Abstract
Thirty-six samples of well-preserved Precambrian sedimentary rocks have been analyzed for rare-earth elements (REE) as one test of the proposition that the present relative REE abundances at the Earth's surface have developed gradually with time. Except for Eu, the average relative abundances of the REE appear to be the same for the Precambrian sediments as in a composite of Paleozoic shales (taken as representative of younger sedimentary rocks). Abundances of Eu relative to the other REE in all except two of the Precambrian sedimente are significantly higher than in the younger sediments. The extent of enrichment of the Precambrian sediments in Eu over the composite of younger sediments ranges from 0.7 to 3.0. The average abundance of Eu relative to the other REE for all the Precambrian sedimente is the same as that in chondritic meteorites. Several of the oldest Precambrian sedimente have relative abundances of Eu that exceed that of chondrites. Large-scale separations of Eu from the other REE as a result of igneous, metamorphic, or sedimentary processes are considered. Present information does not convincingly support any of the proposed mechanisms.
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