Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators

Relative Na, K, Mg, and Ca contents of thermal waters in full equilibrium with a thermodynamically stable mineral system derived through isochemical recrystallisation of an average crustal rock are, at a given temperature and salinity, uniquely fixed. Together with the compositions of waters resulting from isochemical rock dissolution, they provide valuable references for the assessment of the degree of attainment of fluid-rock equilibrium. In geothermal systems, the pair K-Na reaches its equilibrium contents as governed by Lkn = log (cKcNa) = 1.75 − (1390T) most slowly (c1 in mg/kg, T in K). The quotient c2KcMg adjusts much faster and to low temperatures (<100°C) according to Lkm = log (c2KcMg) = 14.0 − (4410T). The system K-Ca is sensitive to variations in fco2 with Lkc = log (c2KcCa) = log fco2 + 3.0. These subsystems are combined to obtain graphical techniques for the evaluation of deep temperatures and CO2-partial pressures by use of Na, K, Mg and Ca contents of geothermal water discharges.