Abstract
Two Fe–Mn crusts among 35 samples, from six seamounts in the Canary Island Seamount Province, were selected as representatives of the endpoint members of two distinct types of genetic processes, i.e., mixed diagenetic/hydrogenetic and purely hydrogenetic. High-resolution analyses pursued the main aim of distinguishing the critical elements and their association with mineral phases and genetic processes forming a long-lived Fe–Mn crust. The Fe–Mn crust collected on the Tropic Seamount is composed of dense laminations of Fe-vernadite (>90%) and goethite group minerals, reflecting the predominance of the hydrogenetic process during their formation. Based on high-resolution age calculation, this purely hydrogenetic crust yielded an age of 99 Ma. The Fe–Mn crust collected on the Paps Seamount shows a typical botryoidal surface yielding an age of 30 Ma. electron probe microanalyzer (EPMA) spot analyses show two main types of manganese oxides, indicating their origin: (i) hydrogenetic Fe-vernadite, the main Mn oxide, and (ii) laminations of interlayered buserite and asbolane. Additionally, the occurrence of calcite, authigenic carbonate fluor-apatite (CFA) and palygorskite suggests early diagenesis and pervasive phosphatization events. Sequential leaching analysis indicated that Co, Ni, Cu, Ba and Ce are linked to Mn minerals. Therefore, Mn-oxides are enriched in Ni and Cu by diagenetic processes or in Co and Ce by hydrogenetic processes. On the other hand, Fe-oxides concentrate V, Zn, As and Pb. Moreover, the evidence of HREE enrichment related to Fe-hydroxides is confirmed in the mixed hydrogenetic/diagenetic crust.
Highlights
IntroductionFerromanganese crusts (hereafter Fe–Mn crusts) grow on seamounts and plateaus worldwide, where currents are strong enough to keep the rock substrate clear of sediments
Ferromanganese crusts grow on seamounts and plateaus worldwide, where currents are strong enough to keep the rock substrate clear of sediments
“Cobalt crusts” are defined as hydroxide/oxide deposits of cobalt-rich iron/manganese concretions formed by direct precipitation of minerals from seawater onto hard substrates [1]. The interest in these deposits began in the early 1980s when they were considered for the first time as a potential Mn, Co, Ni and Cu source [2,3,4,5]. The attention on this deposit rose when recent investigations highlighted that Fe–Mn crusts are enriched in several critical metals and rare earth elements plus yttrium (REYs) [6,7,8,9,10,11,12,13,14]
Summary
Ferromanganese crusts (hereafter Fe–Mn crusts) grow on seamounts and plateaus worldwide, where currents are strong enough to keep the rock substrate clear of sediments. “Cobalt crusts” are defined as hydroxide/oxide deposits of cobalt-rich iron/manganese concretions formed by direct precipitation of minerals from seawater onto hard substrates [1] The interest in these deposits began in the early 1980s when they were considered for the first time as a potential Mn, Co, Ni and Cu source [2,3,4,5]. Previous studies of Fe–Mn nodules and crusts showed that the enrichment in a specific metal, basically depends on the type of genetic process responsible for their growth.
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