FE ISOTOPE SYSTEMATICS OF HYDROTHERMAL WATERS FROM THE YELLOWSTONE HYDROTHERMAL SYSTEM

Active hydrothermal systems provide a natural laboratory in which to study the relationship between water-rock reactions, hydrothermal mineralization, and microbiology. Fe isotopes have been used to study these relationships in more detail, but as of yet have mostly focused on seafloor hydrothermal systems involving peridotites and/or basalts. However, Fe isotopes have been used in Yellowstone to evaluate the process of microbial Fe reduction in certain hot springs. Here we present new Fe isotope data on a variety of hot springs that cover a nearly complete spectrum of hydrothermal fluid geochemical compositions found in Yellowstone. δ⁵⁶Fe values in hydrothermal waters range from ~ - 1.0 ‰ to + 1.2 ‰, and generally form three groups reflecting three different hydrogeobiological processes occurring in the Yellowstone hydrothermal system. The dominant isotopic composition ranges from ~+ 0.30 to 0.50 ‰, and represents bulk leaching of Fe from the volcanic substrate. A second group contains higher δ⁵⁶Fe values up to ~1.2 ‰, and reflects hydrothermal mineralization in the subsurface. We propose pyrite or another Fe bearing geothermal mineral is causing significant Fe fractionation. The third group contains samples that have lower δ⁵⁶Fe values down to ~ - 1.0 ‰, including Chocolate Pots, the previously studied hot spring with confirmed microbial Fe fractionation. The origin of the isotopically light Fe is likely microbial Fe reduction in other hot springs as well. Overall, Fe isotopes provide a unique perspective on a range of processes occurring throughout the Yellowstone Hydrothermal System.