Chlorine isotope fractionation during serpentinization and hydrothermal mineralization: A density functional theory study

Abstract

Because of the large-scale recycling of volatile chlorine from the interior to the surface of the Earth, it is possible to use the isotopic composition of this element (δ37Cl) in serpentinite to study mantle-crust interactions in subduction zones. It is also possible to use chlorine isotopes to track the evolution of fluids (through fluid inclusions/minerals) in hydrothermal ore-forming systems. Here, we report the results of a study of equilibrium chlorine isotope fractionation during serpentinization and hydrothermal mineralization based on density functional theory (DFT) and ab initio molecular dynamics simulations (AIMD). The chlorine isotope fractionation between lizardite and brine under variable thermodynamic conditions is described by the relationship 1000lnαlizardite-fluid = 0.4170 × (1000/T)2–0.0281 × (1000/T) + 0.0582, which yielded Δ37Cllizardite-fluid values of +0.49 to +4.46‰, +0.41 to +0.59‰ and + 0.49 to +3.57‰ for conditions at the seafloor, in the mantle wedge and in subduction zones. As a proxy for the diversity of metal-chloride complexes in hydrothermal fluids, the stable configurations of ferrous chloride complexes were acquired from long trajectories of AIMD simulation. Using this information, the chlorine isotope fractionation between minerals (i.e., apatite-group minerals, muscovite, phlogopite, tremolite, lizardite, marialite and metal halides) and ore-forming fluid was estimated. The relatively large chlorine isotope fractionation (Δ37Cl minerals-hydrothermal fluid values from −1.99 to +2.18‰) might partly explain the large variation of δ37Cl in individual fluid inclusions observed in hydrothermal ore deposits. Because of the limited chlorine isotope fractionation between apatite and hydrothermal fluid (i.e., Δ37Cl apatite-ore-forming fluid of 0.06‰–0.69‰), apatite-group minerals might be an alternative to fluid inclusions for constraining the origin and evolution of hydrothermal fluids using δ37Cl values. The theoretical constraints provided in this paper will facilitate the use of chlorine isotopes in tracking the sources of chloride in subduction zones and the origin of mineralizing fluids in ore deposits.