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Journal of the Geological Society; 1991; v. 148; issue.6; p. 985-992;
DOI: 10.1144/gsjgs.148.6.0985
© 1991 Geological Society of London

Article

Fluid disturbed hornblende K-Ar ages from the Dalradian rocks of Connemara, Western Ireland

W. M. MILLER1,3, A. E. FALLICK2, B. E. LEAKE1, R. M. MACINTYRE2 & G. R. T. JENKIN2

1 Department of Geology and Applied Geology, The University, Glasgow G128QQ, UK
2 Isotope Geology Unit, The Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G750QU, UK

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, {delta}D 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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