Exsolution in hornblende and its consequences for 40Ar39Ar age spectra and closure temperature

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