Chakraborty, Sumit; Dingwell, Donald B.; Chaussidon, Marc
Chemical diffusivity of boron in melts of haplogranitic composition.
In: Geochimica et Cosmochimica Acta, Vol. 57, No. 8: pp. 1741-1751
Chemical diffusivities of B in synthetic melts of haplogranitic composition have been measured by the diffusion couple technique at 1 atm between 1200–1600°C. The compositional profiles were measured by ion microprobe and modelled using the Boltzmann-Matano formalism to retrieve compositionally dependent interdiffusion coefficients.
At the experimental conditions, B2O3 is found to exchange primarily with SiO2 and the interdiffusion coefficient increases with increasing replacement of Si by B in the melt. No isotopic fractionation of boron was observed in the diffusion zone at the experimental conditions. The compositional dependence of diffusivity increases with decreasing temperature. The activation energy of diffusion (~70 kcal) is similar to that for viscous flow in melts of the same composition and is relatively insensitive to B content between 1–10 wt% B2O3 in the melt. However, the addition of the initial 1 wt% B2O3 to a haplogranitic melt appears to dramatically lower the activation energy for these processes from ~ 100 kCal to ~70 kCal. Thus, common geochemical concentrations of B may affect petrogenesis of granitic rocks by their influence on these transport properties. Some implications of these results for crystal growth and dissolution in B-bearing melts and boron isotopic variation of granitic melts have been discussed. If diffusion is the rate-limiting process, boron isotopic heterogeneity may be maintained in granitic melts at magmatic temperatures on time scales of millions of years on a millimeter scale. The influence of small amounts of B on transport properties may also contribute toward resolution of an enigma regarding emplacement mechanisms of peraluminous granites.