Transcrustal, volatile-charged silicic melts revealed by zircon-hosted melt inclusions

The volatile contents of silicic crustal magmas inform models for volcanism, crustal growth, degassing behaviour, and hydrothermal ore formation. Volatile saturation pressures of phenocryst-hosted melt inclusions are generally restricted to late-stage, shallow magmas that have undergone significant, ascent-driven degassing of H₂O, CO₂ and SO₂. As an alternative approach, we analyse the volatile contents of rhyolitic melt inclusions (10–30 µm diameter) in the accessory mineral zircon. We have developed a technique for the reheating of microcrystalline melt inclusions at elevated temperature (950 °C) and pressure (100 MPa) prior to analysis, to form homogeneous glassy inclusions without fracturing the host zircon. Analyses of volatiles were performed using secondary ion mass spectrometry and calculated volatile saturation pressures were used to obtain minimum melt inclusion trapping depths. Our results reveal that zircons often grow over an exceptional crustal depth range; many zircons trap melts deeper than 20 km. Significantly, the deepest melt inclusions from porphyry copper deposit-related magmas have high CO₂ contents (up to 4000 ppm), indicating CO₂-rich sources (fluid molar fraction of CO₂ up to 0.95). Zircon crystallisation and the trapping of melt inclusions testify to silicic melt generation over the entire crustal depth range. Deep crustal zircons can be transported to shallow levels by silicic melts percolating through transcrustal mush systems and/or in rapidly ascending volatile-charged magmas.