Chlorine isotope chemistry of serpentinites from Elba, Italy, as an indicator of fluid source and subsequent tectonic history

Abstract

Chlorine concentrations and δ37Cl and δD values are reported for serpentinites in different tectonic positions on Elba, Italy. Serpentinites from high in the nappe sequence contain 0.01–0.1wt% Cl with δ37Cl values from −0.9 to +1.4‰. Samples with positive δ37Cl values occur beneath gabbro and basalt and are interpreted as having been serpentinized by interaction with seawater. Samples with negative δ37Cl values are in direct contact with sedimentary rocks. These samples were juxtaposed against and/or buried by sediments in response to low‐angle normal faulting on the seafloor, and serpentinization probably occurred by subsequent interaction with sedimentary pore fluids. Most samples from a structurally lower nappe and from areas affected by contact metamorphism lost their original Cl during heating. Low δD values for all samples suggest postserpentinization interaction with meteoric water, but neither Cl nor O isotope ratios were affected by this interaction. Our results demonstrate that chlorine can be retained in obducted serpentinites up to low greenschist conditions and that the Cl isotope composition of such serpentinites preserves a record of seafloor tectonic processes.