Zircon Saturation and Zr Diffusion in Rhyolitic Melts

Abstract:

Zircon is a ubiquitous accessory mineral in silicic igneous rocks. We have carried out new zircon dissolution experiments at 1270-1890 K and 0.5-1.5 GPa to refine our understanding of Zr diffusion and zircon solubility in several rhyolitic melts containing different H₂O contents. Zr diffusivity in rhyolitic melts depends strongly on temperature and H₂O content, and weakly on pressure and anhydrous melt composition. The diffusion data for each individual melts follows the Arrhenius relation. For melts with different composition including H₂O content, Zr diffusivity depends also on the Si+Al cation mole fraction in the anhydrous melts and can be expressed as follows:

lnD_Zr = –14.707 – [34089(Si+Al) – 1559w – 1726P]/T,

which can reproduce experimental data to within 0.72 lnD units (or a factor of 2 in D). Zr diffusion data for different rhyolitic melts in this study and in the literature are roughly expressed as a function of the Eyring diffusivity. This relation can predict Zr diffusivity to within about 1 lnD unit (or within a factor of 2.7 in D). Zr concentration at zircon saturation in silicic melts depends strongly on temperature, and weakly on pressure, anhydrous melt composition and H₂O content. Existing geothermometers based on zircon saturation has large uncertainties although our effort to improve it has not been successful. The dissolution or growth rate of a freely falling zircon crystal is modeled. The controlling factors are mostly the temperature and Zr concentration in the melt. Typical zircon growth rate in wet rhyolitic melt is 0.01 to 1.0 µm/yr. Zircon growth has the potential to serve as a geospeedometer.