Mercury isotope fractionation in fossil hydrothermal systems

The Hg isotopic compositions of samples throughout the vertical extent of two fossil hydrothermal systems were analyzed by multicollector inductively coupled plasma-mass spectrometry. Results show > 5 parts per thousand (delta Hg-202/Hg-198; relative to NIST 3133) fractionation, more than 50 times greater than the 0.1 parts per thousand (2 sigma) external reproducibility of the analyses. The Hg isotope compositions from both hydrothermal systems can be grouped by dominant mineralogy and position; delta Hg-202/Hg-198 values at the tops of the systems are -3.5 parts per thousand to -0.4 parts per thousand in cinnabar-dominant sinter and -0.2 parts per thousand, to +2.1 parts per thousand in metacinnabar-dominant sinter, and the underlying veins have delta Hg-202/Hg-198 values of -1.4 parts per thousand to +1.3 parts per thousand. These differences probably resulted from the combination of boiling of the hydrothermal fluid, oxidation near the surface, and kinetic effects associated with mineral precipitation. The natural variation in Hg isotopic compositions observed in this study is higher than that expected from the trend of decreasing mass-dependent fractionation with increasing mass extrapolated from stable isotope systems up to Z = 26 (Fe), confirming that even the heaviest elements undergo significant stable isotope fractionation in hydrothermal systems.