Enrichment and sources of REY in phosphate fractions: Constraints from the leaching of REY-rich deep-sea sediments

Abstract

Rare earth elements and yttrium (REY)-rich deep-sea sediments, regarded as a potential resource, have triggered extensive investigations over the last decade. The conclusion that the REY enrichment is closely related to the phosphate components contradicts the low REY content (ΣREY) of marine phosphorites, leading to questions about the control of deep-sea phosphate on the enrichment and cycling of REY. Herein, we performed a series of chemical leaching experiments on a group of REY-rich samples (up to 5983 ppm) from two sediment cores obtained from the Pigafetta Basin in the western Pacific to investigate the phosphate and non-phosphate fractions. We found that phosphate components in deep-sea sediments, termed REY-rich phosphates, contain a mean ΣREY of 27,635 ppm and ΣREY/P2O5 of >0.75, which are 1–2 orders of magnitude higher than that of marine phosphorites. The 87Sr/86Sr (0.7083–0.7092) and εNd (–6.41 to –5.15) of the phosphate fractions exhibited strong terrigenous attributes and fell within the range of that of bottom water since 25 Ma. Notably, the non-phosphate components, primarily phillipsite and clay, exhibited stronger terrigenous attributes, confirming extremely low sedimentation rate in the study area. The development and accumulation of REY-rich phosphate, as a consequence of low sedimentation rate in deep water together with erosion and sorting by bottom current, control the content and patterns of REY in deep-sea sediments. Fe-Mn oxides that migrated to the seafloor from the water column were limited and consumed less REY, although their strong Ce enrichment led to the loss of Ce in seawater. The phosphate components in sediments inherited the primary REY pattern of seawater and became the major REY budget on the seabed. In this study, we propose that the mineralization during the sinking of biological particles not only controls the REY distribution in the water column but also becomes an important REY source migration to the seabed and rapidly releases at the sediment–water interface. Consequently, REY can be effectively concentrated by the phosphate components under low sedimentation rates, yielding weak fractionated seawater REY patterns with lower Y/Ho ratios.