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 semi-empirical technique which allows the calculation of 0-isotope fractionation factors between all silicates, provided their structure and chemical composition is known. For the temperature dependency, the experimentally calibrated high-pressure mineral-calcite fractionation factors of Chiba ci al. (1989) are taken as a basis. Then this method is applied to granulite-facies parageneses from Sri Lanka including quartz, 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 to static recrystallization. Locally, retrograde re-equilibration occurred also at much lower temperatures due to late D4. In most cases, however, 0-isotope disequilibria resulted from retrograde oxygen intracrystalline 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 isotopic compositions. The fractionation patterns allow the discrimination between open- and closed-system behaviour. The regional distribution of open- and closed-system patterns corresponds to the frequency of occurrence of late-stage migmatization (western part) and to differences in amount and composition of fluid inclusions.