Resolving the chlorine isotope composition of Earth’s depleted mantle

The chlorine isotope ratio (δ37Cl value) of Earth’s mantle has implications for volatile exchange between Earth’s surface and mantle, as well as for the source of volatile delivery to Earth. However, there is disagreement about this value, with estimates ranging from −3 ‰ to +0.9 ‰. To resolve this, we examine the δ37Cl values of mid-ocean ridge basalt (MORB) glasses from several ridge segments. We find that the δ37Cl value of the depleted MORB-source mantle (DMM) is ∼ −0.5 ‰, and deviation from that value results from incorporation of subducted material in the DMM. MORB samples that have shallowly assimilated Cl extend towards δ37Cl values of −0.6 ‰ to −1.5 ‰, suggesting assimilation of hydrothermal brines will not result in seawater-like δ37Cl values (0 ‰). The calculated Bulk Silicate Earth δ37Cl value is −0.04 ± 0.13 ‰, statistically indistinguishable from chondrites. This similarity suggests that >91–99 % of Earth’s Cl was inherited from chondrites with little contribution from ingassing of the solar nebula.