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

Understanding argon release mechanisms in K-bearing minerals is essential i n interpreting the Ar-40/Ar-39 data and their application to geological stu dies. The release mechanisms of argon in vacuo have been examined in a seri es of Ar-40/Ar-39 isothermal heating experiments on two biotite specimens w ith Fe/(Fe + Mg) (Fe-#) = 0.50 and 0.87 respectively. The crystal structure of the biotite was also monitored during in vacuo heating by an in-situ hi gh temperature X-ray diffractometer (HTXRD), and also examined by scanning electron microscopy (SEM). At temperatures greater than 600 degrees C, argo n release is mainly controlled by the structural decomposition of the bioti te crystal arising from oxidation and dehydroxylation, whereas at temperatu res less than 600 degrees C, argon release appears to be controlled by a mu ltipath-diffusion mechanism, with effective D/a(2) values about 2-4 orders of magnitude higher than those extrapolated from hydrothermal data. Both th e argon diffusivity and Ar release patterns are strongly related to biotite composition, in which the Fe-rich biotite has a higher argon diffusivity a nd degasses at lower temperatures than the Mg-rich biotite, Unless contamin ated by other phases, biotites will tend to yield flat age spectra for temp erature steps higher than 600 degrees C, regardless of the initial distribu tion of argon isotopes in the crystal structure, since the argon released a t T > 600 degrees C is strongly correlated with the decomposition process. At temperature steps lower than 600 degrees C, however, biotite age spectra can exhibit discordant dares 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 sp atial distribution of Ar will not lead to accurate interpretations of geolo gic histories, unless the potential effects of short-circuit diffusion are well-constrained. (C) 2000 Elsevier Science B.V. All rights reserved.