Diffusion of40Ar in hornblende

✍ Scribed by T. Mark Harrison

• Book ID: 104757174
• Publisher: Springer
• Year: 1982
• Tongue: English
• Weight: 821 KB
• Volume: 78
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00398927

✦ Synopsis

Measured radiogenic h~ loss from two compositionally contrasting hornblendes following isothermal-hydrothermal treatment have provided model diffusion coefficients in the temperature range of 750~ to 900~ Eight experiments using a hornblende (77-600) with a Mg/(Mg + Fe) ratio of 0.72 yield a linear array on an Arrhenius plot with a slope corresponding to an activation energy of 66.1kcal-mo1-1 and a frequency factor of 0.061 cm2-sec -1, assuming spherical geometry for the mineral aggregate. Five experiments undertaken on a hornblende (MMhb-1) with a Mg/(Mg+Fe) ratio of 0.36 show similar behavior to the Mgrich sample, suggesting that the diffusivity of Ar in hornblendes is not sensitive to the Mg/Fe ratio.

These data are consistent with kinetic information obtained from a geological experiment using the thermal effect of a granitoid intrusion. Together these data yield an activation energy of 64.1_+1.7 kcal-mo1-1 and a frequency factor of 0.024 + 0.053 -0.011 cm2"sec-1. For a hornblende with an effective diffusion radius of 80 pm, these diffusion parameters predict closure temperatures between 578~ and 490~ for cooling rates in the range 500 to 5 ~ C-Mr-1.