THE ARGON RELEASE MECHANISMS OF HORNBLENDE INVACUO

Widespread use of Ar-40/Ar-39 incremental heating of hornblende as a g eochronometer has prompted a detailed investigation of the mechanisms of Ar release from this mineral in vacuo in order to accurately interp ret the isotopic data. A combination of high-temperature, in situ, in vacuo X-ray diffraction, and Ar-40/Ar-39 isothermal and step-heating r uns was conducted on three hornblende samples with distinct compositio ns. A comparison of Ar retentivity in hornblende with chemical composi tion suggests that Mg-rich samples will lose Ar in vacuo at higher tem peratures than Fe-rich samples; this may indirectly be a result of the smaller ionic radius of Mg2+ in comparison to Fe2+, as well as intern al stress caused by Fe2+ oxidation. The oxidation of Fe2+ as a result of dehydrogenation appears to play an important role in controlling th e release of Ar from the crystal structure by retarding structural dec omposition of the mineral, and may also explain the one- vs. two-pulse nature characterizing the release of Ar from various hornblendes. Iso thermal Ar-40/Ar-39 runs at different temperatures indicate that: (a) at temperatures at or above the structural decomposition of the hornbl endes, Ar is not released via a volume diffusion- or reaction-controll ed mechanism; and (b) D/a2-values from the low-temperature isothermal runs decrease exponentially with time, yielding estimated diffusivitie s that do not differ greatly from those obtained from hydrothermal exp eriments. This decrease in diffusivity with time also cannot be explai ned by either volume-diffusion or chemical-reaction processes, but mig ht be explained by the presence of a diffusion mechanism much faster t han volume diffusion, termed short-circuit (SC) diffusion. Very rough estimates of SC diffusion parameters for Ar yield a pre-exponential co efficient D0 almost-equal-to 5.55 . 10(-3) cm2 s-1 and activation ener gy E almost-equal-to 45.0 kcal mol-1. More importantly, these results demonstrate that, in general, the incremental heating of hornblende in Ar-40/Ar-39 experiments will not necessarily reflect the true Ar dist ribution within the sample, because structural change and decompositio n initiated by Fe oxidation and dehydroxylation will tend to homogeniz e any spatial gradients that originally exist in the mineral. For ther mochronological studies, this means that plateau dates inferred from A r-40/Ar-39 hornblende age spectra may not accurately reflect the time at which the mineral passed through its Ar closure temperature.