Fluids in the upper continental crust

Abstract

Geofluids (2010) 10, 241–253AbstractThe brittle upper continental crust predominantly consists of granite and gneiss. Fractures form an interconnected network of water‐conducting structures with an appreciable permeability also providing substantial fluid‐saturated fracture porosity. The chemical composition of fluids in the fracture porosity of granite and gneiss changes with depth. Near the surface Ca–Na–HCO3 waters dominate. With increasing depth, water contains increasing amounts of alkalis and sulfate and grade into chloride‐rich waters at greater depth. Total dissolved solids (TDS) of 105 mg l−1 are common at 5‐km depth in most basement rocks. All reported deep fluids from the upper crust contain predominantly NaCl and CaCl2. The brines vary from NaCl‐rich in granites to CaCl2‐rich in mafic reservoir rocks such as amphibolites and gabbros. In regions of the crust with strong topography, fluid flow is important and recharge water may have flushed the basement efficiently, thereby removing old brine components from the granites. Water samples from the new Gotthard Rail Base Tunnel of the Alps represent this type of basement fluid. Analyzed fluids from up to 2.5‐km depth differ from basement fluids from areas with less‐extreme topography in the following ways. Such waters have relatively low TDS of some 100 mg l−1 and are typically of the Na2CO3–Na2SO4 type. pH tends to be high and varies from 9 to more than 10. Low Ca and ultra‐low Mg of such waters result from efficient deposition of secondary Ca–Mg minerals as coatings on fracture walls. Reduction of CO2 to CH4 provides the oxidation capacity for sulfate production from primary rock sulfides. The composition of fluids in fractured continental crust at depths below 1–2 km depends strongly on the topography of the erosion surface. In crust with rugged alpine topography fluids at this depth are low‐TDS high‐pH waters that derive its composition from fluid–rock interaction alone. In crust with low‐to‐moderate topography, basement fluids are normally near neutral high‐TDS Na–Ca chloride brines that derive the solutes not only from the rock matrix but also from external sources.