Zircon Solubility, Metamict ZrSiO4 Replacement, and Hydrothermal Zircon Formation at Upper Crustal Pressures

Abstract

Stable and inert under most conditions, zircon can be dissolved and precipitated by aqueous fluids in the upper crust. Geochemical models using currently available thermodynamic properties for Zr aqueous species at 0.2 GPa predict that zircon solubility increases with temperature from 400 to 900°C in fluids saturated with quartz or baddeleyite. Zircon solubility is low in near-neutral pH fluids and enhanced in acidic and alkaline fluids. Adding NaOH and to a lesser extent NaF to the solution significantly increases the solution pH values and Zr concentrations at zircon saturation. Modeled Zr concentrations are often orders of magnitude different from zircon solubilities measured experimentally under similar conditions. Metamict (amorphous) ZrSiO₄ is more soluble than crystalline zircon and is replaced through a coupled dissolution-precipitation process. Reaction path kinetics models were constructed to simulate experiments described in the literature and extract rate constants for replacement of metamict ZrSiO₄. Replacement is rate limited by zircon precipitation and is nearly complete after 1 week when fluid is present at 600°C, with the rate of replacement increasing with temperature. In a closed system, hydrothermal zircon may form by replacement of radiation-damaged zircon but not fully crystalline zircon. Replacement of metamict ZrSiO₄ forms characteristic porosity. Geochemical models identify the conditions that promote zircon solubility, metamict ZrSiO₄ replacement, and the formation of hydrothermal zircon, and provide constraints on the interpretation of zircon U-Pb dates of hydrothermal events.