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Zhongrong Qiu, Yanhui Dong, Weilin Ma, Weiyan Zhang, Kehong Yang, Hongqiao Zhao. Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean[J]. Acta Oceanologica Sinica, 2020, 39(8): 34-42. doi: 10.1007/s13131-020-1616-y
Citation: Zhongrong Qiu, Yanhui Dong, Weilin Ma, Weiyan Zhang, Kehong Yang, Hongqiao Zhao. Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean[J]. Acta Oceanologica Sinica, 2020, 39(8): 34-42. doi: 10.1007/s13131-020-1616-y

Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean

doi: 10.1007/s13131-020-1616-y
Funds:  China Ocean Mineral Resources R&D Association (COMRA) Project under contract Nos DY135-C1-1-05, DY135-N1-1-06 and DY135-C1-1-02; the Scientific Research Fund of the Second Institute of Oceanography, MNR under contract No. JT1304.
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  • Corresponding author: E-mail: dongyh@sio.org.cn
  • Received Date: 2020-02-20
  • Accepted Date: 2020-03-05
  • Available Online: 2020-12-28
  • Publish Date: 2020-08-25
  • Polymetallic nodules and cobalt (Co)-rich crusts are enriched in platinum-group elements (PGEs), especially platinum (Pt) and may be important sinks of PGEs. At present, little information is available on PGEs in polymetallic nodules, and their geochemical characteristics and the causes of PGEs enrichment are unclear. Here, PGEs of polymetallic nodules from abyssal basin in the Marcus-Wake Seamount area of the Northwest Pacific Ocean are reported and compared with the published PGEs data of polymetallic nodules and Co-rich crusts in the Pacific. The total PGEs (ΣPGE) content of polymetallic nodules in study area is 258×10–9 in average, markedly higher than that of Clarion-Clipperton Zone (CCZ) nodules (ΣPGE=127×10–9) and lower than that of Co-rich crusts in the Marcus-Wake Seamount (ΣPGE=653×10–9), similar to that of Co-rich crusts in the South China Sea (ΣPGE=252×10–9). The CI chondrite-normalized PGEs patterns in different regions of polymetallic nodules and cobalt-rich crusts are highly consistent, with all being characterized by positive Pt and negative Pd anomalies. These results, together with those of previous studies, indicate that PGEs in polymetallic nodules and Co-rich crusts are mainly derived directly from seawater. Pt contents of polymetallic nodules from the study area are negatively correlated with water depth, and Pt/ΣPGE ratios in nodules there are also lower than those of the Co-rich crusts in the adjacent area, indicating that sedimentary water depth and oxygen fugacity of ambient seawater are the possible important controlling factors for Pt accumulation in crusts and nodules.
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  • [1] Astakhova N V. 2017. Noble metals in ferromanganese crusts from marginal seas of the Northwest Pacific. Oceanology, 57(4): 558–567. doi:  10.1134/S0001437017040014
    [2] Azami K, Hirano N, Machida S, et al. 2018. Rare earth elements and yttrium (REY) variability with water depth in hydrogenetic ferromanganese crusts. Chemical Geology, 493: 224–233. doi:  10.1016/j.chemgeo.2018.05.045
    [3] Bau M, Schmidt K, Koschinsky A, et al. 2014. Discriminating between different genetic types of marine Ferro-manganese crusts and nodules based on rare earth elements and yttrium. Chemical Geology, 381: 1–9. doi:  10.1016/j.chemgeo.2014.05.004
    [4] Bonatti E, Kraemer T, Rydell H. 1972. Classification and genesis of submarine iron–manganese deposits. In: Horn D R, ed. Ferromanganese Deposits on the Ocean Floor. Washington: National Science Foundation, 149–166
    [5] Brownlee D E, Bates B A, Wheelock M M. 1984. Extraterrestrial platinum group nuggets in deep-sea sediments. Nature, 309(5970): 693–695. doi:  10.1038/309693a0
    [6] Cabral A R, Sattler C D, Lehmann B, et al. 2009. Geochemistry of some marine Fe-Mn nodules and crusts with respect to Pt contents. Resource Geology, 59(4): 400–406
    [7] Dymond J, Lyle M, Finney B, et al. 1984. Ferromanganese nodules from MANOP Sites H, S, and R-Control of mineralogical and chemical composition by multiple accretionary processes. Geochimica et Cosmochimica Acta, 48(5): 931–949. doi:  10.1016/0016-7037(84)90186-8
    [8] Fujinaga K, Yasukawa K, Nakamura K, et al. 2016. Geochemistry of REY-rich mud in the Japanese exclusive economic zone around Minamitorishima Island. Geochemical Journal, 50(6): 575–590. doi:  10.2343/geochemj.2.0432
    [9] Glasby G P, Ren Xiangwen, Shi Xuefa, et al. 2007. Co-rich Mn crusts from the Magellan Seamount cluster: the long journey through time. Geo-Marine Letters, 27(5): 315–323
    [10] Guan Yao, Sun Xiaoming, Ren Yingzhi, et al. 2017. Mineralogy, geochemistry and genesis of the polymetallic crusts and nodules from the South China Sea. Ore Geology Reviews, 89: 206–227. doi:  10.1016/j.oregeorev.2017.06.020
    [11] Halbach P, Kriete C, Prause B, et al. 1989. Mechanisms to explain the platinum concentration in ferromanganese seamount crusts. Chemical Geology, 76(1–2): 95–106. doi:  10.1016/0009-2541(89)90130-7
    [12] Halbach P, Scherhag C, Hebisch U, et al. 1981. Geochemical and mineralogical control of different genetic types of deep-sea nodules from the Pacific Ocean. Mineralium Deposita, 16(1): 59–84
