Geothermometry and water-rock interaction in selected thermal systems in the Cascade Range and Modoc Plateau, western United States

The sulfate—water isotope geothermometer indicates higher equilibrium temperatures than the silica or cation ratio geothermometers for the Na—Cl and Na—Ca—Cl thermal waters of the Cascade Range, Oregon and the Na—SO₄ thermal waters of the Modoc Plateau, northeastern California. Sulfate concentrations in the Na—Ca—Cl waters decrease as σ¹⁸O values of the dissolved sulfate become more depleted; conversely, sulfate concentrations in the Na—SO₄ waters increase as σ¹⁸O values of the sulfate become more depleted. Calculations show that the thermal waters are theoretically saturated with respect to anhydrite (CaSO₄) at temperatures very close to those calculated from the sulfate—water isotope geothermometer.