RADIOGENIC SR AND U ISOTOPES AS NATURAL TRACERS OF SPATIAL AND TEMPORAL FLOW OF THERMAL WATERS IN YELLOWSTONE NATIONAL PARK, USA

Thermal features within the Yellowstone caldera depend on interactions between young, cool recharge to the shallow subsurface and upwelling plumes of older, deeper-circulating thermal water. Radiogenic isotopes of Sr (⁸⁷Sr/⁸⁶Sr) and U (²³⁴U/²³⁸U) are important natural tracers of hydrologic flow and mixing, but have not been widely applied to waters at Yellowstone National Park (YNP). We report new isotope data (mostly ⁸⁷Sr/⁸⁶Sr) for thermal waters and rivers from a number of basins across YNP, as well as time-series results for several features in the Upper Geyser Basin and volcanic rocks constituting the shallow subsurface in that area.

Concentrations of U in thermal waters are typically <30 ppt reflecting the low solubility of U(IV) under reducing conditions. Consequently, reliable ²³⁴U/²³⁸U measurements were difficult to make, and only a subset of samples (n=25) were analyzed (activity ratios of 0.86–2.11). Results for Sr determined on 105 thermal waters and 27 river samples show wide ranges of Sr concentrations (0.5 to ~4000 ppb) and ⁸⁷Sr/⁸⁶Sr values (0.7058 to 0.7375). Linear arrays are not apparent on mixing diagrams plotting reciprocal Sr concentrations against ⁸⁷Sr/⁸⁶Sr. However, spatial distributions indicate that the lowest ⁸⁷Sr/⁸⁶Sr values are from the east side of the caldera relative to higher values on the west side. Samples of thermal water from the Upper Geyser Basin have the highest ⁸⁷Sr/⁸⁶Sr values, mostly between 0.714 and 0.719. Data for 5 geysers monitored between 2007 and 2009 (Aurum, Daisy, Grand, Oblong, and Old Faithful) indicate similar groundwater sources for several geysers, with small, but significant, differences for others. More notably, samples from late Fall through early Spring of 2007–2008 had much higher ⁸⁷Sr/⁸⁶Sr values in all 5 geysers (0.720–0.737), after which, compositions returned to normal values. The lower values are consistent with ⁸⁷Sr/⁸⁶Sr in volcanic rocks constituting the shallow subsurface; however, the higher values, assuming they are not sampling artifacts, imply a transient component of groundwater that interacted with Precambrian-sourced Sr in the subsurface. Data from this study show that ⁸⁷Sr/⁸⁶Sr provides a sensitive indicator of inter- and intra-basin variability as well as means of assessing flowpaths and mixing of different hydrologic components.