    [13] He Gaowen, Sun Xiaoming, Yang Shengxiong, et al. 2006. Platinum group elements (PGE) geochemistry of polymetallic nodules in CC zone, east Pacific Ocean. Mineral Deposits (in Chinese), 25(2): 164–174
    [14] Hein J R, Koschinsky A. 2014. Deep-ocean ferromanganese crusts and nodules. Treatise on Geochemistry, 13: 273–291
    [15] Hein J R, Koschinsky A, Bau M, et al. 2000. Cobalt-rich ferromanganese crusts in the Pacific. In: Cronan D S, ed. Handbook of Marine Mineral Deposits. Boca Raton: CRC Press, 239–279
    [16] Hein J R, Mizell K, Koschinsky A, et al. 2013. Deep-ocean mineral deposits as a source of critical metals for high-and green-technology applications: comparison with land-based resources. Ore Geology Reviews, 51: 1–14. doi:  10.1016/j.oregeorev.2012.12.001
    [17] Hein J R, Spinardi F, Okamoto N, et al. 2015. Critical metals in manganese nodules from the Cook Islands EEZ, abundances and distributions. Ore Geology Reviews, 68: 97–116. doi:  10.1016/j.oregeorev.2014.12.011
    [18] Hodge V F, Stallard M, Koide M, et al. 1985. Platinum and the platinum anomaly in the marine environment. Earth and Planetary Science Letters, 72(2–3): 158–162. doi:  10.1016/0012-821X(85)90002-0
    [19] Iijima K, Yasukawa K, Fujinaga K, et al. 2016. Discovery of extremely REY-rich mud in the western North Pacific Ocean. Geochemical Journal, 50(6): 557–573. doi:  10.2343/geochemj.2.0431
    [20] Koide M, Goldberg E D, Niemeyer S, et al. 1991. Osmium in marine sediments. Geochimica et Cosmochimica Acta, 55(6): 1641–1648. doi:  10.1016/0016-7037(91)90135-R
    [21] Le Suave R, Pichocki C, Pautot G, et al. 1989. Geological and mineralogical study of Co-rich ferromanganese crusts from a submerged atoll in the Tuamotu Archipelago (French Polynesia). Marine Geology, 87(2–4): 227–247. doi:  10.1016/0025-3227(89)90063-7
    [22] Machida S, Fujinaga K, Ishii T, et al. 2016. Geology and geochemistry of ferromanganese nodules in the Japanese Exclusive Economic Zone around Minamitorishima Island. Geochemical Journal, 50(6): 539–555. doi:  10.2343/geochemj.2.0419
    [23] Manheim F T. 1986. Marine cobalt resources. Science, 232(4750): 600–608. doi:  10.1126/science.232.4750.600
    [24] McDonough W F, Sun Shensu. 1995. The composition of the Earth. Chemical Geology, 120(3–4): 223–253. doi:  10.1016/0009-2541(94)00140-4
    [25] McLennan S M. 1989. Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. Reviews in Mineralogy and Geochemistry, 21(1): 169–200
    [26] Nozaki Y. 1997. A fresh look at element distribution in the North Pacific Ocean. EOS Transactions, 78(21): 221. doi:  10.1029/97EO00148
    [27] Okamoto N, Usui A. 2014. Regional distribution of Co-Rich ferromanganese crusts and evolution of the seamounts in the Northwestern Pacific. Marine Georesources & Geotechnology, 32(3): 187–206
    [28] Qiu Zhongrong, Ma Weiling, Zhang Xiaoyu, et al. 2019. Geochemical characteristics of sediments in Southeast sea area of Minamitorishima Island and their indication of rare earth elements resources. Geological Science and Technology Information (in Chinese), 38(4): 205–214
    [29] Ren Xiangwen, Glasby G P, Liu Jihua, et al. 2007. Fine-scale compositional variations in a Co-rich Mn crust from the Marcus-Wake Seamount cluster in the western Pacific based on electron microprobe analysis (EMPA). Marine Geophysical Researches, 28(2): 165–182. doi:  10.1007/s11001-007-9024-7
    [30] Ren Jiangbo, He Gaowen, Yao Huiqiang, et al. 2016. Geochemistry and significance of REE and PGE of the cobalt-rich crusts from west Pacific Ocean seamounts. Earth Science (in Chinese), 41(10): 1745–1757
    [31] Sager W W, Duncan R A, Handschumacher D W. 1993. Paleomagnetism of the Japanese and Marcus-Wake Seamounts, western Pacific Ocean. In: Pringle M S, Sager W W, Sliter W V, et al., ed. The Mesozoic Pacific: Geology, Tectonics and Volcanism, Geophysical Monograph. Washington: American Geophysical Union Press, 401–435
    [32] Stüben D, Glasby G P, Eckhardt J D, et al. 1999. Enrichments of platinum-group elements in hydrogenous, diagenetic and hydrothermal marine manganese and iron deposits. Exploration and Mining Geology, 8(3–4): 233–250
    [33] Sun Xiaoming, Xue Ting, He Gaowen, et al. 2006. Platinum group elements (PGE) and Os isotopic geochemistry of ferromanganese crusts from Pacific Ocean seamounts and their constraints on genesis. Acta Petrologica Sinica (in Chinese), 22(12): 3014–3026
    [34] Takaya Y, Yasukawa K, Kawasaki T, et al. 2018. The tremendous potential of deep-sea mud as a source of rare-earth elements. Scientific Reports, 8(1): 5763. doi:  10.1038/s41598-018-23948-5
    [35] Wilson G C, Rucklidge J C, Kilius L R, et al. 1997. Precious metal abundances in selected iron meteorites: in-situ AMS measurements of the six platinum-group elements plus gold. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 123(1–4): 583–588. doi:  10.1016/S0168-583X(96)00415-6
    [36] Wiltshire J C, Wen Xiyuan, Yao De, et al. 1999. Ferromanganese crusts near Johnston Island: geochemistry, stratigraphy and economic potential. Marine Georesources & Geotechnology, 17(2–3): 257–270
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Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean

