Redox processes at subduction zones: the role of intensive and extensive variables during fluid/rock interaction

The slab-mantle interface is a key location where redox reactions govern the mobility of slab-derived elements transported by fluids. At pressures up to 3 GPa the slab-mantle interface is composed by the mixing of slab and suprasubduction mantle slices in a metasedimentary or ultramafic matrix to form mélange zones. This geodynamic environment is characterised by high fluid fluxes that allow chemical exchange within the mélange materials forming hydrated and low-viscosity layers atop the subducting plate. At higher pressures and warm subduction zones, the slab produces silicate supercritical liquids from dehydration or melting of the subducted oceanic or continental crust. The element mobility occurs through porous flow or focused network of veins that minimize the effective fluid/rock ratio inducing almost instantaneous reactions. This means that the redox reactions are rock-controlled. The oxidising or reducing capacity of a rock is determined by the amount and by the oxidation state of redox-sensitive major elements and by the composition of solid solutions of mineral assemblages of the rock. Therefore, oxygen fugacity is likely very inhomogeneous in a subducting slab, reflecting the different bulk chemical–mineralogical compositions of the slab lithologies. We can therefore say that it is more likely that oxygen fugacity is a variable that is governed by the mineral assemblages in the rock, rather than one that is imposed from the environment. The question arises whether an intensive variable such as oxygen fugacity is a dependent or independent variable in these rock systems. At high fluid-rock ratios oxygen is probably transported along fractures and veins, possibly through mechanisms of dissolution– reprecipitation of O-enriched oxides and silicates, or by advective processes. At these conditions the redox reactions are fluid-buffered, and oxygen can be considered as a perfectly mobile component. In other words, is the redox condition of the fluid that controls the mineral associations of the hosting rocks.