Insights into Lithium Movement in Ascending Silicic Systems: Variations Recorded in Melt Embayments from Four Volcanic Centers

The enrichment of lithium (Li) in rhyolitic deposits is a critical process to constrain given its increasing economic importance. Although research has focused on its post-depositional movements, the origins of Li are magmatic, and therefore it is also necessary to refine its behavior during magma ascent and degassing. In addition, better understanding of how Li behaves in silicic systems during magma ascent could allow its use as an ascent speedometer. Here, we present Li concentration gradients measured in quartz-hosted melt embayments from four silicic centers where decompression rates exist from modeled H2O and CO2 gradients. These melt embayments respond to changing external conditions, preserving records of magma degassing and ascent when paired with appropriate diffusion coefficients. Of the 41 embayments measured, Li concentrations range from 20-120 ppm, with tighter concentrations ranges within each center (Bishop Tuff, CA: 20-55 ppm; Huckleberry Ridge, WY: 25-95 ppm; Bandelier Tuff, NM: 60-120 ppm; Mt. Pinatubo, PH: 20-50 ppm). Lithium is enriched near the rim (reverse profile) in 60% of cases, especially in embayments from the Huckleberry Ridge, Yellowstone (87%). Although a less common observation, normal profiles were most frequently found for embayments from Mt. Pinatubo. However, embayments are found to contain interior Li values lower than co-erupted melt inclusions, suggesting an earlier stage of resetting prior to the final profile development. Application of a simplified diffusion model with published diffusion coefficients for Li in hydrous rhyolite finds that all preserved gradients form astonishingly fast, within 1-201 or 2-552 seconds, providing timescales orders of magnitude faster than what is obtained from H2O and CO2 modeling (minutes to hours). This suggests that this Li movement is not triggered at the same time as H2O and CO2 degassing, but rather is responding to an alternative process that may differ by volcanic system. As the reverse gradients are the most common phenomena observed, one possibility is that the late-stage breakdown of a fluid phase is releasing Li back into the system3.

1 Holycross, et al. (2018). Geochem Perspect Lett, 6, 39-42.

2 Spallanzani, R., et al. (2022). Contr Min Pet 177(8), 1-17.

3 Charlier, B. L. A., et al. (2012). Earth Planet Sci Lett, 319, 218-227.