doi: 10.1007/s13131-020-1616-y
Funds:  China Ocean Mineral Resources R&D Association (COMRA) Project under contract Nos DY135-C1-1-05, DY135-N1-1-06 and DY135-C1-1-02; the Scientific Research Fund of the Second Institute of Oceanography, MNR under contract No. JT1304.

Abstract: Polymetallic nodules and cobalt (Co)-rich crusts are enriched in platinum-group elements (PGEs), especially platinum (Pt) and may be important sinks of PGEs. At present, little information is available on PGEs in polymetallic nodules, and their geochemical characteristics and the causes of PGEs enrichment are unclear. Here, PGEs of polymetallic nodules from abyssal basin in the Marcus-Wake Seamount area of the Northwest Pacific Ocean are reported and compared with the published PGEs data of polymetallic nodules and Co-rich crusts in the Pacific. The total PGEs (ΣPGE) content of polymetallic nodules in study area is 258×10–9 in average, markedly higher than that of Clarion-Clipperton Zone (CCZ) nodules (ΣPGE=127×10–9) and lower than that of Co-rich crusts in the Marcus-Wake Seamount (ΣPGE=653×10–9), similar to that of Co-rich crusts in the South China Sea (ΣPGE=252×10–9). The CI chondrite-normalized PGEs patterns in different regions of polymetallic nodules and cobalt-rich crusts are highly consistent, with all being characterized by positive Pt and negative Pd anomalies. These results, together with those of previous studies, indicate that PGEs in polymetallic nodules and Co-rich crusts are mainly derived directly from seawater. Pt contents of polymetallic nodules from the study area are negatively correlated with water depth, and Pt/ΣPGE ratios in nodules there are also lower than those of the Co-rich crusts in the adjacent area, indicating that sedimentary water depth and oxygen fugacity of ambient seawater are the possible important controlling factors for Pt accumulation in crusts and nodules.

Zhongrong Qiu, Yanhui Dong, Weilin Ma, Weiyan Zhang, Kehong Yang, Hongqiao Zhao. Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean[J]. Acta Oceanologica Sinica, 2020, 39(8): 34-42. doi: 10.1007/s13131-020-1616-y
Citation: Zhongrong Qiu, Yanhui Dong, Weilin Ma, Weiyan Zhang, Kehong Yang, Hongqiao Zhao. Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean[J]. Acta Oceanologica Sinica, 2020, 39(8): 34-42. doi: 10.1007/s13131-020-1616-y
    • Hydrogenetic polymetallic nodules and cobalt (Co)-rich crusts enriched in useful metal elements such as Cu, Ni, Co, and rare earth elements including yttrium (REY) have very important economic value (Hein et al., 2013; Manheim, 1986), and serve as sinks of some trace elements in seawater. Platinum-group elements (PGEs), particularly Pt, are also relatively enriched in nodules and crusts, and are receiving increasing research attention (Astakhova, 2017; Cabral et al., 2009; Halbach et al., 1989).

      Sources and enrichment mechanisms of PGEs in polymetallic nodules and Co-rich crusts are still debated. Halbach et al. (1989) suggested that PGEs in crusts are derived mainly from seawater and extraterrestrial materials, and Pt2+ in seawater co-precipitated with MnO2 though redox reaction, which lead to enrichment of PGEs in crusts. Stüben et al. (1999) and Le Suave et al. (1989) found significant Pd negative anomalies in the CI chondrite-normalized PGEs patterns of hydrogenetic ferromanganese nodules and crusts, and suggested that meteorite spheroids were not the main source of PGEs for these deposits. Ren et al. (2016) and Hodge et al. (1985) suggested that PGEs in nodules and crust should be derived directly from seawater, and selective adsorption of PGEs by seawater in nodules and crusts resulted in positive Pt anomalies. Sun et al. (2006) and He et al. (2006) suggested that PGEs in nodules and crusts are perhaps derived mainly from submarine seawater-basalt interactions, partly from extraterrestrial materials. They explained the greater enrichment of PGEs in crusts than in nodules as being due to the crusts being located near the Oxygen Minimum Zone (OMZ), which gave a more advantageous marine chemical environment for PGEs precipitation.

