Accumulation of Platinum Group Elements in Hydrogenous Fe – Mn Crust and Nodules from the Southern Atlantic Ocean

Distribution of platinum group elements (Ru, Pd, Pt, and Ir) and gold in hydrogenous ferromanganese deposits from the southern part of the Atlantic Ocean has been studied. The presented samples were the surface and buried Fe–Mn hydrogenous nodules, biomorphous nodules containing predatory fish teeth in their nuclei, and crusts. Platinum content varied from 47 to 247 ng/g, Ru from 5 to 26 ng/g, Pd from 1.1 to 2.8 ng/g, Ir from 1.2 to 4.6 ng/g, and Au from less than 0.2 to 1.2 ng/g. In the studied Fe–Mn crusts and nodules, Pt, Ir, and Ru are significantly correlated with some redox-sensitive trace metals (Co, Ce, and Tl). Similar to cobalt and cerium behaviour, ruthenium, platinum, and iridium are scavenged from seawater by suspended ferromanganese oxyhydroxides. The most likely mechanism of Platinum Group Elements (PGE) accumulation can be sorption and oxidation on δ-MnO2 surfaces. The obtained platinum fluxes to ferromanganese crusts and to nodules are close and vary from 35 to 65 ng·cm−2·Ma−1. Palladium and gold do not accumulate in hydrogenous ferromanganese deposits relative to the Earth’s crust. No correlation of Pd and Au content with major and trace elements in nodules and crusts have been identified.


Introduction
Ferromanganese crusts and nodules are considered to be the most important resource of metals in the ocean; mining of these deposits has been widely discussed, for example, in [1][2][3].Platinum accumulates in nodules and crusts, and its content amounts up to 1-3 ppm [4,5].Palladium concentrations in seawater are similar to platinum, but Pd does not accumulate in ferromanganese crusts and nodules [6,7].There is much less data on the distribution and behaviour of ruthenium and iridium [8,9].Relative to the platinum group element (PGE) concentrations in seawater and the Earth's crust, the ferromanganese crusts are enriched in these elements in the following order: Ir > Ru > Pt > Pd [10].Platinum accumulates in the oxyhydroxide minerals of the nodules [11].Depending on the mechanism of formation, Fe-Mn nodules can be classified as hydrogenous and diagenetic.Hydrogenous nodules grow slowly due to direct precipitation of colloidal particles of Fe-Mn oxyhydroxides from seawater.In diagenetic nodules, the source of manganese, iron, and the minor elements is an accretion of Fe-Mn oxyhydroxide colloids dissipated in sediments (oxic diagenesis) or pore water of underlying sediments due to the destruction of organic matter (suboxic diagenesis).Ferromanganese nodules form on the sediment-water interface and always contain diagenetically reworked material [12].Unlike the nodules, the main source of hydrogenous ferromanganese crusts is Fe-Mn oxyhydroxides precipitated from the water column.Fe-Mn oxyhydroxides supplied with hydrothermal fluid to the bottom water compose hydrothermal crusts.Hydrogenous deposits consist primarily of Fe-vernadite.Their typical Mn/Fe ratio is close to 1 and they are enriched with Co, rare earth elements, Y, and Te.In contrast, the main minerals in diagenetic nodules and hydrothermal crusts are asbolane-buzerite, birnessite, and todorokite.These deposits typically demonstrate increased contents of Ni, Cu, and Ba [13].The Mn/Fe ratio in diagenetic nodules is usually higher than 2.5.It is well known that platinum accumulates in various types of ferromanganese deposits differently.Its concentrations decrease in the following order: hydrogenous crusts > hydrogenous nodules > diagenetic nodules > hydrothermal crusts [14,15].
Recent advances in analytical methods have led to an increasing number of studies related to PGE distribution in ferromanganese crusts [10,[16][17][18][19][20][21].Relatively few publications concern the PGE content in ferromanganese nodules [14,15,17].Until recently, there has been no consensus about the sources of platinum group elements and mechanism of their accumulation.Seawater is assumed to be the main source of platinum for hydrogenous ferromanganese deposits.It is suggested that these elements are incorporated due to sorption and subsequent redox reactions on ferromanganese oxyhydroxides [6,10,14,22,23].Also, it was assumed that the enrichment of Fe-Mn crusts with platinum results from the incorporation of fine-grained particles of noble metals [24][25][26].Another reason for Pt accumulation in the hydrogenous crusts is its association with organic ligands in seawater [27].
In the case of PGE accumulation on ferromanganese oxyhydroxides from seawater via oxidative reaction, their distribution should be linked with other elements with the same source and supply mechanism (Mn, Co, Tl, and Ce).In contrast, incorporation of noble metal particles would result in the irregular distribution of PGE among ferromanganese samples and in the absence of a relationship between PGE and other trace and major elements.In this work, the behaviour of platinum group elements (Ru, Pd, Ir, and Pt) and gold in hydrogenous ferromanganese crusts and nodules from the Cape and Brazil Basins of the Atlantic has been considered.Samples were chosen to show PGE variations within the sample, station, basin, and oceanic region.Also, we included Fe-Mn crusts from the same stations to compare the accumulation of platinum group elements between different types of ferromanganese deposits.New PGE content data were presented together with mineralogical, chemical, and geochronological analysis to discuss the reason for platinum enrichment in Fe-Mn deposits.

