Kinetics of zircon dissolution and zirconium diffusion in granitic melts of variable water content

Abstract

The experimental dissolution of zircon into a zircon-undersaturated felsic melt of variable water content at high pressure in the temperature range 1,020° to 1,500° C provides information related to 1) the solubility of zircon, 2) the diffusion kinetics of Zr in an obsidian melt, and 3) the rate of zircon dissolution. Zirconium concentration profiles observed by electron microprobe in the obsidian glass adjacent to a large, polished zircon face provide sufficient information to calculate model diffusion coefficients. Results of dissolution experiments conducted in the virtual absence of water (<0.2% H₂O) yield an activation energy (E) for Zr transport in a melt ofM=1.3 [whereM is the cation ratio (Na+K+2Ca)/(Al·Si)] of 97.7±2.8 kcal-mol⁻¹, and a frequency factor (D ₀) of 980 ^+1,390₋₅₈₀ cm²-sec⁻¹. Hydrothermal experiments provide an E=47.3±1.9 kcal-mol⁻¹ andD ₀=0.030 ^+0.030_−0.015 cm²-sec⁻¹. Both of these results plot close to a previously defined diffusion compensation line for cations in obsidian. The diffusivity of Zr at 1,200° C increases by a factor of 100 over the first 2% of water introduced into the melt, but subsequently rises by only a factor of five to an apparent plateau value of ∼2×10⁻⁹ cm²-sec⁻¹ by ∼6% total water content. The remarkable contrast between the wet and dry diffusivities, which limits the rate of zircon dissolution into granitic melt, indicates that a 50 μm diameter zircon crystal would dissolve in a 3 to 6% water-bearing melt at 750° C in about 100 years, but would require in excess of 200 Ma to dissolve in an equivalent dry system. From this calculation we conclude that zircon dissolution proceeds geologically instantaneously in an undersaturated, water-bearing granite. Estimates of zircon solubility in the obsidian melt in the temperature range of 1,020° C to 1,500° C confirm and extend an existing model of zircon solubility to these higher temperatures in hydrous melts. However, this model does not well describe zircon saturation behavior in systems with less than about 2% water.