Argon release mechanisms of biotite in vacuo and the role of short-circuit diffusion and recoil

Understanding argon release mechanisms in K-bearing minerals is essential in interpreting the ArrAr data and their application to geological studies. The release mechanisms of argon in vacuo have been examined in a series of ArrAr Ž . Ž a. isothermal heating experiments on two biotite specimens with Fer FeqMg Fe s0.50 and 0.87 respectively. The crystal structure of the biotite was also monitored during in vacuo heating by an in–situ high temperature X-ray Ž . Ž . diffractometer HTXRD , and also examined by scanning electron microscopy SEM . At temperatures greater than 6008C, argon release is mainly controlled by the structural decomposition of the biotite crystal arising from oxidation and dehydroxylation, whereas at temperatures less than 6008C, argon release appears to be controlled by a multipath-diffusion mechanism, with effective Dra values about 2–4 orders of magnitude higher than those extrapolated from hydrothermal data. Both the argon diffusivity and Ar release patterns are strongly related to biotite composition, in which the Fe-rich biotite has a higher argon diffusivity and degasses at lower temperatures than the Mg-rich biotite. Unless contaminated by other phases, biotites will tend to yield flat age spectra for temperature steps higher than 6008C, regardless of the initial distribution of argon isotopes in the crystal structure, since the argon released at T)6008C is strongly correlated with the decomposition process. At temperature steps lower than 6008C, however, biotite age spectra can exhibit discordant dates Ž . since the gas release is controlled mainly by defect-enhanced short-circuit diffusion mechanisms. Consequently, models using such low-T steps with the intent of extracting information on the spatial distribution of Ar will not lead to accurate interpretations of geologic histories, unless the potential effects of short-circuit diffusion are well-constrained. q 2000 Elsevier Science B.V. All rights reserved.