The effect of fluid flow on mixed-volatile reactions in metamorphic
rocks is described by an expression derived from the standard equation
for coupled chemical-reaction and fluid-flow in porous media. If local
mineral-fluid equilibrium is assumed, the expression quantitatively
relates the time-integrated flux at any point in a flow-system to the
progress of devolatilization reactions and the temperature- and
pressure-gradients along the direction of flow. Model calculations
indicate that rocks are generally devolatilized by fluids flowing
up-temperature and/or down-pressure. Flow down-temperature typically
results in hydration and carbonation of rocks. Time-integrated fluid
fluxes implied by visible amounts of mineral products of
devolatilization reactions are on the order of 5.10(2)-5.10(4) mol/cm2.
The model was applied to regionally metamorphosed impure carbonate rocks
from south-central Maine, USA, to obtain estimates of fluid flux,
flow-direction, and in-situ metamorphic-rock permeability from
petrologic data. Calculated time-integrated fluxes are 10(4)-10(6)
cm3/cm2 at 400-degrees-450-degrees-C, 3,500 bars. Fluid flowed from
regions of low temperature to regions of high temperature at the peak of
the metamorphic event. Using Darcy's Law and estimates for the duration
of metamorphism and hydrologic head, calculated fluxes are 0.1-20.10(-4)
m/year and minimum permeabilities are 10(-10)-10(-6) Darcy. The range
of inferred permeability is in good agreement with published laboratory
measurements of the permeability of metamorphic rocks.