ROCK CONTROLS ON FLUID REGIMES IN THE CRUST

Both the chemical composition and the pressure of fluids in the deep crust are constrained by the properties of the rocks that host them. Chemical buffering dominates deep fluid chemistry when fluid flow rates are slow relative to the rate of fluid reaction with wall rock. Fluid pressure can likewise be controlled by rock characteristics within defined limits. The upper limit for fluid pressure is given by hydrofracturing, but even in rocks that are undergoing devolatilisation, this limit may not be reached if fluid escapes without failure. There is no lower limit to fluid pressure in the crust, and the values of fluid fugacities in crystalline basement rocks are generally controlled or limited by mineral equilibria.

Three types of fluid domain occur in the crust: approximately hydrostatically pressured, overpressured, and at very low fluid pressure. Near-hydrostatic domains are primarily present in the upper crust, and large fluid fluxes are possible, if recharge is available, but are not always present (e.g. at KTB). Possible overpressured settings include deep sedimentary basins, zones of active burial and metamorphism, and crystallising igneous bodies. Underpressuring is the normal case for old crystalline rocks where their present temperature is lower than their peak metamorphic temperature. Two types of fluid pressure regime may coexist side by side, as when fluid-filled and mineral-lined fractures occur within fluid absent basement wall rocks.

Interactions between these domains may involve mixing between fluids where both are near hydrostatic pressure, but otherwise involve escape of fluids at higher pressures into lower pressure domains. In practice, escape of overpressured fluids is usually slow because they are hosted by low permeability rocks, but skarns provide an important exception. Interaction of normally-pressured fluid from sediments with dry, underpressured basement rocks is likely an important cause of mineralisation, and may also play an important role in tectonic remobilisation.