The pressure dependence of the zirconium‐in‐rutile thermometer

TLDR

The solubility of ZrO₂ in rutile is strongly temperature‑dependent and has been proposed as a thermometer, but while 10 kbar data showed no pressure effect, the larger Zr⁴⁺ ion suggests its rutile content should decline with increasing pressure. The authors sought to determine whether the ZrO₂‑in‑rutile thermometer exhibits a pressure dependence. They performed piston‑cylinder experiments at 10, 20 and 30 kbar and 1 atm furnace runs on the ZrO₂–TiO₂–SiO₂ system to assess this. They found that ZrO₂ solubility in rutile reverses at each pressure, quantified the thermodynamics of the ZrSiO₄ = SiO₂ + ZrO₂ reaction, identified a secondary pressure effect on Zr content, derived new thermometer equations for the α‑quartz, β‑quartz and coesite fields, and applied these equations to various geological settings.

Abstract

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.