Phillipsite in pelagic REY-rich sediments and ferromanganese nodules from the Western Pacific: Geochemical characteristics and implications for REY enrichments

Abstract

Recent studies have shown that the rare earth elements and yttrium (REY)-rich deep-sea sediments often contain abundant phillipsite, which is considered as an indicator mineral for REY-rich sediments. However, the phillipsite contains very low REY content, and the coexistence of abundant phillipsite and REY enrichment is generally explained by the low sedimentation rate in deep sea environment. Whether the phillipsite has the capacity to enrich REY and why it is REY-poor is still unclear. Phillipsite is also present in ferromanganese nodules in the surface of the deep-sea sediments, yet whether there is any compositional/genetic difference between the sediment-hosted and nodule-hosted phillipsite is unclear. Here, we study the phillipsite in pelagic REY-rich sediments and ferromanganese nodules from the Western Pacific Marcus‐Wake seamount chain. Mineralogical, morphologic, and geochemical analyses of the sediment-/nodule-hosted phillipsite were carried out. Our results show that the XRD patterns and Si/Al ratios of the sediment-/nodule-hosted phillipsite are the same, and they are both K-phillipsite. However, the consistent Fe/Ti, Zr/Hf and Nb/Ta ratios as well as similar Zr/Nb and Zr/Ta ratios of sediment-hosted phillipsite indicate that the sediment-hosted phillipsite has a homogeneous parent rock and has undergone the same degree of low-temperature alteration. In contrast, the nodule-hosted phillipsite may have inhomogeneous parent rock and have undergone different degree alteration because of the varying Fe/Ti, Zr/Hf and Nb/Ta ratios, as well as the wide range of Zr/Nb and Zr/Ta ratios in the nodule-hosted phillipsite. Both sediment-/nodule-hosted phillipsite have similar REY patterns to those of the pore water and have low total REY contents, indicating that they do obtain REYs from the pore water. In deep-sea REY-rich sediments, the phillipsite does not enrich REY, although the REY can enter the phillipsite lattice. This may be caused by the low cation exchange selectivity for REYs in phillipsite, which can be attributed to high hydration energy. Our study indicates that phillipsite does not participate in REY-enrichment. The implication of phillipsite for REY enrichment may be attributed to the consistency of the environmental conditions between the phillipsite formation and REY enrichment.