A. Project Summary
Scientific Impacts
We propose to develop 2- and 3-D numerical models of phase separation and hydrothermal
circulation at the Main Endeavour Field (MEF) on the Juan de Fuca Ridge that integrate available vent
fluid temperature, salinity, and heat flow data, along with all other relevant constraints, such as
magma lens geometry and depth, extrusive layer thickness, and vent field area. A particular goal
is to understand the reason for the southwest to northeast gradients in temperature and salinity
across the MEF. In addition to developing a quasi-steady state circulation model that represents
the behavior of the MEF between 1988 and 1995, we also plan to use the available data to model
the response of the system to the magmatic event of 1999, and what appears to be the decline in
hydrothermal heat output since that time. Although currently available data are sufficient to
provide meaningful constraints on the models, additional vent temperature, salinity, and heat
flow data that is expected to become available will provide important constraints on the models.
In particular, the collaboration with Paul Johnson to interpret and his thermal blanket heat flow
data set and to incorporate these data into the numerical model will be especially useful for
constraining shallow recharge and circulation within the upper crust (layer 2A). These data will
provide important constraints on the 3-D nature of hydrothermal circulation at MEF. Preliminary
modeling by the PIs shows that vent temperature, salinity and heat output are controlled
primarily by crustal permeability distribution and the bottom boundary conditions, which control
the temperature and areal extent of the two-phase zone at depth. The research program involves
three inter-related tasks: (1) the development of 2-D single-pass models in order to match
southwest to northeast temperature and salinity gradients across the MEF; (2) the development of
2-D models involving dike emplacement to understand the response to the 1999 event and the
subsequent system decline; (3) the development of 3-D single pass single phase and two-phase
models of hydrothermal circulation that includes the effect of induced circulation in layer 2A,
constrained by all available heat flow data. These will be the first 2- and 3-D numerical models
of phase separation for a mid-ocean ridge hydrothermal system and the first numerical models
for a particular vent field that are constrained by time series data of vent temperature,
hydrothermal heat output, and vent salinity. These will also be the first fully numerical models
of phase separation and hydrothermal circulation of a NaCl-H2O fluid near a dike. Finally, the
results will provide important insight into how a major hydrothermal system decays in time. The
results of this modeling will thus provide important insights into the process of phase separation
in seafloor hydrothermal systems and provide constraints on the permeability structure of the
ridge axis. The results will help determine whether the decay of hydrothermal heat output is
controlled by magmatic crystallization and cooling at depth or by the evolution of crustal
permeability.
Broader Implications
Although these models will be specific to the MEF, the results will lead to increased
understanding of phase separation and hydrothermal circulation at mid-ocean ridges, which is a
broad field of interdisciplinary study. The development of the 3D two-phase code FISHES will
become an important community tool for hydrothermal modeling. The 2-D and 3-D versions of
FISHES, and a user’s manual, will be placed on the PIs’ personal websites when they are available for
general use. This research will train a new graduate female student in an important area of
interdisciplinary research and train the student in the use of numerical codes. The research will
also advance the scientific development of a young female researcher. The results of this
research will be incorporated into courses on fluid processes taught by the PI.
A. Project Summary
1