OXYGEN AND HYDROGEN ISOTOPE COMPOSITIONS IN CLAY MINERALS: A POTENTIAL SINGLE-MINERAL GEOTHERMOMETER

Abstract

The opposite slopes of the isotopic fractionation of hydrogen and oxygen in the clay-water system and the well-known relationship between δD and δ18O in meteoric waters allow the isotopic composition of clay minerals to be used as a ?single-mineral? geothermometer. Equations can be derived which are valid for continental systems where meteoric waters are common. The following equation was derived for smectite: 3.54 * 106T?2 = δ18Osm ? 0.125δDsm + 8.95 (0-150o). This equation was used to determine the isotopic composition and temperature of the waters involved in the transformation of volcanic rock to bentonite at the Cortijo de Archidona (Southeastern Spain) site. The values obtained (?6%% ? δ18O ? ?4%%; 9oC > T ? 21oC, i.e., environmental temperatures) confirm the reliability of this equation.