AN EXPERIMENTAL CALIBRATION OF OXYGEN-ISOTOPE FRACTIONATION BETWEEN CALCITE AND FORSTERITE IN THE PRESENCE OF A CO2-H2O FLUID

Oxygen isotope fractionation between calcite and forsterite has been e xperimentally calibrated in the presence of small amounts of a supercr itical CO2-H2O fluid at temperatures from 600-degrees to 900-degrees-C and pressures from 3 to 12 kbar. The experiments have been carried ou t in the stability field of the calcite-forsterite assemblage based on phase equilibrium relationships in the system CaO-MgO-SiO2-CO2-H2O. A ppropriate proportions of carbon dioxide to water have been used to me et this requirement. Large ratios of mineral to fluid have been used t o ensure that the isotopic exchange between calcite and forsterite in the presence of a fluid is close to that without fluid. In order to ex trapolate partial exchange data to equilibrium values, a data processi ng method for isotopic exchange in a three-phase system has been devel oped. The determined fractionation factors between calcite and forster ite are expressed as: 10(3) ln alpha(Cc-Fo) = 3.17.10(6)/T2+0.44 with an error estimate of approximately +/- 0.3-0.4%0. The present calibrat ion is in good agreement with data from both experimental and theoreti cal determinations. This suggests that oxygen isotope fractionation be tween calcite and a silicate can be calibrated in the presence of smal l amounts of a CO2-H2O fluid and thus within the P-T-X(CO2) stability field of the respective mineral pair.