Negligible fractionation between fluorine and chlorine during magma ocean crystallization and implications for the origin of Earth's volatiles

Halogens are powerful tracers for the accretion of Earth's volatiles. Their sensitivity to Earth's differentiation, such as core formation, magma ocean crystallization, and planetary evaporation allows us to track such major events and the origin of Earth's building blocks. Here we examine the role of magma ocean crystallization in fractionating F/Cl ratio of the BSE by experimentally determining the partitioning behavior of F and Cl between silicate melt and crystallized minerals. Previous experimental data, due to technical challenges, are only available at pressures <25 GPa. To simulate relevant conditions for deep magma oceans, we perform laser-heated diamond anvil cell experiments to determine partition coefficients of F and Cl between bridgmanite and silicate melt at temperatures of similar to 4000-4500 K and pressures of 33-55 GPa. Our data show that F and Cl are both strongly favored in silicate melt, with partition coefficients of F and Cl in the range of 0.12-0.16 and 0.007-0.017, respectively. Given plausible magma ocean scenarios, our results indicate that F and Cl are not significantly fractionated during crystallization of a deep magma ocean. Neglecting other potential processes that may fractionate F and Cl, the F/Cl ratio in the BSE should reflect that of Earth's building blocks. It indicates Earth's volatiles may be largely derived from inner solar system, in alignment with other isotopic systems such as nitrogen and hydrogen.

Automated summary

Halogens are useful tracers for understanding the formation of Earth's volatiles. Through experiments, it has been found that fluorine and chlorine are not significantly fractionated during the crystallization of deep magma oceans.