P-T-X EFFECTS ON EQUILIBRIUM CARBOLATE-H2O-CO2NACL DIHEDRAL ANGLES - CONSTRAINTS ON CARBONATE PERMEABILITY AND THE ROLE OF DEFORMATION DURING FLUID INFILTRATION

Fluid-solid-solid dihedral angles in the NaCl-H2O-CO2-calcite-dolomite -magnesite system have been determined at pressures ranging from 0.5 t o 7 kbar and temperatures from 450 degrees C to 750 degrees C. At 1 kb ar and 650 degrees C, both dolomite and magnesite exhibit a dihedral a ngle minimum for intermediate H2O-CO2 fluids similar to that previousl y determined by the present authors for calcite, but the depth of the minimum is smaller, being above the critical value of 60 degrees for b oth dolomite and magnesite for all fluid compositions. Calcite-calcite -brine dihedral angles at 650 degrees C have been determined in the pr essure range 1-5 kbar. Angles decrease with increasing salt content of the fluid, tending towards a constant value of about 65 degrees for s trong brines at pressures above 2 kbar. There is a general increase of angle with increasing pressure which is most marked for strong brines . A positive correlation of angle with pressure is also observed in ca lcite-H2O-CO2 fluids, the position of the minimum moving towards highe r angles and towards H2O-rich fluids with increasing pressure. The per meability window previously observed by the present authors at 1 kbar and intermediate fluid compositions closes at about 1.5 kbar. The resu lts demonstrate that the permeability of carbonates to grain edge flui d flow is only possible at low pressures and for fluids of restricted H2O-CO2-NaCl compositions. However, geochemical evidence from metamorp hic terrains suggests that pervasive infiltration does occur under con ditions where impermeability is predicted. From examination of publish ed studies of infiltrated carbonates we conclude that deformation play s a critical role in enhancing carbonate permeability. Possible mechan isms for this include shear-enhanced dilatancy (micro-cracking), fluid inclusion drag by deformation-controlled grain boundary migration, an d dynamically maintained transient grain boundary fluid films.