Fractionation of germanium and silicon during scavenging from seawater by marine Fe (oxy)hydroxides: Evidence from hydrogenetic ferromanganese crusts and nodules
Introduction
Germanium (Ge) and silicon (Si) are both members of group 13 of the Periodic Table of Elements. In the first drafts of the periodic table by Dmitri Mendeelev Ge was named “eka‑silicon” which refers to the position of Ge right below Si and implies some kind of relationship between these two elements. Both elements are predominantly tetravalent in natural systems and have very similar ionic radii (Si: 1.46 Å and Ge: 1.52 Å, in 4-fold coordination) and oxide bond lengths (Si-O: 1.63 Å, Ge-O: 1.64 Å) (Martin et al., 1996). This allows Ge to substitute Si in the crystal lattice of silicates, following the basic principles of Goldschmidt (1958). Therefore, Ge and Si show coherent behaviour during those igneous geochemical processes that are mainly controlled by charge and size of the cations (similar to the “CHARAC” behaviour of the geochemical twins YHo and Zr-Hf; Bau, 1996). This is corroborated by the largely uniform Ge/Si ratios that characterise the vast majority of igneous and clastic sedimentary rocks (oceanic crust: Ge/Siwt: 6.5 × 10−6, Ge/Simol: 2.5 × 10−6; continental crust: Ge/Siwt: 4.7 × 10−6, Ge/Simol: 1.8 × 10−6; marine deep-sea clay: Ge/Siwt: 5.4–8.1 × 10−6, Ge/Simol: 2.1 × 10−6–3.1 × 10−6 (Rouxel et al., 2006) and upper continental crust: Ge/Siwt: 4.5 × 10−6, Ge/Simol: 1.7 × 10−6 (Rudnick and Gao, 2003); note that Ge/Siwt ratios (used for solids) are given as mass ratios [g/g] and Ge/Simol ratios (used for solutions) are given as molar ratios [mol/mol]). Although there seems to be a slight fractionation of Ge and Si during igneous fractionation processes resulting in higher Ge/Si ratios of mafic as compared to felsic magmas, their ratios are close to the average Ge/Si ratio for the continental crust.
Slight fractionation of the Ge/Si ratio during continental weathering, leading to somewhat lower Ge/Siwt ratios of river waters (0.8–3.1 × 10−6; Ge/Simol: 0.3–1.2 × 10−6), had already been described by Mortlock and Froelich (1987) and was more recently confirmed by Aguirre et al. (2017) and Ameijeiras-Mariño et al. (2018). Further work by Murnane and Stallard (1990) and Froelich et al. (1992) suggested that the Ge/Si ratios in river waters depend on the intensity of weathering: (i) low weathering intensities result in incongruent alteration of rocks and the formation of secondary clay minerals with enhanced Ge/Si ratio compared to the source rocks, while the related river waters show lower Ge/Si ratios; (ii) during intense (chemical) weathering, Ge-enriched secondary minerals are re-dissolved, leading to an increase of the Ge/Si ratio in such river waters. These results were later confirmed by Kurtz et al. (2002) who described a high Ge/Si reservoir (Ge/Siwt: 16.0–42.7 × 10−6; Ge/Simol: 6.2–16.4 × 10−6) in Hawaiian soils, and argued for retention of Ge in clays during incongruent silicate weathering; they also observed the highest Ge/Si ratios in the oldest soils. Besides inorganic fractionation of the Ge/Si ratio, complexation of Ge and Si by organic ligands can also influence the Ge/Si ratio in aqueous media. Pokrovski and Schott (1998) observed that the complexation of Ge by humic acids may increase the Ge/Si ratio of organic-rich surface waters, because in contrast to Ge, the complexation of Si with such humic acids is minute. This is supported by data from an organic-rich river in Cameroon (Viers et al., 1997), that shows elevated Ge/Si ratios compared to rivers with less organic material. Furthermore, Viers et al. (1997) used stepwise filtration to show that the Ge concentration is directly linked to the dissolved organic carbon (DOC) content, while the Si concentration is not. The enrichment of Ge in coal and other lignitized organic material described by Bernstein (1985), also argues for preferential association of Ge relative to Si with organic ligands.
