FRACTIONATION OF THE STABLE ISOTOPES OF OXYGEN AND CARBON IN CARBON-DIOXIDE DURING THE REACTION OF CALCITE WITH PHOSPHORIC-ACID AS A FUNCTION OF TEMPERATURE AND TECHNIQUE

The fractionation factor (alpha) between the O-18/O-16 ratio in calcite and CO2 gas produced as a result of reaction with phosphoric acid has been determined as a function of temperature for two of the most widely used variations of the dissolution technique, reaction in a sealed vessel and reaction in an acid bath. Although these methods show approximately the same relationship between temperature and alpha, they are offset from one another by between 0.2 and 0.4 parts per thousand over the temperature range 25-90-degrees-C. There appears to be no temperature- or technique-dependent fractionation on carbon over the range of temperatures examined in this study. The difference between these two methods arises from three main processes: (1) isotopic exchange between the CO2, the phosphate polymers in the acid and water produced in the reaction; (2) dissolution of CO2 in the acid; and (3) incomplete removal of CO2 during the extraction process. Of these two methods the "acid bath" method probably provides the most accurate isotopic composition of the calcite. As a result of the fact the original fractionation factors were determined using the "sealed vessel" method, we have calculated our temperature [T(K)] - dependent fractionation factor (alpha-T) taking into account the difference between the two methods at 25-degrees-C. For the "sealed vessel" method: (a) alpha-T = [5.60 (+/- 0.33) X 10(2)]T-2+1.003943 (+/- 0.000067). For the "common acid bath" technique: (b) alpha-T = [5.25 (+/- 0.08) X 10(2)]T-2+1.003970 (+/- 0.000034).