Zircon solubility in alkaline aqueous fluids at upper crustal conditions

Field evidence and thermodynamic data at ambient conditions suggest that complexing of Zr with hydroxyl ions at high pH enhances the solubility of zircon. We tested this hypothesis by measuring the solubilities of the assemblages zircon (ZrSiO4) + baddeleyite (Z + B) and zircon + quartz (Z + Q) in neutral to alkaline fluids at 0.2 GPa and 450-750 degrees C. In neutral to alkaline fluids (0, 0.1, 1m NaOH) zircon dissolved incongruently in quartz-undersaturated fluids to form baddeleyite. At 450 degrees C various Na-Zr-silicates precipitated from fluids saturated and undersaturated in quartz. We observed no significant dependence of solubility on temperature. The measured solubilities of zircon in pure H2O at 600 and 750 degrees C are only slightly higher than the procedural blank of 3.3 x 10(-7)m Zr. However, the measured solubility of zircon increases with increasing aqueous silica, suggesting that zirconium complexes with silica in the fluid. At 600 degrees C linear regression of experimental results yielded: ln (Zr) = 0.25 + 2.9 (*) ln (Si) where terms in parentheses represent molal concentrations. Zircon solubility also seems to increase with increasing hydroxyl concentration in the fluid. Enhanced solubility in silica-rich, alkaline fluids may cause dissolution-reprecipitation of zircon and resetting of its isotopic clocks, suggesting that some zircon U-Pb ages may correspond to fluid events. (C) 2012 Elsevier Ltd. All rights reserved.