Enrichment of rare earth elements in siliceous sediments under slow deposition: A case study of the central North Pacific

Enrichment of rare earth elements in siliceous sediments under slow deposition: A case study of the central North Pacific

Ore Geology Reviews 94 (2018) 12–23 Contents lists available at ScienceDirect Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeore...

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Ore Geology Reviews 94 (2018) 12–23

Contents lists available at ScienceDirect

Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev

Enrichment of rare earth elements in siliceous sediments under slow deposition: A case study of the central North Pacific ⁎

T



Rina Saa,e, Xiaoming Suna,b,c,d, , Gaowen Hee, Li Xub,c, , Qingqing Panb,c, Jianlin Liaob,c, Kechao Zhue, Xiguang Denge a

School of Earth Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China School of Marine Sciences, Sun Yat-Sen University, Guangzhou 51006, China c Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China d State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210046, China e Guangzhou Marine Geological Survey, Guangzhou 510075, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Rare earth elements Clarion-Clipperton Fracture Zone Deep-sea sediment Biological apatite Phillipsite

As an important submarine rare earth elements (REE) exploration target, the REE-rich deep-sea mud in the Pacific Ocean has recently attracted much research attention, yet its metallogenic mechanism has not been sufficiently addressed. In this study, we conducted detailed grain size analysis, mineral identification and geochemical measurement for the MG026 sediment core in the central North Pacific. The > 63 μm fraction of the samples mainly contains siliceous bioclastics, phillipsite accretions, fish teeth and bones and ferromanganese micro-nodules; the 4–63 μm fraction mainly contains the abovementioned biological detritus, together with ilmenite, quartz and zeolite; the < 4 μm fraction mainly contains barite, clay minerals, carbonate-fluorapatite and amorphous ferric hydroxide. The rare earth elements and yttrium (ΣREY) contents in the samples can reach 810.4 ppm, and are mainly concentrated in the biological apatite (fish teeth and bones) and the Fe-Mn oxidehydroxide on the micro-nodule surface. The grain size analysis suggests that the smaller the Mz (mean grain size), the higher the REY enrichment. The post-Archean Australian shale-normalized REE patterns are slightly HREE-enriched with significant negative Ce anomalies, indicating that the rare earth elements of the samples are mainly seawater-derived with minor terrigenous input. We conclude that the REY content is controlled by the grain size, the amount of fish teeth and sedimentation rate: With low sedimentation rate, REYs from the seawater may have mainly replaced the Ca2+ ions of biological apatite lattice in form of isomorphism, and minor REYs may have also adsorbed on Fe-Mn micro-nodules due to the scavenging effect. As a concurrent result of the low sedimentation rate, the mean grain size of sediment may have decreased, and abundant phillipsite may have been formed.

1. Introduction Rare earth elements (esp. heavy rare earth elements such as Gd to Lu) and yttrium (REY) are very important for the development of information technology, biotechnology and energy technology (Service, 2010). China contains the world’s highest REY reserves on land (Gambogi, 2016), and many countries (e.g., Japan) have invested immensely in exploring REE resources on the seafloor. Previously, REEs on the seafloor were considered to be largely hosted by ferromanganese nodules and crusts (Pattan et al., 2001; Baturin and Yushina, 2007; Xue et al., 2008; Jiang et al., 2011; Kolesnik and Kolesnik, 2015). Nonetheless, Kato et al. (2011) reported that the

REY-rich deep-sea mud in the Pacific Ocean also hosts significant REY resources, with its REY content comparable or even exceed that of the ion-absorbed-type REY deposits in South China. Compared to the comprehensive researches on REY deposits on land (Gao et al., 1999; Xu et al., 2002, 2004; Ni et al., 2003; Tian et al., 2003, 2006), studies of marine REE mineral resources are very insufficient due to the sampling difficulties. To ensure stable REE supply for the future generations, investigation of the metal source and oreforming mechanism of these seafloor REE resources are necessary. It was long considered that phillipsite is the main controlling factor for the REE composition of deep sea sediments (Piper, 1974; Shen, 1990). However, geochemical data compilation of 2000 seafloor

