Fractionation Trends and Variability of Rare Earth Elements and Selected Critical Metals in Pelagic Sediment from Abyssal Basin of NE Pacific (Clarion-Clipperton Fracture Zone)

Dominik Zawadzki,Kevin Mccartney,Łukasz Maciąg,Tomasz Abramowski

doi:10.3390/min10040320

Abstract

The geochemical and mineralogical characteristics of pelagic sediments collected from the Interoceanmetal Joint Organization (IOM) claim area, located in the eastern part of the Clarion-Clipperton Fracture Zone (CCFZ; eastern tropical Pacific), are described in this paper. The concentrations of rare earth elements (REE), as well as other selected critical elements contained in 135 sediment samples of siliceous clayey silts, are presented. The vertical and spatial variabilities of elements, with particular emphasis on REE as well as metals of the highest economic interest such as Cu, Ni, and Co, are detailed. The applied methods include grain size analysis by laser diffraction, geochemistry examination using ICP-MS, XRF, AAS, and CNS spectrometry, and XRD analysis of mineral composition (Rietveld method). Additionally, statistical methods such as factor analysis (FA) and principal components analysis (PCA) were applied to the results. Finally, a series of maps was prepared by geostatistical methods (universal kriging). Grain size analysis showed poor sorting of the examined fine-grained silts. ICP-MS indicated that total REE contents varied from 200 to 577 ppm, with a mean of 285 ppm, which is generally low. The contents of critical metals such as Cu, Ni, and Co were also low to moderate, apart from some individual sampling stations where total contents were 0.15% or more. Metal composition in sediments was dominated by Cu, Ni, and Zn. A mineral composition analysis revealed the dominance of amorphous biogenic opaline silica (27–58%), which were mostly remnants of diatoms, radiolarians, and sponges associated with clay minerals (23% to 48%), mostly Fe-smectite and illite, with mixed-layered illite/smectite. The high abundance of diagenetic barite crystals found in SEM−EDX observations explains the high content of Ba (up to 2.4%). The sediments showed complex lateral and horizontal fractionation trends for REE and critical metals, caused mostly by clay components, early diagenetic processes, admixtures of allogenic detrital minerals, or scavenging by micronodules.

Highlights

Rare earth elements (REE), called rare earth metals, comprise the 15 lanthanides with atomic numbers from 57 to 71, i.e., lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, as well as two nonlanthanide elements that have properties that are similar enough to allow them to be included in this set, i.e., scandium and yttrium
This capacity increases with the reduction of ion beams from HREE to LREE, which results in increased HREE content in standard charts, including sea water
A total of 135 samples from 45 sampling sites were collected in 2009 using a Reineck box corer (Ocean Instruments, Fall City, WA, USA), during the Interoceanmetal Joint Organization (IOM) research R/V “Yuzhmorgeologiya”, in the IOM H11 polygon (Figure 2)

Summary

Introduction

General Physical and Chemical Behavior of Rare Earth Elements. REEs have unique chemical similarities, being usually divided into light (LREE = Light Rare Earth Elements; La to Gd) and heavy (HREE; from Tb to Lu and Y). Due to small REE amounts in rocks and waters, lanthanide mobility in natural processes is more often controlled by the ability to form compounds from organic and inorganic ligands than by dissolving components. This capacity increases with the reduction of ion beams from HREE to LREE, which results in increased HREE content in standard charts, including sea water. Average concentrations in porewater do not show this trend [3]

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