Isotope transport and exchange within metamorphic core complexes

Field observations from the Shuswap metamorphic core complex in British Columbia indicate that meteoric fluids were focused along a sub-horizontal shear zone at a depth of at least 7 km. Fluid-rock interactions associated with this flow system resulted in oxygen isotope depletion of mylonitic rocks up to 4 permil in a region less than 900m wide. Dating of the recrystallized shear zone fabric and deformation-assisted fluid flow indicates that this paleo-fluid flow system was relatively short lived, (<1 Ma). Here we present idealized numerical representations of a metamorphic core complex system to assess the hydrologic and thermal controls on fluid-rock isotopic exchange. Our analysis focuses on understanding the relative importance of fault versus matrix controlled fluid flow, reactive-surface area, crustal permeability structure, and isotopic composition of the recharging fluids. The analysis permits us to bracket the possible permeability and surface area conditions that are consistent with field observations. We conclude that downward fluid flow along brittle fault systems and isotope exchange patterns could only be produced by a fracture flow dominated system. We found the fault permeability had to be greater than 10^-16 m² but less than or equal to 10^-15 m². Upper plate crystalline rocks adjacent to the fault zone had to have a permeability less than 10^-17 m². The above findings are valid assuming a lateral water table gradient of 5 percent, a shear zone surface area of 3.0×10^-4 m²/mole, crustal rock surface area of 1.0×10^-5 m²/mole, total duration of flow of 200,000 years, and a basal heat flux of 90 mW/m². Fault zone surface areas are much too small to be consistent with pervasive grain boundary fluid-rock isotope interactions. Rather, the best fit surface areas were consistent with a fracture spacing of 0.25 m for the shear/fault zones and a 5 m spacing for surrounding upper and lower plate rocks. We found that fracture aperture widths of about 0.02 mm for the fault/shear zone units and 0.002 mm for the surrounding upper and lower plate rocks were consistent with the permeability values obtained from our generic modeling exercise. Imposing a more strongly ¹⁸O-depleted oxygen isotope composition for the meteoric recharge was directly reflected in lower computed δ¹⁸O rocks. However, the effects were non-unique and to some degree, masked by the large oxygen reservoir within the crustal rocks. Computed rock isotopic values consistent with field observations could have been produced with either heavier δ¹⁸O fluids in the recharge area over a longer period of infiltration or lighter δ ¹⁸O fluid compositions in the recharge region over shorter periods of time.