      To improve our understanding of possible sources and enrichment mechanisms of PGEs in polymetallic nodules and Co-rich crust, and to increase the available data on PGEs in polymetallic nodules. Here, we report PGEs contents of 23 hydrogenetic polymetallic nodule samples from the abyssal basin in the Marcus-Wake Seamount area of the Northwest Pacific Ocean and compare PGEs characteristics of polymetallic nodules and Co-rich crusts in different regions of the Pacific Ocean.

    • The study area is located in the Marcus-Wake Seamount area, Northwest Pacific, ~1200 km from the eastern Mariana Trench (Fig. 1). There are NE-trending and EW-trending secondary structures in the area and the water depth at the top of the seamount is 1 200–1 800 m (Ren et al., 2016). The seamounts in study area are formed on the ancient oceanic crusts, and their ages are generally younger than those of lower oceanic crusts (Glasby et al., 2007; Sager et al., 1993). Marine geological survey data and studies related to paleomagnetism, petrochronology, and isotopic compositions indicate a seamount age of 90–120 Ma (Ren et al., 2016). Previous studies have shown that since 23 Ma, the entire Western Pacific Seamount, including the study area, has drifted in a WNW direction (Ren et al., 2007). The study area is currently considered the most potentially rich area for ferromanganese oxide deposits in the Pacific (Okamoto and Usui, 2014). China, Russia, Australia, and New Zealand have conducted abundant research on Co-rich crusts and polymetallic nodules in the area (Okamoto and Usui, 2014). REY-rich sediments in the area have also been the subject of recent research attention (Azami et al., 2018; Qiu et al., 2019; Fujinaga et al., 2016; Iijima et al., 2016; Machida et al., 2016; Takaya et al., 2018).

      Figure 1.  The location of study area.

    • A total of 23 polymetallic nodules from 15 sampling stations collected by the box cores of the R/V Xiangyanghong No. 10 in 2016 were analyzed. The sampling depth was 5 200–5 600 m, and all polymetallic nodules are of the exposed-type. Most nodules are large, medium-sized and spherical, with most being concentric layered structure with obvious nucleus. Bedrocks are the most common nucleus type. X-ray Powder Diffractmeter (XRD) analysis indicates that the predominant mineral composition of nodules includes vernadite (δ-MnO2), detrital mineral (quartz), authigenic minerals (zeolite) and so on. The nodules were ground into powder by ball mill for bulk sample geochemical analysis.

      PGEs analyses were conducted at the National Geological Experimental Testing Center, Chinese Academy of Geological Sciences, Beijing, China. Samples were crushed to 200 mesh and weighed into a crucible. Osmium spike and solvents, including sodium carbonate, sodium borate, borax, nickel hydroxide, sulfur, and flour were added and mixed with the sample powder. The mixture was melted in a muffle furnace over 1–1.5 h at 1 100°C. The melt was poured into an iron mold, and picked up sulfur-nickel buckle after cooling. Then the broken buckle was crushed and dissolved with HCl. Te co-precipitant and SnCl2 were then added to dissolve the precipitate and filter out the insoluble matter, dissolved with aqua regia and transferred to a colorimetric tube. Finally, Elemental contents were determined by inductively coupled plasma-mass spectrometry (ICP-MS). The China National Standard Substance GBW07290 was used for quality-control purposes, and the analysis precisions obtained were as follows: Pt 1.25%, Pd 1.74%, Rh 18.47%, Ru 4.08%, Ir 9.30%, and Os 7.92%. Results and related parameters are listed in Table 1.

      Sample No.Pt/10–9Pd/10–9Rh/10–9Ru/10–9Ir/10–9Os/10–9ΣPGE/10–9Pt/Pt*Pt/PdFe2O3/%MnO/%
      S011641.8713.415.53.690.861998.6787.724.225.0
      S021833.7615.418.65.042.392286.3748.722.623.9
      S032403.3319.320.45.221.272907.9372.122.425.6
      S042787.2822.822.16.111.603385.7138.222.326.0
      S052544.6921.716.94.900.933036.6754.220.325.0
      S062303.1416.419.34.521.132748.4973.222.527.4
      S0727111.50 19.620.45.411.163294.7823.618.423.5
      S082222.5018.518.84.931.342688.6488.823.625.6
      S092282.9019.319.55.211.242768.0778.624.426.2
      S102363.2217.719.85.011.212838.2873.324.626.0
      S111942.4317.518.15.131.542397.8879.824.226.1
      S122622.5320.621.35.580.893139.6110424.526.0
      S131922.2415.416.64.751.762338.6585.723.225.0
      S141834.5816.117.44.380.952265.6440.021.922.0
      S151892.1715.819.04.791.702328.5587.124.425.6
      S162402.4820.018.94.770.722879.0296.821.725.5
      S1720613.50 16.914.75.081.542583.6115.322.425.3
      S181832.3315.015.93.851.162218.2078.522.725.1
      S191961.9515.916.94.171.022369.3210123.625.6
      S201734.4614.414.93.880.892125.7238.823.625.2
      S212264.4018.718.44.670.962736.6051.420.524.3
      S221973.6216.616.04.200.882386.7354.422.825.4
      S231561.6512.314.03.511.241899.1794.523.126.9
      Mean2134.0217.418.04.731.232587.4968.022.825.3
      Maximum value27813.50 22.822.16.112.393389.6110424.627.4
      Minimum value1561.6512.314.03.510.721893.6123.618.422.0
      CI chondrite1 010 5501307104554903 3451.001.84
      Primitive mantle7.103.900.905.003.203.4023.51.001.82
      Note: Data for CI chondrite and primitive mantle are from McDonough et al. (1995). – represents the primitive mantle and chondrite are used as reference for PGEs composition, as they are not nodules, and the values of Fe and Mn are not marked.

