OXYGEN-ISOTOPE FRACTIONATION BETWEEN CALCITE AND TREMOLITE - AN EXPERIMENTAL-STUDY

Oxygen isotope partitioning between calcite and tremolite was experime ntally calibrated in the presence of small amounts of a supercritical CO2-H2O fluid at temperatures from 520 to 680 degrees C and pressures from 3 to 10 kbar. The experiments were carried out within the stabili ty field of the calcite-tremolite assemblage based on phase equilibriu m relationships in the system CaO-MgO-SiO2-CO2-H2O, so that decomposit ion of calcite and tremolite was avoided under the experimental condit ions. Appropriate proportions of carbon dioxide to water were used to meet this requirement. Large weight ratios of mineral to fluid were em ployed in order to make the isotopic exchange between calcite and trem olite in the presence of a fluid close to that without fluid. The data processing method for isotopic exchange in a three-phase system has b een applied to extrapolate partial equilibrium data to equilibrium val ues. The determined fractionation factors between calcite (Cc) and tre molite (Tr) are expressed as: 10(3) In alpha(Cc-Tr) = 3.80 x 10(6)/T-2 - 1.67 By combining the present data with the experimental calibratio ns of Clayton et al. (1989) on the calcite-quartz system, we obtain th e fractionation for the quartztremolite system: 10(3) In alpha(Qz-Tr) = 4.18 x 10(6)/T-2 - 1.67 Our experimental calibrations are in good ag reement with the theoretical calculations of Hoffbauer et al. (1994) a nd the empirical estimates of Bottinga and Javoy (1975) based on isoto pic data from naturall assemblages. At 700 degrees C good agreement al so exists between our experimental data and theoretical values calcula ted by Zheng (1993b). With decreasing temperature, however, an increas ing difference between these data appears. Retrograde isotopic reequil ibration by oxygen diffusion may be common for amphibole relative to d iopside in metamorphic rocks. However, isotopic equilibrium in amphibo le can be preserved in cases of rapid cooling.