Distribution of rare earth elements and implication for Ce anomalies in the clay-sized minerals of deep-sea sediment, Western Pacific Ocean

Abstract

Recent studies reveal that bioapatite and FeMn micro-nodules are predominantly hosts of rare earth elements and yttrium (REYs) in bulk deep-sea sediments by in-situ analysis and chemical extraction. However, individual clay-sized fraction (<2 μm; CSFS) could also be an important REY carriers, in which chemical species and distribution of REYs are unclear. In order to clarify the REY migration or cycle and light/heavy REE enrichment mechanism in different clay-sized minerals, this study undertook clay-sized separation, sequential chemical extraction, X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF) and inductively coupled plasma–mass spectrometry (ICP-MS) analysis of REY-rich sediment samples from two sites in the western Pacific Ocean. The results show the REY in CSFS are mainly hosted in the amorphous Fe phase (Feox1), with lesser amounts in carbonate and the crystalline Fe phase (Feox2), with average proportions of 68.0%, 27.5%, and 2.9% at site 30, and 75.2%, 16.2%, and 7.1% at site 64, respectively. The bulk sediments and CSFS show no obvious Eu anomalies, suggesting there was limited hydrothermal input. Carbonate may be an authigenic phase that inherited the REY characteristics of seawater by substituting for Ca2+, and organic degradation resulted in larger negative Ce anomalies than in seawater. The ΣREY contents increase and the heavy REEs are more readily adsorbed onto the Feox1 under oxidizing conditions. The Feox2 phase may be transformed from Feox1, which facilitates the release of light REEs. The negative Ce anomalies in Fe phase minerals are not solely caused by the depositional reduce conditions because an obvious change of Ce anomaly in different iron phase minerals.