Geochemistry of phillipsite in abyssal sediments from eastern pacific: indications of false rare earth element enrichment

Phillipsite, a well-documented marine authigenic mineral, is widely recognized as a product of volcanic material alteration. Although phillipsite was historically considered a potential rare earth element and yttrium (REY) carrier in abyssal sediments, subsequent studies have shown that its REY content is generally insignificant, casting doubt on this interpretation. Nevertheless, the precise role of phillipsite in REY-enriched sedimentary systems remains incompletely understood. In this study, we combined morphological characterization and geochemical analyses to investigate phillipsite recovered from box cores collected from the eastern Pacific. Our results indicate that phillipsites occur as single grains, crossed-twinned grains and multiple-twinned grains, with iron (Fe)-manganese (Mn) oxides can be clearly observed along crystal margins and at intercrystalline junctions. Phillipsite consists predominantly of Si, Al, which account for 85–90 wt% of the total composition. The average REY content of phillipsite is 54 ppm, ruling out its role as a potential REY host phase in marine sediments. Through comprehensive comparisons of morphology, elemental ratios (Fe/Ti, Zr/Hf, Nb/Ta, Zr/Nb, and Zr/Ta), and REY distribution patterns, we demonstrate that nodule-hosted phillipsites may share a common origin with the sediment-hosted phillipsites. This indicates that nodule-hosted phillipsites was captured during nodule growth, distinctly differing from phillipsites formed endogenously within the nodules. The pronounced positive Ce anomaly, coupled with the positive correlations among REY, Fe, and Ce in phillipsites, indicates that Fe oxides, rather than phosphorus, serve as the predominant REY hosts within these mineral grains. Our study proposes that REY can hardly enter the phillipsite lattices due to the low cation exchange selectivity for REYs in phillipsite. During early diagenetic stage at the sediment–water interface (SWI), the porous structure of phillipsite exhibits strong adsorption capacity, facilitating the surface adherence and internal incorporation of Fe-Mn oxides and phosphorus. Such incorporation of Fe oxides and phosphorus significantly alters the REY distribution patterns of phillipsite, leading to a pronounced REY enrichment that significantly deviates from the mineral’s inherent. Our study proposed that the abundance of phillipsites in abyssal sediments does not exhibit an absolute positive correlation with REY enrichment.