⁎ Corresponding authors at: School of Earth Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China (X. Sun). School of Marine Sciences, Sun Yat-Sen University, Guangzhou 51006, China (L. Xu). E-mail addresses: [email protected] (X. Sun), [email protected] (L. Xu).

https://doi.org/10.1016/j.oregeorev.2018.01.019 Received 29 March 2017; Received in revised form 10 January 2018; Accepted 17 January 2018 0169-1368/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 3. Classification of sediment samples based on ternary “Folk” diagram (S- Sand; cSclayey Sand; mS- muddy Sand; zS- silty Sand; sC- silty Clay; sM- sandy Mud; sZ- Sandy silt; C- Clay; M- Mud; Z- Silt). Base diagram is from Zhang et al. (2013a) and modified after Folk et al. (1970) and Folk (1980).

Fig. 1. Satellite image showing the geographic location of the sampling site. Satellite image from Google Earth. The pathways of bottom current are branches of Antarctic Bottom Water (AABW) modified after Glasby (2006).

oxyhydroxides precipitated from hydrothermal plumes and phillipsite are the two main carriers of REYs. Dubinin (2000) examined the REE concentrations of the phillipsite samples (> 50-μm-fraction) from the Southern Basin of the Pacific, and found that massive rounded

sediment samples from 78 sites across the Pacific Ocean indicated that REY-rich mud (including metalliferous sediments, zeolitic clay and pelagic red clay) is mainly distributed in the eastern South Pacific and central North Pacific (Kato et al., 2011), and that hydrothermal iron-

Fig. 2. Photographs of bulk sediment samples.

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Fig. 4. Comparison of X-ray powder diffraction patterns of bulk-sediment samples in different layers. Patterns were arranged according to ΣREY: A026-47: 417.8 ppm; A026-56: 485.6 ppm; A0264: 522.5 ppm; A026-22: 633.0 ppm; A026-25: 756.8 ppm: A026-34: 810.4 ppm. Depth below seafloor: A026-07: 90–105 cm; A026-13: 180–195 cm; A026-22: 315–330 cm; A026-34: 495–510 cm; A026-47: 690–705 cm; A026-56: 825–847 cm. X-ray powder diffraction patterns are indexed after Yu and Jiang (1984), Moore and Reynolds (1989), Lin et al. (1990), Yu et al. (2011).

the Hawaiian Ridge and the Mid-Pacific Mountains to the north, and the Line Islands Ridge to the southwest (Fig. 1). The NW-trending Line Islands Ridge southwest of the sampling site was likely formed in the late Early Cretaceous to Late Cretaceous by the hot spot (Haggerty and Silva, 1995; Bergersen, 1995). The Clarion Fracture is one of the four ENE-trending, thousands-km long fractures developed in the East Pacific Basin. It contains oceanic troughs and ridges formed during the Cenozoic Pacific plate expansion (He et al., 2011). The volcanic activities in the western part of the belt are more intense than that in eastern part, and formed a number of bead-like seamounts scattered over the flat seafloor. The CCFZ is mainly 4500–5500 m deep (BSL), with the eastern part containing abyssal plains and western part (where the sampling site is located) containing abyssal hills. Oceanic crust of the northwestern CCFZ comprises Late Cretaceous (100–74 Ma) tholeiite (Eittreim et al., 1992), and is featured by multiphase faulting and intensive volcanism caused by the oceanic crust uplift. This area has a deposition rate of 1.7 m/Ma since Pliocene (Huang et al., 1996). The seafloor is covered with the remains of oceanic organisms, advantageous for polymetallic nodule development. Major sediments below the CCD (carbonate compensation depth) are radiolarian ooze, siliceous clay and zeolitic clay (Xiao et al., 1991). The Antarctic Bottom Water (AABW) formed in the South Pole is the most important water mass that affects this area. The cold and heavy AABW settles down onto the seabed and flows northwards, and of which a minor branch turns northeastward to cross the Line Islands Ridge at the Horizon and Clarion Passage and into the CCFZ (Gordon and Gerard, 1970; Edmond et al., 1971; Johnson, 1972) (Fig. 1). The AABW brings in dissolved oxygen and causes sediment decomposition, and therefore hiatus in sedimentation is often present in this area.

