Experimental constraints on metal-silicate partitioning of chlorine and implications for planetary core formation

Abstract

Chlorine (Cl) is critical for Earth’s habitability and an important tracer for volatile accretion processes. Yet its chemical behavior during core formation, one of the major events throughout planetary formation, is still poorly understood. This is primarily hindered by experimental challenges of reproducing such extreme pressure and temperature conditions and characterizing chemical compositions of recovered samples. Here we perform experiments on Cl partitioning between iron-rich metallic melt (core analog) and silicate melt (mantle analog) at temperatures of 1900–2400 K and pressures of 1–18 gigapascals to simulate core-mantle differentiation of terrestrial planets. More importantly, to avoid likely complications of Cl loss due to wet or oil polishing, we find it is critical to apply dry polishing to recovered samples as shown in previous work focusing on halogens. Our determined partition coefficients of Cl between metallic melt and silicate melt range from <0.003 to 1.38, which shows its siderophile (iron-loving) behavior for the first time. Moreover, we find Cl gradually prefers metallic melt as the increase of pressure, while confirming a positive effect of oxygen contents in metallic liquid. Considering plausible core formation scenarios for Earth and Mars, our results indicate that Cl abundance in Mars’ mantle could be explained by a single-stage core formation scenario. While for the Cl budget in Earth’s mantle, multi-stage core formation with partial core-mantle equilibrium may be required, and this would provide further constraints for dynamics of core formation and volatile accretion.