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Published November 1, 2012 | public
Journal Article

Zircon solubility in alkaline aqueous fluids at upper crustal conditions

Abstract

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 (ZrSiO_4) + baddeleyite (Z + B) and zircon + quartz (Z + Q) in neutral to alkaline fluids at 0.2 GPa and 450–750 °C. In neutral to alkaline fluids (0, 0.1, 1m NaOH) zircon dissolved incongruently in quartz-undersaturated fluids to form baddeleyite. At 450 °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 H_2O at 600 and 750 °C are only slightly higher than the procedural blank of 3.3 × 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 °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.

Additional Information

© 2012 Elsevier Ltd. Received 22 November 2011; accepted in revised form 20 August 2012; available online 28 August 2012. Associate editor: Steven B. Shirey. This material is based upon work supported by the National Science Foundation under Grant Nos. EAR-0510092 and EAR-0838391 to Ayers. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Thanks to Rossane Delapp for help with ICP-MS analyses and Alan Wiseman for help with the cold seal pressure vessel systems. Reviews by Craig Manning, two anonymous reviewers, and associate editor Steven B. Shirey helped to make the manuscript more focused.

Additional details

Created:
August 19, 2023
Modified:
October 20, 2023