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

Abstract

Understanding argon release mechanisms in K-bearing minerals is essential in interpreting the 40 Ar/ 39 Ar data and their application to geological studies. The release mechanisms of argon in vacuo have been examined in a series of 40 Ar/ 39 Ar isothermal heating experiments on two biotite specimens with 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 high temperature X-ray diffractometer (HTXRD), and also examined by scanning electron microscopy (SEM). At temperatures greater than 600°C, argon release is mainly controlled by the structural decomposition of the biotite crystal arising from oxidation and dehydroxylation, whereas at temperatures less than 600°C, argon release appears to be controlled by a multipath-diffusion mechanism, with effective D/ a 2 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 600°C, regardless of the initial distribution of argon isotopes in the crystal structure, since the argon released at T>600°C is strongly correlated with the decomposition process. At temperature steps lower than 600°C, however, biotite age spectra can exhibit discordant dates 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.