Compositional evolution and immiscibility processes in fractionated felsic magmas and its implications on Nb-Ta-Sn mineralisation
Rare-element pegmatites host high concentrations of gems, industrial minerals and strategic metals. The determination of the composition of pegmatite magmas remains one of the most challenging problems in the fields of petrology and metallogeny. During cooling of these magmas, crystals precipitate and the remaining melt enriches in elements that are unsuitable to get incorporated in the major mineral phases (e.g. Li, Nb, Ta, Sn etc.). More and more geochemical and experimental studies indicate that during these conditions, the remaining melt can exsolve an aqueous fluid or even separate into two melts with contrasting chemical affinities. The only direct method to examine the exsolution of melts and fluids and their compositions in natural magmas is by quantifying the composition of melt and fluid inclusions trapped inside minerals. The reliability of inclusions in pegmatite research is debated due to descriptive and analytical problems. In this study we will develop an innovative LA-ICP-MS analysis method to critically examine and quantify inclusions in pegmatites. In combination with mineral analysis and Raman spectroscopy, we will be able to quantify the full range of processes acting during and after cooling of rare-element pegmatites and identify their role on the distribution of economic Nb-Ta-Sn mineralisation. This will provide valuable information about the crystallisation dynamics of felsic magmas and the enrichment processes for strategic metals.