Regional Setting
Brazil Basin is located between the continental slope of Brazil and the Mid-Atlantic Ridge (MAR) and extends from north to south for 3000 km at a water depth of more than 4000 m (Figure 1).Formation of the Basin was followed by strong volcanic activity.Relief of the Brazil Basin floor includes seamounts, ridges, depressions, and valleys.Stratified sediments have a thickness of 100-400 m and fill up depressions and valleys between ridges.The Cape Basin is situated between the Walvis and Agulhas Ridges and borders the MAR on the west.On the floor of the Cape Basin, there are numerous seamounts [28].
Both Basins are situated under the anticyclone gyre formed by the Benguela, South Equatorial, Brazil, and South Atlantic Currents.Three major subsurface water masses are governed by Circumpolar Water.Antarctic surface water masses descend to form Antarctic Intermediate Water (AAIW) that spreads to the north between 500 and 1000 m water depth.Above this, warm and salty South Atlantic Central Water is located.Upper Circumpolar Deep Water (UCDW) is the deepest water mass of the upper ocean, with a net northward transport [29].The deep and abyssal water masses are the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW).The NADW is transported into the South Atlantic predominantly in the deep western boundary current (DWBC) along the Brazilian continental slope.The NADW is located underneath the Upper Circumpolar Deep Water between about 1200 m and 3900 m in the tropics and between 1700 and 3500 m in the subtropics.The AABW enters from the south in the Cape Basin, and in the Brazil Basin, it penetrates through Rio Grande gaps.AABW is distributed in both Basins at depths greater than 3800 m [30].AABW is cold and undersaturated with calcium carbonate.Carbonate compensation depth (CCD) is located at the depth of 4800-5000 m in the Brazil Basin and at the depth of 5100 m in the Cape Basin.Anticyclone circulation leads to water descending, which results in a low biological productivity of its surface water and low sedimentation rates [31].Below the CCD, in both Basins, red pelagic clays are widely distributed.Sediments of the Brazil Basin have high iron and manganese contents, which could result from volcanic activity in the past [32].

Materials and Methods
Samples were collected during cruise 18 of the Research Vessel Akademik Sergey Vavilov (2004) and cruise 29 of the Research Vessel Academik Ioffe in 2009 (Figure 1, Table 1).Cores of sediments were collected using a gravity corer in the Brazil Basin at stations 1536 and 1541.Ferromanganese nodules were found on the sediment-water interface and at the depth of 418 and 83 cm below the seafloor, respectively (Figure 2, Table 2).At station 1538, a crust on basalt substrate was recovered.Sampled at station 2188 in the Cape Basin were a spherical ferromanganese nodule, three Fe-Mn nodules with fish teeth in their nuclei, and a thin crust on the honeycombed substrate.The sizes of the nodules and the crust are shown in Table 2.The spherical nodule was subdivided into two samples: the outer black layer (0-3 mm) and the inner grey layer (3-15 mm).From the biomorphous nodules, the teeth forming the nuclei were withdrawn, and only the oxyhydroxide layers were analysed in this work.The honeycombed substrate of crust Cr 2188 was pumice changed to clayey minerals yellow-whitish in colour.
Mineral composition was determined by X-ray diffraction using a Rigaku D/MAX 2200 diffractometer with CuKα radiation (Rigaku Corporation, Tokyo, Japan).Diffraction patterns were The AABW enters from the south in the Cape Basin, and in the Brazil Basin, it penetrates through Rio Grande gaps.AABW is distributed in both Basins at depths greater than 3800 m [30].AABW is cold and undersaturated with calcium carbonate.Carbonate compensation depth (CCD) is located at the depth of 4800-5000 m in the Brazil Basin and at the depth of 5100 m in the Cape Basin.Anticyclone circulation leads to water descending, which results in a low biological productivity of its surface water and low sedimentation rates [31].Below the CCD, in both Basins, red pelagic clays are widely distributed.Sediments of the Brazil Basin have high iron and manganese contents, which could result from volcanic activity in the past [32].

