Thallium behavior during high-pressure metamorphism in the Western Alps, Europe

Thallium (Tl) is a highly incompatible element which can be redistributed during various igneous and hydrothermal events such as mineral dehydration and low-temperature hydrothermal alteration. Despite the strong influence fluid-rock interactions can have on Tl redistribution and the importance of fluids in subduction-zone volatiles cycling, the geochemical behavior of Tl during high-P/T metamorphism has not yet been quantified. Here we provide new Tl isotope composition (Ε205Tl) and concentration ([Tl]) data for a suite of 18 metapelite and metacarbonate rocks from the Schistes Lustrés Complex and the nearby Lago di Cignana exposure in the western Alps. This suite represents metamorphism over a range of peak metamorphic P-T conditions of 300–550 °C and 1.5–3.0 GPa (similar to the range of P-T experienced in most modern subduction zones). With increasing metamorphic grade, [Tl] shows no statistical variation while Ε205Tl initially remains fairly constant (in order of increasing grade, Fraiteve (unit A) Ε205Tlavg = −2.1 (± 0.3); Assietta (unit B) Ε205Tlavg = −1.6 (± 0.6); Albergian (unit C) Ε205Tlavg = −1.9 (± 0.5)) before shifting to more negative values during higher grade metamorphism (Finestre (unit D) Ε205Tlavg = −2.4 (± 0.3); Cignana (unit E) Ε205Tlavg = −3.5 (± 0.3)). Although it appears likely that Tl is largely retained in such rocks to depths approaching those of arc magma generation, the mineralogical hosts for Tl change during prograde devolatilization. This may contribute to a significant shift in Ε205Tl related to fractionation induced by only minor amounts of loss due to fluid-mineral partitioning. Prograde metamorphism results in the breakdown of chlorite and other minor mineralogical constituents, releasing chlorite-borne Tl that could be in part incorporated into phengite and in part lost to fluids. This study emphasizes the need for prograde mineral reaction history to be considered in studies of the retention or loss of trace elements such as Tl. © 2021 Elsevier B.V.