OXYGEN-ISOTOPE THERMOMETRY BASED ON A REFINED INCREMENT METHOD AND ITS APPLICATION TO GRANULITE-GRADE ROCKS FROM SRI-LANKA

In this paper we first present a revision of the increment method, a s emi-empirical technique which allows the calculation of 0-isotope frac tionation factors between all silicates, provided their structure and chemical composition is known. For the temperature dependency, the exp erimentally calibrated high-pressure mineral-calcite fractionation fac tors of Chiba ci al. (1989) are taken as a basis. Then this method is applied to granulite-facies parageneses from Sri Lanka including quart z, K-feldspar, sillimanite, garnet, pyroxenes, amphiboles, cordierite, magnetite and ilmenite. It can be shown that 0-isotope equilibration occurred at granulite-facies conditions, at about 830-degrees-C, due t o static recrystallization. Locally, retrograde re-equilibration occur red also at much lower temperatures due to late D4. In most cases, how ever, 0-isotope disequilibria resulted from retrograde oxygen intracry stalline diffusion. It can be proved by model calculations that the 0- diffusion data for water-saturated systems reported in the literature, are too high by two orders of magnitude to explain the measured isoto pic compositions. The fractionation patterns allow the discrimination between open- and closed-system behaviour. The regional distribution o f open- and closed-system patterns corresponds to the frequency of occ urrence of late-stage migmatization (western part) and to differences in amount and composition of fluid inclusions.