Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites

Chlorine isotopes rethink Solar System prehistory may need to be rewritten following a reappraisal of the chlorine isotope content of chondritic (stony) meteorites, and samples from the Earth's crust and mantle. Large differences in the ratio between 37 Cl and 35 Cl in meteoritic, mantle and crustal materials have, for years, been taken as evidence for distinct reservoirs in the solar nebula, the dusty gas cloud left over after the formation of the Sun which eventually formed the Solar System planets. But the new analyses reveal that the large isotopic differences do not exist. In fact, carbonaceous chondrites, mantle and crust all have the same 37 Cl/ 35 Cl ratios. So there were no nebular reservoirs with distinct isotopic compositions, no isotopic fractionation during differentiation of the Earth and no late chlorine-bearing volatile additions to the crust. This paper reports that large isotopic differences in the stable chlorine isotope ratios of meteoritic, mantle and crustal materials do not exist, that carbonaceous chondrites, mantle and crust all have the same 37 Cl/ 35 Cl ratios, establishing that there were no nebular reservoirs with distinct isotopic compositions, no isotopic fractionation during differentiation of the Earth and no late Cl-bearing volatile additions to the crust. 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 37 Cl/ 35 Cl 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 δ 37 Cl 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.