Equilibrium Isotope Fractionation Factors Research Articles

The use of heat capacity data to calculate carbon isotope fractionation between graphite, diamond, and carbon dioxide: a new approach

A new approach for calculating the β-factors of elements (substances consisting of single-element atoms) from calorimetric data on the specific heat capacities has been established. The approach is based on first order thermodynamic perturbation theory and harmonic approximation. It has been applied to calculate the β-factors of graphite and diamond. The polynomial expansions for them have been evaluated: ▪ A comparison with values of the β-factors obtained by Bottinga (1969b) has shown a disagreement that seems to be essential for carbon isotope geothermometry. The carbon dioxide β-factors have been calculated in the harmonic approximation from spectroscopic data on fundamental frequencies. The method of calculation is based on the group theory technique. The calculated equilibrium isotope fractionation factors between carbon dioxide and graphite are significantly lower than those obtained from partial exchange experiments by Scheele and Hoefs (1992). Combining the Chacko (1991) data on the calcite β-factors with the present data on the β-factors of graphite, the isotope equilibrium fractionation curve between calcite and graphite has been evaluated. This curve shows somewhat better coincidence with the Valley and O'Neil (1981) empirical fractionation curve than the curve obtained by Chacko (1991). The excellent agreement between the theoretical and the empirical calibrated isotope fractionation curves has been achieved by taking into account a pressure effect on the calcite-graphite equilibrium isotope fractionation factor. Nevertheless, significant disagreement with appropriate data of Scheele and Hoefs (1992) is observed.

Biogeochemical implications of the isotopic equilibrium fractionation factor between the oxygen atoms of acetone and water

Carbonyl oxygens of organic molecules undergo isotopic exchange with water during reversible hydration reactions. The equilibrium isotopic fractionation factors between the carbonyl oxygen of acetone and water at 15°, 25°, and 35°C are 1.028, 1.028, and 1.026 respectively. The differences between the δ 18 O values of the carbonyl oxygen of acetone and of the water with which it is in equilibrium are similar to the differences that have been observed between the δ 18 O values of cellulose and the water used in its synthesis by a variety of aquatic plants and animals. Additionally, the identity of the acetone-water fractionation factors at 15° and 25°C parallels the observation that the difference between the δ 18 O values of cellulose and water shows no temperature dependence for individual species of plants grown over the same temperature range. These results are discussed in relation to the proposal that the oxygen isotopic relationship between cellulose and water is established by isotopic exchange occurring during the hydration of carbonyl groups of the intermediates of cellulose synthesis.

Low temperature $$\lg f_{O_2 } - pH$$ diagrams of sulfur isotope evolution in an equilibrium hydrothermal system

According to Sakai-Ohmoto's theory regarding the evolution of sulfur isotopes in hydrothermal systems, in conjunction of new data on chemical resaction equilibrium constants and equilibrium isotopic fractionation factors as well as on individual ion activity coefficients of aqueous sulfur species, the following lgfo2.-pH diagrams are constructed: a) mole fractions of aqueous sulfur species (X i ), b) stability fields of some minerals in the Fe-S-O system, c) diagram depicting the oxidation-reduction-state ratio for aqueous sulfur species (R′) d) isotopic compositions of sulfur compounds (\(\delta S_1 ^{34} \)).