REACTIVE FLOW MODELING OF SUBMARINE HYDROGEOLOGY, ENDEAVOUR RIDGE

Based on published seismic sections and other tectonic and stratigraphic data on the Endeavour Ridge a 2-D numerical computer model was designed (L: 8 km, H: 3 km) with a typical seafloor stratigraphy (basalt, sheeted dikes) to test the influence of a range of physical parameters (permeability, thermal conductivity) and geometrical features (distribution and extent of faults, layer thickness) on the recharge and discharge behavior of seawater in close proximity to the ridge crest. Results indicate that the permeability distribution within the basalt layer (anisotropy) is the main factor controlling the magnitude and location of seawater recharge, fluid migration and fluid discharge; the location and nature of fault structures are of less importance. Major fluid recharge close to the vent orifice determines predicted fluid velocities at the ridge crest (> 200 m/y) for the whole width of the fault (25 m) and discharge temperatures of ~ 250°C; seawater recharge elsewhere is significantly smaller. Reactive transport simulations investigating the fate of Ca2+ during seawater recharge, heating and reaction with basalt indicate that Ca-bearing minerals other than anhydrite (clays, zeolites) may precipitate in significant amounts. The distribution and amounts of Ca-bearing phases is mostly a function of mass water-rock ratio. Below water-rock ratios of ~ 10 no anhydrite is predicted to form; in its place other Ca-bearing minerals such as smectite or prehnite occur. In addition, results from several databases show that pressure and salinity may affect the absolute amount of anhydrite formed at lower temperatures; above 160°C there are only minor differences. Results provide valuable constraints for the modeling of seawater recharge and anhydrite formation in the oceans crust using reactive transport simulations with a smaller numerical domain (L: 2.4 km, H: 1.8 km) including rock and fluid chemistry.