The dynamics of two-phase hydrothermal systems at a seafloor pressure of 25 MPa

摘要

We present 2-D numerical simulations of two-phase flow in seafloor hydrothermal systems using the NaCl-H2O numerical code Fully Implicit Seafloor Hydrothermal Event Simulator to better understand phase separation and the evolution of the temperature and salinity of vent fluids in seafloor hydrothermal systems. We consider a fixed seafloor pressure of 25 MPa, a range of homogeneous and isotropic permeabilities, and various constant bottom temperatures to represent a subaxial magma chamber. The goal is to investigate how permeability and maximum bottom temperature affect vent fluid temperature and salinity. The simulations show that hydrothermal heat output increases nearly linearly with permeability, or Rayleigh number, but maximum bottom temperature has a greater effect on vent fluid temperature and salinity than the permeability. Although plume structures are relatively stable, the high Rayleigh numbers considered here result in temporal and spatial variations in temperature and salinity of vent fluids. The frequency of the fluctuations in the temperature and salinity of vent fluids increases with Rayleigh number. Vapor- and brine-derived fluids can vent simultaneously in close proximity and at different times and locations throughout a simulation. The simulations also show that vent fluids are complex mixtures between phase separated fluids formed near the base of the system and seawater. Consequently, neither the spatial and temporal variability, nor the temperature and salinity of vent fluids can be used to uniquely determine P-T conditions or indicate temporal changes in such conditions at depth.