The modern marine system mainly receives its element inventory from three sources: (i) hydrothermal fluids which predominantly influence the direct vicinity of vent sites and show Ge/Siwt ratios between 10.1 and 134.3 × 10−6 (Ge/Simol: 3.9–51.7 × 10−6; e.g., Mortlock et al., 1993; Escoube et al., 2015); (ii) riverine input (avg. Ge/Siwt: 1.6 × 10−6, Ge/Simol: 0.6 × 10−6; Mortlock and Froelich, 1987) which is the major source in coastal waters; and (iii) atmospheric dust (Ge/Siwt of loess: 3.6–3.8 × 10−6, Ge/Simol: 1.4–1.5 × 10−6; Ge/Siwt of mineral aerosols: 2.2–6.0 × 10−6, Ge/Simol: 0.8–2.3 × 10−6; Mortlock and Froelich, 1987) which is of particular importance in pelagic ocean areas at large distance from any coastline. In the marine water column, Ge and Si behave very similar to each other: The dissolved concentrations of both elements in the Pacific, Atlantic and Southern Oceans increase with depth (e.g., Froelich et al., 1985; Sutton et al., 2010), indicating continuous removal of Ge and Si from particles. Dissolved Ge and Si concentrations in the Atlantic are lower compared to those in the South Pacific or the Southern Oceans, while Antarctic Bottom Water (AABW) also shows elevated Ge and Si concentrations. Dissolved Ge/Si ratios, however, are very similar throughout the ocean basins and throughout the water column (Sutton et al., 2010; Guillermic et al., 2017). In general, the correlation between dissolved Ge and Si concentrations in the oceans can be described by a linear equation and only at very low Si concentrations (<10 μmol/L; ~0.28 mg/kg) the relationship between Ge and Si is better described by a 2nd-order polynomial function (Sutton et al., 2010). Residence times for Ge and Si were estimated to be ~8000 years and 15,000–17,000 years, respectively (Hammond et al., 2004; Tréguer and De La Rocha, 2013).
As the dominant source of the marine element inventory is derived from the continents via riverine input (e.g., Hammond et al., 2004), the Ge/Si ratios of modern seawater (Ge/Siwt: 1.7 × 10−6, Ge/Simol: 0.7 × 10−6; Mortlock and Froelich, 1987) which are similar to those of the source rivers, suggest only minor fractionation during estuarine processes. However, although the input of Ge (and Si) into the oceans is rather well understood, this is different for the removal of Ge removal and the apparent mismatch is commonly referred to as the “missing Ge sink” (King et al., 2000).
Hydrogenetic ferromanganese crusts (FeMn crusts) are marine chemical sediments which form encrustations on rocks and other substrates in the deep sea (400–7000 m; Hein et al., 2000). In contrast to diagenetic and hydrothermal FeMn crusts, the chemical inventory of hydrogenetic crusts is exclusively derived from ambient seawater with small but variable contributions from detrital aluminosilicates (e.g., Bau et al., 2014, and references therein). They mainly consist of Fe and Mn (oxyhydr)oxides and grow at very slow rates of only very few mm/Ma on submarine plateaus and flanks of seamounts, where sedimentation rates are low enough to not exceed these slow growth rates. During their formation, hydrogenetic FeMn crusts very effectively scavenge particle-reactive trace elements from ambient seawater. This results in a strong enrichment of, for example, Rare Earths and Yttrium (REY), Cu, Ni, Co, Bi, Mo, W, Te, Pt, Ti, Zr, Hf, Nb, Ta, Th, and U (e.g., Hein et al., 2013, Hein et al., 2000; Hein and Koschinsky, 2013; Schmidt et al., 2014) compared to their seawater source.
