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40Ar ∗ loss in experimentally deformed muscovite and biotite with implications for 40Ar/ 39Ar geochronology of naturally deformed rocks
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 granit...
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Published in: | Geochimica et cosmochimica acta 2011-12, Vol.75 (24), p.7759-7778 |
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Main Authors: | , , , |
Format: | Article |
Language: | English |
Subjects: | |
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Online Access: | Get full text |
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Summary: | 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 situ
40Ar/
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 |
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ISSN: | 0016-7037 1872-9533 |
DOI: | 10.1016/j.gca.2011.10.012 |