Diffusion of 40 Ar in biotite: Temperature, pressure and compositional effects

Details Diffusion of 40 Ar in biotite: Temperature, pressure and compositional effects Diffusion of 40 Ar in biotite: Temperature, pressure and compositional effects

Nov 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49.2461h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 49, Issue 11, pp.2461-2468

Mathematics

Logic

113

Scientific paper

The measured radiogenic 40 Ar loss from sized biotite (56% annite) samples following isothermalhydrothermal treatment have provided model diffusion coefficients in the temperature interval 600°C to 750°C, calculated on the assumption that Ar transport proceeds parallel to cleavage. These data yield an array on an Arrhenius plot with a slope corresponding to an activation energy 47.0 ± 2 kcal-mol -1 and a frequency factor of 0.077 +0.21 -0.06 cm 2 -sec -1 . Together with previous diffusion data for micas in the annitephlogopite series, our results indicate a strong compositional effect, with increasing Fe / Mg ratio corresponding to an increase in diffusivity. An effective diffusion radius of about 150 m for biotite is inferred from the experimental data which compares favorably with that estimated from geological studies. A pressure effect on activation energy corresponding to an activation volume of about 14 cm 3 -mol -1 is observed. These data yield closure temperature estimates for this biotite composition cooling at rates of 100°C-Ma -1 , 10°C-Ma -1 and 1°C-Ma -1 of 345°C, 310°C and 280°C, respectively. 40 Ar / 39 Ar age-spectrum analysis of a hydrothermally treated biotite yields a complex release pattern casting doubt on the general usefulness of such measurements for geochronological purposes.

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