Yellowstone subsurface should host a two-phase hydrothermal
system containing liquid water and bubbles of a mixed
steam–CO2 vapor phase. The relative abundance of the two
phases can be estimated for a common pressure–temperature
regime at Yellowstone: 1 km depth and 250°C (red
square in FIGURE 4). Under those conditions, a 95% H2O–5%
CO2 fluid cannot exist as a single phase, but would unmix
into a vapor phase (50% CO2) and liquid water with minor
dissolved gas. The vapor phase would have a volume
approximately equal to that of the liquid. In reality, the two
phases are unlikely to remain together and would separate,
so that vapor-dominated hydrothermal reservoirs could
form above areas of deep boiling and degassing (White et al.
1971; Fournier 1989). The key point is that the considerable
flux of CO2 almost certainly requires that large portions of
the hydrothermal system beneath Yellowstone are gas saturated,
plausibly down to depths of 4 km or more.