Materials and Methods
Samples were collected during cruise 18 of the Research Vessel Akademik Sergey Vavilov (2004) and cruise 29 of the Research Vessel Academik Ioffe in 2009 (Figure 1, Table 1).Cores of sediments were collected using a gravity corer in the Brazil Basin at stations 1536 and 1541.Ferromanganese nodules were found on the sediment-water interface and at the depth of 418 and 83 cm below the seafloor, respectively (Figure 2, Table 2).At station 1538, a crust on basalt substrate was recovered.Sampled at station 2188 in the Cape Basin were a spherical ferromanganese nodule, three Fe-Mn nodules with fish teeth in their nuclei, and a thin crust on the honeycombed substrate.The sizes of the nodules and the crust are shown in Table 2.The spherical nodule was subdivided into two samples: the outer black layer (0-3 mm) and the inner grey layer (3-15 mm).From the biomorphous nodules, the teeth forming the nuclei were withdrawn, and only the oxyhydroxide layers were analysed in this work.The honeycombed substrate of crust Cr 2188 was pumice changed to clayey minerals yellow-whitish in colour.
Mineral composition was determined by X-ray diffraction using a Rigaku D/MAX 2200 diffractometer with CuKα radiation (Rigaku Corporation, Tokyo, Japan).Diffraction patterns were collected from 2θ = 5 • to 60 • .Determination of platinum group elements (Ru, Pd, Ir, and Pt) and gold has been carried out by the method of mass spectrometry with inductively coupled plasma (ICP-MS) using Agilent 7500a (Agilent Technologies, Santa Clara, CA, USA) after preliminary concentration on anionite Dowex 1 × 8 [33].The standard addition method has been applied to eliminate losses during chromatography.The accuracy and precision of PGE concentration data have been checked using reference sample NOD-P-1.Analysis of magnesium and rare earth and trace elements has been carried out using the ICP-MS method after sample acid digestion [34].Fe, Mn, Ti, and Al have been analysed applying the method of atomic absorption spectrometry (AAS) using SpectrAA 220 (Varian) (Varian Australia Pty Ltd, Victoria, Australia).The P content was determined by the spectrophotometric method with a precision of 3%.The precision of ICP-MS and AAS analyses varied within the limits of 3-5%.The accuracy of determination methods was controlled using reference samples: basalt (BCR-1, BCR-2), andesite (AGV-1), and Fe-Mn nodule (NOD-P-1, OOPE-601 and OOPE-602).The ages of the teeth from nodule 2188-Th4 and from surface sediments were determined using the method of strontium isotopic stratigraphy (SIS) [35] described in previous works [36,37].The rate of ferromanganese crust growth was estimated by cobalt chronometer in accordance with the equation: V (mm•Ma −1 ) = 0.68/[Co] 1.67  [38,39].collected from 2θ = 5° to 60°.Determination of platinum group elements (Ru, Pd, Ir, and Pt) and gold has been carried out by the method of mass spectrometry with inductively coupled plasma (ICP-MS) using Agilent 7500a (Agilent Technologies, Santa Clara, CA, USA) after preliminary concentration on anionite Dowex 1 × 8 [33].The standard addition method has been applied to eliminate losses during chromatography.The accuracy and precision of PGE concentration data have been checked using reference sample NOD-P-1.Analysis of magnesium and rare earth and trace elements has been carried out using the ICP-MS method after sample acid digestion [34].Fe, Mn, Ti, and Al have been analysed applying the method of atomic absorption spectrometry (AAS) using SpectrAA 220 (Varian) (Varian Australia Pty Ltd, Victoria, Australia).The P content was determined by the spectrophotometric method with a precision of 3%.The precision of ICP-MS and ААS analyses varied within the limits of 3-5%.The accuracy of determination methods was controlled using reference samples: basalt (BCR-1, BCR-2), andesite (AGV-1), and Fe-Mn nodule (NOD-P-1, OOPE-601 and OOPE-602).The ages of the teeth from nodule 2188-Th4 and from surface sediments were determined using the method of strontium isotopic stratigraphy (SIS) [35] described in previous works [36,37].The rate of ferromanganese crust growth was estimated by cobalt chronometer in accordance with the equation: V (mm•Ma −1 ) = 0.68/[Co] 1.67 [38,39].

Results
The mineral composition of the studied crusts and nodules consists predominantly of Fe-vernadite and X-ray amorphous iron oxyhydroxide (feroxyhyte).Some nodules contain a moderate amount of 10 Å manganate minerals (buserite, asbolane-buserite), which could reflect diagenetic Mn contribution.Mineral composition of the buried nodule 1541_83 was described by Dubinin et al. [36].Manganese minerals were replaced along with the periphery of columnar framboids by low crystallized goethite FeOOH (Figure 3) with relict growth structures.

