Excess argon (40ArE) uptake during slate formation: A 40Ar/39Ar UV laserprobe study of muscovite strain-fringes from the Palaeozoic Welsh Basin, UK

Syn-kinematic pure muscovite strain-fringes grew around pyritized graptolites in response to the early Devonian (Acadian) closure and uplift of the Peri-Gondwanan Welsh Basin. They record Ar-isotope information on the timing of low-temperature deformation and the mudrock to slate transition. Here we have applied three different 40Ar/39Ar laserprobe dating methods to strain-fringes from the central Welsh Basin to test for variations in age and to explore the technical limitations of the method. Infrared laserprobe step-heating analyses yielded only occasional plateau ages (varying between ca. 403 Ma and 422 Ma) and strong evidence for discordance in the Ar-system. Likewise, ages from infrared laserprobe total fusion analysis are scattered, and range from ca. 390 Ma to 651 Ma. By contrast, ultraviolet laserprobe intra-fringe age profiles revealed a complex pattern of ages across individual strain-fringes. This is the highest spatial resolution technique available, and the resulting age profiles consistently showed an oscillatory pattern comprising troughs, or ‘baseline’ ages that are all within error of ca. 397 Ma, and peaks, or ‘apparent’ ages, ranging up to ca. 550 Ma. These ‘apparent’ ages we interpret as the results of repeated, periodic assimilation of excess 40Ar (40ArE) during deformation and the mudrock to slate transition. Here, basinal fluids carrying 40ArE are driven through bulk mudrock during contractional phases of the Acadian orogen, and 40ArE is taken up by muscovite strain-fringes. The concentration of 40ArE varies with metamorphic grade, and is inversely proportional to the temperature of metamorphism, and we suggest is controlled by the solubility of Ar in water, which has potentially quite severe implications for 40Ar/39Ar dating of K-bearing minerals in any mudrock. These results provide a first record of Ar-isotope information in strain-fringes, and a first record of orogenically-driven fluid-flow through bulk deforming mudrock. They also demonstrate how the high spatial resolution ultraviolet laserprobe 40Ar/39Ar method can access Ar-isotope information that is obscured by lower spatial resolution laser analysis, or bulk sampling methods.