      Table 1.  PGEs, Fe and Mn contents in polymetallic nodules from the Northwest Pacific Ocean

      Major and trace elements analyses were completed at the Analytical Laboratory of Beijing Research Institute of Uranium Geology, Beijing, China. Trace elements compositions were determined by ICP-MS using an ELEMENT XR instrument, with analytical uncertainties of <10%. REY and trace elements data for samples are listed in Table 2. Major elements such as Fe and Mn were analyzed by X-ray fluorescence spectrometer using an AxiosmAX instrument, with analytical uncertainties of <5%.

      Sample No.LaCePrNdSmEuGdTbDyYHoErTmYbLuΣREYδCeYSN/HoSNNiCoCu
      S012021 34844.819039.09.8140.58.7241.91497.8322.83.6724.53.801 9873.270.704 0373 9231 939
      S021861 01543.915137.18.7540.47.9538.81477.8522.03.8025.63.761 5922.590.693 3753 3661 928
      S031821 29340.717732.89.1038.27.0037.91387.4521.44.0726.74.271 8823.470.683 8984 1311756
      S042241 29153.220847.211.050.28.8344.11789.0023.24.3324.53.922 0022.730.733 9684 0971 888
      S051921 51943.321343.69.9745.07.7341.91797.7521.54.1122.73.912 1753.840.854 4943 6651 953
      S062011 12542.017838.49.1139.67.6739.91508.4121.03.9723.23.721 7412.820.654 5803 6882 056
      S071431 04630.712227.06.8428.45.3624.41245.3114.92.6415.42.951 4753.640.864 1753 7721 955
      S082221 60948.321141.610.743.37.7340.81558.4322.94.1723.73.912 2983.580.673 8174 1101 404
      S092211 47648.119439.99.7242.08.2638.11518.6623.54.1922.64.012 1403.300.644 1243 7671 808
      S102431 43752.021741.610.341.88.0340.01518.2520.83.8823.74.192 1522.950.673 8403 9881 495
      S112441 55549.720847.111.145.98.0242.31638.5823.24.1923.84.182 2753.250.703 4554 5981 177
      S122181 50950.821644.712.145.98.7741.51608.8624.44.2222.64.242 2113.310.663 8904 5181 785
      S132251 54546.320839.310.443.68.0440.31577.9822.54.0124.54.322 2293.490.723 8713 9461 597
      S142171 32943.518240.49.1739.67.0935.31387.5418.93.4619.73.581 9563.150.673 1803 4741 529
      S152501 62252.022645.111.547.18.4341.81608.9324.14.5224.84.512 3713.280.664 1524 3531 677
      S162171 58647.220443.310.945.08.0138.51668.5122.54.0723.44.262 2633.610.724 5603 7202 096
      S172151 59544.418741.69.4342.97.4435.71417.6120.33.5821.54.112 2363.760.683 7324 3241 599
      S181991 42442.118741.79.8540.97.5337.11457.9420.03.8120.93.942 0463.590.673 9423 8431 467
      S192151 43545.320141.210.342.87.8138.21547.8921.24.0821.64.062 0953.350.724 3244 1642 063
      S202171 36345.819742.710.542.87.9638.71568.2121.24.0823.64.222 0273.150.703 9513 8241 853
      S212001 34142.618937.89.7040.47.4937.91587.9121.03.9821.54.241 9653.350.733 9833 5731 947
      S222161 57447.319942.510.047.47.8041.41618.3621.74.2122.94.142 2473.590.714 5674 1861 965
      S232051 48542.719438.69.3842.57.1838.01457.7819.73.9121.53.802 1193.660.684 6414 5821 858

      Table 2.  REY (10–6) and some trace elements (10–6) data for samples in study area

    • Total PGEs (ΣPGE) content of polymetallic nodules range from 189×10–9 to 338×10–9 with an average of 258×10–9. The Pt content is the highest among the PGEs with an average of 213×10–9, followed by Ru (18.0×10–9) and Rh (17.4×10–9). Ir and Pd contents are slightly lower, while the Os contents are the lowest (Table 1). PPGEs (Rh+Pt+Pd) contents are higher than IPGEs (Os+Ir+Ru) contents, with an average PPGEs/IPGEs ratio of 9.79.

