Discriminating between different genetic types of marine ferro-manganese crusts and nodules based on rare earth elements and yttrium

Abstract

Marine ferro-manganese (Fe–Mn) crusts and nodules are metal-rich chemical sediments that are archives of paleoceanographic proxies and potential metal resources and targets of deep-sea mining. Traditionally, crusts and nodules are subdivided into hydrogenetic, diagenetic, and hydrothermal types. Because these are characterized by different compositions and hence economic resource potential, a useable genetic classification is not only a tool for geochemical studies but may also help to characterize potential exploration targets. We propose two easy-to-use, yet robust discrimination diagrams based on geochemical relationships controlling the rare earths and yttrium (REY) inventory of marine Fe–Mn (oxyhydr)oxide deposits. The REY are a set of trace elements that show coherent behavior, are routinely determined in geochemical studies, and for which the analytical quality can easily be evaluated. Graphs of Ce anomaly vs Nd concentration and Ce anomaly vs Y anomaly effectively discriminate between the different types of precipitates, regardless of their diverse mineralogical composition. Both hydrogenetic crusts and nodules show positive Ce anomalies, negative Y anomalies and high Nd concentrations of >100mgkg−1, although nodules tend to have slightly lower Nd concentrations than crusts. In marked contrast, hydrothermal deposits generally yield negative Ce anomalies, positive Y anomalies, and low Nd concentrations of <10mgkg−1, in spite of their large geochemical, mineralogical and sedimentological diversity. Diagenetic nodules show negative Ce anomalies, negative Y anomalies, and intermediate Nd concentrations, between 10 and 100mgkg−1. We discuss the geochemical background of these diagrams, the respective sources of REY in the different precipitates, and address the processes that control their REY inventory. Besides the three end-members, we also use REY systematics to define mixed-type diagenetic–hydrogenetic nodules and classify well-studied international reference standards.