Chlorine isotope mantle heterogeneity: Constraints from theoretical first-principles calculations

Abstract

The processes that caused the heterogeneity of δ37Cl in the Earth's mantle are unclear. Here we report theoretical estimates of equilibrium chlorine isotope fractionation among common chlorine-bearing minerals, namely apatite-group minerals (Cl-Ap, F-Cl-Ap, OH-Cl-Ap), muscovite, phlogopite, tremolite, lizardite, marialite and metal halides, based on first-principles calculations, and use them to provide an explanation for this heterogeneity. Our results show that at ambient P-T conditions, the reduced isotopic partition function ratio (β-factor) is strongly correlated with the metal-Cl bond length/bond strength of the above minerals and that phlogopite and muscovite are more enriched in 37Cl than other minerals. As a result of a number of factors, including the adjacent atomic environment, the sites occupied by chlorine atoms in crystals, the Cl-metal coordination and the crystal density, the sensitivity of 103lnβ to pressure follows the sequence: halite > fluoro-chlorapatite > chlorapatite > marialite > hydroxyl-chlorapatite > lizardite > tremolite > muscovite > phlogopite. Estimates of the chlorine isotope fractionation between chlorine-bearing minerals and aqueous fluid at the pressure and temperature prevailing during subduction indicate that the δ37Cl values of mantle minerals could vary between −6‰ and + 3‰, assuming equilibrium. In contrast, the average δ37Cl value of the bulk mantle is −0.53 ± 0.16‰. Thus, large-scale recycling of volatile Cl from the deep mantle to the continental/oceanic crust and ocean, and the isotopic fractionation of chlorine with increasing metamorphic grade during subduction, could explain the heterogeneity of δ37Cl values observed in mantle materials.