Calculation of oxygen isotope model temperature in minerals.

The conventional mineral-pair isotope geothermometry is improved by simulat ing isotope exchange trajectories of minerals in terms of diffusion-control led isotope exchange. An independent approach is established by combining t he calculation of oxygen isotope model temperature (OIMT) in minerals with the semi-empirical sequence for oxygen diffusion blocking. The calculation method of model temperature is better than the conventional method in the f ollowing aspects: (1) oxygen isotope exchange in minerals due to diffusion is taken into account; (2) mass balance equation is satisfied, which fits t o describe oxygen isotope exchange between minerals within a finite reservo ir; (3) the record of high temperature can be recovered from high-temperatu re rocks; (4) it is independent of concrete diffusion models; and (5) its g eological meaning is more explicit, and calculation method can be widely ap plied. The tests with the ideal granite, San Jose tonalite and the troctoli te from Sybille quarry have demonstrated that the OIMT concept and numerica l method are promising to isotope geothermometry. The application to the tw o types of gneisses from Shuanghe in the Dabie Mountains obtains the model temperatures of 695 to 930 degrees C for garnet and titanite from the parag neiss, and 560 to 750 degrees C for garnet and magnetite from the orthognei ss. It appears that the paragneiss may have a different cooling history fro m that of the orthogneiss in the high-temperature stage, and that both of t hem may have experienced the metamorphism of eclogite-facies temperatures.