Page 421 - Caldera Volcanism Analysis, Modelling and Response

396 Micol Todesco

(favouring a ductile rock behaviour), lithostatic pressures and abundant mineral
deposition at temperatures in excess of 350–4001C(Fournier, 1987). However,
petrological studies of contact metamorphism provide clear evidence of fluid
transfer across this transition region. Upward and outward fluid migration from
cooling plutons has been shown to occur at temperatures in excess of 5001C and
pressures up to 220 MPa, preferentially along bedding or lithological contacts
(Ferry et al., 2002; Buick and Cartwright, 2002). The presence of fluids, and of
considerable mass transport via fluid phases, is known to take place through the
deep crust, at depths greater than 15 km and under granulite facies conditions
(Ague, 2003). Hydrofracturing may occur through the transition zone as pressure of
exsolved volatiles increases above the tensile strength of the surrounding rocks
(Tait et al., 1989; Jamtveit et al., 1997). This may lead to transient high flux across
the transition region, lasting as long as the source is capable of sustaining it.
Metamorphic devolatilisation reactions can also intervene and reduce the total solid
volume, generating secondary porosity (Jamtveit et al., 1997).
Once hot magmatic gases escape the magma chamber, buoyancy forces prompt
natural convection, provided that rock permeability is high enough (Ingebritsen
and Sanford, 1998). Hot fluids then propagate through pores and/or fractures,
interact with shallow groundwater and eventually reach the surface generating
fumaroles, hot springs or geysers. Hydrothermal circulation depends on fluid
properties, which in turn change as a function of system conditions. Hydrothermal
systems encompass a wide range of physical conditions, spanning from almost
magmatic to surface settings. As a consequence hydrothermal fluids may exist as
single-phase, two-phases or supercritical fluids depending on their pressure and
temperature. Physical and transport properties may change accordingly, favouring
or hindering fluid motion and heat transport. If gas and liquid coexist, the mobility
of each phase is reduced by the presence of the other. In this case, the gas phase
tends to occupy larger pores and fractures, whereas liquid water is preferentially
held into smaller pores, due to capillary pressure and surface effects (Corey et al.,
1956; Elder, 1981). Above the critical point (374.151C and 22 MPa for pure water)
gas and liquid phases become indistinguishable and the resulting supercritical fluid is
characterised by very high compressibility, thermal expansion and heat capacity and
by low viscosity. Strong buoyancy forces and low viscous resistance favour fluid
motion and greatly enhance convective heat transport as the critical transition is
approached. High pore pressure is easily generated across the critical transition and,
in general, the entire hydrothermal circulation is affected (Elder, 1981; Norton,
1984). The composition of circulating fluids, as well as the presence and nature of
dissolved salts, also affect fluid properties and modify the temperature and pressure
(and hence depth) at which phase transitions occur (Elder, 1981; Bowers and
Hegelson, 1983a, 1983b; Bishoff and Pitzer, 1984; Bishoff et al., 1986; Pitzer and
Pabalan, 1986; Bishoff, 1991). Highly saline fluids (or brine) are known to evolve
from crystallising magma bodies, together with exsolved volatiles (Fournier, 1987).
Denser brine tends to concentrate in the deeper region of hydrothermal systems
and separates from the dilute gas phase that rises buoyantly. Double diffusive effects,
driven by temperature, density and salinity gradients may develop between
the dense and dilute convective systems (Elder, 1981; Bishoff and Pitzer, 1984;