Results
The mineral composition of the studied crusts and nodules consists predominantly of Fe-vernadite and X-ray amorphous iron oxyhydroxide (feroxyhyte).Some nodules contain a moderate amount of 10 Å manganate minerals (buserite, asbolane-buserite), which could reflect diagenetic Mn contribution.Mineral composition of the buried nodule 1541_83 was described by Dubinin et al. [36].Manganese minerals were replaced along with the periphery of columnar framboids by low crystallized goethite FeOOH (Figure 3) with relict growth structures.The chemical composition of the studied ferromanganese samples is presented in Table 3. Surface ferromanganese nodules from the Brazil Basin and the buried nodule from station 1536 are close in their compositions.Mn contents vary from 17.8% to 19.7%; Fe from 13.7% to 15.1%; Co from 0.16% to 0.21%; Ni from 0.54% to 0.77%; Cu from 0.26% to 0.29%.The buried nodule nod 1541_83 is unique in its composition with Mn/Fe ratio of 0.41 together with Fe enrichment (23.5%).This nodule is depleted in copper, nickel, cobalt, and zinc relatively to the nodule on the surface sediments [36].The Ce anomaly value reaches 41 (normalized to Post Archean Australian Shale (PAAS)), because the contents of trivalent rare earth elements are too low (Figure 4).The crust from station 1538 has a Mn/Fe ratio = 1.2; it is also enriched with cobalt and cerium.Mn/Fe ratios in the nodules of the Cape Basin vary from 1.0 to 1.6.Cobalt, nickel, and copper contents are lower than in Brazil Basin nodules.The crust from station 2188 is similar to nodules in its composition.As compared with nodules, it contains less lithium, copper, and nickel.The cerium anomaly (Ce an) is 3.4.On the ternary diagram in coordinates of Fe-Mn-10(Co + Ni + Cu) [40], the studied ferromanganese deposits from the Cape and Brazil Basins are located in the area of deep-sea pelagic nodules and crusts (Figure 5).The buried nodule from station 1541 and crust 2188 are the exceptions.Due to the low contents of Co + Ni + Cu, the buried nodule falls into a hydrothermal deposit area.Figure 6 shows the composition of rare-earth elements (REE) and yttrium in coordinates of Ce an−Nd and Ce an−(Y/Y PAAS )/(Ho/Ho PAAS ) [41].The studied ferromanganese crusts and nodules are located in hydrogenous ferromanganese deposits area on the diagram, while the buried nodule from station 1541 is located off from the highlighted areas due to high Ce an value.The chemical composition of the studied ferromanganese samples is presented in Table 3. Surface ferromanganese nodules from the Brazil Basin and the buried nodule from station 1536 are close in their compositions.Mn contents vary from 17.8% to 19.7%; Fe from 13.7% to 15.1%; Co from 0.16% to 0.21%; Ni from 0.54% to 0.77%; Cu from 0.26% to 0.29%.The buried nodule nod 1541_83 is unique in its composition with Mn/Fe ratio of 0.41 together with Fe enrichment (23.5%).This nodule is depleted in copper, nickel, cobalt, and zinc relatively to the nodule on the surface sediments [36].The Ce anomaly value reaches 41 (normalized to Post Archean Australian Shale (PAAS)), because the contents of trivalent rare earth elements are too low (Figure 4).The crust from station 1538 has a Mn/Fe ratio = 1.2; it is also enriched with cobalt and cerium.Mn/Fe ratios in the nodules of the Cape Basin vary from 1.0 to 1.6.Cobalt, nickel, and copper contents are lower than in Brazil Basin nodules.The crust from station 2188 is similar to nodules in its composition.As compared with nodules, it contains less lithium, copper, and nickel.The cerium anomaly (Ce an) is 3.4.On the ternary diagram in coordinates of Fe-Mn-10(Co + Ni + Cu) [40], the studied ferromanganese deposits from the Cape and Brazil Basins are located in the area of deep-sea pelagic nodules and crusts (Figure 5).The buried nodule from station 1541 and crust 2188 are the exceptions.Due to the low contents of Co + Ni + Cu, the buried nodule falls into a hydrothermal deposit area.Figure 6 shows the composition of rare-earth elements (REE) and yttrium in coordinates of Ce an−Nd and Ce an−(Y/Y PAAS )/(Ho/Ho PAAS ) [41].The studied ferromanganese crusts and nodules are located in hydrogenous ferromanganese deposits area on the diagram, while the buried nodule from station 1541 is located off from the highlighted areas due to high Ce an value.As a rule, platinum contents in hydrogenous ferromanganese deposits are higher by one to two orders of magnitude than those of other platinum group elements (Table 4).In the crust and nodules from the Brazil Basin, Pt concentrations vary from 110 ng/g to 247 ng/g.Comparison between the surface and buried nodules from station 1536 shows that the platinum and iridium contents are practically the same.The buried nodule from station 1541 is enriched with platinum by more than  As a rule, platinum contents in hydrogenous ferromanganese deposits are higher by one to two orders of magnitude than those of other platinum group elements (Table 4).In the crust and nodules from the Brazil Basin, Pt concentrations vary from 110 ng/g to 247 ng/g.Comparison between the surface and buried nodules from station 1536 shows that the platinum and iridium contents are practically the same.The buried nodule from station 1541 is enriched with platinum by more than As a rule, platinum contents in hydrogenous ferromanganese deposits are higher by one to two orders of magnitude than those of other platinum group elements (Table 4).In the crust and nodules from the Brazil Basin, Pt concentrations vary from 110 ng/g to 247 ng/g.Comparison between the surface and buried nodules from station 1536 shows that the platinum and iridium contents are practically the same.The buried nodule from station 1541 is enriched with platinum by more than twofold compared to the surface nodule.Its Pt content is higher than that in the crust from station 1538.In Brazil Basin ferromanganese deposits, Ru contents vary from 9.5 to 19 ng/g and Ir from 1.2 to 4.6 ng/g.For the samples from the Cape Basin, the highest contents of PGEs have been found in the spherical nodule from station 2188.In the outer (black) and inner (grey) layers, the contents of platinum, ruthenium, iridium, and palladium are practically the same.The nodules with the teeth in the nuclei contained 78-80 ng/g of platinum.The lowest platinum content was found in the crust from station 2188 (47 ng/g).Palladium contents vary in the studied crusts and nodules from 1.1 to 2.8 ng/g, reaching maximum values in the crust substrate of 3.1 ng/g.Gold varies from <0.2 to 1.2 ng/g; no relation of it with major and trace elements has been revealed.The age of the surface sediments of station 1541 is evaluated as 24.1 Ma [36].Assuming the age of nodule 1541_0 does not exceed the age of underlying sediments, its growth rate equals 1.2-2.4mm•Ma −1 .The age of the tooth inside nodule 2188-4 (SIS) is estimated as 5.2 ± 0.2 Ma [35].We have calculated the minimum possible growth rate of ferromanganese nodules (Table 5).Based on the growth rate, Pt fluxes (F) in ferromanganese deposits were calculated using the formula F = CGD, where C is concentration, D is density in situ (accepted as 1.6 g/cm 3 ), and G is the growth rate [50].

