widespread diffuse flow and that black smokers develop as subsurface precipitation reduces the permeability of the upper section of the crust.
