Origin of negative Ce anomalies in Barberton sedimentary rocks, deduced from La–Ce and Sm–Nd isotope systematics

Abstract

The origin of sedimentary rocks with negative Ce anomalies among several samples derived from the Barberton Greenstone Belt (3250 Ma), South Africa, was investigated using La–Ce and Sm–Nd isotopic systematics, together with light rare Earth element (LREE) abundances. Negative Ce anomalies were recognized in two sedimentary rocks, a banded ferruginous chert and a shale. Their Ce/Ce * values are 0.40 and 0.81, respectively. Initial Ce isotopic ratio calculated for the ferruginous chert, 138Ce/ 142Ce (3250 Ma) = 0.0225059, is far lower than CHUR value at 3250 Ma and even the value at Earth formation, 138Ce/ 142Ce CHUR (4560 Ma) = 0.0225409. The calculated initial Ce isotopic ratio for the shale with a Ce anomaly is also lower than that of other rocks without Ce anomalies. These results indicate that the negative Ce anomalies were not inherited from an Archean redox change, but occurred following later episodes of alteration. Two possible formation models were examined to estimate a plausible occurrence age for the negative Ce anomalies. One is characterized by Ce extraction and the other is by an addition of La, Nd and Sm. Both models indicate that negative Ce anomalies arose before 1100 Ma. This study suggests that La–Ce isotopic systematics provides a unique tool to constrain the occurrence age of negative Ce anomalies observed in Archean sedimentary rocks and gives some hints about the redox condition in the Archean ocean and the evolution of the Earth.