Variations of PGE Contents in Nodules and Crusts
In hydrogenous nodules of the Pacific Ocean, platinum content varies from 83 ng/g [15] to 674 ng/g [14].The PGE concentrations in the nodules of the Atlantic Ocean are within the same range, but they are lower than average values for the Pacific.Platinum concentrations in the two crusts are lower than average concentrations in the crusts from the Pacific, Indian, and Atlantic oceans [10,[15][16][17][18][19][20][21]51].It should be noted that the number of studied samples from the Atlantic Ocean is not sufficient to make conclusions on the contents of platinum elements in ferromanganese ores of the Atlantic.Nevertheless, the observed PGE concentrations are consistent with the general trend of the lower redox-sensitive element concentration in Fe-Mn oxyhydroxides of the Atlantic compared to the Pacific Ocean [52].In general, PGE contents in crusts are higher than in nodules.Gold and palladium do not accumulate in the oxyhydroxide part of ferromanganese crusts and nodules because their contents in crust substrate are equal or even higher than in crusts.
There is no significant difference in the contents of platinum group elements in the nodules between the Cape and Brazil Basins.Nevertheless, PGEs have smaller variations in the samples from one Basin than in samples from different Basins.Correlation analysis carried out for all studied nodules (except buried nodule Nod 1541_83) shows the significant correlation coefficients for the following pairs of elements: Pt-Co (R = 0.99), Ir-Co (R = 0.74), Ru-Tl (R = 0.79), and Ru-Ce (R = 0.74) (Table A1). Figure 7a shows platinum content dependence on cobalt in the studied ferromanganese crusts and nodules.
Minerals 2018, 8, x FOR PEER REVIEW 12 of 21 make conclusions on the contents of platinum elements in ferromanganese ores of the Atlantic.Nevertheless, the observed PGE concentrations are consistent with the general trend of the lower redox-sensitive element concentration in Fe-Mn oxyhydroxides of the Atlantic compared to the Pacific Ocean [52].In general, PGE contents in crusts are higher than in nodules.Gold and palladium do not accumulate in the oxyhydroxide part of ferromanganese crusts and nodules because their contents in crust substrate are equal or even higher than in crusts.
There is no significant difference in the contents of platinum group elements in the nodules between the Cape and Brazil Basins.Nevertheless, PGEs have smaller variations in the samples from one Basin than in samples from different Basins.Correlation analysis carried out for all studied nodules (except buried nodule Nod 1541_83) shows the significant correlation coefficients for the following pairs of elements: Pt-Co (R = 0.99), Ir-Co (R = 0.74), Ru-Tl (R = 0.79), and Ru-Ce (R = 0.74) (Table A1). Figure 7a shows platinum content dependence on cobalt in the studied ferromanganese crusts and nodules.For all nodule samples, with the exception of the buried nodule from station 1541, a strong correlation between Pt and Co (R = 0.99) is observed.As has been already noted, platinum correlates with redox-sensitive elements (Co and Ce) which is highly enriched in hydrogenous ferromanganese deposits [10,14].Co and Ce are incorporated in Fe-Mn oxyhydroxides during oxidation in surface seawater.Mn oxides oxidize Co(II) and Ce(III) to poorly soluble Co(III) and Ce(IV); also, FeOOH can oxidize Ce(III).In the ocean, platinum exists in two oxidation states, Pt(II) and Pt(IV), in the form of two complexes: PtCl4 2− and PtCl5OH 2− [53].Although sorption of negatively charged particles on the negatively charged surfaces of δ-MnO2 contradicts the electrochemical model [54], experimental studies confirm sorption and subsequent oxidation of platinum (Pt(II) → Pt(IV)) on manganese oxide [23,55].We have observed a significant correlation (R = 0.95) between Pt/Pd and Mn/Fe ratios (Figure 7b) in our nodule and crust samples.The Pt/Pd ratio can be considered as the extent of platinum accumulation respectively to that of palladium and reflects the process of these elements' fractionation during hydrogenous accumulation.Thus, in seawater, Pt/Pd equals approximately 1, and grows in the following order: hydrothermal crusts < diagenetic nodules < hydrogenous nodules < hydrogenous crusts, from 7 to 407 [14].Generally, the Mn/Fe ratio is a geochemical indicator of the source of matter in ferromanganese deposits.This ratio is sensitive for the Mn supply of hydrothermal or diagenetic origin.Yet, within the narrow range of values typical for hydrogenous ferromanganese deposits (Mn/Fe = 0.8-1.5),this value also determines the ratio of Mn and Fe oxyhydroxides in the hydrogenous material.Platinum accumulation grows with an increase of the manganese oxide component in the hydrogenous ferromanganese deposits, which can implicitly indicate a possible association of Pt exactly with Mn.Experiments fulfilled by Kubrakova [27] showed that platinum (II) For all nodule samples, with the exception of the buried nodule from station 1541, a strong correlation between Pt and Co (R = 0.99) is observed.As has been already noted, platinum correlates with redox-sensitive elements (Co and Ce) which is highly enriched in hydrogenous ferromanganese deposits [10,14].Co and Ce are incorporated in Fe-Mn oxyhydroxides during oxidation in surface seawater.Mn oxides oxidize Co(II) and Ce(III) to poorly soluble Co(III) and Ce(IV); also, FeOOH can oxidize Ce(III).In the ocean, platinum exists in two oxidation states, Pt(II) and Pt(IV), in the form of two complexes: PtCl 4 2− and PtCl 5 OH 2− [53].Although sorption of negatively charged particles on the negatively charged surfaces of δ-MnO 2 contradicts the electrochemical model [54], experimental studies confirm sorption and subsequent oxidation of platinum (Pt(II) → Pt(IV)) on manganese oxide [23,55].We have observed a significant correlation (R = 0.95) between Pt/Pd and Mn/Fe ratios (Figure 7b) in our nodule and crust samples.The Pt/Pd ratio can be considered as the extent of platinum accumulation respectively to that of palladium and reflects the process of these elements' fractionation during hydrogenous accumulation.Thus, in seawater, Pt/Pd equals approximately 1, and grows in the following order: hydrothermal crusts < diagenetic nodules < hydrogenous nodules < hydrogenous crusts, from 7 to 407 [14].Generally, the Mn/Fe ratio is a geochemical indicator of the source of matter in ferromanganese deposits.This ratio is sensitive for the Mn supply of hydrothermal or diagenetic origin.Yet, within the narrow range of values typical for hydrogenous ferromanganese deposits (Mn/Fe = 0.8-1.5),this value also determines the ratio of Mn and Fe oxyhydroxides in the hydrogenous material.Platinum accumulation grows with an increase of the manganese oxide component in the hydrogenous ferromanganese deposits, which can implicitly indicate a possible association of Pt exactly with Mn.Experiments fulfilled by Kubrakova [27] showed that platinum (II) sorbs more effectively on a mixture of ferromanganese oxyhydroxides than on iron oxyhydroxides and manganese oxides separately.Apparently, the positively charged iron oxyhydroxides are involved in the Pt sorption process.
Cobalt and cerium behaviour in ferromanganese deposits is studied well, and the incorporation mechanism has been confirmed by experiments [56,57].To compare platinum accumulation relatively to Co and Ce in different types of ferromanganese nodules, we have plotted their compositions onto the ternary diagram in Co-Ce-Pt × 10 4 coordinates (Figure 8).As well as our data, it shows published data on the contents of cobalt, platinum, and cerium in other nodule samples [14,15,17], including reference samples (see the legend).Based on the values of the Mn/Fe ratio in the nodules, we have distinguished the diagenetic and hydrogenous nodules (hydrogenous nodules: Mn/Fe < 2.5; diagenetic: Mn/Fe > 2.5).Separately, the nodules of marginal seas have been also plotted.They included the nodules from the South China Sea [17], reference sample NOD-A-1, and two nodules obtained in the Tasmanian Sea and described by [15].According to the Mn/Fe ratio, these nodules are hydrogenous.Ferromanganese nodules of the Brazil and Cape Basins are located in the same field on the diagram, which we have determined as the nodule area of the Atlantic Ocean.The diagram shows the hydrogenous nodules of the Atlantic and Pacific Oceans in the different areas, since ferromanganese deposits of the Pacific Ocean contain more cobalt [52].
Minerals 2018, 8, x FOR PEER REVIEW 13 of 21 sorbs more effectively on a mixture of ferromanganese oxyhydroxides than on iron oxyhydroxides and manganese oxides separately.Apparently, the positively charged iron oxyhydroxides are involved in the Pt sorption process.
Cobalt and cerium behaviour in ferromanganese deposits is studied well, and the incorporation mechanism has been confirmed by experiments [56,57].To compare platinum accumulation relatively to Co and Ce in different types of ferromanganese nodules, we have plotted their compositions onto the ternary diagram in Co-Ce-Pt × 10 4 coordinates (Figure 8).As well as our data, it shows published data on the contents of cobalt, platinum, and cerium in other nodule samples [14,15,17], including reference samples (see the legend).Based on the values of the Mn/Fe ratio in the nodules, we have distinguished the diagenetic and hydrogenous nodules (hydrogenous nodules: Mn/Fe < 2.5; diagenetic: Mn/Fe > 2.5).Separately, the nodules of marginal seas have been also plotted.They included the nodules from the South China Sea [17], reference sample NOD-A-1, and two nodules obtained in the Tasmanian Sea and described by [15].According to the Mn/Fe ratio, these nodules are hydrogenous.Ferromanganese nodules of the Brazil and Cape Basins are located in the same field on the diagram, which we have determined as the nodule area of the Atlantic Ocean.The diagram shows the hydrogenous nodules of the Atlantic and Pacific Oceans in the different areas, since ferromanganese deposits of the Pacific Ocean contain more cobalt [52].[14,15], reference samples GSPN-2, OOPE-601, OOPE-603; 4-diagenetic nodules [14,15], reference samples JMn-1, NOD-P-1, GSPN-3, OOPE-602; 5-marginal sea nodules [15,17].
All the diagenetic nodules shown are from the Pacific Ocean.They form an elongated area, which is related to the variable contents of platinum respective to the constantly high value of the Co/Ce ratio.Ferromanganese nodules of marginal seas show high platinum contents at relatively low cobalt and cerium concentrations.In the nodule samples, iridium-like platinum is significantly linked with cobalt, while ruthenium correlates better with cerium and thallium, which is oxidized on Fe-Mn oxyhydroxides from Tl(I) to Tl(III) (Table A1).As consistent with the stability constants, iridium and  [14,15], reference samples GSPN-2, OOPE-601, OOPE-603; 4-diagenetic nodules [14,15], reference samples JMn-1, NOD-P-1, GSPN-3, OOPE-602; 5-marginal sea nodules [15,17].
All the diagenetic nodules shown are from the Pacific Ocean.They form an elongated area, which is related to the variable contents of platinum respective to the constantly high value of the Co/Ce ratio.Ferromanganese nodules of marginal seas show high platinum contents at relatively low cobalt and cerium concentrations.In the nodule samples, iridium-like platinum is significantly linked with cobalt, while ruthenium correlates better with cerium and thallium, which is oxidized on Fe-Mn oxyhydroxides from Tl(I) to Tl(III) (Table A1).As consistent with the stability constants, iridium and ruthenium can be in two states of oxidation: +3 and +4 [8].The possible mechanism of enrichment for these elements might be oxidative sorption on suspended iron and manganese oxyhydroxides: Ir(III) → Ir(IV) and Ru(III) → Ru(IV).

