PRESSURE EFFECTS ON THE ISOTOPIC PROPERTY OF WATER

A quantitative understanding of the isotopic properties of water and other fluids are of great importance to the interpretation of isotopic behavior in crustal systems at elevated pressures and temperatures. Currently, conventional, statistical-mechanical calculations of molecules provide information on isolated water molecules, i.e. ideal-gas at infinitely low pressures. Furthermore, only very simplistic models are available in the literature for calculating the pressure effect on the reduced isotope partition function ratio (RIPFR) of waters at elevated pressures. We have developed a simple, yet accurate theoretical method for calculating the RIPFR of water at elevated pressures. This approach requires only accurate equations of state (EOS) for pure isotopic end-members (H₂O and D₂O), which are available in the literature. The effect of pressure or density on the RIPFR of water was calculated relative to that of ideal-gas water at infinitely low pressure for the temperature range, 0 to 527°C. For gaseous and low-pressure (ca. <15 MPa) supercritical phases of water, the RIPFR increases slightly (1 – 1.3 ‰) with pressure or density in a fashion similar to those of many other geologic materials. However, in liquid and high-pressure (>20 MPa) supercritical phases, the RIPFR of water decreases (2 – 5 ‰) with increasing pressure (or density) to 100 MPa. This rather unique phenomenon is ascribed to the inverse molar volume isotope effects (MVIE) of water, V(D₂O)>V(H₂O), while other substances show the normal MVIE, including minerals. For example, at 500°C we predict that D/H fractionation between a hydrous mineral (or other H-bearing substances) and water decreases by as much as 6‰ between 20 MPa and 100 MPa. These theoretical predictions were experimentally confirmed by Horita et al. (2002) for the system brucite – water. Although the P – T ranges for the EOS of normal and heavy waters are rather limited, our modeling indicates that the RIPFR of water continues to decrease with pressure above 100 MPa. These results have important implications for the interpretation of isotopic partitioning at high-pressures. Research sponsored by Division of Chemical Sciences, Geosciences, and Biosciences, U.S. DOE under contract DE-AC05-00OR22725, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.