Pegmatite systems can host a variety of Li- and B-rich mineral assemblages. Lithium and B are light metals that occupy contrasting roles in the structural melt network and their diffusion rates differ by several orders of magnitude in melts. Nevertheless, the complex processes during pegmatite crystallization can lead to an enrichment of both, Li and B.

To shed more light on the interaction between a pegmatite-forming melt and a coexisting fluid phase, we developed a novel experimental setup. Two glasses, one Li- and B-bearing, the other Li- and B-free, were placed at both ends of a ca. 7 cm long Au-capsule. The two melts were physically separated by an inert solid material (ground quartz or zircon). The exchange of Li and B between the two melts was made possible by adding a fluid with either NaCl or CsCl concentrations in the range of 1–3 wt %. Thus, the fluid phase was the transport medium for Li and B. Experimental runs were carried out in a rapid-heat/rapid-quench cold seal pressure vessel at 100 MPa. The duration was systematically varied between 1 h and 96 h. The temperature was either set to 850 °C, or a temperature gradient of ca. 50 K was applied over the entire length of the capsule.

The presence of Li or B in the initially Li- and B-free melt proves that they were transported via the fluid phase. We found that the mass flux highly depends on the composition of the fluid phase, but also on the material used to separate the two melts. While the sink had almost identical δ11B as the source melt, it was significantly heavier in δ7Li. This shows that the Li isotopes fractionated by equilibrium processes during the transport (i.e. differences in bond strength between melt and fluid).