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