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Interpreting high-pressure phengite <sup>40</sup>Ar/<sup>39</sup>Ar laserprobe ages: an example from Saih Hatat, NE Oman

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Abstract

New single grain fusion and core-rim 40Ar/39Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite 40Ar/39Ar ages are due either to internally derived 40Ar inherited from a K-bearing precursor, or externally derived 40Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.

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