Chemical diffusivity of boron in melts of haplogranitic composition
نویسندگان
چکیده
A~~~-~erni~ Asiatic of B in synthetic melts of hapl~~tic 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 interdiffision coefficients. At the experimental conditions, &OS is found to exchange primarily with SiO, 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 di~sivity increases with decreasing temperature. The activation energy of Dillon ( -70 kcal) is similar to that for viscous flow in melts of the same composition and is relatively insensitive to B content between l-10 w-t% B203 in the melt. However, the addition of the initial 1 wt% B203 to a haplogranitic melt appears to dramatically lower the activation energy for these processes from 100 kCa1 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 mte-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.
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