Critical metal enrichment in atypical hydrogenetic ferromanganese nodules: A case study in the Central Basin Ridge of the West Philippine Basin

Abstract

Typical hydrogenetic ferromanganese nodules, which are formed by colloidal aggregation of hydrated FeMn oxides and scavenge multiple critical metals from seawater, exhibit high Co contents (>0.4 wt%) and low Mn/Fe ratios (<2.5). Based on a systematic study of the microstructures, mineralogy, and elemental, Nd and Hf isotope compositions of 49 nodules from the Central Basin Ridge of the West Philippine Basin, we identified a subtype of hydrogenetic ferromanganese nodules exhibiting lower Co (<0.4 wt%) and Ti contents, higher Pb and REY contents, and higher Fe/Co and Fe/Mn ratios than typical hydrogenetic nodules. These nodules have elemental, Nd and Hf isotope compositions of hydrogenous origin. Geochemical analysis and regional mapping show that such subtype nodules are widely distributed off the continental margin of the adjacent western Pacific and are labeled “atypical hydrogenetic ferromanganese nodules” in this study. Nevertheless, their significant selective enrichment of metal elements in seawater is similar to that of typical hydrogenetic ferromanganese nodules. They both have relatively consistent REY and PGE patterns; although Nd and Os isotopes indicate changes in material sources in historical periods. They also control scavenged-type elements in seawater, such as Co, Mn, Ce, and Pb. Notably, the mineral compositions of Fe-vernadite (43.6 ± 15.5 wt%), 10 Å vernadite (17.2 ± 10.1 wt%) and the abundant detritus in the nodules indicate an oxidized and strong bottom current environment. The lower Co and Ti contents of the nodules can be attributed to dilution caused by higher growth rates or detrital contents. Importantly, Fe, Pb and REY with short-lived residence times can be well recharged through the Asian continent to provide the metal fluxes favorable for the atypical hydrogenetic ferromanganese nodules. Coincidentally, the strong bottom currents hinder further accumulation of detrital components, making the ridge a favorable place for nodule growth since the mid-Miocene.