CHEMICAL DIFFUSIVITY OF BORON IN MELTS OF HAPLOGRANITIC COMPOSITION

Chemical diffusivities of B in synthetic melts of haplogranitic compos ition have been measured by the diffusion couple technique at 1 atm be tween 1200-1600-degrees-C. The compositional profiles were measured by ion microprobe and modelled using the Boltzmann-Matano formalism to r etrieve compositionally dependent interdiffiision coefficients. At the experimental conditions, B2O3 is found to exchange primarily with SiO 2 and the interdiffusion coefficient increases with increasing replace ment of Si by B in the melt. No isotopic fractionation of boron was ob served in the diffusion zone at the experimental conditions. The compo sitional dependence of diffusivity increases with decreasing temperatu re. The activation energy of diffusion (approximately 70 kcal) is simi lar to that for viscous flow in melts of the same composition and is r elatively insensitive to B content between 1-10 wt% B2O3 in the melt. However, the addition of the initial 1 wt% B2O3 to a haplogranitic mel t appears to dramatically lower the activation energy for these proces ses from approximately 100 kcal to approximately 70 kCal. Thus, common geochemical concentrations of B may affect petrogenesis of granitic r ocks by their influence on these transport properties. Some implicatio ns of these results for crystal growth and dissolution in B-bearing me lts and boron isotopic variation of granitic melts have been discussed . If diffusion is the rate-limiting process, boron isotopic heterogene ity may be maintained in granitic melts at magmatic temperatures on ti me 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 peralumin ous granites.