Diffusion geospeedometry in natural and experimental shear zones

A new method for determining the timescales of ductile deformation utilizes diffusion profiles around oriented mineral inclusions. This geospeedometer was tested with rutile needle inclusions in mylonitized quartz grains because they are sensitive to the kinematics of deformation and behave as diffusion couples that can be either a source or sink for Ti diffusion in the host quartz. The method was applied to experimental samples, in which the time–temperature conditions are known, and to natural samples, in which the time–temperature conditions are determined using independent geochronometers and geothermometers. In our experimental quartzite, rutile needles become included in quartz during isostatic annealing, rotate into the elongation direction of shearing during imposed deformation, and act as a source of Ti for the surrounding undersaturated quartz. Diffusion modeling of Ti enrichment halos yields appropriate trends in experiment duration. In a natural quartzite mylonite from the extensional detachment zone of the Pioneer Core Complex (Idaho, USA), quartz grains are densely and pervasively rutilated. Titanium depletion halos in quartz around lineated and boudinaged rutile needles indicate that rutile exsolved during deformation. By estimating temperature from quartz microstructures and Ti-in-quartz thermometry, diffusion modeling results suggest that ductile deformation in the shear zone lasted 5–10 Myr, consistent with geologic and thermochronologic data defining exhumation of the complex. Considering structural offsets across the detachment, extrapolating our lab-derived diffusivities to the conditions of natural samples equates to strain rates of ∼1×10⁻¹² s⁻¹. Tests of geospeedometry in both naturally and experimentally deformed rutilated quartz samples indicate that this method can provide reliable information on shear zone longevity and strain rate directly from the phase controlling the rheological behavior of deformation.