Where Has the Water Been? Combining Geochemical Data to Trace Hydrothermal Fluid Flow and Water-Rock Reactions

The Yellowstone hydrothermal system produces wide-ranging fluid compositions that support exceptional microbial diversity, contributing to the greater Yellowstone ecosystem. Initial fluid compositions and complex subsurface interactions, from soil processes to deep hydrothermal reactions, define surficial hot spring environments. Previous studies used stable water isotopes to identify signatures associated with meteoric sources, fractionation due to boiling and evaporation, as well as the extent of water-rock reactions. Comparing concentrations of sulfate to chloride has led to insights about the composition of deep hydrothermal fluids, shallow waters with meteoric signatures, and the effect of magmatic gas input on those endmember sources. Ratios of total boron and chloride were used to infer the effects of meteoric dilution on a single source water. Here we combine these three approaches to interpret an extensive dataset from Rabbit Creek; a geographically condensed thermal area in Yellowstone National Park, containing diverse hot spring compositions. The results indicate that when major anions show a deep hydrothermal signature, water-rock reactions can shift stable water isotopes off of the meteoric water line toward heavier oxygen values, which are then fractionated by surface evaporation. In contrast, when major anions indicate a meteoric signature, stable water isotopes preserve a meteoric signature at some sites and exhibit extensive fractionation at others. By using the same aliquot of water in all analyses, the identification of a deep signature in major anions defines deep signatures in stable water isotopes and boron concentrations. Applying all three methods simultaneously reveals that neighboring springs can have exceptionally different proportions of deep to surface-derived fluids and contrasting histories of water-rock reactions.