Episodic negative anomalies of cerium at the depositional onset of the 2.65-Ga Itabira iron formation, Quadrilátero Ferrífero of Minas Gerais, Brazil

• Truly negative anomalies of Ce found in magnetite bands in the uppermost part of the Batatal Formation.
• The Batatal Formation grades upward into the 2.65-Ga Itabira iron formation.
• Low Th/U ratios with and without oxidative decoupling of Ce from the other REE.
• Mild oxygenation of sea water 200 Ma before the Great Oxidation Event.

Magnetite bands in sericitic phyllite of the Batatal Formation mark the transition to the overlying 2.65-Ga Itabira iron formation, or Cauê Itabirite, in the Quadrilátero Ferrífero of Minas Gerais, Brazil. The magnetite bands have low (non-crustal) Th/U ratios, suprachondritic Y/Ho ratios and sporadic anomalies of Ce that are characterised as truly negative. The data point to mild oxygenation of sea water at the depositional onset of the Itabira iron formation some 200 Ma before the Great Oxidation Event.