Some important Ge isotope equilibrium fractionation coefficients in organic-rich fluids

Germanium(Ge) isotope has a great potential that has not been well recognized in the field of geochemistry.Our ability to explore the unknown world for Ge isotope, however, has been severely limited by the lack of the basic isotope fractionation parameters.In this study, we provided some important Ge isotope equilibrium fractionation coefficients between the dominant Ge(OH)4, GeO(OH)-3 species in solutions and several Ge-bearing organic complexes(including Ge-catechol, Ge-oxalic acid, and Ge-citric acid) in organic-rich fluids, based on the Urey model(or Bigeleisen and Mayer equation) and the quantum chemistry calculations.The calculations were made using Gaussian03 at the B3LYP/6-311+ G(d,p) level.Solvent effect was modeled using the" water droplet"method.The accuracy of the parameters are estimated to be around 0.2‰ .The △Ge(OH)4-GeO(OH)-3 is 0.6‰ in pure water, and is slightly lower(about 0.3‰ ) in sea water.In comparison, the △Ge(OH)4-Ge-catechol,,△Ge(OH)4-Ge-oxalic acid,△Ge(OH)4-Ge-citric acid(c) and △Ge(OH)4-Ge-citric acid(d) are much larger, around 4.4‰ , 3.5‰ , 3.7‰ and 3.9‰ , respectively.The large fractionations might suggest a new way to distinguish the possible bio-interference process.Furthermore, our calculations indicated that light Ge isotopes should be enriched in organic-rich environments like the coal-bearing series and blank shale series formed under anoxic conditions, therefore such environments might represent a reservoir of light Ge isotopes.