The results of K-Ar age determinations and hydrogen and oxygen stable isotope measurements on hornblendes from the poly-metamorphic Dalradian schists of Connemara, western Ireland indicate that the hornblende K-Ar systems have been significantly disturbed by low temperature fluid reaction. Correlations between K-Ar ages, δD values, structural water contents and the occupancy of the hornblende structural ‘A’ sites suggest that excess water as H3O+ occupies vacant ‘A’ site positions during stable isotope exchange and displaces Ar* causing lowering of the K-Ar ages. This process occurs below the normally accepted closure temperature for Ar in hornblende and is not detectable by optical or X-ray investigation of the mineral. This process may not be unique to Connemara hornblendes and so is of potential importance to geochronology in other metamorphic terranes.

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