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.