A Global Marine Sediment Compilation and a Cerium Anomaly Perspective on Metasomatized Mantle Sources for REE-Mineralized Carbonatites

Abstract

Rare earth elements (REE) are vital for powerful permanent magnets used in electric motors and wind turbines. These elements are chiefly sourced from carbonatites and their weathering products. The economic attractiveness of carbonatites is explained by the 10,000-fold enrichment of REE in their mineralized portions relative to the average continental crust. Carbonatites form from mantle-derived melts, but the ultimate origin of their REE is not completely clear. One widely cited model invokes subduction of marine sediments which accumulate REE-rich material, priming the mantle to produce REE-rich carbonatite melts which subsequently form deposits in the upper crust. Here we examine a global marine sediment compilation, revealing a wide variety in REE abundances and patterns. We use the sensitive lambda method that separates REE pattern curvature from redox-related element anomalies to examine both marine sediments and presumably derived carbonatite rocks. We find that the most REE-rich marine sediments are characterized by strongly negative Ce anomalies, which if recycled via subduction, mineralized carbonatites are expected to inherit. In contrast, we find that mineralized carbonatite rocks do not contain Ce anomalies. This indicates that the REE from the most REE-rich marine sediments are not recycled into carbonatite deposits, and a different REE source is needed to explain carbonatite fertilities. We also find evidence that raises questions on whether any sediment-derived REE are present in carbonatite deposits to a significant amount. We suggest that a REE-rich source may not be required and REE enrichment occurs primarily during crustal magmatic differentiation.