Magmatic degassing as the primary source of salt in Archean oceans

The salinification of Earth’s early oceans impacted both the climate and the evolution of life. However, available halogen data of Archean seawater samples are at apparent odds with a conventionally assumed mantle origin of sea salt, highlighting a critical lack of mechanistic understanding of how the Archean oceans became salty. Here, we present new triple halogen (Cl-Br-I) data from high temperature (~750–840 °C) fluid inclusions from a gabbroic intrusion in Iceland. Our data show that the magmatic degassing fractionates the halogens and generates fluids with high Br/Cl and I/Cl that match with Archean seawater. These observations suggest that seawater salinity in the Archean oceans was primarily regulated via fluids exsolved from cooling intrusions during genesis of early crust, and that the mantle had a modern-like halogen composition by 3.5 Ga. Our findings highlight the strong connection between the chemistry of the mantle, magmatic fluids and the oceans, with implications for understanding the chemical environments that guided the evolution of early marine organisms.