Oxygen isotope exchange processes and disequilibrium between calcite and forsterite in an experimental C-O-H fluid

Oxygen isotope exchange between calcite and forsterite were investigated in the presence of a large amount of supercritical fluid. The experiments wer e conducted in standard cold-seal pressure vessels at 680 degrees C and 500 MPa in the T-P-X-CO2 stability field of the calcite-forsterite assemblage for 2, 5, 10, 20, 40 and 80 days, respectively. The weight ratio of mineral to fluid in the starting mixture is 1.46; the fluid was a mixture of H2O a nd CO2 with the mole fraction of CO2 being 0.1. The results show that the o xygen isotope exchange between the minerals was accomplished via mineral-fl uid exchange by a dual-mechanism, i.e. initial rapid exchange due to Ostwal d ripening of both calcite and forsterite, followed by a slower diffusion-c ontrolled process. Furthermore, for the given fluid composition, calcite sh ows a greater rate of dissolution-recrystallization and oxygen isotope exch ange with fluid than forsterite. As a result, oxygen isotope fractionations between calcite and forsterite and between the minerals and the fluid can simply pass the equilibrium fractionations with time and even lead to cross over behavior. Once diffusion becomes a primary mechanism for further isoto pe exchange in the three-phase system, the rate of oxygen diffusion in calc ite is equal to, or slightly less than that in forsterite. Copyright (C) 19 99 Elsevier Science Ltd.