Abstract
Oxygen isotope fractionation between calcite and forsterite has been experimentally calibrated in the presence of small amounts of a supercritical CO 2H 2O fluid at temperatures from 600° to 900°C and pressures from 3 to 12 kbar. The experiments have been carried out in the stability field of the calcite-forsterite assemblage based on phase equilibrium relationships in the system CaOMgOSiO 2CO 2H 2O. Appropriate proportions of carbon dioxide to water have been used to meet this requirement. Large ratios of mineral to fluid have been used to ensure that the isotopic exchange between calcite and forsterite in the presence of a fluid is close to that without fluid. In order to extrapolate partial exchange data to equilibrium values, a data processing method for isotopic exchange in a three-phase system has been developed. The determined fractionation factors between calcite and forsterite are expressed as: 10 3ln α Cc − Fo= 3.17 · 10 6 T 2 + 0.44 with an error estimate of ∼ ±0.3–0.4‰. The present calibration is in good agreement with data from both experimental and theoretical determinations. This suggests that oxygen isotope fractionation between calcite and a silicate can be calibrated in the presence of small amounts of a CO 2H 2O fluid and thus within the P- T- X CO 2 stability field of the respective mineral pair.
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