STABLE-ISOTOPE FRACTIONATION BETWEEN CARBON-DIOXIDE AND CALCITE AT 900-DEGREES-C

Equilibrium stable isotope fractionation factors between carbon dioxid e and calcite were determined by direct equilibration of the two phase s at 900 degrees C and 12.5 kbar. The results of six three-day runs ga ve Delta(13)C(CO2-calcite) and Delta(18)O(CO2-calcite) fractionations of +2.70 +/- 0.36 parts per thousand and +3.30 +/- 0.16 parts per thou sand, respectively (2 sigma). Runs of 22.5, 45, and 90 min; and 3, 6, 12, and 24 h duration were also conducted. Rapid initial recrystalliza tion accompanied by extensive carbon and oxygen isotope exchange (grea ter than or similar to 40%) and a very high oxygen/carbon exchange rat e ratio (similar to 5) was seen in runs using both natural and synthet ic calcite. Subsequent exchange slowed drastically, and the rate of ox ygen exchange decreased relative to that of carbon to a constant value of similar to 3, implicating the involvement of carbonate ion migrati on in the later, slower stages of exchange. The lower limits of bulk c arbon and oxygen diffusion coefficients determined from the partial ex change experiments were 4 X 10(-13) and 9 x 10(-13) (cm(2)/s), respect ively.