LITHIUM MOBILITY DURING DEHYDRATION, FACT OR FICTION?: INSIGHTS FROM LI ISOTOPES IN ALPINE METASEDIMENTARY ROCKS

Understanding element mobility during prograde dehydration is key for constraining geochemical cycling at convergent margins. As a nominally fluid-mobile element, Li can be a powerful tracer of dehydration and other fluid-related processes during subduction (e.g., partial melting, seismicity), yet its behavior remains poorly constrained. Paleo-accretionary wedge metasedimentary rocks from the Schistes Lustres (SL; Italian Alps) record an increase in peak P-T (1.2GPa-350°C to 3.0GPa-550°C), making this suite a natural laboratory for isolating the geochemical effects of prograde dehydration. Whole-rock Li isotope analyses of these rocks were undertaken to constrain Li mobility during subduction. Previous work on Catalina Schist (CA) metasedimentary rocks showed that Li concentrations vary with Chemical Index of Alteration (CIA), an indicator of protolith weathering, suggesting retention of Li to depths of 40km, even along relatively warm P-T paths (Penniston-Dorland et al., 2012). In this study, we examined Li behavior to greater depths (~90km) along cool P-T gradients like those experienced by the low-grade units of the Catalina Schist.

Lithium concentration shows no obvious whole-rock loss with increasing grade, suggesting that Li remains immobile during dehydration to even greater depths than previously thought. Unlike in the Catalina Schist, δ⁷Li in the SL varies with metamorphic grade, however the trend is inconsistent with experimentally-derived fluid-rock fractionations. Both [Li] and δ⁷Li vary with CIA in the SL suggesting that the weak relationship between δ⁷Li and metamorphic grade is sampling-related. These findings are consistent with previous work in the SL demonstrating closed-system redistribution of Li during prograde metamorphism and up to 20% loss of H₂O (Bebout et al., 2013). Variations with CIA suggest a preservation of protolith Li compositions to 90km in a cold subduction zone, with any dehydration loss minimal relative to protolith heterogeneity. The retention of Li during subduction is consistent with other studies of the same traverse demonstrating little loss of fluid mobile or volatile elements during subduction along cooler P-T paths (e.g., Bebout et al., 2013; Cook-Kollars et al., 2014).