      ΣPGE contents of the polymetallic nodules are significantly lower than those of chondrites, but are an order of magnitude higher than that of primitive mantle (Mcdonough and Sun, 1995). Their Pt, Rh, and Ru contents are much higher than mantle values. The contents of other elements, such as Pd, Ir and Os, are similar to primitive mantle values (Table 1). The total REY (ΣREY) contents of polymetallic nodules range from 1 475×10–6 to 2 371×10–6, and with an average value of 2 064×10–6. The δCe values of all samples are greater than 1, indicating that the nodules have positive Ce anomalies, and the YSN/HoSN (SN means normalized by Post-Archean Australian Shale from McLennan (1989)) ratios of all the samples are less than 1 (Table 2).

      The PGEs compositions of polymetallic nodules in the study area are compared with those of nodules and Co-rich crusts from other areas in Fig. 2. The Pt content displays a strong positive correlation with Ir and Rh contents in both polymetallic nodules and Co-rich crusts (Figs 2a, e), but no obvious correlation with Os contents (Fig. 2c). Pt contents are well correlated with Ru contents in polymetallic nodules in the study area, but not in other polymetallic nodules and Co-rich crusts from other areas (Fig. 2d). In the Pt-Pd diagram (Fig. 2b), Pt and Pd in polymetallic nodules and Co-rich crusts display no obvious correlations, and Pd is significantly enriched in polymetallic nodules compared to Co-rich crusts (Fig. 2b).

      Figure 2.  Relationships among PGEs elements in nodule and crusts. Co-rich crusts in West-central Pacific after Sun et al. (2006), nodules in Clarion–Clipperton Zone (CCZ) after He et al. (2006), Co-rich crusts in Marcus-Wake Seamount after Ren et al. (2016), nodules in Cook Islands, Exclusive Economic Zone (EEZ) after Hein et al. (2015), and Co-rich crusts in South China Sea after Guan et al. (2017).

      Figure 3 shows CI chondrite-normalized PGEs patterns of the samples in study area. The PGEs distribution curves of the samples are highly consistent, and obviously left leaning with gradual enrichment from Os to Pt and strong depletion of Pd.

      Figure 3.  CI chondrite-normalized PGEs patterns of the samples.

      All samples exhibit obvious positive Pt anomalies. Pt/Pd ratios range from 23.6 to 104 with an average of 68.0. The Pt anomaly (Pt/Pt*; Table 1) was calculated as follows:

      where the PtN, RhN, and PdN are CI chondrite-normalized values, and Pt/Pt* range from 3.61 to 9.61 with an average of 7.49.

    • Deep-sea polymetallic nodules and Co-rich crusts comprise mainly poorly crystallized and amorphous Fe-Mn (oxyhydr) oxides (Hein and Koschinsky, 2014; Hein et al., 2013). Bonatti et al. (1972) classified such nodules according to their Mn/Fe ratios, with ratios <2.5 indicating a hydrogenetic type, ratios >5 a diagenetic type, and ratios of 2.5–5.0 a mixed type. Hydrogenetic nodules are generally formed by precipitation of Fe and Mn (oxyhydr) oxides from seawater, and comprise predominately δ-MnO2 containing FeOOH·xH2O (Dymond et al., 1984; Halbach et al., 1981), with their mineralizing substances being derived directly from seawater. Here, the Mn/Fe ratios of polymetallic nodules range from 1.01 to 1.28 with an average of 1.11, which, together with the Mn-Fe-(Cu+Co+Ni)×10 differentiation diagram and REY discrimination diagrams (Figs 4, 5), confirm that they are of the hydrogenetic type.

      Figure 4.  Ternary major element geochemical discrimination diagram of the Fe-Mn nodules genetic types (modified after Bonatti et al. (1972)).

      Figure 5.  Data for nodules of our study plot in the hydrogenetic fields in discrimination graphs (Bau et al., 2014) of CeSN/CeSN* ratio vs. YSN/HoSN ratio. Subscript SN stands for Post Archean Australian Shale (PAAS) normalized.

      The major characteristics of PGEs in polymetallic nodules and Co-rich crusts (Table 3) can be summarized as follows:

      Sample typePtPdRhRuIrOsΣPGEPPGEs/IPGEsPt/Pt*Pt/Pd
      Our studyhydrogenetic2134.0217.418.04.731.232589.797.4968.0
      Marcus-Wake Seamount crustshydrogenetic6002.0527.315.67.590.7565326.7022.70366.0
      CCZ nodulesmixed-type1015.17 4.5512.52.531.061277.085.6119.7
      Cook Islands noduleshydrogenetic2547.7317.819.55.642.0030610.405.9235.9
      West-central Pacific crustshydrogenetic2521.1212.319.12.402.0628911.6018.00238.0
      South China Sea crustshydrogenetic2062.2422.513.76.281.6225210.607.7898.3
      Note: All values are averaged. The quantity and source of samples are consistent with Fig. 2.

      Table 3.  PGEs content (10–9) and major parameters of PGEs in Co-rich crusts and polymetallic nodules

      (1) The Pt content of PGEs in polymetallic nodules and Co-rich crusts is obviously the highest, followed by Rh and Ru. The Pd content in polymetallic nodules is higher than that of crusts, indicating that Pd is more enriched in nodules than crusts. The contents of other PGEs in nodules are not absolutely higher or lower than those of crusts, noting that the Pt content of polymetallic nodules in our study is significantly lower than that of Marcus-Wake Seamount crusts.

