CALCULATION OF OXYGEN ISOTOPE FRACTIONATION IN METAL OXIDES

Abstract

A theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200-degrees-C on the basis of the modified increment method. The following order of O-18-enrichment is obtained: cassiterite > rutile > magnetite > ilmenite > ulvospinel > hematite > uraninite > chromite > corundum > spinel. The results are expressed as 10(3) ln alpha = A x 10(6)/T2 + B x 10(3)/T + C, which compare well with existing theoretical, experimental, and empirical calibrations on quartz-oxides and oxides-water systems. The present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available, but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks.