Chlorine and lithium behavior in metasedimentary rocks during prograde metamorphism: A comparative study of exhumed subduction complexes (Catalina Schist and Schistes Lustrés)
Résumé
Lithium and chlorine concentrations and isotopic compositions of well-characterized metasedimentary suites from the Schistes Lustrés and Lago di Cignana (Western Alps) and the Catalina Schist (southern California, USA) were compared to determine Li and Cl behavior during prograde subduction-zone metamorphism. New data were obtained for Cl from both suites and for Li from the Western Alps. The Catalina Schist contains lawsonite-albite to amphibolite grade rocks recording peak temperatures of 200 to 750 °C and depths of 15 to 45 km, with the higher-grade units representing relatively warm subduction P-T paths. Bulk Cl concentrations decrease from 100-500 μg/g to 10-25 μg/g across metamorphic grades, largely due to Cl loss from chlorite and white mica, with most of this loss occurring at the lower grades. This loss mirrors the previously documented decreases in N, B, Cs, As, and Sb concentrations, with relatively little loss at higher metamorphic grade, and contrasts with the behavior of Li showing only modest loss and change in δ7Li values across all grades. Metapelitic rocks from the Western Alps representing a relatively "cool" geothermal gradient of 8 °C/km, record peak temperatures of 350 to 550 °C and depths of up to 90 km. In this suite, both Cl and Li concentrations are uniform across grade, showing no obvious change in concentration. Lithium concentrations range to somewhat higher values compared to those previously measured in the Catalina Schist. Chlorine isotope compositions of the Schistes Lustrés overlap with those measured in the Catalina Schist, whereas Li isotope compositions of the Schistes Lustrés samples range to somewhat higher values than those from the Catalina Schist. Both suites show significant isotopic heterogeneity within single metamorphic grades and no systematic change in δ37Cl or δ7Li values with increasing grade. These heterogeneities are interpreted to be inherited from the protolith. Lithium isotope compositions correlate well with the chemical index of alteration (CIA), an observation also supporting minimal metamorphic alteration of the isotopic composition reflecting the protolith. These data demonstrate that, although some Cl is retained to great depths during subduction, most Cl is lost from metasedimentary rocks at shallow depths, similar behavior to that of B, Cs, As, and Sb. In contrast, Li loss is minimal across the full range of grade, more similar in behavior to Rb, Ba, and K.