OXYGEN ISOTOPIC FRACTIONATION IN THE SYSTEM QUARTZ-ALBITE-ANORTHITE-WATER

Oxygen isotopic fractionations have been determined between quartz and water, albite and water, and anorthite and water at temperatures from 300 to 825°C, and pressures from 1.5. to 25 kbar. The equilibrium quartz-feldspar fractionation curves can be approximated by the following equations: 1000ln αQ-PI = (0.46 + 0.55β)106T-2 + (0.02 + 0.85β) between 500 and 800°C 1000ln αQ-PI = (0.79 + 0.90β)106T-2 - (0.43 - 0.30β) between 400 and 500°C where β is the mole-fraction of anorthite in plagioclase. Application of these isotopic thermometer calibrations to literature data on quartz and feldspar gives temperatures for some metamorphic rocks which are concordant with quartz-magnetite temperatures. Plutonic igneous rocks typically have quartz-feldspar fractionations which are substantially larger than the equilibrium values at solidus temperatures, indicating substantial retrograde exchange effects. © 1979.