Deep-fluids: Neptune meets Pluto

Excerpt

Groundwater in fractured crystalline rocks such as granite and gneiss has a remarkable compositional variability and the crystalline hard rock aquifers show a wide range of hydrogeological properties (Gustavson and Krasny 1993; Stober and Bucher 1999a). With increasing depth both the total load of dissolved solids (TDS) and the temperature of groundwater in the crystalline basement increases. Groundwater gradually changes into a hot saline brine that is better termed a crustal deep-fluid. The hydrochemistry of deep-fluid has been recently reviewed by Frape et al. (2004) and their data compilation shows that at depths greater than about 500 m almost all deep waters are essentially binary NaCl-CaCl₂ mixtures. Few data on fluid composition and in situ hydraulic properties are available from below a depth of 3 km in the continental crystalline crust. Such “ultra” deep-fluids have been reported from research boreholes and from wells drilled for geothermal energy (Stober and Bucher 2004). At 4–5 km depth, the temperature of these deep-fluids ranges from about 100 to 200°C. Two deep wells, 12.5 km Kola, Russia (Kozlovsky 1984) and 9.1 km KTB, Germany (Möller et al. 1997) had bottom hole temperatures of 240 and 270°C. At still greater depth, there is a gradual and continuous transition to so-called hydrothermal fluids (Barnes 1995). The depth range of some hundreds to thousands of meters is truly the venue where Neptune, the god of the waters and the sea, meets Pluto, the god of the underworld, heat and fire; this is the world of hot saline deep-fluids. …