Ultra-oxidized rocks in subduction mélanges? Decoupling between oxygen fugacity and oxygen availability in a Mn-rich metasomatic environment

The manganese ore of Praborna (ItalianWestern Alps) is embeddedwithin a metasedimentary sequence belonging to a subduction mélange equilibrated at high-pressure (HP) conditions (ca. 2 GPa) during the Alpine orogenesis. The pervasive veining of the ore and the growth of “pegmatoid” HP minerals suggest that these Mn-rich rocks strongly interacted with slab-derived fluids during HP metamorphism. These rocks are in textural and chemical equilibriumwith the veins and in contactwith sulphide- andmagnetite-bearing metabasites at the bottom of the sequence. They contain braunite (Mn2+Mn3+ 6SiO12), quartz, pyroxmangite (Mn2+SiO3), and minor hematite, omphacite, piemontite and spessartine-rich garnet. Sulphides are absent in theMn-rich rocks,whereas sulphates (barite, celestine) occur togetherwith As- and Sb-oxides and silicates. This rock association provides an excellent natural laboratory to constrain the redox conditions in subducting oceanic slab mélanges at HP and fluid-present conditions. Similarly to Fe-bearing minerals, Mn oxides and silicates can be regarded as natural redox-sensors. A thermodynamic dataset for these Mn-bearing minerals is built, using literature data as well as newthermal expansion parameters for braunite aud pyrolusite, derived fromexperiments. Based on this dataset and the observed assemblages at Praborna, thermodynamic calculations show that these mélange rocks are characterised by ultra-oxidized conditions (ΔFMQ up to +12.7) if the chemical potential of oxygen (or the oxygen fugacity fO2) is accounted for. On the other hand, if the molar quantity of oxygen is used as the independent state variable to quantify the bulk oxidation state, the ore appears onlymoderately oxidized and comparable to typical subduction-slab mafic eclogites. Such an apparent contradiction may happen in rock systemswhenever oxygen is improperly considered as a perfectly mobile component. In the Earth's mantle, redox reactions take place mainly between solid oxides and silicates, because O2 is a negligible species in the fluid phase. Therefore, the description of the redox conditions of most petrological systems requires the introduction of an extensive variable, namely the oxygen molar quantity (nO2). As a consequence, the oxygen chemical potential, and thus fO2, becomes a dependent state variable, not univocally indicative of the redox conditions of the entire rock column of a subduction zone, from the dehydrating oceanic crust to the overlying mantle wedge. On a more general basis, the comparison of fO2 retrieved fromdifferent bulk compositions and different phase assemblages is sometimes challenging and should be undertakenwith care. From the study of mélange rocks at Praborna, the distribution of oxygen at subduction zones could be modelled as an oxidation gradient, grading froma maximum in the subducted altered oceanic crust to a minimum in the overlying peridotites of the mantle hanging-wall.