Equilibrium and Kinetic Isotopic Fractionation Processes Recorded in δ7Li Values of Highly Evolved Granitic Pegmatites

In geologic settings, Li isotopes are significantly influenced by both equilibrium and kinetic fractionation mechanisms. This has the potential to make δ7Li values valuable in identifying geological processes or tracing source rocks. Lithium isotopic analysis is becoming an increasingly popular geochemical tool, however, a better understanding of the mechanisms involved in Li isotopic fractionation is necessary if this method is to achieve its full potential. This study combined δ7Li values, trace element geochemistry, mineralogy and primary textural evidence from rock-forming minerals (quartz, albite, spodumene and mica) and whole rock samples taken from a coeval swarm of rare element pegmatite dikes, to look at the extent and mechanisms of Li isotopic fractionation during pegmatite formation. Pegmatite crystallization can be extremely rapid, potentially on a similar timescale to Li diffusion, as a result the influence of variable, non-equilibrium conditions during consolidation was assessed. Rock-forming minerals (given above) from the Little Nahanni Pegmatite Group (Northwest Territories, Canada) display δ7Li values that correlate with textural evidence supporting consolidation of the dikes under non-equilibrium conditions. Two examples of spodumene from different pegmatite samples have comparable δ7Li values of +3.5 and +3.7‰. In contrast, the δ7Li value of mineral separates from co-precipitated mineral assemblages varies from sample to sample. Very uniform δ7Li values for co-precipitated minerals from one sample (muscovite at +7.9‰, plagioclase at +7.9‰ and quartz at +8.7‰), contrast with very different δ7Li values for the same mineral assemblage from a different sample (muscovite at +2.2‰, plagioclase at +3.4‰ and quartz at +15.7‰). Whole rock samples from the same outcrops suggest strong Li isotope fractionation in peraluminous magma is associated with F build up in the late stages of magmatic differentiation. Pegmatites derived from the majority of the melt fraction (85%) are geochemically, relatively poorly evolved and display a restricted range of low δ7Li values (-0.94 to +2.9‰), similar to those of nearby granitoids (-0.4 to +2.2‰) and other examples of S-type magmatism (e.g., Lachlan Fold Belt, Bryant et al. 2004; -0.4 to +2.1‰). In contrast, consolidation of the final, flux component-rich, ~15% fraction of the melt resulted in pegmatite dikes with δ7Li values of up to +11.36‰. We propose that late stage build up of F in the melt is indicated by the Li isotopic signature of the whole rock, and that due to the variation in Li isotope diffusion rates isotopic signatures from minerals of these rapidly crystallised, peraluminous granitic rocks provide a qualitative assessment of the state of chemical equilibrium.