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 18O enrichment obtained in common rock-forming minerals is: pyrophyllite > kaolinite > tourmaline ⩾ talc > prehnite ⩾ topaz > illite > phengite > lepidolite ⩾ muscovite ⩾ staurolite > epidote > glaucophane > serpentine ⩾ chlorite > tremolite > hornblende > phlogopite ⩾ biotite > humite > norbergite > ilvaite. Hydroxyl-bearing silicates are enriched in 18O relative to hydroxyl groups but depleted in 18O 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°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.