This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average La_N/Sm_N = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.

Ferromanganese nodules are a potential energy resource because of their high contents of economically interesting elements (i.e. Mn, Ni, Cu, and Zn). These are higher in diagenetic layers than in hydrogenetic layers. The study of the causes of elemental accumulation in the diagenetic layer is useful for the exploration metal-rich nodules. A diagenetic-dominant ferromanganese nodule, from the central Clarion–Clipperton Fracture Zone of the eastern Pacific Ocean was studied from core to rim. It was divided into four layers and seven sublayers, of the four typical diagenetic sublayers (L1, L2-1, L2-2, and L3-1). Differences were observed in these diagenetic sublayers. L1 presents the highest Mn/Fe ratio (54), the lowest Co content (2262 ppm), and a positive Ce anomaly. L2-1 exhibits high Co (3122 ppm) and Ba contents (4020 ppm), a positive Ce anomaly, and an obvious peak for 10 Å manganate minerals. L2-2 contains the lowest Ni+Cu contents (3.2 wt%), the highest Ba and Co contents (5110 ppm), and the strongest positive Ce anomaly. In L2-2, the δCe value can be positively correlated to the Mn/Fe ratio and a pronounced peak for 10 Å manganate minerals indicates that this layer has the highest mineral crystallinity. L3-1 shows the highest Ni+Cu contents (5.4 wt%), the lowest Ba (1247ppm), and Co (1725 ppm), a weakly positive Ce anomaly, and the poorest mineral crystallization. Diagenetic- and hydrogenetic-endmember mixing models reveal that hydrogenetic input contributes minimally to these chemical changes, whereas diagenetic input contributes greatly. The changes in diagenetic input may be caused by the changes in primary productivity brought about by movement of tectonic plates and the intense activity of the diagenetic pore fluid. The activity may provide a metal source for the diagenetic sublayer (anomalously high Co and Ce content) via the incorporation of metals released from dissolved buried nodules and micronodules under a suboxic or reducing environments. Graphic Abstract

Highly depleted peridotites from the Yap Trench in the western Pacific Ocean have been studied for Re-Os elements and Re-Os isotopes. These peridotites have a low Re-Os content and variable ¹⁸⁷Os/¹⁸⁸Os ratios (0.12043–0.14867). The highest ¹⁸⁷Os/¹⁸⁸Os ratio is far higher than that of the primitive upper mantle and the lowest ¹⁸⁷Os/¹⁸⁸Os ratio is comparable to the most unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratio (0.11933) discovered in subduction zone peridotites. The suprachondritic ¹⁸⁷Os/¹⁸⁸Os ratios of the Yap Trench peridotites results from modification of the mantle wedge by slab-derived fluid and melt. This is consistent with the observation that high ¹⁸⁷Os/¹⁸⁸Os ratios generally occur in oceanic peridotites with low Os content (<2 ppb) since Os may be reduced during late processes such as fluid alteration and melt refertilization. The sub-chondritic ¹⁸⁷Os/¹⁸⁸Os ratios of the Yap Trench peridotites correspond to a Re depletion age of 0.24–1.16 billion years, which means that these peridotites represent old mantle residue of ancient melting events. This ancient melting, combined with probable back-arc melting and forearc melting during subduction initiation, indicates that the Yap Trench mantle has a complex evolutionary history. The amount of old mantle residue in the oceanic asthenosphere was underestimated because the ¹⁸⁷Os/¹⁸⁸Os ratio in mantle peridotites is elevated during late processes. Therefore, old depleted mantle fragments may contribute substantially to the chemical heterogeneity of the oceanic mantle.