Transport and reactions of light elements in pegmatitic systems under thermal disequilibrium – implications for magmatic/ hydrothermal ore deposits

Lithium has played an ever-increasing role in various industrial branches during the past decades, with lithium-ion batteries consuming the major amount of produced Li. Brines carry a much larger reservoir of lithium than pegmatites, however, pegmatites advantageously offer a wider geographic distribution and a more Li-dominant composition (Kesler et al., 2012). Thus, there is a large interest in understanding the enrichment mechanisms of Li in pegmatites. Due to the relatively large mass difference of the most common isotopes of Li and boron (7Li/6Li; 11B/10B), large fractionation effects may occur, driven either by equilibrium partitioning between coexisting phases or by kinetic (non-equilibrium) processes. Therefore, Li and B and their respective isotopes can be utilized as geochemical tracers for hydrothermal and magmatic differentiation processes and possibly reveal information on fluid–rock interactions (Li et al., 2018; Maner and London, 2018). While knowledge about the partitioning of Li and B isotopes between coexisting phases has made significant progress in the last decades, little is known about the behaviour of these elements under disequilibrium conditions.

The key questions we want to investigate in our study are: How fast is diffusion of Li and B in pegmatitic to granitic melts under magmatic to hydrothermal conditions? Does diffusion in the melt and transport in the fluid produce significant isotope fractionation for both elements? What are the effects of temperature, pressure and composition on these transport phenomena? To answer these questions, we want to use an experimental approach involving a source for the diffusing species, a porous medium filled with a fluid and a sink for Li and B. We plan to place pre-fractured quartz crystals in the porous medium to potentially entrap the fluid during transport. We will use a pegmatitic model composition that was previously studied at our institute with respect to viscosities. To tackle the questions about the behaviour of Li and B during kinetic processes, we will use a three-zone furnace, where experiments will be conducted under thermal gradients. Subsequently, Li and B isotope compositions will be analysed by femtosecond LA-MC-ICP-MS.

Figure 1: Preliminary investigation of Li and B diffusion according to the diffusion couple technique.

Figure 1: Preliminary investigation of Li and B diffusion according to the diffusion couple technique. Experimental conditions are 850 °C, 100 MPa and 12 min duration. Top: Photomicrograph of a Li diffusion couple. The right part (blue colour) is Li-rich and the left part is initially Li-free. Bottom: Li concentration plotted against distance with the zero-point marking the interface between the initially Li-rich and Li-free parts. Error bars refer to 2σ error.

 

References

Kesler, S. E., Gruber, P. W., Medina, P. A., Keoleian, G. A., Everson, M. P., and Wallington, T. J.: Global lithium resources: Relative importance of pegmatite, brine and other deposits, Ore Geology Reviews, 48, 55–69, doi.org/10.1016/j.oregeorev.2012.05.006, 2012.

Li, J., Huang, X.-L., Wei, G.-J., Liu, Y., Ma, J.-L., Han, L., and He, P.-L.: Lithium isotope fractionation during magmatic differentiation and hydrothermal processes in rare-metal granites, Geochimica et Cosmochimica Acta, 240, 64–79, doi.org/10.1016/j.gca.2018.08.021, 2018.

Maner, J. L. and London, D.: Fractionation of the isotopes of boron between granitic melt and aqueous solution at 700 °C and 800 °C (200 MPa), Chem. Geol., 489, 16–27, doi.org/10.1016/j.chemgeo.2018.05.007, 2018.