Rare earth evidence in iron-formations for changing Precambrian oxidation states

Abstract

Rare earth element distributions in Precambrian iron-formations show evidence of evolving oxidation states for the elements europium and cerium. Chemical sediments older than about 1900m.y. are enriched in Eu compared to their contemporaneous clastic sediments. This indicates that at some time during the weathering, transportation and/or deposition of the rare earth elements, a considerable proportion of the Eu was present as Eu 2+ rather than totally as Eu 3+. Sea water prior to 1900m.y. was probably enriched in Eu because of this. At some time or interval between 1900 and about 800 m.y. Eu was totally converted under surficial conditions to the +3 state and its behavior ceased to be anomalous. The Eu-enriched nature of Precambrian sediments may be partly explained by Eu enrichment of sea water prior to 1900 m.y. Cerium exhibits anonalous abundances in Proterozoic chemical sediments as old as 2300 m.y., but is not anomalous in Archean rocks. Oxidation of Ce to the +4 state is thus indicated during the early Proterozoic and implies the presence of Mn nodules in early Proterozoic seas. Mn distributions, however, would appear to contradict the presence of Mn nodules as there is no evidence for Mn oxidation states higher than +2 at this time. Rare earth element contents in Precambrian iron-formations are very low and may indicate that ferric iron precipitation was not accompanied by trace element scavenging as it is today.