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

The processes that caused the heterogeneity of ?37Cl 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 (?-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 103ln? 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 ?37Cl values of mantle minerals could vary between -6%o and + 3%o, assuming equilibrium. In contrast, the average ?37Cl value of the bulk mantle is -0.53 ? 0.16%o. 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 ?37Cl values observed in mantle materials.

Automated summary

This study provides theoretical estimates of equilibrium chlorine isotope fractionation among common chlorine-bearing minerals, and explains the heterogeneity of ?37Cl in the Earth's mantle. The results show that the isotopic fractionation is strongly correlated with the metal-Cl bond length of minerals, and the ?37Cl values of mantle minerals could vary between -6‰ and +3‰. This heterogeneity is likely due to large-scale recycling of volatile Cl from the deep mantle and isotopic fractionation during subduction.