Behavior of thallium during subduction-zone devolatilization in the Western Alps HP/UHP metasedimentary rocks: The role of phengite

Abstract

Recent studies of thallium (Tl) in various metamorphic and magmatic settings have indicated its usefulness in tracking redox conditions, fluid behavior, and sedimentary sources, with mineral residency and fluid-mineral isotopic fractionation potentially dictating patterns resulting from prograde metamorphism. Previous work has identified phengite as the dominant host for Tl in subduction zone metamorphic suites; thus, Tl behavior deep in subduction zones appears to be strongly tied to the stability of this phyllosilicate. Any isotopic fractionation occurring in forearc regions should dictate the Tl isotope composition of sedimentary sources contributing fluids or melts to the subarc mantle wedge. Toward greater understanding of subduction-zone Tl cycling, phengite separates were isolated from the well-studied Schistes Lustrés/Lago di Cignana suite exposed in the Western (French/Italian) Alps, which experienced peak metamorphism at 1.5–3.0 GPa and 300–550 °C, with ∼20% loss of H2O at the higher grades (peak temperature range of ∼450–550 °C) resulting from breakdown of hydrated mineral phases including chlorite and chloritoid. In the Schistes Lustrés, Tl concentrations (noted as [Tl]) in phengite show no obvious trend across metamorphic grade, hovering mostly between 500 and 1000 ng/g; however, phengite shows a statistically significant increase to ∼1500 ng/g in the highest-grade Cignana (Grade E) rocks. Accompanying this increase in concentration is a significant shift in ε205Tl to lower values, with an average ε205Tl = –2.2 ± 2.9 (2σ) for the lowest-grade metapelites within the Schistes Lustrés to ε205Tl = –4.1 ± 0.6 (2σ) for the highest-grade metapelites at Lago di Cignana. These observations are consistent with the redistribution of Tl from destabilizing Tl hosts, specifically chlorite, during prograde devolatilization, resulting in increased phengite [Tl] at higher grades. Release into fluids of some isotopically heavy Tl during devolatilization could reflect fluid-mineral partitioning of Tl between phengites and the H2O-rich fluid released over the 450–550 °C temperature range. Additional contributing factors could be variability in the [Tl] and ε205Tl composition of the protoliths in this suite and, for the Cignana metapelites, overprinting of mineral assemblages during early-stage exhumation, with or without metasomatism by an infiltrating fluid. The behavior of Tl in this metasedimentary suite, which somewhat resembles that of Li in the same rocks, highlights the importance of phengite as a key mineral host that warrants further exploration.