Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites

Abstract

Chlorine in the Earth is highly depleted relative to carbonaceous chondrites and solar abundances(1). Knowledge of the Cl concentrations and distribution on Earth is essential for understanding the origin of these depletions. Large differences in the stable chlorine isotope ratios of meteoritic, mantle and crustal materials have been used as evidence for distinct reservoirs in the solar nebula(2) and to calculate the relative proportions of Cl in the mantle and crust(3). Here we report that large isotopic differences do not exist, and that carbonaceous chondrites, mantle and crust all have the same Cl-37/Cl-35 ratios. We have further analysed crustal sediments from the early Archaean era to the Recent epoch and find no systematic isotopic variations with age, demonstrating that the mantle and crust have always had the same delta Cl-37 value. The similarity of mantle, crust and carbonaceous chondrites establishes that there were no nebular reservoirs with distinct isotopic compositions, no isotopic fractionation during differentiation of the Earth and no late (post-core formation) Cl-bearing volatile additions to the crustal veneer with a unique isotopic composition.