Theoretically, if a rock consists of n phases in equilibrium, it would have n-1 isotopic geothermometers that should all give the same T of equilibration. This relationship is shown in Fig. However, O-isotope geothermometry suffers from the ease with which some systems are reset by retrogressive processes, with the d 18 O values of minerals strongly influenced by the last fluid with which they have equilibrated. Moreover, exchange between many pairs has not been calibrated in the laboratory, and changes in the strength of some bonds as a result of some types of solid solution add a further complication, as more experiments are needed to cover larger compositional ranges. The tedious process of mineral separation has also limited the use of this method, although this is now changing with the advent of laser techniques.
Photosynthesis therefore results in the concentration of the light isotope in organic compounds.
Low values of d 13 C reflects some organic origin.