Intrusion, Deformation and Degassing at the Yellowstone Caldera

Jacob B. Lowenstern

USGS, Menlo Park, California

The Yellowstone caldera is well known for its cycles of uplift and subsidence over both historic and geologic timescales. Most models for deformation assume sources due to transport of magma or hydrothermal brine streaming through ruptured permeability barriers. Recent investigations of chemical mass balance at Yellowstone provide critical insights into potential sources of both deformation and heat. Volatile fluxes from the Yellowstone caldera have been calculated by summing the flux of Cl–, F, SO42–, and HCO3– through the major rivers leaving the Yellowstone Plateau. Long-term studies show that Cl–, the primary non-H2O component of geothermal brines has not changed appreciably in output during recent periods of subsidence and uplift. Instead, Cl– flux is dominated by recharge constraints, increasing during periods of greater precipitation. Carbon is much more abundant than sulfur in Yellowstone’s waters, but is even more dominant when combined with data on gas flux from fumaroles and diffuse degassing. In fact, CO2 is about 300 times more abundant than Cl– on a molar basis as an effluent from the Yellowstone hydrothermal system. Similarly sulfur flux exceeds Cl– by about 25 times what one would expect from the concentrations in degassed volcanic rocks that could be leached. Phase equilibrium constraints imply that the shallow subsurface at Yellowstone (the upper two km) should be saturated with a CO2-rich vapor phase under most conceivable P-T conditions. This volumetrically significant (even dominant) phase should have an important role in pressurization of the hydrothermal system and may contribute to ongoing cycles of deformation within the caldera. The volatile “signature” from Yellowstone strongly suggests that gas discharge is controlled not by the crustal granitic magma chamber but by subjacent basaltic intrusions that provide both heat and mass to the overlying system.