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[SMP32-01] Argon release mechanism of deformed phengite
Keywords:deformation of phengite, duration of deformation, in-situ Ar-Ar ages, exhumation rate, argon release mechanism, effective diffusion length scale
Cosca et al. (2011) described the effect of deformation on radiogenic argon retentivity from high pressure experiments (10kb and 600 °C over a period of 29 hours, giving ca. 10% shortening) performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Intragrain in-situ 40Ar/39Ar analyses for deformed muscovite provided 309 to 264 Ma, consistent with 0-16 % argon loss relative to the undeformed muscovite (311 Ma). They interpreted that the reduction in the effective diffusion length scale in naturally deformed rocks occurs most probably through productions of mesoscopic and submicroscopic defects such as stacking faults. The extreme size reduction of phengite observed at domains close to the rigid garnet in pelitic schists of the Sanbagawa belt are identical to the reduction in the effective diffusion length scale described by Cosca et al. (2011), likely enhancing the argon release. The Sanbagawa pelitic schists suffered extremely long deformation (longer than 31 million years) during the exhumation and had the extremely slow exhumation rate (slower than 0.7 mm/y). In this case, the phengite could grow up with “Ostwald ripening phenomenon” consuming the extremely fine-grained phengite close to the rigid garnet. The size reduction and “Ostwald ripening” growth would continue to become final stage of static phengite crystal size (0.02-0.06 mm in width and 0.4-1.0 mm in length) observed in matrix. During these processes phengite could experience chemical change depending on local bulk chemistry under the P/T conditions changing from moment to moment during the exhumation, making heterogeneous chemistry. On the other hand, Lago di Cignana UHP unit in western Alps suffered relatively quick deformation (shorter than 5 million years) during the exhumation and had the extremely fast exhumation rate (15 - 26 mm/y). In this case, the phengite crystals could record heterogeneously deformed domains in a crystal as well as among the grains, resulting in heterogeneous argon release. In fact, the laser step heating 40Ar/39Ar analyses of single phengite crystal in the UHP schists of Lago di Cignana provided significantly irregular age spectrum in comparison with the phengite from the Sanbagawa schists of which the age spectra show the perfect plateau for both the phengite separates and a phengite crystal.
A muscovite in metagranitoid from Grand Paradiso Massif in western Alps yielded heterogeneous age values, giving 54 to 218 Ma (Beltrando et al., 2013). The dated points by the crystal edge are significantly younger than the ages in the part far from the edge. These features suggest that the submicrosopic defects occurring preferentially near the crystal edges are affected by the rubbing with other phases, enhancing preferentially the argon release.