From the perspective of pure continuum theory, permeability in rocks can be treated as a statistical problem. In a recently published monograph, Turcotte [1992] provided an overview of this probabilistic view of permeability. This sort of analysis predicts that there will be a sudden onset of macroscopic, inter-REV permeability at some critical value of porosity. Accordingly, metamorphic fluid flow is expected to be episodic if porosity is altered by reactions and strain.
Identification of the physical features that actually give rise to metamorphic permeability at sub-REV scales remained an active area of research (see Brenan [1991] for a summary through 1991). Connected grain edges, microcracks, and fractures are all posited as being potent determinants of permeability. The effects of fluid composition on formation and persistence of these features was recognized as an issue of cardinal importance in the last quadrennium.
The role of intergranular permeability depends on the extent to which
fluid-filled grain edges are connected. It is now well established
that upon attainment of textural equilibrium, grain-edge connectivity
occurs in isotropic porous media when the solid-solid-fluid dihedral
angle 
is less than 
(for realistic porosities).
Assessment of the efficacy of pervasive intergranular fluid flow
during metamorphism is underway and entails identification of the
factors that control grain-edge fluid connectivity in different
mineral-fluid systems.
Following original experiments by Watson and Brenan
[1987], Lee et al. [1991], Laporte and
Watson [1991], Holness [1992], and
Holness [1993] examined the effects of pressure, temperature, and fluid
composition on intergranular connectivity of H
O-CO
-alkali-halide
salt fluids in polycrystalline SiO
. In concert, the experimental data
show a broadly negative correlation between pressure and 
, a
positive correlation between the mole
fraction of CO
in the fluid phase (
) and
, and a negative correlation between concentrations of
dissolved alkali-halide salt and 
. One concludes that
quartzose lithologies in textural equilibrium will be permeable to
intergranular flow of H
O and aqueous brines in the upper and middle
crust (
0.6 GPa) if
temperatures are less than ca. 400
to 500
C,
respectively. Pervasive intergranular flow of aqueous fluid during
high-grade metamorphism in upper- and middle-crustal quartzites is
apparently precluded. Intergranular flow of CO
-rich fluid
(
) in quartzite at all metamorphic conditions
also appears unlikely.
Holness and Graham [1991] demonstrated that 
for
calcite and aqueous fluid decreases with increasing NaCl concentration,
consistent with the results obtained previously for quartz and
water-rich fluid [e.g., Watson and Brenan, 1987]. In
contrast to quartz-fluid experiments, however, Holness and
Graham [1991] also
found that calcite-calcite-(H
O-CO
) fluid 
does not
increase monotonically with 
in the fluid phase.
Instead, 
exhibits a minimum at 
for a
temperature and pressure of 650
C and 
GPa, respectively.
The critical condition for connectivity, 
, was
found to occur for 
between 
and 
. These
data suggest that the effects of fluid composition on 
can be
highly mineral specific. Relations in the calcite-H
O-CO
system are as yet unresolved, however, as Koga [1993]
reports a monotonic increase in calcite-calcite-(H
O-CO
) fluid
with 
at higher pressures.
The implications of these data are far reaching. Permeability enhancement or diminution is revealed to be chemical and not simply mechanical. Feedback effects in which buffering of infiltrated fluid composition alters the permeability of the host rock must now be considered.
Anisotropic interfacial energies will affect solid-fluid dihedral
angles. The 60
critical value is therefore not necessarily
applicable in polymineralic metamorphic
[4]
rocks. In order to circumvent
reliance on dihedral angles, and to assess the robustness of the
criterion in anisotropic media, measurements of bulk
transport properties of fluid-bearing rock materials at relevant P-T
conditions are being used to determine intergranular fluid
connectivity. Diffusivities of oxygen and chlorine in porous
polycrystalline quartz measured by Farver and Yund [1992]
and Brenan [1993], respectively, exhibit positive
correlation with textural evidence for grain-edge fluid connectivity. The
diffusion data
corroborate many of the relations between fluid composition and
intergranular fluid connectivity previously derived mainly from
measurements of 
, although the 
-criterion was shown
to be fallible at very low porosities [ Brenan, 1993].
Where fluid composition or ambient conditions prevent intergranular
flow, microcracks and fractures must accommodate fluid advection.
Brantley [1992] showed that, in general, fluid
compositions that
promote grain-edge connectivity (e.g., aqueous brines) also promote
healing of microcracks in quartz. These data suggest a possible
delineation of intergranular flow and microfracture flow regimes in
quartzose rocks on the basis of fluid composition. Healing in all
fluid compositions was shown by Brantley to be virtually
instantaneous (
100 years), however, and so it appears that
fluid overpressures are required to maintain flow along connected
microfracture networks during metamorphism regardless of fluid
composition. Dutrow [1994]
concluded that such overpressures will
produce diachronous and episodic fluid flow during metamorphism.
Episodic fracture flow was also predicted by Germanovich
and Lowell [1992].
Fisher and Brantley [1992] used textural observations,
vein distributions, and kinetic model calculations to investigate the
time scale of fracture flow in slates from Kodiak and Afognak Islands,
Alaska. They concluded that crack and seal events lasted
approximately 
to 
years with a cumulative duration of
to 
years. Some veins show textural evidence for having
remained open for 
to 
years.
The potential extent of fluid-rock interaction afforded by fracture flow is controlled by the spatial distribution of fractures. Manning [1994] showed that the distributions of subparallel veins (representing fluid pathways) in several amphibolite to greenschist facies metamorphic terranes are self similar. Manning suggested that the fractal dimension of each vein set can therefore be used to quantify the degree to which fracture flow was channelized or pervasive.