ASSESSING THERMAL DISCHARGE FROM YELLOWSTONE HOT SPRINGS AND IMPLICATIONS FOR BASALTIC MAGMA RECHARGE RATES

The Yellowstone Caldera, WY, USA, hosts a world-famous hydrothermal system, where >10,000 surficial thermal features are spread over an area of nearly 6000 km². The hydrothermal system has been active for at least the past 400,000 years. Both acid-sulfate and circumneutral hydrothermal fluids discharge from the springs. The hydrothermal system is driven by a mid-crustal magma and appears to be the primary mechanism for heat transport from the magma to the surface. Although direct measurements of advective flux from individual hot springs would be beneficial for adding constraints on the total rate of heat transport associated with the Yellowstone Caldera and its magmatism, these measurements are seldom made due to the difficulty of obtaining mass flow rates to support them. Typically, discharge from thermal springs migrates through the shallow subsurface, making accurate flow rate assessments problematic. By adding small amounts of nearly pure D₂O to four springs in the Morning Mist area of Lower Geyser Basin in Yellowstone National Park, USA, we have made direct measurements of mass and thermal discharge. The springs ranged in temperature from 74 to 95°C, and we analyzed time series δD values to determine the volumes and discharge rates of the test springs. D₂O was chosen to limit the ecological and/or visual impacts of other common tracers, such as NaCl or fluorescein dyes. We calculated spring volumes to range between 560 and 27,400 L, and estimated mass and heat discharge at 0.08 to 1.35 L/s and 0.0199 to 0.335 MW, respectively. The volumes calculated by deuterium doping were larger in every case than those estimated by field inspection, suggesting the volume participating in shallow fluid circulation is generally larger than is visible from the surface. The heat flow data, when paired with conductive heat loss estimates in the vicinity of the springs, suggest current estimates of thermal discharge at Yellowstone may require revision, and offer insights on the rate of magma supplied by the mantle. Thermal flux estimates, although highly uncertain, suggest that a minimum of 3.2 to 6.3 x 10^-2 km³ of basalt magma enters the base of the crust annually, about half the rate of historic basalt eruptions from Kilauea.