APPARENT ARGON DIFFUSIVE LOSS AR-40 AR-39 AGE SPECTRA IN AMPHIBOLES/

Detailed investigations have been made of two amphibole samples where Ar-40/Ar-39 age spectra have been interpreted to show argon diffusive loss. Both samples display complex compositional zoning that reflects partial major and minor element chemical re-equilibration during later thermal events that is associated with the loss of radiogenic Ar-40. The apparent diffusive loss age spectra in these samples are an artefa ct of this chemical re-equilibration process. One sample additionally shows the effect of potassium gain and contamination with a potassium- rich phase (biotite). The resulting ages in this sample are geological ly meaningless and the apparent fit to a theoretical diffusive loss cu rve is coincidental. This study shows that volume diffusive loss of ar gon appears to operate at slower rates than argon loss by chemical re- equilibration in amphiboles.