Oxidation and hydration states of porphyry-copper ore-forming magmas recorded by trace elements in zircon

Trace elements in magmatic zircon preserve a record of the hydration state as well as oxidation state of the ore- forming silicate melts. The zircon trace-element ratio (Eu/ Eu*)/Yb-N is formulated as an indicator of silicate melt hydration state and is empirically validated by comparison with hygrometry and geobarometry on coexisting hornblende phenocrysts. Zircon records tandem rise of oxidation state and hydration state of arc magmas during fluid-undersaturated magmatic differentiation at lower-crustal depths. Zircon records dehydration and further oxidation of ascending silicate melts as exsolving hydrothermal fluid segregates at lower-to upper-crustal depths. Porphyry-copper ore-forming melts have distinctive values and trends on zircon oxybarometer vs hygrometer plots. Our zircon oxybarometer and hygrometer are applicable to detrital zircons, permitting identification of watersheds that contain a copper-fertile igneous complex ( exploration target) upstream from the sediment sampling site.