Carbon retention during subduction of ophicarbonate rocks from the Zermatt-Saas unit (Western Alps)

Abstract Serpentinite-hosted carbonate rocks (i.e., ophicarbonates) are an important rock type for the deep C cycle because they can occur either in the slab or in the mantle wedge. We present a case study of ophicarbonate rocks from the Zermatt-Saas unit, Western Alps, that were subducted up to eclogite facies conditions at 2.5 GPa, 560 °C. In the study area, ophicarbonates overlie a large body of partially dehydrated serpentinites. This allows us to understand whether fluids released from the serpentinites infiltrated the ophicarbonates or not, and to what extent decarbonation reactions occurred in an open or closed system. We investigated three carbonate-bearing rock types: ophicarbonates, olivine-carbonate veins, and a talc-magnesite reaction rind at the contact between ultramafic and mafic/felsic lithologies. Our petrological and geochemical investigation, as well as thermodynamic modeling, reveal that the metamorphic evolution of the ophicarbonate was in a closed system, where calcite/aragonite was replaced by metamorphic dolomite and diopside, and that this reaction is nearly CO 2 conservative, with the released fluid composition close to pure water. Limited carbonate mobility is indicated by the occurrence of minor olivine-carbonate veins. In situ LA-ICP-MS trace element analysis shows that carbonate in veins is most likely sourced from the ophicarbonates suggesting CO 2 transport on the 10 m scale. The silicate-oxide-sulfide redox buffering assemblage indicates that both ophicarbonates and olivine-carbonate veins are equilibrated at redox conditions at or below FMQ. Field and thermodynamic modeling show that C-rich fluids circulated during exhumation along major structures and/or lithological interfaces. This leads to the formation of metasomatic talc-magnesite rocks during early exhumation between 9 and 13 kbar and 530–460 °C, at X CO2 between 0.007 and 0.009. Our study demonstrates that in the absence of external fluid infiltration, carbonates in ultramafic lithologies are stable at subduction zone conditions and can efficiently return C to the deep mantle.