Chlorine isotope behavior during prograde metamorphism of sedimentary rocks

摘要

Chlorine stable isotope compositions of two sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n = 25) were collected from a 50 m section of Triassic fluvial and playa-lake strata and Jurassic (Liassic) marine black shales in a well-characterized quarry. Low greenschist to middle amphibolite facies equivalents (n > 80) were collected from the Glarus Alps, Urseren Zone, and Lucomagno region. Bulk delta Cl-37 values are constant within individual sedimentary layers, but vary from -2.0 to +2.4 parts per thousand in Triassic rocks and from -3.0 to 0 parts per thousand in the black shales. Dolomitic and gypsiferous samples have positive delta Cl-37 values, but mans and shales are isotopically negative. Bulk Cl contents show only small declines during the earliest stages of metamorphism. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in delta Cl-37 as their protoliths. Samples with highly correlated bulk compositions but different metamorphic grade show no statistically significant difference in delta Cl-37. These data lead to the following conclusions: (1) Terrestrial and marine sedimentary rocks display large primary heterogeneities in chlorine isotope composition. As a result, an unambiguous "sedimentary signature" does not exist in the chlorine stable isotope system. (2) No isotopic fractionation is discernable during metamorphic devolatilization, even at low temperatures. Alpine-style metamorphism thus has little to no effect on bulk chlorine isotopic compositions, despite significant devolatilization. (3) Cl is largely retained in the rocks during devolatilization, contrary to the normally assumed hydrophilic behavior of chlorine. Continuous release of mixed-volatile C-O-H fluids likely affected Cl partitioning between fluid and minerals and allowed chlorine to remain in the rocks. (4) There is no evidence for fluid communication across (meta)sedimentary layers except within the Urseren shear zone. These data contribute to a relatively closed-system view of metamorphic devolatilization from anchizone through mid-amphibolite facies conditions. (C) 2015 Elsevier B.V. All rights reserved.