Scientific Drilling to Investigate the Physical and Chemical Nature of Fluids in the Earth’s Crust at 400–500 °C

Abstract

The physical and chemical natures of pore-fluids deep in the earth’s crust are of great interest from geological, geochemical, and geophysical points of view. Many geophysicists have speculated that high pore-fluid pressure in some places deep in the crust may account for extremely high electrical conductivity (e.g., Shankland and Ander 1983), for increased seismic reflectivity (e.g., Walder and Nur 1984; Matthews 1986), and for relatively low seismic velocities (e.g.. Berry and Mair 1977; Walder and Nur 1984). However, the geophysical community, in general, seems reluctant to accept models in which a free fluid phase is widespread in the crust at depths greater than about 10–15 km. In contrast, geochemists and metamorphic petrologists have used fluid inclusions, found in metamorphic minerals that formed deep in the crust, to document large ranges in compositions of discrete fluid phases existing at high temperatures and high pressures (Crawford and Hollister 1986, and references therein). There is general agreement that water and CO₂ play key roles in metamorphic processes occurring throughout a wide range of temperatures and pressures up to magmatic conditions (Fyfe et al. 1978; Valley 1986; Walther and Wood 1986). There is less agreement about the relative importance of various possible sources of these and other volatile constituents, about the variation in composition of pore fluids in space and time, about fluid-rock ratios in metamorphic processes, and about the nature of convective fluid flow where the pore-fluid pressure, P_f, is greater than hydrostatic, P_h.