Fig. 5. SEM images of a granular zeolite accretions contained in radiolarian skeleton: (a) rounded zeolite accretions granular; (b) crossed twinning phillipsite; (c) compound twinning phillipsite.

phillipsite accretions are depleted in REEs, whereas pseudorhombic phillipsite accretions are REE-rich and marked by positive Ce anomalies. He suggested that oceanic phillipsites do not accumulate REEs or inherit the REE signature of volcaniclastic materials or oceanic deep water. Toyoda and Tokonami (1990) reported REE zoning in fish teeth from the Pacific sediments, and suggested that the REE enrichment there was likely derived from the surrounding post-burial sediment pore fluids. Kon et al. (2014) conducted in-situ mineral geochemical analyses of the REY-rich mud from the Minami-Torishima area in the Pacific. The authors reported that the apatite is REY-rich (9300–32,000 ppm) and is characterized by negative Ce anomalies and HREE enrichment. They concluded that apatite is the main REY-hosting phase in the mud. Some other studies proposed that clay minerals (Liu, 1992; Yin et al., 2002) and hydrothermal ferromanganese oxyhydroxides (Zhang et al., 2013b; Ren et al., 2015) can also host REYs. Among the many persisting questions about the seafloor REY mineralization, the nature of REY-bearing minerals and the ore-forming mechanism have attracted most attention. In this paper, we present new mineralogical and geochemical data of a sediment core from the central North Pacific, and discuss the potential REY sources and enrichment mechanism.

3. Samples and methods The core MG026 was collected in July 2013 using a gravity-piston type corer at the site marked in Fig. 1 in CCFZ with a water depth of 5 486 m (Fig. 1). The core is 8.47 m long with a diameter of 7 cm. The core is dark brown sediment with some fawn spots (Fig. 2). The sediment is homogenous clayey soft mud with no visible sand grains and a mean grain size of 4–10 μm. There are very few black micro-nodules with < 0.5 mm diameter through the whole core. White particles with < 1 mm diameter are occasionally seen in sediments. Calcareous organism is not found in sediment. The sediment contains minerals like quartz, zeolite and albite. The core MG026 was sampled at 15 cm intervals and 56 specimens in total were collected for mineralogy and geochemistry studies. They were sealed immediately in clean polyethylene bags and were placed in −18 °C freezer storage.

2. Geological background The sampling site MG026 is located in the northwestern part of the Clarion-Clipperton Fracture Zone (CCFZ) in the central North Pacific, near the western end of the Clarion Fracture. This area is bounded by 14

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Fig. 6. SEM images of biologic detritus and minerals in the samples. (a) radiolarian; (b) diatom; (c) fish tooth; (d) ferro-manganese micro-nodule with irregular shape; (e) montmorillonite; (f) fargite; (g) microcrystalline barite; (h) ilmenite; (i) spherical amorphous ferric hydroxide. Fig. 7. TEM images of nano-minerals: (a) goethite in ferromanganese micro-nodules; (b) todorokite in ferromanganese micro-nodules; (c) carbonate-fluorapatite in ferromanganese micro-nodules; (d) fibrous biological apatite from fish teeth (Si and Al signals in spectra a, b may come from the micro-nodule salic core).

