Young oxygenation of the Archean Keonjhar Palaeosol, India, from 138La-138Ce chronometry

Though there is increasing evidence from different geochemical proxies for intermittent oxygenation prior to the Great Oxidation Event (GOE), it remains ambiguous whether post-depositional processes altered these proxy signals. In this regard, the ca. 3.0 Ga old Keonjhar Palaeosol in the Singhbhum Craton in India has received particular attention, as the presence of Ce anomalies in this Archean palaeosol has been interpreted to reflect oxic surface weathering conditions during the Mesoarchean. Trace element analyses from this study revealed only small or no Ce anomalies and depleted REE patterns relative to the protolith. Most importantly, 138La-138Ce chronometry constrains a 138La-138Ce age of <620 Ma. This result precludes previous claims for Archean atmospheric oxygenation preserved in the Keonjhar palaeosol. Corresponding 147Sm-143Nd data yielded an isochron age of 1765 ± 180 Ma and 176Lu-176Hf data do not define an isochron. Altogether, these results indicate post-Archean REE mobilisation. Our findings emphasise the importance of combining palaeo-redox-proxies with radiogenic isotope analysis to validate their significance.