Oxygen isotope fractionation between zoisite and water

Oxygen isotope fractionations between zoisite and water have been studied at 400-700°C, PH₂O = 13.4 kbar, using the three-isotope method described by Matsuhisa et al. (1978) and Matthews et al. (1983a). The zoisite-waier exchange reaction takes place extremely slowly and consequently direct-exchange calibration of equilibrium ¹⁸O ¹⁶O fractionation factors was possible only at 600 and 700°C. Fractionation factors at 400-600°C were determined from samples hydrothermally crystallized from a glass of the anhydrous zoisite composition. At 600°C, both exchange procedures gave identical fractionations within experimental error. Scanning electron microscope studies showed that the zoisite-water exchange reaction occurs largely by solution-precipitation mass-transfer mechanisms. The slow kinetics of zoisite-water exchange may be typical of hydrous silicates, since additional experiments on tremolite-water and chlorite-water exchange also showed very low rates. When the zoisite-water fractionation factors determined in this study are combined with the quartz and albite-water data of Matsuhisa et al. (1979) and the calcite-water data of O'Nell et al. (1969), mineral-pair fractionations are obtained for which the coefficients "A" in the equation 1000 In α = A × 10⁶T^-2 are: {A table is presented} Fractionation data on coexisting quartz-zoisite (epidote) ± calcite in metamorphic rocks yield temperatures which do not appear to represent peak metamorphic conditions and possibly reflect the complexity of the chemistry of the epidotes and/or the complexity of zoisite and epidote parageneses.