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Table 1 Major components and minerals types in the bulk-sediment samples of the core MG026. Grain size

> 63 μm 4–63 μm < 4 μm

Oxysalt

Oxides & Hydroxides

Silicate

Sulfate

Phosphate

Albite, Phillipsite, Fargite, Clinoptilolite, Merlinoite Albite, Phillipsite, Fargite, Clinoptilolite, Merlinoite Montmorillonite, Illite, Phillipsite

– – Barite

Fish tooth, Fish bone (biological apatite) Debris of fish tooth and fish bone Carbon fluorapatite

Siliceous bone (biological SiO2), Fe-Mn micro-nodule Siliceous debris (biological SiO2), Quartz, Ilmenite Fe-Mn oxide-hydroxide

3.1. Grain size measurement

4. Results

All samples were dried in the oven and pretreated by adding 0.1 mol/L sodium hexametaphosphate, stirring and standing for 12 h for particle dispersion. The grain size measurement was performed at the Guangdong Key Laboratory of Marine Resources and Coastal Engineering, Sun Yat-Sen University (SYSU) by using a Bettersize BT9300H laser particle size analyzer. Each sample was tested at least 3 times to calculate the average value. The instrument has a particle size measuring range of 0.1–340.0 μm, repeatability error and accuracy error are both less than 1%.

4.1. Granularity characteristics, mineral composition and micromorphology Grain size analyses show that the mud samples are dominated by fine particles and no gravels (> 2000 μm), and grain size variation is insignificant. The average contents of sand (63–2000 μm), silt (4–63 μm) and clay (< 4 μm) are 5.80%, 53.22% and 40.97%, respectively. The mean grain size (Mz) of the samples range from 4.68 μm to 9.35 μm (average: 6.84 μm) (Fig. 7). According to Folk’s sediment ternary classification diagram (Folk et al., 1957), most samples belong to M (mud) class (Fig. 3). All granularity parameters are calculated using the formulae in Folk and Ward (1957). Bulk-sediment XRD analyses only show minerals with good crystallinity and in considerable content (Fig. 4). The samples shown in Fig. 4 were from different depths of the core MG026, yet they contain very similar mineral assemblage of phillipsite, merlinoite, clinoptilolite, quartz and albite (There is a marked peak of halite in the diagram which is owing to that the samples were not processed by distilled water to remove the salt.). Among these minerals, phillipsite is the most widely distributed (Fig. 5b, c). SEM analysis reveals that the zeolite is mostly sand-/silt-sized pale-yellow rounded accretions formed by radiolarian skeleton growth (Fig. 5a). There is a small amount of individual clayey zeolite crystals (Fig. 6f). X-ray powder diffraction patterns have a bulge between 20° and 40°, implying that the samples contain a large amount of amorphous components (Takahashi et al., 2000; Rancourt, 2005). Microscopic and SEM observations also show that the > 63 μm fraction comprises mainly bioclastics. Most of the bioclastics are radiolarian (Fig. 6a), diatom (Fig. 6b) and other siliceous bones, shells and debris composed main of opaline SiO2. In some fish teeth and bone bioclastics (Fig. 7c), the main component is calcium phosphate. TEM analysis shows that the fish teeth surface is featured by being fibrous and uneven (Fig. 7d). Amorphous components in the samples also include the irregularlyshaped, black ferromanganese micro-nodules (size: 100–500 μm) (Fig. 6d) and ferromanganese oxide-hydroxide (size: ∼500 nm) (Fig. 6i). The micro-nodule surface is covered with black goethite (Fig. 7a) and todorokite microcrystals (Fig. 7b) attached on stumpy nanocrystalline carbon fluorapatite aggregates (Fig. 7c). The ferromanganese oxide-hydroxide also exists in the form of nm-size colloidal granule (Fig. 6i). Our SEM and EDS analyses indicate that the 4–63 μm fraction is mainly composed of siliceous biological debris and terrigenous detrital minerals such as ilmenite and quartz (Fig. 6h). The < 4 μm fraction includes mainly clay minerals (Fig. 6e) and minor zeolite (Fig. 6f), barite (Fig. 6g) and ferromanganese oxide-hydroxide micelles (Fig. 6i). The major components and mineral types identified in the samples are listed in Table 1.

