Rare earth elements in the phosphatic-enriched sediment of the Peru shelf

Apatite-enriched materials from the Peru shelf have been analyzed for their major oxide and rare earth element (REE) concentrations. The samples consist of (1) the fine fraction of sediment, mostly clay material, (2) phosphatic pellets and fish debris, which are dispersed throughout the fine-grained sediment, (3) tabular-shaped phosphatic crusts, which occur within the uppermost few centimeters of sediment, and (4) phosphatic nodules, which occur on the seafloor. The bulk REE concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction. Analysis of the fine-grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale REE values; the analysis of the fish debris suggested that the apatite fraction might have seawater values. The seawater contribution of REE's is negligible in the nodules and crust, in which the apatite occurs as a fine-grained interstitial cement. That is, the concentration of REE's and the REE patterns are predominantly a function of the amount of enclosed fine-grained sediment. By contrast, the REE pattern of the pelletal apatite suggests a seawater source and the absolute REE concentrations are relatively high. The REE/P₂O₅ ratios of the apatite fraction of these samples thus vary from approximately zero (in the case of the crust and nodules) to as much as approximately 1.2 × 10⁻³ (in the case of the pellets). The range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the REE content of this authigenic fraction of the sediment.

Pelletal glauconite was also recovered from one sediment core. Its REE concentrations closely resemble those of the fish debris.