Zircon solubility in aqueous fluids at high temperatures and pressures

Abstract

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°C and 20kbar by in situ observation of dissolving zircon grains in the hydrothermal diamond anvil cell. Zircon solubilities in H2O with silica activity buffered by quartz are very low, from 1.0 to 3.3ppm Zr, and weakly increase with temperature and pressure. Experimental results were fitted to the following fluid density model:logc(±0.10)=(3.45±0.92)-(3803±1098)T-1+(1.52±0.63)logρwhere c is the Zr concentration in the fluid (ppm by weight), T is temperature (K) and ρ is the fluid density (gcm−3). An additional experiment with a saline fluid (15wt.% NaCl) revealed an increase in zircon solubility by a factor of 3 (4.8±1.6ppm Zr at 890°C and 14kbar) whereas addition of 4.5wt.% albite as solute increased solubility by about a factor of 5. The Zr solubility at the forsterite–enstatite silica buffer appears to be slightly higher than that at the quartz buffer and it further increases at baddeleyite saturation (48±15ppm Zr at 930°C and 16kbar). These observations are consistent with the stability of zircon relative to ZrO2+SiO2 and suggest that Zr–Si complexes are not abundant in the fluid. During slab dehydration, the Zr content in the aqueous fluid is predicted to be 1–4ppm. Mass balance calculations imply that the high field strength element concentrations in primary arc melts will slightly decrease due to the dilution effect of infiltrating fluid. By contrast, mobile lithophile elements are predicted to increase their abundances in the melt by orders of magnitude. Our results suggest that the high abundance of large ion lithophile elements relative to high field strength elements in arc magmas is related to different solubilities of these elements in aqueous fluids migrating from the slab to the magma source regions.