The redox budget of crust-derived fluid phases at the slab-mantle interface

Forward modelling fO2 calculations indicate that the garnet orthopyroxenites record ΔFMQ = −1.8 ÷ −1.5, therefore resulting apparently more reduced with respect to metasomatised supra-subduction garnet-peridotites. On the other hand, oxygen mass balance, performed both on the Maowu hybrid orthopyroxenite and on metasomatised supra-subduction garnet peridotites, indicate that the excess of oxygen (nO2) is the same (10 mol m−3). The oxygen mass balance of the crust-derived fluids (multiphase inclusions) also indicates that the fluid precipitates are more oxidised than the host rock, reaching up to 400 mol m−3 of nO2. This suggests that even after their interaction with the metasomatic orthopyroxenites, the residual fluid phases could be potentially carrier of oxidised components when it escapes the slab-mantle interface. Because of this gradient in nO2, a metasomatic front develops from the oxidised slab to the overlying lithospheric mantle wedge passing through a transitional layer of hybrid rocks at the slab-mantle interface.