Nanoparticles and colloids of Fe and Mn (oxyhydr)oxides form in the water column wherever oxygen-rich bottom water gets in contact with the water of the metal-enriched oxygen-minimum-zone (e.g., Koschinsky and Halbach, 1995; Hein et al., 2000). The oxidic Fe and Mn nanoparticles may aggregate to colloids and interact with dissolved trace elements. Following a very simplistic “electrostatic model” (e.g., Li, 1991; Koschinsky and Halbach, 1995; for its limitations see, e.g., Bau and Koschinsky, 2009), positively charged dissolved metal cations associate with the negatively charged surface of the Mn oxides, while negatively charged and neutral metal complexes attach to the slightly positively charged surface of the Fe (oxy)hydroxides. This process continues after, eventually, these Fe and Mn colloids are deposited at the sea floor for as long as they are exposed at the sediment/water interface during the slow growth of the FeMn oxide encrustations. Hydrogenetic FeMn crusts are characterized by high porosities and specific surfaces areas of up to 60% and ~ 325 m2/g, respectively (e.g., Hein et al., 2000).
Considering the quest for a “missing Ge sink in seawater” (King et al., 2000) combined with the analytical challenges of determining Ge concentrations in Fe-rich sample matrices and the resulting lack of data on Ge concentrations and Ge/Si ratios of marine hydrogenetic FeMn crusts, we investigated the Ge-Si-Fe systematics of these marine chemical sediments. Moreover, FeMn crusts and nodules have become targets of deep-sea mining of unconventional resources (e.g., Koschinsky et al., 2018) for which sound knowledge of their inventory of critical metals such as Ge is a prerequisite. Here we provide the results of the first study on Ge and Ge/Si ratios in marine hydrogenetic FeMn crusts (and also Ge and Ge/Si data for FeMn nodule reference standards).
Section snippets
Samples
During the 66th cruise of the German research vessel FS SONNE (SO66) FeMn crusts were sampled in the Central Pacific in an area between Kiribati and Wallis and Futuna (0° – 10° S and 175° E – 175° W). Bau et al., 1996, Bau et al., 2014 described and analysed 23 FeMn crust samples, 14 of which were classified as purely hydrogenetic and non-phosphatized. From those 14 samples, eleven were selected for the reconnaissance study reported here. In addition to these eleven FeMn crust samples, the FeMn
Reference materials NOD-P1, NOD-A1 and JMn-1
Compared to the analysis of many other major and trace elements, analysis of Ge and Si by ICP-MS is rather challenging. During the standard three-acid-digestion (HCl + HNO3 + HF) Ge and Si evaporate as fluoride complexes. Therefore, published data sets for rock samples and RMs determined by analytical techniques that require sample dissolution often lack Ge and Si data, and the Si data that is reported has often been determined by X-ray fluorescence (XRF). For the RM NOD-P1 there are two Ge
Sample purity
For geochemical investigations of chemical sedimentary rocks it is of utmost importance to identify and assess possible “contamination” of the chemical precipitate by detrital material, as trace element concentrations of potential detritus are often significantly higher than those of the chemically precipitated autochthonous component (e.g., Schier et al., 2021a, b; and references therein). In FeMn crusts, detrital contamination could originate from the ubiquitous background sedimentation
Conclusions
We here provide for the first time Ge/Si ratios determined by LA-ICP-MS measurements on nano-particulate pellets prepared from marine hydrogenetic FeMn crusts and nodules. Data quality was evaluated and confirmed by comparison of these new data to reference values published for the well-characterized FeMn nodule RM NOD-P1, and attests to the good analytical quality of the results (Ge/Siwt = 16.79 × 10−6, Ge/Simol: 6.47 × 10−6; RSDEV of 8.5% for all 24 conducted measurements). We also report the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to Samuel Müller and Ulrike Westernströer (both CAU Kiel University) who helped with the production of the pressed powder pellets and the operation of the high-resolution LA-ICP-MS, respectively. We acknowledge the many fruitful discussions with colleagues in DFG Priority Program SPP-1833 and appreciate funding from the Deutsche Forschungsgemeinschaft (grant BA-2289/8-1). The reviews of two anonymous reviewers are greatly appreciated and substantially improved the manuscript.
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