OXYGEN ISOTOPE FRACTIONATION BETWEEN OXYGEN OF DIFFERENT SITES IN ILLITE MINERALS - A POTENTIAL SINGLE-MINERAL THERMOMETER

The experimental results of Hamza and Epstein mark internal oxygen isotope fractionations of hydrosilicates as potential single-mineral thermometers. In this study methodical investigations were made to determine the oxygen isotope ratios of hydroxyl groups in silicate minerals. As a reference material a commercial kaolinite was examined by vacuum extraction and by use of a modified partial fluorination technique first deseribed by Hamza and Epstein. The concordance of the results argue against oxygen isotope fractionation during dehydroxylation. Consequently, vacuum extraction can be used to determine the internal fractionation of minerals, which contain no ferrous iron. For calibration of the internal oxygen isotope fractionation, hydrothermally formed illites from the "Lone Gull" uranium deposit in Canada and from the "Leuggern" exploration drill site in Switzerland were investigated. Formation temperatures of the hydrothermal mineralization were estimated by mineral paragenesis, illite crystallinity and by oxygen isotope fractionations on coexisting mineral phases. the oxygen isotope fractionation between oxygen of different sites in several selected illites from both regions has been analysed. The results indicate a linear correlation between the illite-OH oxygen isotope fractionation and temperature. The fractionation can be expressed by the following equation: {Mathematical expression} (i equals temperature in °C). Using this first calibration of the illite-OH fractionation, it is possible to determine formation temperatures of hydrothermal mineral assemblages. To test the calibration curve, the internal fractionation of an illite sample from the Nama-Series (Namibia) was measured. The temperature calculated from the determined fractionation is in agreement with estimates from mineral paragenesis and illite crystallinity. Concordant illite-OH fractionations have also been calculated by use of the increment method according to Schütze. © 1990 Springer-Verlag.