Chronology and critical metals enrichment mechanism of ferromanganese nodules from the Parece Vela Basin, Philippine Sea

Ferromanganese nodules are potential economic metal resources enriched with Mn, Co, Ni, Cu, Ti, and rare earth elements plus yttrium (REY). The mineralogy, magnetic scanning, and geochemistry of ferromanganese nodules from the Parece Vela Basin in the Philippine Sea were documented to verify critical metals variations during nodule growth. Based on in situ magnetic scanning, the nodules from the basin were found to initially form during the late Pliocene. At ∼1.6 Myr, the growth structure of the nodules changed remarkably from lamellar to columnar. The inner part has a dense lamellar structure with unstable buserite I, whereas the outer layers are columnar with stable todorokite (or buserite II). The growth rate of the nodule decreased from 4.39 to 2.82 mm/Myr, which may have been related to the persistently weakening of deep-sea sedimentary dynamics in the Philippine Sea since ∼1.6 Myr. The slower growth rate facilitates the enrichment of Ce, resulting in a higher REY content and strong positive Ce anomalies in the columnar structure. The mineralogical compositions and Mn/Fe ratios indicate that the nodules were primarily hydrogenetic with low diagenetic influences. The contents of Ni and Cu increase rapidly, but the trends of Co and REY display an inflection point from the hydrogenetic to diagenetic. REY was primarily enriched in Mn mineral phases, and hydrogenesis dominated the enrichment of REY. Hydrogenesis is beneficial for the enrichment of Ti, Ge, Se, Nb, and Sn. However, diagenesis facilitated the enrichment of Ga, Cd, Li, Tl, Zn, Mo, and W in the nodules of the Parece Vela Basin.