THERMOELASTIC PROPERTIES OF ZIRCON

Zircon (ZrSiO₄) is an important and widespread mineral in the Earth’s crust and upper mantle and is commonly found trapped as inclusions in host minerals such as garnet. With a reliable equation of state (EoS), zircon inclusions have the potential to be used in piezobarometry, in which the residual stress or pressure in the inclusions, arising from the contrast in the elastic properties of the host garnet and zircon, can be used to infer entrapment conditions. In this study, we have determined a thermal-pressure equation of state for zircon that characterizes its thermoelastic behavior at metamorphic conditions. New pressure-volume (P-V) data from a “Mud-Tank” zircon (donated by John W. Valley, Univ. of Wisconsin) have been collected from 1 bar to 8.47(1) GPa using single crystal X-ray diffraction, and elastic moduli were measured from room temperature up to 1172 K by resonance ultrasound spectroscopy (RUS). These data were fitted simultaneously with temperature-volume (T-V) data from the literature in EoSFit7c using a new scaling technique. The parameters of a third order Birch-Murnaghan EoS combined with a Mie-Grüneisen-Debye model for thermal pressure results in compressional EoS parameters: K_0T = 224.5(1.2) GPa, K_0T' = 4.90(31) with a fixed initial unit-cell volume V₀ = 39.26 cm³/mol and thermal parameters: γ₀ = 0.868(15), q = 2.37(80), and θ_D = 848(38) K. This new EoS confirms that zircons are stiffer than garnets but exhibit a much lower thermal expansion. This results in steep isomekes between zircon and garnets, which makes zircon trapped as inclusions in garnets at metamorphic conditions a good piezothermometer.

This work was supported by NERC grants (NE/B505738/1, NE/F17081/1) and an EPSRC grant (EP/I036079/1) to M.A. Carpenter, the ERC under the European Union’s Horizon 2020 program TRUE DEPTHS (714936) to Matteo Alvaro, and the NSF MRI (1726077) to N.L. Ross.