Extreme lithium isotope fractionation in quartz from the Stewart pegmatite

Abstract

Pegmatites are magmatic bodies consisting of centimeter to meter sized crystals and can sometimes be a source for critical economic resources such as lithium (Li). Determining the chemistry and kinetics involved in pegmatite formation may be important for understanding the element enrichment process. Here, we analyzed the Li isotope compositions of quartz crystals from the Stewart pegmatite in southern California, USA. We find large Li isotopic fractionations: >40‰ between different crystals and >20‰ from core to rim in a single crystal. Two mechanisms for these extreme fractionations were considered: rapid crystal growth rate and Rayleigh fractionation. We find that although rapid crystal growth rate (1–10 m/day) can explain elemental variations, rapid growth alone is unable to explain the most extreme isotopic fractionations. Rayleigh fractionation can account for the largest isotopic fractionations if 96–99.9% of the Li is removed from the system through crystallization of lepidolite and spodumene, but alone cannot explain the observed elemental variability in the quartz. We thus suggest that both processes operated. Trace element enrichments may be more sensitive to growth rate while Li isotope ratios may be more sensitive to changes in the isotopic composition of the pegmatitic fluids during crystallization of the quartz.