Earth's "Missing" Chlorine May Be in the Core

L Yuan and G Steinle-Neumann, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 129, e2023JB027731 (2024).

DOI: 10.1029/2023JB027731

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 log10DClm/s $\log _10D_\textCl<^>\mathrmm/\mathrms$ 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 log10DFm/s $\log _10D_\mathrmF<^>\mathrmm/\mathrms$ values between -3.61 +/- 0.81 and -3.37 +/- 0.80. The shift in log10DClm/s $\log _10D_\textCl<^>\mathrmm/\mathrms$ 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. Chlorine plays a critical role within Earth's systems: balancing cellular charge and osmotic potential; regulating ocean salinity; transporting ore-forming metal in geological fluids; degrading stratospheric ozone. Despite its importance, the amount of chlorine in the planet has been poorly understood due to uncertainty in the composition of Earth's core, likely the largest reservoir that contains most of the Earth's volatile elements, for example, hydrogen and carbon. Here, we demonstrate that chlorine becomes increasingly compatible with metal compared to silicate at pressure-temperature conditions at which Earth's core formed. Conversely, fluorine as another halogen element continues to show a strong affinity for rocks. Therefore, substantial amounts of chlorine may exist within the Earth's core. Using quantum mechanical calculations, we find that chlorine changes behavior from lithophile to siderophile in the deep Earth Fluorine, on the other hand, remains strongly lithophile for all pressure-temperature conditions relevant to planetary core formation Earth's superchondritic fluorine/chlorine ratio can plausibly be explained by chlorine sequestration into the core

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