The effects of deformation on radiogenic argon (
40Ar
∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO
2) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined
40Ar/
39Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating
40Ar
∗ loss of 0-35% in muscovite and 2-3%
40Ar
∗ loss in biotite. Intragrain
in situ ultraviolet (UV) laser ablation
40Ar/
39Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16%
40Ar
∗ loss relative to the undeformed muscovite. The
in situ UV laser ablation
40Ar/
39Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32%
40Ar
∗ loss. No spatial correlation is observed between
in situ40Ar/
39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed
40Ar
∗ loss in the experimentally treated muscovite can be utilized to predict average
40Ar
∗ diffusion dimensions. Maximum
40Ar/
39Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for
40Ar
∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental
40Ar/
39Ar ages indicating
40Ar
∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale
40Ar
∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for
40Ar
∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both
40Ar
∗ retention and intragrain distribution in deformed mica. Intragrain
40Ar/
39Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas.
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