White Paper Title: 
Modeling hydrothermal circulation at the East Pacific Rise ISS site

One of the most important issues concerning hydrothermal circulation at ocean ridge axes is the location, size and orientation of the recharge zone. Mathematical models argue that anhydrite precipitated from heated sea water would rapidly reduce porosity and permeability of the high temperature recharge zone and thus affect the thermal characteristics of high temperature venting, unless recharge occurs over a wide area. On the other hand seismicity data from the fast-spreading East Pacific Rise (EPR) have been used to argue that hydrothermal recharge occurs over a relatively small region that is strongly aligned along the ridge axis and that strains generated by seismicity could maintain porosity and permeability even in the event of anhydrite precipitation.

We are attempting to reconcile these two points of view by applying more advanced mathematical and numerical models to the EPR hydrothermal cell at 9°50' N to determine whether the seismicity distribution observed at EPR near 9°50' N delineates the recharge zone for the overlying hydrothermal system. Using a combination of thermal, geochemical, and seismic data on the depth to the subaxial magma lens as constraints, we developed a preliminary model of hydrothermal circulation along approximately 2 (km) of ridge axis from TWP to the Bio9 vent complex. Using Mg and Si data collected at EPR as conservative tracers and the observed partitioning of heat transport between high temperatures focused flow and low temperature diffuse flow provides additional constraints on the hydrothermal system behavior. These analyses show that at least 80% of the total heat flow comes from magma sources. We find that the sub-axial magma chamber must be actively replenished on a decadal time scale, which is consistent with recent petrological data. Although a simple 1-D model indicates that the seismically inferred recharge zone may not be large enough to carry all the flow without clogging as a result of anhydrite precipitation, preliminary results from the 2-D numerical model suggest that anhydrite precipitation might occur over broader regions and not have as strong a sealing effect.

Future numerical studies will incorporate temperature and time-dependent permeability by assuming a relationship between porosity and permeability and will include a model for the solubility of anhydrite as a function of temperature in order to investigate the feedback of anhydrite precipitation on the circulation. We also hope to devise a method to estimate the effects of seismicity on temporal changes in porosity and thus determine whether seismicity data can be used to estimate rates and scales of crack generation and opening of new flow paths as anhydrite seals old ones.