Effect of water on δ18O in zircon

Abstract

The mineral zircon is stable across a wide range of temperature and pressure conditions, and the most widely used geochronometer in felsic rocks. The oxygen isotope ratio recorded by zircon crystals is a sensitive tracer for crustal recycling in magmatic rocks and is a commonly used tool to address zircon petrogenetic questions (e.g. in studies on the evolution of geodynamics and continents). However, secondary processes (i.e. water uptake facilitated by radiation damage) can alter the oxygen isotopic composition of zircon. We present in-situ oxygen isotopic and 16O1H/16O data of natural zircon samples in combination with zircon trace element concentrations, and parameters that quantify crystal damage (i.e. Raman spectral parameters and alpha dose). In agreement with previous studies, our results demonstrate that the uptake of water into the zircon crystal structure strongly influences its recorded oxygen isotope ratio. Two distinct secondary processes are recorded which shift zircon δ18O values towards isotopically heavier and lighter values, respectively. Lower δ18O values associated with broadening of the zircon Raman bands and increasing 16O1H/16O are the result of a local charge-balance process during the interaction of radiation-damaged zircon with meteoric water. In contrast, an equilibrium fractionation process between zircon and percolating water leads to higher δ18O values with increasing 16O1H/16O. The disturbance of the oxygen isotopic system in zircon can be assessed through monitoring zircon 16O1H/16O; a parameter that is highly sensitive to the influx of water into radiation-damaged areas of the zircon grain, providing a powerful tool to access the validity of primary δ18O signatures.