Vn. Balashov et Bwd. Yardley, MODELING METAMORPHIC FLUID-FLOW WITH REACTION-COMPACTION-PERMEABILITYFEEDBACKS, American journal of science, 298(6), 1998, pp. 441-470
Existing models of metasomatic now do not allow for the effect that re
action has on the now patterns. Instead, it is assumed that the volati
les produced are negligible in volume compared to those infiltrated an
d that reaction does not modify permeability. This is clearly unlikely
to be true for infiltration-driven decarbonation reactions. The rates
of porosity creation by reaction and porosity loss by creep have been
calculated for a representative volume of calcite-quartz-wollastonite
marble, and it is found that, even for a weak calcite matrix, the rat
e of porosity generation by reaction is likely to outstrip the collaps
e of porosity, as long as the system is out of equilibrium. We have ap
plied a self-consistent ID finite-difference model to the reaction of
calcite + quartz to wollastonite in a 10 m thick marble, in response t
o influx of H2O rich fluid, with fixed boundary conditions. The model
allows us to evaluate the effect of reaction on the porosity structure
and fluid pressure variation across the layer, from which local Darcy
fluxes can be evaluated. The progress of reaction that we model is co
nstrained by hydrological considerations, with the requisite parameter
s recalculated as reaction progresses, assuming creep compaction of ro
ck under the stress difference between lithostatic and fluid pressures
. We find that the volume of fluid released by decarbonation, driven b
y influx of H2O, is sufficient to create a back-now, so that further a
dvancement of the reaction front is only possible as a result of diffu
sion of water against the Darcy flux. The effect of creep driven by di
fferences between fluid pressure and lithostatic pressure is to reduce
the permeability of the layer and especially reduce the secondary por
osity developed in the zone at and behind the advancing reaction front
. We predict that in a 3D situation, the porous zone of reacted marble
becomes a conduit for layer-parallel now, and the secondary porosity
is infilled by calc-silicate minerals due to silica metasomatism.