Theoretical prediction for several important equilibrium Ge isotope fractionation factors and geological implications

摘要

This study estimates equilibrium fractionation factors in the Ge isotope system, including the dominant aqueous Ge(OH)(4) and GeO(OH)(3)(-) species in seawater, Ge-bearing organic complexes (e.g. Ge-catechol, Ge-oxalic acid and Ge-citric acid), and Ge in quartz- (or opal-), albite-, K-feldspar-, olivine- and sphalerite-like structures. Estimations are based on Urey model (or Bigeleisen-Mayer equation) and high level quantum chemistry calculations. All calculations are made at B3LYP/6-311 + G(d,p) theory level. Solvation effects are treated by explicit solvent model ("water-droplet" method), and mineral structures are simulated using cluster models, in which the clusters are cut from the X-ray structures of those minerals. In addition, a number of different conformers are used for aqueous complexes in order to reduce the possible errors coming from the differences of configurations in solution. The "salt effect" on GeO(OH)(3(aq))(-) species is also carefully evaluated. We estimate the accuracy of these fractionation calculations at about +/- 0.3 parts per thousand. Excitedly, very large isotope fractionations are found between many Ge isotope systems. The Ge-containing sulfides (e.g. sphalerite) can extremely enrich light Ge isotopes (more than 10 parts per thousand) compared with 4-coordinated Ge-O compounds (e.g. Ge(OH)(4(aq)) or quartz). The fractionations between Ge(OH)(4(aq)) and 6-coordinated Ge-bearing organic complexes can be also up to 4 parts per thousand at 25 degrees C. These results give a good explanation for the experimental observations of Rouxel et al. (2006). It also suggests a great potential for broad application of Ge isotope method in various geological systems.