Platinum Group Elements in Buried Nodules
Contents of PGEs in the two buried nodules from the Brazil Basin differ almost twofold.The buried nodule from the depth of 418 cm below the seafloor at station 1536 has a chemical composition close to that of the nodule from surface sediments.The plots of Pt versus Co show that the buried nodule 1536_418 composition lies along the trend line with the surface nodules (Figure 7a).Platinum content in nodule 1541_83 is significantly higher than in the buried nodule 1536_418 and in the surface nodules (Table 4).By its chemical composition, this buried nodule does not belong to any of genetic types of ferromanganese nodules.Diagenetic redistribution of matter that occurred after the nodule burial affected its composition significantly [36].This led to partial Mn reduction and replacement of manganese minerals with goethite.
If one assumes that the ratios of cobalt and Mn/Fe to platinum in the buried nodule 1536_418 were the same as in other nodules, then the cobalt content in it had reached 0.3% and the Mn/Fe ratio was equal to 1.2 (Figure 7a,b).Such a chemical composition of the nodule is similar to other nodules of the Brazil Basin.Obviously, platinum accumulated in the nodules during their growth on the sediment surface.After the burial, manganese and trace elements related to manganese were lost, while platinum content remained unchanged.This passive platinum accumulation can indicate that it is not mobile under the early diagenesis conditions.Terashima et al. [44] did not reveal significant migration of Pt during early diagenesis, except in rare cases.Colodner et al. [58] showed redistribution of platinum and iridium during iron and manganese oxyhydroxide reduction in pelagic sediments.Apparently, platinum reduction occurs in sediments only at complete Mn and Fe oxyhydroxide reduction.The remaining manganese (IV) phases did not allow migration of platinum.In the case of nodule 1541_83, platinum turned out to be less mobile than cobalt during the partial dissolution of manganese oxyhydroxides and their replacement with goethite.Relatively low Pt concentrations observed in diagenetic nodules indicate both limited platinum migration during oxic diagenesis and the relatively high growth rate of diagenetic nodules [14].

