Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS-H2O-CO2 system

The type and kinetics of metamorphic CO2-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO2 cycle. This paper focuses on CO2 production by garnetforming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO-FeO-Al2O3-SiO2-CO2-H2O (CFAS-CO2-H2O) system are investigated using P-T phase diagrams at fixed fluid composition, isobaric T-X(CO2) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS-CO2-H2O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO2 source during prograde heating and/or early decompression. However, if the system remains closed, fluid-rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO2-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO2 cycle.