Controls on the mobilisation of Li, Be, Sn, Cs, Ta and W during the melting of metapelites

Elements such as Li, Be, Cs, Sn, Ta, and W are critical for transitioning to green energy supplies and are needed for modern technologies. These metals are commonly mined from peraluminous granitoids and pegmatites, which form from sedimentary protoliths through mica melting reactions (Linnen et al., 2012). While previous studies on the genesis of deposits of these elements have focused on igneous processes, increasing attention is being given to how metamorphic minerals and melting conditions affect the mobilisation of Li, Be, Cs, Sn, Ta, and W. For instance, recent studies have shown that (1) there is a preference of critical elements for either muscovite or biotite and that concentrations in these minerals are often temperature-dependent (Kunz et al., 2022); (2) W and Sn mobilisation at different temperatures is related to the stability fields of muscovite and biotite, respectively (Zhao et al., 2022); and (3) Li and Be concentrations are affected by the peritectic phase that forms during biotite dehydration melting (Ballouard et al., 2023; Costa et al., in review). However, further data and/or constraints are needed to explore numerous still unclear aspects, such as the behaviour of critical elements in different minerals and melting reactions. In this study, we investigated critical element concentrations in various minerals from high-grade terranes across the Variscan orogen in Europe. Also, we coupled our data with existing global datasets, phase equilibria and trace element modelling to understand what controls Li, Be, Cs, Sn, Ta, and W mobilisation to melt during the partial melting of metapelites. Our preliminary data show that: (1) cordierite is the main host of Li and Be, ilmenite and rutile are the main hosts of Ta, Sn and W, and biotite is the main host of Cs, and a secondary host of Li, Sn, Ta, and W (no muscovite present); (2) inclusions and matrix Ti-oxides show variable trace element concentrations; (3) Li concentrations in cordierite are temperature-dependent; and (4) biotite has higher W concentrations in low-pressure samples. In combination with existing literature data, our results have important implications for Li, Be, Cs, Sn, Ta, and W mobilisation during partial melting, particularly in the light of melting reaction sequences at different pressures. These findings underscore the importance of metamorphic minerals and melting conditions and also highlight uncertainties and open questions for future research.