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

[object Object]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.