The pressure dependence of the zirconium‐in‐rutile thermometer

Abstract

The solubility of ZrO 2 in rutile is strongly temperature‐dependent and has been identified as a potentially powerful thermometer when the rutile coexists with an appropriate buffer assemblage, e.g. zircon + quartz. In combination with experimental data at 10 kbar, previous consideration of data on natural rutile has not identified a pressure dependence for the thermometer. However, the expected volume change as a result of substitution of the larger Zr 4+ cation for Ti 4+ suggests that the Zr content of rutile should decrease with increasing pressure. To investigate the pressure dependence of the thermometer, piston cylinder (at 10, 20 & 30 kbar) and 1 atm furnace experiments were performed in the system ZrO 2‐TiO 2‐SiO 2. The solubility of ZrO 2 in rutile, in the presence of zircon and quartz was reversed at each pressure value. From these experiments, the thermodynamics of the end‐member reaction ZrSiO 4 = SiO 2 + ZrO 2 (in rutile) have been determined. There is a secondary pressure effect accompanying the primary temperature dependence of the Zr content of rutile. New thermometer equations are, in the α‐quartz field: image in the β‐quartz field image and in the coesite field image in which φ is ppm Zr, P is in kbar and R is the gas constant, 0.0083144 kJ K−1. Thermometric results using these equations are shown for a range of geological settings.