IRON ISOTOPE CHARACTERISTICS OF HOT SPRINGS AT CHOCOLATE POTS, YELLOWSTONE NATIONAL PARK

Chocolate Pots Hot Springs in Yellowstone National Park is a hydrothermal system that contains high aqueous ferrous iron (~0.1 mM Fe(II)) at circumneutral pH conditions. This site provides an ideal field environment in which to test our understanding of Fe isotope fractionation factors as well as mechanisms derived from laboratory experiments. As spring fluids flow downhill to the Gibbon River, Fe(II) is oxidized through contact with oxygen in the atmosphere, resulting in precipitation of hydrous ferric oxides. The Fe(III) oxides have high ⁵⁶Fe/⁵⁴Fe ratios compared with the aqueous Fe(II), as expected for redox-driven Fe isotope fractionation. However, the degree of fractionation is less than the value of ~3‰ expected in a closed system at isotopic equilibrium. We suggest two explanations for the observed Fe isotope compositions. One is that light Fe isotopes partition into a sorbed component and precipitates out on the Fe(III) oxide surfaces in the presence of silica. The other explanation is internal regeneration of isotopically heavy Fe(II) via dissimilatory Fe(III) reduction further down the flow path, as well as deeper within the mat materials. These findings provide evidence that silica plays an important role in governing Fe isotope fractionation factors between reduced and oxidized Fe. Under conditions of low ambient oxygen, such as may be found in early Earth or Mars, significantly larger Fe isotope variations are predicted, reflecting the more likely attainment of Fe isotope equilibrium associated with slower oxidation rates under low-O₂ conditions.