Oxygen isotope fractionations involving diopside, forsterite, magnetite, and calcite: Application to geothermometry

Oxygen isotope fractionations between diopside, forsterite, magnetite, and calcite have been studied experimentally at high pressures (P = 15-16 kbar) and temperatures (T = 600-1300°C) with the carbonate-exchange technique of Clayton et al. (1989). The fractionations determined for these minerals can be combined with the data of Clayton et al. (1989) to give an internally consistent set of mineralmineral fractionations of the form 1000 ln α = A × 10⁶T^-2, where the coefficient A is given in the following table: {A table is presented} The diopside-calcite and forsterite-calcite fractionations of the present study are in excellent agreement with the theoretically-derived fractionations of Kieffer (1982). Mineral-mineral fractionations obtained by the carbonate-exchange technique are also in fair agreement with those derived from hydrothermal experiments except where the fractionations involve quartz or calcite. In those cases, the results of the present study indicate that the experimentally-determined quartz-water and calcite-water fractionations are systematically too small. Application of the present calibrations to natural samples yields reasonable crystallization temperatures for volcanic rocks. In plutonic igneous rocks and granulites, however, thermometers involving magnetite indicate extensive retrograde re-equilibration. Using the quartz-pyroxene thermometer, it may be possible in favorable cases to recover high temperature data from granulites.