DIFFUSION OF CHLORINE IN FLUID-BEARING QUARTZITE - EFFECTS OF FLUID COMPOSITION AND TOTAL POROSITY

To assess further the role of pore fluids in enhancing mass transport in deep-seated rocks, bulk diffusion coefficients (D-values) for chlorine have been measured at 1.0 GPa and 1000-degrees-C in texturally-equilibrated quartzites containing varying amounts (0.3-2.8 vol%) of H2O or a CO2-H2O mixture (X(CO2) = 0.4). Experiments were used to monitor transport predominantly through the fluid phase by employing a chemical tracer dissolved in the fluid (chlorine) that is virtually insoluble in the rock matrix (quartzite) but is somewhat soluble in a small fraction of dispersed indicator minerals (fluorapatite). For diffusion in H2O-bearing quartzite, experiments with greater-than-or-equal-to 1 vol.% fluid exhibit a continuous decrease in D-values with decreasing porosity whereas an abrupt drop in the diffusion coefficient is indicated by the experiments with 0.3 vol.% fluid. At a given total porosity, diffusion coefficients obtained from quartzites containing the CO2-H2O fluid range from approximately 80x to > 3000x lower than those from H2O-bearing experiments. Bulk transport measurements were correlated with textural observations and the observed reduction in bulk D-values for quartzites containing the CO2-H2O fluid reflects the overall isolated nature of porosity in such samples. The drop in the bulk D-value for quartzites with 0.3 vol.% H2O probably arises from the elimination of interconnected porosity owing to the presence of a sufficient number of dry grain edges. Textural observations, combined with transport measurements, are consistent with the pore structure predicted by dihedral angle measurements. However, due to anisotropy in the interfacial energy of quartz, long-range fluid connectivity is dramatically reduced in quartzites with low H2O contents, despite a median dihedral angle of less than (but near) 60-degrees. Observed variations in chlorine D-values in samples with connected and non-connected porosity are found to be consistent with previous bulk property measurements on texturally-equilibrated, fluid-bearing rocks. Results of this study, combined with prior bulk diffusion measurements for oxygen, provides a general assessment of the effect of small amounts of fluid on the enhancement of mass transport in quartzose lithologies over a range of crustal P and T At conditions of textural equilibrium, it is expected that the fluid phase provides little or no contribution to the long-range diffusive flux of solutes in quartz-rich rocks containing small amounts of H2O or CO2-H2O mixtures.