      (2) The average ΣPGE content of polymetallic nodules in the study area is equivalent to that of crusts in the South China Sea, and significantly higher than that of nodules in the CCZ, East Pacific, but significantly lower than that of Co-rich crusts in Marcus-Wake Seamount. In general, without considering the ΣPGE content of the hydrogenetic crusts in the South China Sea, the ΣPGE content of hydrogenetic Co-rich crusts in the open ocean is higher than that of hydrogenetic polymetallic nodules, whereas the ΣPGE content of polymetallic nodules is obviously higher than that of mixed-type nodules from the CCZ. ΣPGE content of Co-rich crusts is relatively low in the South China Sea, possibly due to the dilution of the terrigenous materials and the high deposition rate (the mean growth rate of South China Sea crusts is 19.20 mm/Ma, higher than most hydrogenetic crusts worldwide (Hein et al., 2000)).

      (3) Polymetallic nodules in our study area as well as polymetallic nodules and Co-rich crusts from other areas are enriched in Pd-group PGEs (PPGEs: Rh+Pt+Pd), but depleted in Ir-group PGEs (IPGEs: Os+Ir+Ru). In general, the PPGEs/IPGEs ratios of nodules are lower than those of crusts, i.e., a greater degree of differentiation of PGEs occurs in crusts than in nodules.

      (4) Consistent with the PPGEs/IPGEs, the average Pt/Pt* of Co-rich crusts is higher than that of polymetallic nodules, indicating that Co-rich crusts are more enriched in Pt than polymetallic nodules.

      (5) The Pt/Pd ratios vary significantly both in polymetallic nodules and Co-rich crusts (19.7−366), but the average Pt/Pd ratios of polymetallic nodules are lower than that of Co- rich crusts.

      In addition, the CI chondrite-normalized PGEs patterns of polymetallic nodules in the study area and those of nodules and Co-rich crusts from other regions are highly consistent (Fig. 6), with all being characterized by positive Pt anomalies, gradual enrichments from Os to Pt, and gradual depletions from Pt to Pd, suggesting a consistent source of PGEs for the polymetallic nodules and Co-rich crusts.

      Figure 6.  CI chondrite-normalized PGEs patterns of the samples and comparison with other geological bodies. The quantity and source of samples are consistent with Fig. 2. The iron meteorites values are from Wilson et al. (1997), the seawater values are from Nozaki (1997), and data for CI chondrite and primitive mantle are from McDonough and Sun (1995). Data sources are consistent with Fig. 2.

    • Sources of PGEs in polymetallic nodules and Co-rich crusts remain under debate (Brownlee et al., 1984; Wiltshire et al., 1999; Koide et al., 1991). Previous studies have suggested that extraterrestrial materials may be partial providers of PGEs in polymetallic nodules and Co-rich crusts, with their PGEs patterns being similar to those of iron meteorites (Sun et al., 2006; He et al., 2006). Halbach et al. (1989) discovered Ni-rich meteorites in the Central Pacific Ocean Seamount area, and suggested that seawater and cosmic particles could be a source of PGEs in Co-rich crusts. However, the ΣPGE content of meteorites is much higher than those of nodules and Co-rich crusts (Fig. 6). Furthermore, iron meteorites have low Pt/Pd ratios (mean 7.87; Wilson et al., 1997) compared with nodules and crusts (Sun et al., 2006; He et al., 2006). Iron meteorites are therefore not the main source of PGEs in crusts and nodules.

      The chondrite-normalized PGEs patterns of seawater display negative Pt anomalies, with gradual depletion from Os to Ir and Rh to Pt and enrichment from Pt to Pd, the converse of trends in nodules and crusts (Fig. 6). This implies that seawater and nodules/crusts have a relationship of source and sink with PGEs in nodules and crusts being derived directly from seawater. Ren et al. (2016) compared the geochemical characteristics of REY and PGEs of Co-rich crusts from West Pacific Seamount and concluded that REY distribution patterns with positive Ce anomalies, and unique PGEs patterns with positive Pt and negative Pd anomalies in Co-rich crusts, are due to the selective adsorption of REY and PGEs from seawater by oxides in the crusts. The preferential absorption of Ce from seawater by oxides in crusts resulted in significant positive Ce anomalies in the NASC-normalized REY patterns of Co-rich crusts (Ren et al., 2016). Besides, Hodge et al. (1985) also suggested that the mechanism of Pt anomaly formation in polymetallic nodules is similar to that of Ce anomalies in rare-earth-element series. Thus, the preference of the oxides for Pt over Pd leads to the unique PGEs patterns, which further indicates that PGEs in polymetallic nodules and crusts are derived directly from seawater.

    • Although the PGEs patterns in polymetallic nodules and Co-rich crusts are very consistent (Fig. 6), the comparison of Pt/Pt* values of hydrogenetic nodules with these of hydrogenetic crusts and mixed-type diagenetic nodules in Section 5.2 indicates that Pt/Pt* values of hydrogenetic crusts > hydrogenetic nodules > mixed-type nodules (Table 3). Based on this and the correlation characteristics of Pt and Pd in nodules (Fig. 2b), we suggest that there is a difference in enrichment of Pt from seawater between nodules and crusts during their growth.

