The Use of Oxygen Isotopes in High-Temperature Geological Thermometry

Recent measurements on the oxygen isotopic fractionation between calcite and water and the oxygen isotope measurements on coexisting minerals permit the estimation of the isotopic fractionation factor as a function of temperature for systems involving any pair of the phases: quartz, calcite, hematite, and water. The use of these calibration curves for geological thermometry is illustrated for several natural samples. The isotopic composition of oxygen in hydrothermal fluids is also estimated from the measured O18/O16 ratios in hydrothermal minerals. The mean value thus estimated is 6%0 greater than the oxygen of mean ocean water.