3.2. Mineralogical analyses Twenty-eight bulk-sediment samples were tested for mineral identification. Each sample was grounded to < 63 µm with an agate mortar. Powders in standard XRD slides were analyzed with Cu-Kα radiation (40 kV, 40 mA) using a PANalytical Empyrean X-ray Diffractometer (XRD) at the Instrumental Analysis and Research Center, SYSU. Whole patterns from 5° to 60° 2θ were scanned at 2°/min for qualitative analysis of major constituent minerals. Microtextural and geochemical analyses of 15 samples were performed at the School of Earth Science and Geological Engineering, SYSU. The samples were mounted on a scanning electron microscope (SEM) conductive double-sided carbon tape. The sample surface was gold-coated for microtextural observation or carbon-coated for energy dispersive spectrometer (EDS) microanalysis. The instrument used was a Carl Zeiss SIGMA 300 field emission (FE)-SEM with a Schottky thermal field emitter, and an Oxford X-Max EDS on the SEM. For nm-scale mineral analysis, two representative samples (A026-06 and A026-34) were analyzed by a JEOL JEM-2010HR transmission electron microscope (TEM) with a maximum voltage of 200 kV, and an Oxford 7718 EDS at the Instrumental Analysis and Research Center, SYSU. Trace amount (pinhead size) of dry powder of the bulk-sediment sample A026-34 was placed inside a small mortar filled with ethanol and carefully crushed (not grinded) to make some debris float. Floated portion of the crushed powder in ethanol was then transported to a holey carbon-coated copper grid with a dropper, and the grid was placed on the machine after drying. A small fish tooth from the sample A026-06 was preprocessed with the same method.

3.3. Major and trace elements analyses Major element compositions of all 56 bulk-sediment samples were analyzed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS), using a Rigaku 100e X-ray fluorescence (XRF) spectrometer, following the procedures described by Goto and Tatsumi (1994). Trace element and REY abundances for all bulk-sediment samples were determined by an Agilent 7700× inductively coupled plasma mass spectrometer (ICP-MS) at the ALS Mineral-ALS Chemex laboratory with analytical uncertainties of < 10%. The samples were digested with perchloric acid, nitric acid and hydrofluoric acid, and then evaporated to nearly dry and dissolved with diluted hydrochloric acid to 100 mL.

4.2. Major, trace and REE geochemistry The REY contents, characteristic parameters and the contents of some REY-related major/trace elements of the 56 samples analyzed are listed in Table 2. SiO2 (60–72 wt%) is the dominant oxide of the samples, followed by Al2O3 (5–10 wt%). Average contents of Na2O, Fe2O3, 16

A026-1 0–15

89.0 63.2 24.2 101.5 23.1 6.5 26.2 4.3 21.6 5.2 14.4 2.1 12.3 2.0 171.0 566.6 307.5 88.1 3.49 0.53 0.31 1.23

61.44 7.64 1.67 0.69 4.41 0.98

292.0 80.6 177.5 108 A026-21

300–315 102.0

60.6 26.1 114.5 27.4 7.0 29.2 4.4 27.7 5.9 16.3 2.3 14.1 2.1 190.0 629.6

Sample No. Depth/cm

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ΣREY ΣLREE ΣHREE LREE/HREE (La/Yb)N δCe δEu

SiO2 Al2O3 CaO P2O5 Fe2O3 MnO

Cu Co Ni Zn Sample No.

17

Depth/cm La

Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ΣREY

60.0 26.0 115.0 27.8 6.7 28.7 4.5 27.2 5.9 15.9 2.2 14.6 2.0 194.0 633.0

315–330 102.5

282.0 74.8 170.5 102 A026-22

63.71 7.35 1.67 0.70 4.29 0.89

86.6 59.6 23.3 97.6 22.7 6.2 25.6 4.3 21.0 5.1 14.4 2.0 12.1 2.0 166.5 549.0 296.0 86.5 3.42 0.53 0.31 1.20

