White Paper Title: 
Modeling Magma-Hydrothermal Processes at Oceanic Spreading Centers

Mathematical and numerical modeling can help provide an integrated, whole-system quantitative understanding of magma-hydrothermal processes at oceanic spreading centers and make predictions that can guide field investigators to new avenues of exploration. Numerous data sets, particularly from the Juan de Fuca and EPR ISS sites, can be incorporated into, or used to constrain these models, at least partially. To various degrees, these data sets include: (a) time series measurements of vent temperature and salinity; (b) advective heat output and fluid flux measurements from focused venting and diffuse flow (and integrated water column measurements); (c) geochemical data from high-temperature vents and adjacent diffuse flow discharge; (d) petrological data that describe the evolution of the sub-axial magma chamber over an eruptive cycle; (e) seismicity data that may delineate hydrological flow paths; (f) active source seismic data that constrains crustal structure and the size, shape and melt content of crustal and sub-crustal magma bodies; (g) detailed seafloor mapping.

We have recently developed models of magma convection and heat transfer as they link with hydrothermal heat output. These results show that magma replenishment on short time scales (~ years) is required to maintain steady heat output and vent temperature. This work is being continued by exploring convection in high silica magmas, the effect of initial intrusion size and periodic magma replenishment, and the effect of a two-component melt composition. Although these models provide useful, first-order insights into coupled magma-hydrothermal processes, advances in these models can be made if they could be linked with realistic models of multi-component magma convection and the evolution of erupted lavas over time. Additional advances can be made if the magma convection-replenishment models are linked with realistic physical mechanisms of magma replenishment, crustal assimilation, and eruption.

We are also using the NaCl-H2O numerical code FISHES to investigate phase separation processes in seafloor hydrothermal systems. Preliminary results with prescribed basal temperatures show that: (a) vent salinity may vary over time, (b) that vents with salinity both greater than and less than seawater may discharge simultaneously in close proximity, even though the fluid source comes from the same depth, and (c) salinity of the vent fluid represents a mixture between phase separated fluid and seawater and thus may not generally provide a unique solution to the P-T conditions at which phase separation occurs. We are planning to couple models of magmatic convection, replenishment and heat transfer, with FISHES to obtain a more integrated picture of magma-hydrothermal processes together with phase separation, and eventually with vent chemistry.