Theses and Dissertations Collection

Recent studies of ductile shear zones in the Josephine Peridotite (SW Oregon) find higher helium concentrations in whole rock samples located where total strain is greatest and recrystallized grain sizes are smallest. Based upon these results, previous workers suggest that dynamic recrystallization may create new inter-grain voids where He can be stored. To assess the feasibility of this mechanism for enhanced He storage, we utilize a combined set of new and previous data from Shear Zones A and B of the Fresno Bench outcrop to constrain a 1D numerical model of a ductile shear zone; the combined data set includes both He concentrations as well as measured total strain across the shear zone. Our numerical model is discretized using finite differences and incorporates a non-linear, temperature-dependent viscosity, shear heating, and dynamic recrystallization. The numerical model results produce similar helium concentration profiles to those of the Josephine shear zones, but comparison of both data sets suggests that deformation occurred much more rapidly than we originally suspected. Our model provides useful insight into the incompatible diffusive behavior of helium in mantle shear zones and suggests possible mechanisms of helium enrichment in high-strain mantle material.