Enrichment of rare earth elements in siliceous sediments under slow deposition: A case study of the central North Pacific

Abstract

As an important submarine rare earth elements (REE) exploration target, the REE-rich deep-sea mud in the Pacific Ocean has recently attracted much research attention, yet its metallogenic mechanism has not been sufficiently addressed. In this study, we conducted detailed grain size analysis, mineral identification and geochemical measurement for the MG026 sediment core in the central North Pacific. The >63 μm fraction of the samples mainly contains siliceous bioclastics, phillipsite accretions, fish teeth and bones and ferromanganese micro-nodules; the 4–63 μm fraction mainly contains the abovementioned biological detritus, together with ilmenite, quartz and zeolite; the <4 μm fraction mainly contains barite, clay minerals, carbonate-fluorapatite and amorphous ferric hydroxide. The rare earth elements and yttrium (ΣREY) contents in the samples can reach 810.4 ppm, and are mainly concentrated in the biological apatite (fish teeth and bones) and the Fe-Mn oxide-hydroxide on the micro-nodule surface. The grain size analysis suggests that the smaller the Mz (mean grain size), the higher the REY enrichment. The post-Archean Australian shale-normalized REE patterns are slightly HREE-enriched with significant negative Ce anomalies, indicating that the rare earth elements of the samples are mainly seawater-derived with minor terrigenous input. We conclude that the REY content is controlled by the grain size, the amount of fish teeth and sedimentation rate: With low sedimentation rate, REYs from the seawater may have mainly replaced the Ca2+ ions of biological apatite lattice in form of isomorphism, and minor REYs may have also adsorbed on Fe-Mn micro-nodules due to the scavenging effect. As a concurrent result of the low sedimentation rate, the mean grain size of sediment may have decreased, and abundant phillipsite may have been formed.