The lithium content of quartz in pegmatites has been widely used as a geochemical proxy to indicate rare-metal mineralization in granitic ore deposits. Despite its significant economic and petrogenetic implications, the partitioning behaviour of lithium between quartz and felsic melts remains poorly constrained. Here, crystallization experiments were performed on metaluminous and peraluminous felsic glasses in an internally-heated pressure vessel at 600-750oC and 300 MPa to better constrain mineral-melt partition coefficients of lithium in granitic systems. Lithium is moderately to highly incompatible in quartz, which varies as a complex function of mineral and melt compositions. The partitioning of lithium is primarily controlled by the coupled substitution of Li+ and Al3+ with Si4+ in the silicate tetrahedra. The aluminium content of quartz is in turn controlled by the activity of Al2O3 in the melt (αAl2O3), which can be monitored using the aluminium saturation index (ASI), defined as the molar ratio of aluminium to alkali metals and calcium (Al2O3/(Li2O+Na2O+K2O+CaO)). Evolved granitic melts are typically considered to be strongly peraluminous; however, the inclusion of lithium in the ASI calculations has a pronounced effect on the aluminosity of glasses above lithium concentrations of ~3000 ppm, wherein the melt becomes metaluminous. Consequently, lithium-rich melts have a lower αAl2O3, which inhibits the substitution of aluminium and lithium into quartz, demonstrating a deviation from Henry's law. The low ASI of enriched granitic pegmatites is consistent with the low partition coefficients of lithium between quartz and melt in lithium-rich felsic system [1]. The effect of lithium on aluminium coordination was tested by performing nuclear magnetic resonance (NMR) on felsic glasses doped with up to 10,000 ppm lithium. The experimental results demonstrate that lithium charge compensates AlO4- tetrahedra, which is interpreted to have a marked effect on αAl2O3 in evolved granitic melts.

[1] Pichavant M (2022) Journal of Petrology 63:1-28