1. Department of Earth and Space Sciences, University of Science and Technology of China, hefei 230026, PR China; 2. Instirute for Mineralogy, Petrology and Geochemistry, University of Tübingen, Wilhelmstraste 56, 72074 Tübingen, Germany
Oxygen isotope fractionation in amphiboles was determined by theoretical calculation using the increment method and by experimental measurement using isotope exchange technique. The theoretical results suggest that there are given oxygen isotope fractionations among chemically distinct amphiboles. The obtained order of 18O——enrichment is as follows: riebeckite M glaucophane> grunerite>actinolite= cummingto’lite > anthophyllite > tremolite De hornblende> gedrite> pargasite. At temperatures above 500t, the amphiboles are depleted in 18O (up to 1-3‰) relative towater.The experiments to determine oxygen isotope fractionation between calcite and tremolite were carried out in the presence of small amounts of a supercritical CO2 H2O fluid under the following conditions: (1) 680 and 640℃, 10kb and Xco2= 0.2, (2) 580 and 560℃, 5kb and Xco2 = 0.4, (3) 520℃ 3kb and Xco2=0.4. All of the experiments was performed within the T-P-Xco2 stability field of the assemblage calcite-tremolite based on the thermodynamic phase equilibrium data, so that thedecomposition of tremolite was avoided under the experimental conditions. The isotopic equilibrium was approached from two directions by varying the isotopic compositions of starting materials. A data processing method for three——phase systemisotope exchange was applied to extrapolate partial equilibrium data to equilibrium values. The experimentally calibrated fractionations are in good agreement with the theoretical calculations. However, both theoretical and experimental data are significantly lower than the known empirical calibrations. This is consistent with oxygen diffusion data on amphiboles. Probably retrograde isotopic reequilibration may be common for amphibole relative to diopside in metamorphic rocks.