Tracing magma water evolution by H2O-in-zircon: A case study in the Gangdese batholith in Tibet

The presence of water has significant influence on magma properties such as the chemistry, viscosity, and crystallization temperature. Water content in the magma can be modified by magma replenishment or crystal fractionation, but technique for direct tracing of these processes are lacking. In this study, we analyzed zircon water content, oxygen isotopes, and trace element contents for a co-genetic granitic suite (one primitive biotite granite and two evolved garnet-bearing two-mica granites) from the Gangdese batholith in Tibet. The biotite granite shows a relatively narrow H2O-in-zircon range with a sharp peak in the density plot, while multiple peaks are present for the garnet-bearing two-mica granites. The wide Hf range indicates that the zircon grains have a protracted magmatic history, and the positive H2O vs. Hf correlation indicates that H2O-in-zircon faithfully recorded the water variation of the evolving magma. These results suggest that H2O-in-zircon is a good indicator to trace magma fluid evolution.