Relatively oxidized fluids fed Earth's earliest hydrothermal systems.

The properties of high-temperature lithospheric fluids within the early Earth are poorly known, yet many origin-of-life scenarios depend upon their characteristics. These fluids represent a key communication pathway between Earth's interior and hydrothermal pools. We use zircon chemistry, experiments, and modeling to infer the character of lithospheric fluids approaching 4 billion years. We constrain oxygen fugacity, chlorine content, and temperature, which allow us to model the solubility and transport of metals that are hypothesized to be crucial for the origin of life. We show that these fluids were more oxidized than the terrestrial mantle during this time and that they were interacting with near-surface aqueous systems, possibly subaerial hydrothermal pools, amplifying redox gradients in a location attractive for prebiotic molecular synthesis or sustained microbial activity.