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REY‐rich mud, consisting of deep‐sea sediments with high concentrations of rare‐earth elements and yttrium (REY), holds significant economic potential. Many studies have been conducted on biogenic apatite, ferromanganese micronodule, and phillipsite within these deposits to ascertain the REY enrichment mechanisms. However, the knowledge of clay minerals in REY‐rich mud, which is the predominant component of pelagic sediments, is still limited. In this study, two adjacent gravity cores (core GC02: REY‐rich mud; core GC03: typical sediments of equatorial Pacific) were collected from the Clarion‐Clipperton Fracture Zone (CCFZ) of the Eastern Pacific to study the role of different clay minerals in REY enrichment. The clay minerals in core GC03 and core GC02 are primarily illite (averaging 60%) and smectite (averaging 63%), respectively, and the smectite in core GC02 was mainly Fe‐rich, which was probably formed via the reaction between opal and FeOOH. Moreover, multiple studies have reported similar smectite enrichment in REY‐rich mud, suggesting that it is a common characteristic. The presumed hydrothermal or volcanic origination of smectite in REY‐rich layers of core GC02 indicates the essential role of hydrothermal and volcanic activities in REY‐rich mud formation during the Oligocene in the western CCFZ. Key Points: Smectite is likely to be commonly enriched in rare‐earth elements and yttrium (REY)‐rich mud of the PacificAuthigenic smectite probably originated from hydrothermal or volcanic activity and concentrated moderate amounts of REYThe REY‐rich mud of the western Clarion‐Clipperton Fracture Zone presumably formed through hydrothermal or volcanic activities during the Oligocene [ABSTRACT FROM AUTHOR]

The mantle source beneath the Southwest Indian Ridge (SWIR) reflects a complex history of contamination. Magmatic volatile contents are vital tracers of these kinds of heterogeneities, which may fractionate otherwise constant volatile/non‐volatile elemental ratios, such as the H2O/Ce ratio. Although several studies have recently used trace element and isotopic data to address mantle source heterogeneity and magmatic processes at SWIR 48°–51°E region, volatile element constraints provide a valuable test of models for the origins of mantle heterogeneities in this region. Here, we present new data for nine rare basaltic glass samples from the 48°–51°E region, which enable careful assessment of the effects of primary versus secondary processes on the glass volatile contents. These samples are strongly affected by variable extents of carbon degassing, and shallow assimilation of Cl‐rich fluid, but also reveal consistently high H2O/Ce ratios (458.8 ± 14.9), among the highest in MORBs, that cannot be explained by late‐stage secondary processes, crustal assimilation, or simple melting of peridotite mantle at variable depths. Instead, the high H2O/Ce ratios are features of the mantle source composition. The 48°–51°E region is notably more depleted in highly incompatible trace elements relative to other regions of the SWIR, although this depletion is not apparent in H2O, which is similarly abundant throughout the SWIR. We link the high H2O/Ce ratios in these glasses with other trace element characteristics diagnostic of subduction and fluid addition, suggesting that the mantle source reflects signatures of a refractory mantle residue that previously melted within a subduction zone. Plain Language Summary: The Southwest Indian Ridge is an ideal place to study mantle heterogeneity within a complex upper mantle that involves mixing of plume‐related and lithospheric materials. Trace elements and radiogenic isotope ratios of mid‐ocean ridge basalts are often used to reveal the origins of mantle heterogeneity, but in the SWIR 48°–51°E region the origins of heterogeneity remain controversial. Magmatic volatile contents (especially H2O) are a vital tracer that provides a valuable test of models for the origins of these kinds of heterogeneities in the mantle. Here, we present new volatile, major, and trace element compositions for basalt samples from the SWIR 48°–51°E region. The volatile data enable careful assessment of the effects of secondary processes (alteration, assimilation) relative to the original magma composition, and enable assessment of the water content of the regional mantle. Abnormally high volatile/non‐volatile elemental ratios (H2O/Ce) of the basalts and the mantle source, paired with trace element signatures usually seen at subduction zones, suggests that the H2O enrichment is tied to a past history of subduction in the regional mantle. Key Points: Glasses from the Southwest Indian Ridge 48°–51°E have extremely high H2O/Ce ratios that are not explained by secondary processesThe high H2O/Ce ratio appears unrelated to the Crozet hotspot, but accompanies geochemical signatures commonly associated with subductionVolatile and trace element characteristics are consistent with previous melting and depletion within a subduction zone mantle [ABSTRACT FROM AUTHOR]

The Okinawa Trough is characterized by enrichment of Ag in hydrothermal precipitates; however, the distribution of this enrichment remains poorly constrained. This study presents the results of a field‐emission scanning electron microscopy and electron‐microprobe analysis based mineralogical and geochemical investigation of the spatial distribution of Ag within Ag‐rich sulfide samples from the Okinawa Trough. The tetrahedrite, covellite, and galena in these samples contain high concentrations of Ag (average values of 1.60, 0.78, and 0.23 wt%, respectively) and also various Ag sulfosalts. Examination of the Ag budget of these samples indicates that most of the Ag is hosted by tetrahedrite followed by galena. The Ag within tetrahedrite is incorporated by substitution into the Cu site, whereas galena becomes Ag‐enriched by the coupled incorporation of monovalent Ag, Tl, and Cu, and trivalent Sb and Bi into Pb lattice sites. Tetrahedrite and galena containing higher concentrations of Sb favor increased Ag substitution. Four sets of Ag host minerals are identified with distinct ore formation temperatures. Tetrahedrite and galena concentrate the majority of Ag at medium temperatures (150–300°C). Other Ag host minerals concentrate only minor or trace amounts of Ag, including massive sphalerite, chalcopyrite, and pyrite at high temperatures (>300°C), colloform pyrite and sphalerite at low temperatures (<150°C), and Ag‐sulfosalts at even lower temperatures (<100°C). [ABSTRACT FROM AUTHOR]