Zircon Solubility in Solute‐Rich Supercritical Fluids and Zr Transfer From Slab to Wedge in the Deep Subduction Process

Abstract

AbstractZircon solubility in aqueous fluids, hydrous melts, and supercritical fluids is important for understanding the high field strength elements (HFSEs) chemical transport in subduction zones. Although zircon solubility was extensively studied in aqueous fluids and hydrous silicate melts, its solubility in solute‐rich fluids or supercritical fluids is poorly known. Here, we experimentally determined zircon solubility in KAlSi3O8 (±K2O ± Al2O3) – H2O supercritical fluids at 2.0–6.0 GPa and 800°C–1000°C, close to the slab‐top conditions at sub‐arc depths. The results show that zircon solubility (expressed as ZrO2 content at zircon saturation) ranges from 65 to 6,400 ppm ZrO2, 10–100 times higher than that in dilute aqueous fluids; it increases with temperature, solute content, and solute alkalinity (molar K/Al ratio) but decreases with pressure. The experiments at 2.0 GPa show that solute alkalinity in addition to temperature and solute content exerts a primary control on zircon solubility, while the experiments at 4.0–6.0 GPa show that the negative effect of pressure on zircon solubility is offset by the increase in solute alkalinity due to the crystallization of Al‐rich phases kyanite and muscovite. We suggest that high‐alkali supercritical fluids during deep subduction could be significant transfer agents for Zr from slab to mantle wedge.