Characteristics of rare earth elements in the surface sediments of Southwest Indian Ridge: implication of grain size for the identification of hydrothermal activity

The characteristics of rare earth elements (REEs) in mid-ocean ridge (MOR) sediments are important indicators of hydrothermal activity. However, these sediments generally have multiple sources and different grain sizes, resulting in variations of the REE characteristics in bulk sediment samples, which create difficulties when interpreting the results. We studied the characteristics of REEs in sediments with different grain sizes collected proximal, a moderate distance, and distal to hydrothermal fields located on the Southwest Indian Ridge (SWIR). The results show that > 250 μm sediments comprised abundant local debris (especially in proximal sediments) and hydrothermal components with minor Fe–Mn oxides and hydroxides, while < 250 μm sediments comprised higher contents of hydrothermal components, Fe–Mn (hydr)oxides, and terrigenous dust. The REE features of > 250 μm sediments were significantly controlled by local mafic and ultramafic debris (heavy REE enrichment), while hydrothermal components, calcareous ooze, and Fe–Mn oxides played a more important role in the features of the 74–250 μm sediment REEs (light REE enrichment and negative Ce anomaly). The REE characteristics of the < 74 μm sediments were partially controlled by the occurrence of Mn oxides and eolian dust. With decreasing grain size, the sediments generally exhibited increasing negative Ce anomalies, probably the result of adsorption of REE onto hydrothermal Fe oxyhydroxides. In addition, with increasing distance from the hydrothermal fields, the influence of the hydrothermal components on the REE features decreased. The relative contributions of material source components to the sediments and feasible grain size for the identification of hydrothermal contribution were discriminated by the δEu–(Gd/Yb) N diagram. Overall, our results showed that the REE features of 74–250 μm MOR sediments are the most feasible for the identification of potential hydrothermal activity in studied segments.