Long-term variations of factors controlling REE-rich mud formation based on ocean–sediment Nd mass balance modeling

Abstract

Deep-sea sediments highly enriched in rare-earth elements (REEs), termed “REE-rich mud,” have recently attracted attention as a potential mineral resource for industrially critical metals. Previous studies of the geochemistry and mineralogy of REE-rich mud have suggested that the sedimentation rate is key for the formation of REE-rich mud, and, therefore, that Earth system dynamics affecting pelagic sedimentation rates control the formation, ore grade, and distribution of REE-rich mud on a geological time scale. However, the environmental factors controlling the formation of REE-rich mud and their secular variations have never been studied systematically nor quantitatively. In this study, to investigate the dominant factors promoting the formation of REE-rich mud in the Pacific Ocean, we constructed a new multi-box neodymium (Nd) mass balance model that considers interactions between the ocean and seafloor sediments. Sensitivity analysis results showed that dilution by eolian dust, which is not enriched in REEs, results in decreased REE contents in REE-rich mud, whereas the discharge of REEs from the continental margin to the ocean can have a strong positive impact on the REE content of REE-rich mud. Long-term simulations through the Cenozoic demonstrated that dust fluxes dominantly control the secular trend of REE contents in REE-rich mud. The calculated REE content range is consistent with the range observed in data previously reported for North and South Pacific REE-rich mud. However, our model could not reproduce the temporary peaks in REE concentrations observed in actual REE-rich muds. This result implies that such REE peaks reflect local or regional processes, as previously proposed.