Platinum Fluxes in Fe-Mn Crusts and Nodules
Possible sources of platinum group elements in the ocean can be riverine fluxes, hydrothermal processes, cosmic dust flux, and halmyrolysis of oceanic basalts.If riverine and cosmic inputs have been estimated in some works [59,60], the influence of hydrothermal process and basalt halmyrolysis are significantly less studied [61].Ferromanganese deposits in the ocean are a significant sink of metals entering the ocean and indicate the source of the metal supply.Figure 9 shows PGEs in ferromanganese crusts and nodules normalised to CI chondrite.For comparison, the composition of platinum group elements in seawater and basalts of the Atlantic Ocean is also shown.
It is assumed that PGEs are supplied to ferromanganese crusts and nodules from seawater.Platinum elements in the ocean are not a chemically coherent group, and the seawater PGE composition is not inherited by ferromanganese deposits.Thus, palladium is mobile in seawater and its residence time in the ocean is longer (10-100 thousand years, [62]) than that of platinum (10-22 thousand years, [60]) and iridium (2-20 thousand years, [9]).Pd does not accumulate in ferromanganese deposits.On the contrary, iridium significantly enriches ferromanganese deposits relative to seawater due to its preferential sorption on ferromanganese oxyhydroxides.Hence, Ru, Ir, and Pt enrich nodules by accumulating in their hydrogenous components.PGE distribution in hydrothermal crusts is similar to MORB composition, and Pd is less depleted relative to Pt in these deposits [14].Platinum fluxes to ferromanganese crusts and nodules were found to be of the same order of magnitude (Table 5), although the methods of age determination were different.Calculated platinum flux into the nodules is lower than into the crusts.This effect is possibly related with a supply of diagenetic matter into the nodules, because the diagenetic matter is depleted of PGEs.The growth rate of crust Cr 2188 was found to be three times higher than that of Cr 1538, which explains the difference in Pt contents between the two crusts (47 versus 184 ng/g).Calculated platinum fluxes to these crusts differ less than by 15%.It confirms that platinum accumulation similar to that of cobalt and cerium depends primarily on the growth rate of ferromanganese crusts and nodules [38,63].The lower the growth rate, the higher the platinum accumulation.Regardless of some differences in PGE contents in these deposits, accumulation of platinum in both crusts and nodules occurred due to the same process of sorption from seawater.Although it does not rule out other sources of PGE to ferromanganese crusts and nodules in the ocean, nevertheless, their influence has not been detected in the samples studied.

Conclusions
This study has investigated the behaviour of platinum group elements (Ru, Pd, Ir, and Pt) and gold in surface and buried deep-sea ferromanganese nodules of the Cape and Brazil Basins, together with the PGE composition of Fe-Mn crusts from both basins.Nodules and crusts show Mn/Fe ratio values ranging from 0.4 to 1.6, they are enriched with Co, display high positive anomaly values of Ce, and are hydrogenous in their origin.Platinum contents in the nodules from the Brazil Basin vary from 110 to 174 ng/g and generally are higher than those in the nodules from the Cape Basin (from 78 to 107 ng/g).There is no significant difference in concentrations of other PGEs and gold between both basins.Ru varies from 9 to 26 ng/g, Pd from 1.1 to 2.1 ng/g, Ir from 1.2 to 3.3 ng/g, and Au from less than 0.2 to 1.2 ng/g.
In the ocean, ruthenium, iridium, and platinum can change their oxidation state: Ru(+3) → Ru(+4), Ir(+3) → Ir(+4), and Pt(+2) → Pt(+4).Within ferromanganese nodules, they correlate with other redox-sensitive elements: Co, Ce, and Tl.This correlation indicates a similar mechanism of their accumulation in ferromanganese deposits, namely surface oxidation of these elements when sorbed onto Fe-Mn oxyhydroxides.For palladium and gold, no relationship with major and trace elements have been revealed.
Platinum contents increase with the growth of the Mn/Fe ratio value in the studied hydrogenous crusts and nodules.It is probably related with platinum oxidation on the MnO2 surface.The highest Platinum fluxes to ferromanganese crusts and nodules were found to be of the same order of magnitude (Table 5), although the methods of age determination were different.Calculated platinum flux into the nodules is lower than into the crusts.This effect is possibly related with a supply of diagenetic matter into the nodules, because the diagenetic matter is depleted of PGEs.The growth rate of crust Cr 2188 was found to be three times higher than that of Cr 1538, which explains the difference in Pt contents between the two crusts (47 versus 184 ng/g).Calculated platinum fluxes to these crusts differ less than by 15%.It confirms that platinum accumulation similar to that of cobalt and cerium depends primarily on the growth rate of ferromanganese crusts and nodules [38,63].The lower the growth rate, the higher the platinum accumulation.Regardless of some differences in PGE contents in these deposits, accumulation of platinum in both crusts and nodules occurred due to the same process of sorption from seawater.Although it does not rule out other sources of PGE to ferromanganese crusts and nodules in the ocean, nevertheless, their influence has not been detected in the samples studied.

