Oxidized Fluids Powered Earth's Ancient Hydrothermal Systems

An analysis of nearly 4-billion-year-old zircons has informed scientists' grasp of the geochemistry of Earth's earliest hydrothermal systems and provided new insights into how minerals and prebiotic organics may have formed in the planet's youngest days. The chemistry of Earth's early hydrothermal systems is important to understanding the formation of minerals and organic compounds, which are thought to be crucial to the eventual development of life on the planet. However, the geological record of early Earth is sparse; the lack of unaltered minerals that date back to the planet's earliest days makes understanding the chemical and physical properties of high-temperature lithospheric fluids and their interactions with Earth's interior difficult. Zircons are extremely resilient minerals and can record the isotopic and elemental signatures of their surroundings when they are formed deep inside the planet. Dustin Trail and Thomas McCollom used ~4 billion-year-old Jack Hills zircons to explore the fluid geochemistry of Earth's ancient hydrothermal systems. Using synthetic zircons crystallized in hydrothermal fluids of varying redox states, Trail and McCollom developed a zircon experimental calibration to quantify the oxygen fugacity of the hydrothermal fluids from which 8 of the Jack Hills zircons had recrystallized nearly 3.9 billion years ago. The authors discovered that the redox state of the ancient hydrothermal fluids was slightly more oxidized than the upper mantle during that time. The findings suggest that these fluids would have reached the surface enriched in catalytic metal ions and reduced gases, creating pools at the surface that may have been ripe for prebiotic organic synthesis or sustained microbial activity. "The calibration approach developed by Trail and McCollom can be applied to characterize the geochemistry of other hydrothermal systems captured within ancient zircons, including those found on other worlds," writes Laura Rodriguez in a related Perspective. "Mars is predicted to have a preponderance of zircons from rocks that had been altered by ancient fluids."