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- Title
Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates.
- Authors
DiMonte, A. A.; Ault, A. K.; Hirth, G.; Meyers, C. D.
- Abstract
Deformation experiments on hematite characterize its slip‐rate dependent frictional properties and deformation mechanisms. These data inform interpretations of slip behavior from exhumed hematite‐coated faults and present‐day deformation at depth. We used a rotary‐shear apparatus to conduct single‐velocity and velocity‐step experiments on polycrystalline specular hematite rock (∼17 μm average plate thickness) at slip rates of 0.85 μm/s to 320 mm/s, displacements of primarily 1–3 cm and up to 45 cm, and normal stresses of 5 and 8.5 MPa. The average coefficient of friction is 0.70; velocity‐step experiments indicate velocity‐strengthening to velocity‐neutral behavior at rates <1 mm/s. Scanning electron microscopy showed experimentally generated faults develop in a semi‐continuous, thin layer of red hematite gouge. Angular gouge particles have an average diameter of ∼0.7 μm, and grain size reduction during slip yields a factor of 10–100 increase in surface area. Hematite is amenable to (U‐Th)/He thermochronometry, which can quantify fault‐related thermal and mechanical processes. Comparison of hematite (U‐Th)/He dates from the undeformed material and experimentally produced gouge indicates He loss occurs during comminution at slow deformation rates without an associated temperature rise required for diffusive loss. Our results imply that, in natural fault rocks, deformation localizes within coarse‐grained hematite by stable sliding, and that hematite (U‐Th)/He dates acquired from ultracataclasite or highly comminuted gouge reflect minor He loss unrelated to thermal processes. Consequently, the magnitude of temperature rise and associated thermal resetting in hematite‐bearing fault rocks based on (U‐Th)/He thermochronometry may be overestimated if only diffusive loss of He is considered. Plain Language Summary: Laboratory deformation experiments on hematite, a common iron‐oxide in the shallow portion of fault zones, characterize its frictional properties and how it deforms in natural fault systems. We conducted experiments on bare surfaces of hematite rock at dominantly low sliding velocity, cm‐scale displacements, and low normal stresses. Experiments showed that, at these conditions, hematite has a friction coefficient similar to many common rocks and minerals. In experiments where slip velocity is varied, hematite strengthens or retains the same strength with increasing slip rate. High spatial resolution microscopy showed experimental faults localize in a thin layer of hematite particles created from grain size reduction during slip. Radiometric dating of hematite via (U‐Th)/He thermochronometry constrains the time when He, a noble gas, is trapped in a crystal, although both thermal or mechanical perturbations can cause He loss. Comparison of hematite (U‐Th)/He dates from the undeformed rock and particles generated during laboratory slip indicates He was lost along newly generated particle surfaces during grain size reduction at slip rates <1 mm/s. Our results imply hematite deforms brittlely in nature at subseismic slip rates in shallow crustal conditions. (U‐Th)/He dates acquired from natural, comminuted hematite may reflect some He loss unrelated to thermal processes. Key Points: We performed rotary‐shear experiments on coarse‐grained hematite rock slabs to assess frictional behavior and Helium loss at slow slip ratesHematite has a moderate to high coefficient of friction and is velocity strengthening to velocity neutral at slow slip ratesHelium loss in gouge occurs in the absence of temperature rise by grain size reduction that exposes Helium along new grain boundaries
- Subjects
HEMATITE; SIZE reduction of materials; RADIOACTIVE dating; FAULT zones; DEFORMATIONS (Mechanics); ROCK deformation; CYCLIC fatigue
- Publication
Journal of Geophysical Research. Solid Earth, 2024, Vol 129, Issue 3, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2023JB027514