Conclusions
This study has investigated the behaviour of platinum group elements (Ru, Pd, Ir, and Pt) and gold in surface and buried deep-sea ferromanganese nodules of the Cape and Brazil Basins, together with the PGE composition of Fe-Mn crusts from both basins.Nodules and crusts show Mn/Fe ratio values ranging from 0.4 to 1.6, they are enriched with Co, display high positive anomaly values of Ce, and are hydrogenous in their origin.Platinum contents in the nodules from the Brazil Basin vary from 110 to 174 ng/g and generally are higher than those in the nodules from the Cape Basin (from 78 to 107 ng/g).There is no significant difference in concentrations of other PGEs and gold between both basins.Ru varies from 9 to 26 ng/g, Pd from 1.1 to 2.1 ng/g, Ir from 1.2 to 3.3 ng/g, and Au from less than 0.2 to 1.2 ng/g.
In the ocean, ruthenium, iridium, and platinum can change their oxidation state: Ru(+3) → Ru(+4), Ir(+3) → Ir(+4), and Pt(+2) → Pt(+4).Within ferromanganese nodules, they correlate with other redox-sensitive elements: Co, Ce, and Tl.This correlation indicates a similar mechanism of their accumulation in ferromanganese deposits, namely surface oxidation of these elements when sorbed onto Fe-Mn oxyhydroxides.For palladium and gold, no relationship with major and trace elements have been revealed.
Platinum contents increase with the growth of the Mn/Fe ratio value in the studied hydrogenous crusts and nodules.It is probably related with platinum oxidation on the MnO 2 surface.The highest platinum content has been found in a buried nodule at the depth of 83 cm at station 1541 (Brazil Basin).During early diagenesis, the buried Fe-Mn nodule changed its composition, having lost a part of the manganese and manganese-related elements (Co, Ni, and Tl), while the platinum and cerium contents were retained.So, at preservation of Mn oxides within a nodule, Pt mobility in diagenesis is very limited.Based on strontium isotope stratigraphy and Co-chronometer methods, platinum fluxes in crusts and nodules have been estimated.Platinum fluxes are from 35-42 ng•cm −2 •Ma −1 in nodules and 49-65 ng•cm −2 •Ma −1 in crusts.Platinum fluxes in hydrogenous nodules were found to be somewhat lower than in crusts, because some admixture of the diagenetic material poor in PGE in Fe-Mn nodules is always contained.Given that the values of platinum fluxes in crusts and nodules are close, we can suggest that PGE accumulation occurs as a result of scavenging these elements from seawater.Bolded values reflect significant correlation of PGE with other elements for n = 8 samples at the 95% confidence level and for n = 6 at the 90% confidence level.For Pt and Ir, n = 8; for Ru and Pd, n = 6.The Ce anomaly is calculated as Ce an = 2Ce/Ce PAAS /(La/La PAAS + Pr/Pr PAAS ), where Ln PAAS is the content in PAAS [42].

Figure 1 .
Figure 1.Location of crust and nodule samples.

Figure 1 .
Figure 1.Location of crust and nodule samples.

Figure 2 .
Figure 2. Photographs of the crust and nodules from the Brazil and Cape Basins.Figure 2. Photographs of the crust and nodules from the Brazil and Cape Basins.

Figure 2 .
Figure 2. Photographs of the crust and nodules from the Brazil and Cape Basins.Figure 2. Photographs of the crust and nodules from the Brazil and Cape Basins.

Figure 3 .
Figure 3. X-ray diffraction analysis pattern of the buried nodule 1541_83 and the reflected light image of its inner part.Inset: Fe-vernadite (1) is replaced by goethite (2).

Figure 3 .
Figure 3. X-ray diffraction analysis pattern of the buried nodule 1541_83 and the reflected light image of its inner part.Inset: Fe-vernadite (1) is replaced by goethite (2).

Figure 5 .
Figure 5. Ternary diagram of Bonatti et al. [40] showing nodule composition in Fe-Mn-10 × (Co + Ni + Cu) coordinates: (I) Field of deep-sea pelagic nodules and hydrogenous crusts; (II) field of hydrothermal Fe-Mn deposits; (III) field of hydrothermal metalliferous sediments and diagenetic nodules, from the margins of the oceanic pelagic zone.

Figure 6 .
Figure 6.Composition of REE and Y in coordinates of Ce an−Nd and Ce an−(Y/Y PAAS )/(Ho/Ho PAAS ) [41] in studied crusts and nodules.

Figure 6 .
Figure 6.Composition of REE and Y in coordinates of Ce an−Nd and Ce an−(Y/Y PAAS )/(Ho/Ho PAAS ) [41] in studied crusts and nodules.

Figure 6 .
Figure 6.Composition of REE and Y in coordinates of Ce an−Nd and Ce an−(Y/Y PAAS )/(Ho/Ho PAAS ) [41] in studied crusts and nodules.

Figure 7 .
Figure 7.The relationship between Pt and Co (a) and Pt/Pd and Mn/Fe (b) in crusts and nodules from the Cape and Brazil Basins.

Figure 7 .
Figure 7.The relationship between Pt and Co (a) and Pt/Pd and Mn/Fe (b) in crusts and nodules from the Cape and Brazil Basins.

Table 1 .
Coordinates, water depth, and exploration tools used for sampling.

Table 1 .
Coordinates, water depth, and exploration tools used for sampling.

Table 3 .
Concentrations of major and trace elements of crust and nodules from the Cape and Brazil Basins.

Table 4 .
PGE content (ng/g) in ferromanganese crust and nodules from the Cape and Brazil Basins.

Table 5 .
Growth rates and Pt fluxes in investigated crust and nodules.