EQUILIBRIUM MINERAL-FLUID STABLE ISOTOPE FRACTIONATION FACTORS IN GRAPHITIC METAPELITES

Abstract

The stable isotopic composition (C, O, H, and S) of a fluid in equilibrium with the common metapelite assemblage graphite + sulfide has been calculated using a modified Redlich-Kwong equation of state at 500°3.5 kbar, and 700°C, 6 kbar, for a range of fO2 and fS2 conditions. The four major fluid species under these conditions are H2O, CO2, CH4 and H2S. Stable isotope fractionation factors among these fluid species remain significant to large, even at temperatures as high as 700°C. At 500°C, 3.5 kbar, Δ18Oquartz-Σfluid ranges from −3.0 to + 3.1‰, Δ13Cgraphite-Σfluid ranges from −10.1 to +4.3‰, and ΔDbiotite-Σfluid (XF, biotite = 0.25) ranges from −31 to + 74‰, depending on the values of fo2 and fS2. At 700°C, 6 kbar, Δ18Oquartz-Σfluid ranges from − 1.7 to + 1.6‰, Δ13Cgaph;te-Σfluid ranges from -7.6 to +1.6‰, and Δ34Sbiotite-Σfluid (XFe biotite = 0.25) ranges from −17 to + 164‰, depending on the values of fo2 and fs2. Values of Δj34Spyrite−Σfluid are small and independent of fO2 and fS2.The results have implications for studies using stable isotopic methods to investigate interaction between a fluid and a graphitic metapelite during metamorphism, especially during devolatilization and fluid-rock interaction. Equilibrium values of Δmineral-Σfluid during metamorphism are dependent upon the composition of the fluid, which in turn is dependent upon fo2 and fs2