Halogens in serpentinites from the Isua supracrustal belt, Greenland: An Eoarchean seawater signature and biomass proxy?

Serpentine in modern seafloor and ophiolitic environments incorporates and often retains high concentrations of atmospheric noble gases and seawater-derived halogens. Ancient serpentinites therefore provide the potential to trace the composition of early surface environments. Antigorite-serpentinites locally carbonated to talc-magnesite schist outcropping in a low strain zone within the Eoarchean Isua supracrustal belt (Greenland) are investigated here, to test the retention of paleo-atmospheric noble gases and Eoarchean seawater halogens, and to further determine the genetic setting and metamorphic history of some of Earth's oldest serpentinites. Based on field relationships, whole rock major and trace element geochemistry, and mineral chemistry, the investigated serpentinites are shown to represent hydrated and variously carbonated magmatic olivine ± orthopyroxene + Cr-spinel cumulates emplaced at the base of a lava flow of boninitic affinity pillowed in its upper portion. In addition, rare zircons extracted from one of the serpentinised cumulates have distinct magmatic trace element signatures and a U-Pb age of 3721 ± 27 Ma indicating the pillowed lava flow erupted on the Eoarchean seafloor. The serpentinites have high concentrations of noble gases, but the presence of parentless radiogenic ‘excess’ ⁴⁰Ar, introduced by crustal-derived metamorphic fluids, obscures the ⁴⁰Ar/³⁶Ar ratio of Eoarchean seawater. Local carbonation of the serpentinites also caused halogen loss and fractionation. However, the least carbonated antigorite serpentinites preserve Br/Cl and I/Cl ratios within the range of modern seafloor serpentinites, which is interpreted as indicating Archaean serpentinising fluids were similar in composition to modern seawater-derived fluids. Importantly, the lowest measured I/Cl ratio of 29 (±2) × 10⁻⁶, taken as a maximum value for the Eoarchean ocean, is an order of magnitude lower than estimates for the primitive mantle I/Cl value. Iodine has a low concentration relative to Cl in modern seawater because it is sequestered by organic matter. If the inferred low I/Cl of Eoarchean seawater is correct, then similar I-sequestration was likely occurring in the Eoarchean, a process requiring the presence of significant biomass in Earth's early oceans. Further constraining the Precambrian evolution of seawater I/Cl via serpentinites or other proxies may provide a novel method to explore the emergence and evolution of terrestrial biomass.