CONSTRAINTS ON THE REMOBILIZATION OF THALLIUM AND FLUID-MINERAL INTERACTIONS DURING HIGH-PRESSURE METAMORPHISM

Subduction is a primary driver for the recycling of Earth materials, including that for various metallic elements. However, for most elements, degrees of loss in forearcs allow for return to the surface via volcanism, while transfer into the deeper mantle remains uncertain. Recently, a number of non-traditional stable isotope systems have been explored toward quantifying material recycling in subduction zone settings. One of these systems, thallium (Tl), is useful as it can track redox conditions, fluid behavior, and sediment sourcing through coupled redistribution and isotopic fractionation patterns. Previous work has identified phengite as the dominant host for Tl in many subduction zone terranes (e.g., Rader et al., 2021, Chem. Geol.), particularly at higher-grade metamorphic conditions. To further investigate this, phengite separates were isolated from the well-studied Schistes Lustrés/Lago di Cignana suite exposed in the Western (French/Italian) Alps and representing peak metamorphism at 1.5-3.0 GPa and 300-550°C. Here, we present Tl concentration ([Tl]) and isotope composition (ε²⁰⁵Tl ) data for 15 phengites from metapelitic rocks of this HP/UHP suite.

With increasing grade, phengite shows a statistically significant increase in average [Tl], from ~460 ng/g Tl at the lowest grade to ~1,500 ng/g Tl in the highest grade Cignana rocks. The same phengites show a significant shift to lower ε²⁰⁵Tl values with increasing metamorphic grade, with an average ε²⁰⁵Tl = -2.2 ± 2.9 (2σ) at the lowest grade to ε²⁰⁵Tl = -4.1 ± 0.6 (2σ) at the highest grade. These data may reflect remobilization and redistribution of Tl from destabilizing phases during prograde devolatilization, with ²⁰³Tl preferentially incorporated into existing and newly forming phengite at higher grades, mobilizing some isotopically heavy fluid. These data have implications for fluid-mineral partitioning of Tl, including the likely Tl isotope compositions of fluids and melts released from subducting slabs in forearcs and beneath arcs.