Radiogenic Isotope Constraints on Fluid Sources in the Yellowstone Hydrothermal System

Abstract:

For decades, researchers in Yellowstone National Park (YNP) have used major and trace element and light stable isotope geochemistry to evaluate fluid sources and geochemical reactions in the Yellowstone hydrothermal system. However, the results can be affected by mixing, boiling and vapor-phase separation. We present new strontium (Sr), neodymium (Nd), and lead (Pb) isotopic data from hydrothermal waters and fumarole condensates that allow us to evaluate fluid sources independent of near-surface mixing and boiling. Our sample set was selected to explore the range of fluid compositions found in the Yellowstone hydrothermal system, including waters/fluids that are thought to be exclusively meteoric, exclusively from the deep hydrothermal system, and those which are a mixture of these end members and/or that have been influenced by various hydrothermal processes such as boiling or gas/water interaction. We have identified at least three isotopic endmembers that persist in various features throughout the YNP hydrothermal system. The first endmember has relatively unradiogenic Pb with Sr, Nd, and Pb isotopic compositions that are consistent with Yellowstone basalts and rhyolites. This endmember is typified by low pH features. We interpret this fluid as surface water and shallow groundwater that has interacted with volcanic rocks associated with the YNP magmatic system, with the acidity derived from oxidation of volcanic gases. The second endmember has relatively radiogenic Pb, radiogenic Sr, and unradiogenic Nd. This endmember is typified by neutral pH features and near neutral fumarole condensates. We interpret this endmember to represent the hypothesized deep hydrothermal reservoir that interacts with and reflects the isotopic composition of the host rock. The third endmember contains radiogenic Pb, unradiogenic Nd, and unradiogenic Sr. We observe this endmember in neutral features, which are interpreted as hydrothermal waters (shallow, deep, or mixtures) that have interacted locally with Madison Limestone near the surface. Collectively, these data provide new constraints on the source of fluids that comprise hot springs in Yellowstone which has implications for interpreting variation in spring geochemistry and microbial diversity.