EQUILIBRIUM MINERAL-FLUID STABLE-ISOTOPE FRACTIONATION FACTORS IN GRAPHITIC METAPELITES

The stable isotopic composition (C, O, H, and S) of a fluid in equilib rium with the common metapelite assemblage graphite + sulfide has been calculated using a modified Redlich-Kwong equation of state at 500 de grees C, 3.5 kbar, and 700 degrees C, 6 kbar, for a range of f(O2) and f(S2) conditions. The four major fluid species under these conditions are H2O, CO2, CH4, and H2S. Stable isotope fractionation factors amon g these fluid species remain significant to large, even at temperature s as high as 700 degrees C. At 500 degrees C, 3.5 kbar, Delta(18)O(qua rtz-Sigma fluid) ranges from -3.0 to +3.1 parts per thousand, Delta(13 )C(graphite-Sigma fluid) ranges from -10.1 to +4.3 parts per thousand, and Delta D-biotite-Sigma fluid (X(Fe) biotite = 0.25) ranges from -3 1 to +74 parts per thousand, depending on the values of f(O2) and f(S2 ). At 700 degrees C, 6 kbar, Delta(18)O(quartz-Sigma fluid) ranges fro m -1.7 to +1.6 parts per thousand, Delta(13)C(graphite-Sigma fluid) ra nges from -7.6 to +1.6 parts per thousand, and Delta D-biotite-Sigma f luid (X(Fe) biotite = 0.25) ranges from -17 to +164 parts per thousand , depending on the values of f(O2) and f(S2). Values of Delta(34)S(pyr ite-Sigma fluid) small and independent of f(O2) and f(S2).The results have implications for studies using stable isotopic methods to investi gate interaction between a fluid and a graphitic metapelite during met amorphism, especially during devolatilization and fluid-rock interacti on. Equilibrium values Of Delta(mineral-Sigma fluid) during metamorphi sm are dependent upon the composition of the fluid, which in turn is d ependent upon f(O2) and f(S2).