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Exsolution in hornblende and its consequences for40Ar 39Arage spectra and closure temperature

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/0016-7037(86)90173-0


Abstract Step-heating studies of hornblende and biotite which coexist in an amphibolite near North Walpole, Vermont, reveal dramatically different 40Ar 39Ar age spectra. The biotite (Mg#59) is characterized by a plateau over 98 percent 39Ar release at 343 ± 5 Ma, in strong contrast to the amphibole which exhibits both a complex age spectrum and a K-Ar age significantly younger than the biotite. The amphibole release pattern contains three complete age cycles which range between extremes of 199 Ma and 408 Ma. Interestingly, the K Ca ratio (calculated from reactor-produced isotopes) also varies with Ar-release, but follows a trend which is antipathetic to the 40Ar 39Ar age pattern. Detailed microstructural examination, including transmission electron microscopy (TEM), reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40Ar ( 40Ar ∗) retentivity in the amphibole. This result underscores the variability of closure temperature ( T c ) for 40Ar ∗ in metamorphic amphiboles and should warn against accepting a single, universal value of T c .

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