GAS SATURATION IN THE
YELLOWSTONE SUBSURFACE
The impressive gas discharge hints that the shallow subsurface
beneath Yellowstone should be gas saturated over a
wide depth–temperature range. An initial understanding is
gained by comparing the CO2 flux to that of the Cl- released
through rivers. If the two volatiles are currently emerging
from the hydrothermal system at their long-term rates,
then the CO2/Cl- mass ratio in the subsurface can be estimated
at ~300 (TABLE 1). Fournier (1989 and references therein)
inferred the existence of a parent thermal water with
400ppm Cl-. If the gas could be dissolved back into that
water, the CO2 concentration would be 400 ppm multiplied
by 300, equaling 12 wt% (or 5 mol%). However, under most
pressure and temperature conditions, the CO2 is too abundant
to dissolve completely, as demonstrated in FIGURE 4,
which displays the phase boundaries in the system 95%
H2O–5% CO2, assuming a hydrostatic pressure gradient.
Model calculations suggest that the upper 4 km of the