Earth's “Missing” Chlorine May Be in the Core

Abstract

The budget of volatile halogens in the bulk silicate Earth, fluorine and chlorine, differs distinctly from that of chondritic meteorites. Arguably, the bulk silicate Earth shows a low abundance of Cl, while the F budget is in line with the expected volatility trend. One hypothesis for the missing Cl is its sequestration into Earth's core during planetary segregation, but experimental data on partitioning between silicate and metallic melts are limited in pressure and remain inconclusive. Here we use computational quantum mechanical methods to study F and Cl geochemistry during core–mantle differentiation over a wide pressure and temperature range. Our calculations reveal that with increasing pressure and temperature, chlorine shows an enhanced affinity for iron metal. The Cl metal–silicate partition coefficient increase from −1.89 ± 0.84 at 10 GPa and 3000 K to 1.62 ± 0.80 at 130 GPa and 5000 K, while corresponding calculations for F show minimal variations in metal–silicate partitioning across the pressure–temperature conditions investigated, yielding values between −3.61 ± 0.81 and −3.37 ± 0.80. The shift in shows a transition from lithophile to siderophile behavior. Further calculations on isotopic partitioning show negligible fractionation of Cl isotopes (37Cl/35Cl) between the mantle and core. Our results suggest that metallic iron within Earth's mantle and core may serve as an important Cl reservoir, potentially accounting for up to 40% of Earth's overall Cl inventory.