RARE-EARTH ELEMENTS IN MARINE-SEDIMENTS AND GEOCHEMICAL STANDARDS

A comparison was made of sediment dissolution methods used for analysis of rare-earth elements (REE) and major elements (Fe, Al, Ti, Zr, P). Dissolution by HF and lithium metaborate fusion were compared in seven Geochemical Standards and twenty marine sediments. Using HF dissolution of shelf and slope sediments, this author previously observed that these marine sediments were significantly depleted in heavy REE (HREE) relative to the crustal abundance (i.e. shale). This observation plus published REE compositions of river particles lead to the conclusion that the riverine input of particles to the oceans cannot be assumed to have the crustal abundance for REE. Comparisons of fusion (total) to HF dissolutions for marine sediments show that the latter method misses between 20% and 100% of the HREE (i.e. Dy, Er, Yb, Lu) and large percentages of Zr. In contrast, fusion and HF dissolution yield similar concentrations for the light (La, Ce, Nd) and middle (Eu, Gd) REE and other major elements. The fusion data show that the marine sediments are more similar in REE composition to shales than previously reported by this author who underestimated the contribution of HF-insoluble residues to the concentration of HREE. The heavy mineral zircon is the most likely cause of this observation. Zircons, known to be enriched in HREE were identified by X-ray diffraction as a major component of the HF-insoluble residues. Geochemical standards also have a significant, but usually smaller fraction of their HREE associated with HF-insoluble phases. These compositional differences have significance with respect to interpreting river inputs, oceanic abundances and diagenesis of REE. © 1990.