As mentioned in Chap. 12 analyses of the most pristine oceanic crust samples available were used to determine the chlorine isotope composition of the mantle. These studies attempted to determine the chlorine isotope composition of the mantle though the analysis of samples that show clear mantle characteristics. The oceanic crust however, is for the largest part contaminated by chloride that descends from assimilation with seawater, and it is generally agreed that the oceanic crust has an average δ37Cl value which is close to 0‰. This follows not only from the data reported by Sharp et al. (2007, 2013), who measured chlorine isotope data close to 0‰ in most of their samples, but this is also inferred from the data reported by Bonifacie et al. (2008) and Magenheim et al. (1995) who both concluded that in samples with higher chlorine concentrations δ37Cl values appear to converge to 0‰.