Venus' geological and atmospheric evolution remains enigmatic, particularly regarding the fate of chlorine. Chlorine is highly incompatible and hydrophilic, and efficient mantle outgassing on Earth has concentrated terrestrial chlorine in surface reservoirs, notably the oceans. Venus was likely subject to similar early magmatic degassing, manifesting in a thick atmosphere and planetary-scale volcanic plains. Nevertheless, the atmosphere of Venus today contains only trace levels of HCl, whilst the extensive NaCl salt deposits observed on Earth and Mars appear absent from the Venusian surface.

Here, we propose chlorine was efficiently outgassed from the primordial Venusian mantle but was later sequestered in crustal minerals as surface temperatures rose. Recent studies have demonstrated chlorine to be highly soluble in silicate melts, and under Venusian conditions its incorporation into alkali-rich, low-degree melts will be efficient. Upon cooling, these Cl-rich melts may crystallize halide-bearing minerals such as sodalite, which act as mineralogical hosts for Venusian chlorine. Previous studies suggest that HCl in the atmosphere of Venus could be buffered by surface mineralogy, and in-situ compositional data and extensive rift-related volcanism suggest alkaline volcanics capable of crystallizing sodalite are widespread on Venus.

To test this hypothesis, we conducted sealed Si-tube box furnace experiments to determine chlorine partitioning between sodalite and alkaline silicate melts representative of the putative Venusian surface. Our results suggest that alkaline partial melts and their differentiates are an efficient petrological ‘sink' for chlorine, with surface over-plating of 1 to 3.75 km of phonolitic or trachytic material capable of sequestering an Earth-like surface chlorine budget. When compared to past volcanic flux estimates, these findings support a model whereby chlorine is efficiently removed from the Venusian surface and sequestered back into its crust. This mechanism offers insights into Venus' volatile evolution and divergence from terrestrial geochemical pathways.