Comment to “Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean” by Wei et al. A review of the Cl isotope composition of serpentinites and the global chlorine cycle

Wei et al. [Wei, W., Kastner, M., Spivack, A., 2008. Chlorine stable isotope and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean. Earth Planet. Sci. Lett. 266, 90–104] published chlorine isotope data for pore fluids and serpentinites from three subduction zones and two obducted serpentinites. The measured δ37Cl values of serpentinites ranged from 1.2 to 6.0‰ (vs. SMOC), far higher than all other published data for serpentinites. The explanation for these high δ37Cl values defies known fractionation mechanisms. Instead, the anomalous δ37Cl values are almost certainly due to analytical artifacts known to exist for the method the authors employed. al. [Wei, W., Kastner, M., Spivack, A., 2008. Chlorine stable isotope and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean. Earth Planet. Sci. Lett. 266, 90–104] concluded that if high δ37Cl serpentinites (∼ 6‰) were subducted into the mantle, then a change in the δ37Cl value of the crustal reservoir would be observed in the evaporite record over the last 200Ma. Because the Cl isotope composition of the crustal reservoirs has been constant over this time period, as constrained by evaporites, they argue that Cl in serpentinites is recycled back to the oceans. This argument is flawed for two reasons. First, the δ37Cl values of serpentinites is close to 0‰, not 6‰ as suggested in their paper. Second, the δ37Cl value of the crustal reservoir would change immeasurably over 200Ma even if serpentinites of 6‰ were subducted into the mantle. In fact, it has been shown that the crust, mantle and chondritic reservoir all have δ37Cl values close to 0‰ [Sharp, Z.D., Barnes, J.D., Brearley, A.J., Fischer, T.P., Chaussidon, M., Kamenetsky, V.S., 2007. Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites. Nature 446, 1062–1065]. No secular variation in the δ37Cl value of the crust is expected, a conclusion that is supported by chlorine isotope data on sediments extending back over 3.5Ga [Sharp, Z.D., Barnes, J.D., Brearley, A.J., Fischer, T.P., Chaussidon, M., Kamenetsky, V.S., 2007. Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites. Nature 446, 1062–1065].