COMPARING SHALLOW GROUND TEMPERATURES IN ACID-SULFATE AND HIGH-CHLORIDE, CIRCUM-NEUTRAL HYDROTHERMAL DISCHARGE AREAS OF YELLOWSTONE NATIONAL PARK, WYOMING

We used shallow ground temperature surveys to observe the temperature field distributions for two hydrothermal discharge areas in Yellowstone National Park, Wyoming. The areas surveyed were the surroundings of spring LCBNN159 in the Morning Mist Springs (MMS) area and a sub-area of the River Group Springs (RGS) between springs LRNN178 and LRNN179. The MMS temperature field was sampled in a 72x72 m area at a resolution of 3x3 m; at the RGS, we used a 48x72 m grid measured at a 4x4 m resolution. We analyzed the data using geostatistical (variogram) analyses and kriging to construct high-resolution maps of the expected temperature fields. The inferred temperature fields show very different behavior between the two areas. The MMS, which are high-chloride circum-neutral springs, present a smoothly-varying aspect, where higher temperature areas generally confined to the vicinity of hydrothermal discharge points (i.e., hot springs). In contrast, the RGS temperature field is highly heterogeneous, with abrupt and discontinuous changes in temperature. The observed areas of elevated temperature, some of which are near the boiling point, are not reliably correlated to the acid-sulfate springs that constitute the surface manifestation of hydrothermal discharge in the area. We interpret the observed temperature fields to reflect differences in the near-surface plumbing and mechanisms of heat transfer between the two areas: while the MMS are likely fed by sub-boiling hydrothermal fluids migrating through silt-rich, low-permeability deposits at the land surface, the temperature fields at the RGS appear to be the result of heating by vapor-phase hydrothermal fluids, the movement of which is controlled by high-permeability fractures in the shallow subsurface. The findings of this study are consistent with conceptual models of previous investigators for vapor-dominated zones in hydrothermal reserviors, and shed light on mechanisms controlling the expression of vapor- and liquid-dominated zones in Model III type hydrothermal systems.