      Previous studies have suggested that PGEs in the ocean are mostly in the form of complexes. Halbach et al. (1989) suggested that Pt in seawater exists mainly in divalent and tetravalent states, and that Pt enrichment in Co-rich crusts may be due to the co-precipitation of Pt2+ with MnO2 in seawater. Sun et al. (2006) proposed that sedimentary water depth and seawater oxygen fugacity are the dominant factors leading to differences in Pt enrichment between crusts and nodules. The negative correlation between Pt and Fe2O3 of nodules and Co-rich crusts, and a lack of correlation with MnO (Fig. 7), indicates that iron mineral phases and manganese mineral phases in polymetallic nodules and crusts do not promote the concentration of Pt from seawater. This is inconsistent with the results of previous studies that Pt enrichment in the crusts is related to manganese mineral phases, whereas Pt enrichment in nodules is related to the iron mineral phases (Sun et al., 2006; He et al., 2006).

      Figure 7.  Correlation between MnO, Fe2O3 and Pt in nodules and crusts. The data of nodules and crusts are the same with Fig. 2.

      Halbach et al. (1989) found that the Pt content in Co-rich crusts is closely related to water depth, the shallower the depth at which crusts grow, the higher their Pt content, and thus the higher their PGEs content. In this study, the Pt in polymetallic nodules of this study shown negative correlation with water depth (Fig. 8), implying that the sedimentary water depth also affects Pt enrichment in polymetallic nodules.

      Figure 8.  Plot of depth vs. Pt content. Pt contents of samples in the same station are averaged.

      In addition, Pt/ΣPGE ratios in the polymetallic nodules in study area vary between 0.80–0.84 and with an average of 0.82, whereas values in Co-rich crusts of the Marcus-Wake Seamount are 0.88–0.94 and with an average of 0.91 (Ren et al., 2016). This indicates that there is a difference during Pt adsorption in seawater by the nodules and crusts, and that crusts may preferentially absorb Pt. If Pt enrichment in nodules and crusts were affected only by sedimentary water depth, then Pt should be preferentially enriched in shallower crusts as depth increases, with Pt contents being lower in deeper seawater where polymetallic nodules grow, and with Pt/ΣPGE ratios of nodules and crusts being similar.

      To improve our understanding of the Pt enrichment mechanism of hydrogenetic nodules and crusts in seawater, we hypothesized the following model (Fig. 9).

      Figure 9.  Enrichment of Pt in polymetallic nodules and Co-rich crusts.

      The shallower Co-rich crusts are closer to the OMZ, where a reduction reaction of [PtCl4]2− occurs in seawater, with Pt2+ being reduced to Pt0. As the sedimentary water depth increases, the oxygen fugacity in the seawater gradually increases, and colloidal particles of hydrous δ-MnO2 adsorb Pt0 that formed near the OMZ and co-precipitate into the crusts (Halbach et al., 1989). Furthermore, few Pt2+ ions in seawater are directly adsorbed and enriched by negatively charged δ-MnO2 with very large specific surface area (Fig. 9). The positively charged amorphous iron hydroxide repels the Pt2+, with Fe2O3 and Pt thus exhibiting a negative correlation (Fig. 7). With the increasing of water depth, the manganese oxides in the shallower Co-rich crusts adsorb large amounts of Pt from the seawater, reducing the Pt content of deeper seawater. Consequently, the total Pt content of hydrogenetic nodules is lower than that those of shallower hydrogenetic crusts, and the percentage of Pt in ΣPGE of the hydrogenetic nodules is also lower than that in the hydrogenetic crusts.

      Diagenetic nodules generally grow inside sediments or are completely buried by them (Fig. 9). The main sources of ore-forming materials in these nodules are provided by cold pore-water, and they are more enriched in Ni and Cu than the hydrogenetic nodules (Dymond et al., 1984; Halbach et al., 1981). Previous studies have shown that Pt in mixed-type nodules is positively correlated with Cu and Ni in the CCZ (He et al., 2006), and the Pt content is significantly lower than that of hydrogenetic nodules both in study area and the EEZ of Cook Islands (Table 3). This may indicate that the Pt content of pore-water is lower than that of seawater, but further research is needed to confirm this.

    • (1) Polymetallic nodules in the study area are of the hydrogenetic type with an average ΣPGE content of 258×10–9, significantly lower than that of Co-rich crusts in Marcus-Wake Seamount, higher than that of mixed-type nodules in the CCZ of the East Pacific Ocean, and similar to that of crusts in the South China Sea.

      (2) The CI chondrite-normalized PGEs patterns of polymetallic nodules and Co-rich crusts are highly consistent, with all being characterized by gradual enrichment from Os to Pt and positive Pt anomalies. These characteristics are the converse of those of PGEs in seawater, which implies that PGEs in the nodules and crusts should be mainly derived directly from seawater.

      (3) The Pt content and Pt/ΣPGE ratios of hydrogenetic nodules are significantly lower than those of hydrogenetic crusts in the same area, likely due to differences in sedimentary water depth and oxygen fugacity of ambient seawater.

    • We thank all members of the DY-40B cruise for their contribution to sampling.

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