Oxidation of magmas during gain and loss of H₂O recorded by trace elements in zircon

Hydroxylation-oxidation reactions of the type H₂O + Fe→2+Fe³⁺ + OH⁻ + 1/2 H₂ oxidise Fe²⁺ in olivine and pyroxenes in oceanic lithosphere prior to subduction and in the supra-subduction-zone mantle wedge and oxidise Fe²⁺ in silicate melts as dissolved H₂O accumulates during magmatic differentiation. Collectively, the coupled hydroxylation-oxidation of melts and of minerals in melt sources tends to produce positive correlations of hydration state and oxidation state of silicate melts, about which there has been much dispute and discussion in literature. We introduce a melt “hygro-barometer” which provides evidence from trace elements in zircon that melt oxidation state rose as dissolved H₂O accumulated in fluid-undersaturated residual melts during mafic-to-felsic differentiation at Moho-vicinity depths. Zircon-based measures are consistent with experimental evidence of melt oxidation during gain of dissolved H₂O. Examples from many arc igneous complexes demonstrate further oxidation of residual melts as hydrothermal fluid exsolved and segregated during trans-crustal ascent of the magmas. Zircons in those suites show that much of the contrast between redox states of dry mid-ocean-ridge and hydrous arc magmas (especially porphyry copper magmas) develops as H₂O is lost during trans-crustal ascent of arc magmas because decompression-induced exsolution and open-system fractional segregation of hydrothermal fluid selectively extracts reduced members of the principal redox couples (H₂–H₂O, S⁴⁺–S⁶⁺, Fe²⁺–Fe³⁺), thereby raising the oxidation state of residual melt.