Rare earth element patterns and crustal evolution—I. Australian post-Archean sedimentary rocks

A suite of Australian shales, greywackes and subgreywackes ranging in age from Proterozoic to Triassic were analyzed for the rare earth elements (REE) in order to detect any secular changes in rare earth distribution. These post-Archean sediments show remarkably similar relative rare earth patterns. They are characterized by negative Eu anomalies of almost constant magnitude (average Eu/Eu* = 0.67 ± 0.05) relative to chondrites and nearly constant ratios of light to heavy rare earths (average ∑LREE/∑HREE = 9.7 ± 1.8).

The REE abundances are generally higher in the younger sediments which may suggest that the absolute abundances of the rare earths in clastic sediments have gradually increased with time. Since no secular change in relative rare earth distribution was detected in the post-Archean sediments, a uniform process of crustal growth and evolution seems to have operated over the past 1500 million years at least.

Australites show rare earth distributions very similar to that of the average clastic sediment. This suggests that the tektite parent material originated in the upper crust.