1999 Volume 108 Issue 2 Pages 158-176
Isotopic geothermometry is an important tool to clarify the thermal history of the metamorphic events because of independence of pressure. When applying the thermometer, it is necessary that the temperature dependence of the fractionation factors among the rock forming minerals be well calibrated.
Carbon isotopic geothermometry between calcite and graphite is the most suitable thermometer to record peak metamorphic temperatures in crystalline limestone, because of the inert property of graphite in an isotopic exchange reaction during retrograde metamorphism. Various authors and approaches have reported fractionation factors in the calcitegraphite system. There is, however, still inconsistency among experimental, theoretical, and empirical results. In particular, the slope of the fractionation curve for temperature dependence still has a discrepancy between theoretical-experimental and empirical data. The causes of this discrepancy may be due to uncertainty in individual approaches, e.g., in experimental studies, uncertainty as to the reaction mechanism due to the inert property of crystalline graphite for isotopic exchange experiments ; in theoretical studies, uncertainty caused by a deficiency of reliable data on the vibrational frequency of calcite and graphite crystals; and, in empirical studies, incomplete crystallinity of carbonaceous matter.
According to empirical data on carbon isotopic systematics, in crystalline limestone that metamorphosed at temperatures higher than 400°C, isotopic equilibria have been mostly attained in many cases. At temperatures lower than 400°C, data sets show an increased disequilibrium. Because the carbon isotopic fractionation between calcite-graphite is more than 3‰ at about 700°C, this thermometer has great potential as a tool for determining ultra-high temperature metamorphism. Furthermore, because of the inert property of graphite in an isotopic exchange reaction during retrograde metamorphism, graphite-calcite assemblage is the most probable candidate for preserving the highest temperature condition. Again, carbon isotopic zonation in graphite crystal could provide another tool to clarify the thermal history of polymetamorphic terrains.
Oxygen isotopic thermometry was long been widely used to determine the temperatures of metamorphic rocks. It has long been recognized that oxygen isotope thermometers often record discordant temperatures during slowly cooling retrograde metamorphism, because of a relatively rapid diffusion rate during retrograde metamorphism. Revised correction techniques for re-equilibration during a retrograde isotopic exchange reaction under the such high temperatures help in using oxygen isotopic thermometry in deducing thermal history.