The origin and role of fluid associated with lower crustal metamorphism

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Fluid in lower crust probably plays an important role on the stability of mineral assemblages in various metamorphic and igneous rocks at high pressure and temperature. Although it is generally known that CO_2 is an abundant fluid component in middle to lower crust, while H_2O is dominant in shallower level, the origin of the fluid and its role during P-T evolution of crustal rocks are not known. In this study we therefore focused on fluid inclusions in granulite to amphibolite facies rocks, identified fluid specimens, and attempted to unravel the role of fluid during high-grade metamorphism. All the analyzed rocks contain CO_2 trapped in various minerals such as garnet and quartz, which confirms thatCO_2 is the dominant fluid in the lower crust. Carbon isotope study on some inclusions suggest that some CO_2 were derived from mantle. H2O bearing fluids were also identified, but they are mostly trapped in a retrograde stage. Phase equilibrium modeling of mineral assemblages on orthogniess from southern India suggest that infiltration of H_2O-bearing reduced fluid (probably H_2O + CH_4) during a retrograde (around 700℃) formed orthopyroxene-bearing granulite-facies assemblages, which indicates oxidation state, as well as H_2O activity, of fluid probably controls mineral paragenesis during high-grade metamorphism in lower crust.