Argon composition of metamorphic fluids: Implications for 40Ar/39Ar geochronology

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Abstract
The concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe, Ar-40/Ar-39 laser microprobe, and fluid inclusion analyses on the same samples. Such information provides constraints on the origin of metamorphic fluids. In our study, fluid in inclusions formed during ductile deformation was probably derived locally from the Paleozoic, Caledonian sequences, whereas brines, emplaced in cross-cutting fractures, were derived from Proterozoic footwall rocks several kilometers from the mylonite. By utilizing Ar-38Cl to correct for ''excess Ar-40,'' we estimate a 370 Ma closure age for the amphibole, which is distinctly younger than the 440-480 Ma plateau ages indicated by step-heating spectra. The ''excess Ar-40'' responsible for the anomalous plateaus probably resulted from diffusive re-equilibration of the amphibole with the fluids present during ductile deformation of the mylonite. The significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region. These ages indicate that the western gneiss terrain, in our study area, records deformation and cooling associated with Siluro-Devonian tectonothermal metamorphism rather than an earlier Caledonian tectonism as proposed in previous Ar-40-/Ar-39 investigations.