Using a Numerical Model to Quantitatively Assess Dynamic Recrystallization as a Mechanism for He Enrichment in Mantle Shear Zones

Abstract:

Recent studies of ductile peridotite shear zones in the Josephine Peridotite in 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 lead to increased storage of He on grain boundaries. To assess the feasibility of this mechanism for enhanced He storage, we utilize a combined set of new and previous data from Shear Zone A (SZA) and B (SZB) of the Fresno Bench of the Josephine Peridotite 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. Existing data within the region of highest strain (0 to ~2.5 m from the center of each shear zone) are sparse and, thus, we strategically sampled locations within this zone to maximize data resolution across a range of total strain. In each sample, we measure helium concentrations in unserpentinized harzburgite bulk rock using mass spectrometry. Analysis of the orientation of pyroxene foliation planes compared to shear planes provides an estimation of shear strain during deformation. Numerically, our model is discretized using finite differences and incorporates a non-linear, temperature-dependent viscosity, shear heating, and dynamic recrystallization. Here, we present our newly compiled collection of helium concentrations relative to total strain within SZA and SZB and measured grain sizes, which are used to constrain the modeled equilibrium grain size and quantitatively test dynamic recrystallization as a mechanism for concentrating He within peridotite shear zones.