Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia

The concentration of halogens (Cl, Br) and sulfate in seawater during the Archaean eon have important implications for the evolution of Earth’s hydrosphere and atmosphere and the development of early life. Insights into the composition of Archaean seawater and hydrothermal fluids can be obtained by direct analysis of fluid inclusions preserved in Archaean sediments and hydrothermal systems. Here, we investigated a suite of well-preserved intrapillow quartz–carbonate pods that formed during oceanic hydrothermal alteration of the 3.49 Ga Dresser Formation, North Pole Dome, Western Australia. Texturally, the pods seems to contain a unique population of primary fluid inclusions which were analyzed individually using microthermometry and synchrotron radiation X-ray microfluorescence (A-SR-XRF) techniques. Bulk chemical analyses were also performed using crush-leach method. Microthermometric data combined with crush-leach and A-SR-XRF analyses yielded a model composition of 1100 mM Na, 2250 mM Cl., and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt.% salt equivalent. This high Cl concentration (ca. four-times present-day value) reflects a typical modern-day seawater evaporation trend in a shallow marine, closed basin environment. Individual fluid inclusion analysis using A-SR-XRF revealed the presence of three main fluid populations: a metal-depleted fluid, a Ba-rich and S-depleted fluid, and a Fe–S-rich end-member. The Cl/Br ratio of metal-depleted fluid inclusions (630) is similar to the modern seawater value (649). By contrast, Baand Fe-rich brines have Cl/Br ratios (350 and 390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. The metaldepleted fluid displays low sulfate concentration (0–8 mM compared to 28 mM in present-day ocean). Sulfur content of the Ferich fluids ranges between 41 and 82 mM.