Chert formation pathways through geological time constrained by triple silicon isotopes

• Triple Si isotopes distinguish kinetic and equilibrium chert formation paths. • Archean chert fractionation laws vary systematically with depositional setting. • Hydrothermal vein cherts record near-equilibrium quartz growth. • Post-Archean cherts converge on a common low-temperature kinetic array. • Triple Si isotopes refine cherts as archives of ancient ocean chemistry. Cherts and their silicon isotope compositions are widely used to reconstruct the evolution of seawater chemistry and the marine silica cycle. Despite this, distinguishing between the silicon isotope composition of the parental silica reservoir and the fractionation processes operating during silica precipitation and later recrystallisation challenges interpretations. To address this problem, we present high-precision triple silicon isotope data for 39 cherts spanning the Archean to the Phanerozoic, measured with an external reproducibility of ∼3 ppm for Δ ’29 Si. All samples fall between the theoretical kinetic and equilibrium mass-dependent fractionation lines, indicating that their triple-silicon isotope systematics are consistent with simple mass-dependent behaviour and have not experienced large multi-step overprinting. The Archean samples show a clear dependence on depositional setting. Bedded, interpillow, and volcaniclastic cherts follow a slope consistent with low-temperature kinetic fractionation during precipitation from seawater-derived silica, whereas hydrothermal vein and upflow cherts define a steeper, near-equilibrium slope consistent with slower quartz growth from evolved hydrothermal fluids. These relationships show that distinct silica precipitation pathways coexisted in Archean environments and were linked to depositional setting. By contrast, Proterozoic and Phanerozoic cherts define similar empirical arrays, close to the experimentally determined low-temperature kinetic law and values reported for modern siliceous sponges. Their shared behaviour suggests isotopic convergence toward a common low-temperature fractionation regime after the Archean, regardless of depositional setting. Their arrays also intersect the equilibrium reference line at higher silicon isotope values than the Archean array, consistent with a shift from comparatively light dissolved silica reservoirs in the Archean toward isotopically heavier reservoirs in later oceans. These results show that triple silicon isotopes provide a powerful means of distinguishing precipitation mechanism from source effects in cherts, identifying hydrothermal, seawater-derived, and detrital silica contributions, and refining the use of cherts as archives of ancient ocean chemistry.