The depletion of high field strength elements such as Zr, Nb and Ta is a characteristic feature of arc magmas and it has been attributed to a low solubility of these elements in slab-derived aqueous fluids. We have determined zircon solubility in aqueous fluids up to 1025 degrees C and 20 kbar by in situ observation of dissolving zircon grains in the hydrothermal diamond anvil cell.