Dual role of seawater and hydrothermal fluids in Early Archean chert formation: Evidence from silicon isotopes

The great variety and abundance of chert deposits in Archean terrains constitute one of the most unusual features that mark Earth's early geological history. Proposed explanations for their origin largely relying on fi eld observations, trace element patterns, or oxygen isotope signatures have not yielded an encompassing model. Here we document silicon isotope systematics in cherts from 3.5-3.0 Ga units in the Pilbara Craton (Western Australia) as evidence of their formation by several distinct processes in Early Archean near-surface environments. Our 30 Si results, in combination with geochemical and mineralogical signatures and fi eld relations, point to three end-member sources of silica derivation. One chert type is inferred to have originated through massive transformation of precursor material by silica added from sea water. At least 2 differences in 30 Si between the two other types, produced by direct chemical precipitation on the seafl oor or in conduits, discriminate seawater from hydrothermal fl uid as a source of silica. A virtually continuous Si isotope trend in cherts from this group is consistent with interaction between silica-carrying fl uids at submarine vent systems.