Chemical diffusivity of boron in melts of haplogranitic composition

Abstract

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. I~ODU~ONTHE GEOCHEMICAL SIGNIFICANCE of B has been receiving increasing attention in recent years. It has been shown that the B content of late-stage fluids during the evolution of granite-pegmatite systems may be as high as 5-10 wt% ( LON-DON, 1986a). A body of experimenta data now exists on some ~uilib~um properties of B-bearing melts of geological relevance. For example, experiments have shown that the addition of B lowers both the solidus and liquidus temperatures of water-saturated granitic melts (CHORLTON and MARTIN, 1978; PICHAVANT, 198 1, 1987), shifts the composition of minimum melting in the haplogranite system to more albite-rich compositions ( PICHAVANT, 1987), and increases the solubility of water in melts in the haplogranitic system ( HOLTZ et al., 1993). The influence of B on the differentiation trends of granite-pegmatite systems has been investigated by LONDON and coworkers (LONDON, 1987; LON-DON et al., 1988.Boron also plays a role in the evolution of geological systems via its influence on transport processes. In particular, because of strong partitioning of B into the fluid phase under many geological conditions, B and its isotopes are used as indicators of fluid-rock interaction and to test for open-system behavior. As an example, VAN BERGEN ( 1980) considers me~irnen~~ ~ndidie~te to be produced by reaction between Al mine...

Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic

“…Until now it is still uncertain in which form the boron exists in the melt and how big is the boron isotope fractionation between the melt and tourmaline. Boron in the melt is thought to be in the form of tetrahedral B(OH) 4 -complexes (Pichavant, 1983;Chakraborty et al, 1993). However, Geisinger et al (1988) suggested that a substantial proportion of boron is in trigonal complexes in Al-rich granitic melts.…”

Beryllium‐boron and aluminum‐magnesium chronology of calcium‐aluminum‐rich inclusions in CV chondrites

“…The diffusion of B in melilite, on the other hand, has not been studied. But generally speaking B diffusion in silicates is expected to be slow because B replaces Si (Chakraborty et al, 1993). Boron diffusion rates in albite and anorthte were reported as 2 x 10-17 = 3 x 10-16 cmVs at about 900-1050 "C (Giletti, 1997).…”

Light elements, volatiles, and stable isotopes in basaltic melt inclusions from Grenada, Lesser Antilles: Inferences for magma genesis

“…We thus suggest that kinetic fractionation during dehydration of both AOC or sediments could strongly control the Li isotopic composition of Grenada MIs, whereas the boron model is based on isotopic equilibrium dehydration. This difference relies on the slower diffusivity of B compared to Li [ Chakraborty et al , 1993].…”