CALCULATION OF OXYGEN-ISOTOPE FRACTIONATION IN HYDROXYL-BEARING SILICATES

The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl-bearing silicate minerals. The order of 180 enrichment obtained in common rock-forming minerals is: pyrophyllite > kaolinite > tourmaline greater than or equal to talc > prehnite greater than or equal to topaz > illite > phengite > lepidolite greater than or equal to muscovite greater than or equal to staurolite > epidote > glaucophane > serpentine greater than or equal to chlorite > tremolite > hornblende > phlogopite greater than or equal to biotite > humite > norbergite > ilvaite. Hydroxyl-bearing silicates are enriched in O-18 relative to hydroxyl groups but depleted in O-18 relative to anhydrous counterparts. Three sets of self-consistent fractionation factors: between quartz and the hydroxyl-bearing silicate minerals, between calcite and the silicate minerals, and between the silicate minerals and water, have been calculated for a temperature range of 0-1200 degrees C. The fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous, metamorphic and sedimentary petrology. They can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest. The difference in oxygen isotope composition between the hydroxyl-bearing mineral and the OH group is quantitatively demonstrated to be temperature dependent and, therefore, can be used as a single-mineral geothermometer.