Heterogeneous 40Ar* distributions in naturally deformed muscovite: in situ UV-laser ablation evidence for microstructurally controlled intragrain diffusion

Abstract

Micas are commonly used in 40Ar/ 39Ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained. This study examines the relationship between deformation and deformation-induced microstructures on radiogenic argon retention in muscovite. A combination of furnace step-heating and high-spatial resolution in situ UV-laser ablation 40Ar/ 39Ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the Siviez–Mischabel Nappe, western Swiss Alps (Penninic domain, Briançonnais unit). The pegmatite forms part of the Variscan (∼350 Ma) Alpine basement and exhibits a prominent Alpine S–C fabric including numerous mica ‘fish’ that developed under greenschist facies metamorphic conditions, during the dominant Tertiary Alpine tectonic phase of nappe emplacement. Furnace step-heating of milligram quantities of separated muscovite grains yields an 40Ar/ 39Ar age spectrum with two distinct staircase segments but without any statistical plateau, consistent with a previous study from the same area. A single (3×5 mm) muscovite porphyroclast (fish) was investigated by in situ UV-laser ablation. A histogram plot of 170 individual 40Ar/ 39Ar UV-laser ablation ages exhibit a range from 115 to 387 Ma with modes at approximately 340 and 260 Ma. A variogram statistical treatment of the 40Ar/ 39Ar results reveals ages correlated with two directions; a highly correlated direction at 310° and a lesser correlation at 0° relative to the sense of shearing. Using the highly correlated direction a statistically generated (Kriging method) age contour map of the 40Ar/ 39Ar data reveals a series of elongated contours subparallel to the C-surfaces which where formed during Tertiary nappe emplacement. Similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish. The observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during Alpine (∼35 Ma) greenschist facies metamorphism. One-dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts, thus providing a network of fast diffusion pathways. During Alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon. The structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion, which is consistent with the heterogeneous nature of deformation.