La-Ce and Sm-Nd systematics of siliceous sedimentary rocks: A clue to marine environment in their deposition

Abstract

La-Ce isotopic data, together with Sm-Nd isotopic data, were determined on siliceous sedimentary rocks (cherts) in order to elucidate the rare earth element (REE) character of their sources and the nature of their depositional environments. The cherts studied are a late Archean chert from the Gorge Creek Group in the Pilbara block of Western Australia, Triassic cherts from central Japan, and Cretaceous and Paleogene deep-sea cherts from the central Pacific and the Caribbean Sea. The Archean chert from the Gorge Creek Group shows chondritic Ce and Nd isotope ratios at its sedimentation age which indicate that its sources had a time-integrated chondritic REE pattern. Triassic cherts from Japan have initial Ce and Nd isotope ratios that show a direct derivation from their continental source. On the other hand, for Cretaceous and Paleogene deep-sea cherts having negative Ce anomalies in their REE patterns, two different sources for Ce and Nd are revealed from their initial Ce and Nd isotope data: Ce from long-term light-REE-depleted oceanic volcanic rocks and Nd from light-REE-enriched continental rocks. The reverse nature observed for deep-sea cherts is considered to be a reflection of their depositional environment far from a continent. These results confirm that the La-Ce isotopemore » system is highly useful in determining the nature and cause of Ce anomalies observed in marine sedimentary rocks such as chert.« less