A026-2 15–30

62.2 26.7 118.0 27.7 7.1 30.6 4.6 28.3 6.1 17.1 2.4 14.6 2.3 198.5 651.7

330–345 105.5

278.0 74.5 165.0 93 A026-23

65.77 7.14 1.57 0.67 4.05 0.89

83.5 58.0 22.4 92.4 21.7 6.1 24.3 4.1 20.3 4.8 13.4 2.0 11.2 1.9 160.5 526.6 284.1 82.0 3.46 0.55 0.31 1.24

A026-3 30–45

58.9 25.6 113.0 27.3 6.8 29.9 4.5 27.7 5.9 16.1 2.3 14.1 2.0 190.0 624.1

345–360 100.0

257.0 72.8 153.0 86 A026-24

65.57 6.85 1.56 0.66 3.85 0.82

83.6 57.0 22.2 92.2 21.1 6.1 23.5 4.0 19.4 4.7 13.1 1.9 11.3 1.9 160.5 522.5 282.2 79.8 3.54 0.55 0.30 1.28

A026-4 45–60

73.2 32.1 135.5 31.2 8.7 35.5 6.0 29.0 7.1 19.3 2.9 16.5 2.8 238.0 756.8

360–375 119.0

259.0 74.5 161.0 91 A026-25

65.43 7.08 1.56 0.67 4.03 0.86

79.7 55.3 21.3 89.8 20.6 5.7 23.2 3.8 19.0 4.6 12.4 1.8 10.9 1.8 153.0 502.9 272.4 77.5 3.51 0.54 0.31 1.21

A026-5 60–75

69.3 31.1 131.0 30.0 8.3 34.5 5.7 28.3 6.7 18.9 2.8 16.1 2.7 232.0 733.9

375–390 116.5

233.0 68.9 155.0 82 A026-26

64.33 6.62 1.55 0.66 3.74 0.81

81.1 54.3 21.7 90.6 20.8 5.7 23.5 3.9 19.5 4.7 12.7 1.9 10.9 1.8 155.0 508.1 274.2 78.9 3.48 0.55 0.30 1.20

A026-6 75–90

67.9 30.4 126.5 29.0 8.3 32.6 5.7 28.3 6.6 18.7 2.8 16.2 2.6 230.0 720.6

390–405 115.0

217.0 68.9 157.5 78 A026-27

66.97 6.59 1.56 0.68 3.64 0.80

84.7 53.2 22.4 93.3 21.5 6.0 24.7 4.1 20.3 5.0 13.7 2.1 11.7 2.0 164.0 528.7 281.1 83.6 3.36 0.53 0.28 1.21

A026-7 90–105

61.6 28.1 118.5 27.6 7.6 30.5 5.4 26.1 6.3 17.7 2.7 14.7 2.5 219.0 676.8

405–420 108.5

246.0 69.2 170.0 81 A026-28

67.00 6.85 1.58 0.70 3.79 0.83

84.1 52.5 22.5 93.4 20.9 6.0 23.6 4.0 20.1 4.8 13.2 2.0 11.4 1.9 162.0 522.4 279.4 81.0 3.45 0.54 0.28 1.26

A026-8 105–120

56.9 27.0 113.0 25.8 7.3 29.3 4.9 24.2 6.0 16.4 2.5 14.2 2.4 205.0 638.4

420–435 103.5

279.0 74.9 179.5 91 A026-29

65.86 7.16 1.74 0.79 4.04 0.93

90.8 54.3 24.2 102.0 23.2 6.6 26.2 4.5 22.0 5.2 14.3 2.2 11.8 2.1 176.0 565.4 301.1 88.3 3.41 0.57 0.27 1.25

A026-9 120–135

Table 2 Bulk-sediment major (wt%), trace (ppm) and REY (ppm) element contents of the samples from the core MG026.

55.0 25.4 108.0 24.8 6.9 27.5 4.8 23.2 5.5 15.5 2.4 13.2 2.2 191.5 602.8

435–450 96.9

283.0 74.0 171.5 92 A026-30

64.61 7.15 1.78 0.81 4.10 0.94

93.5 55.9 25.3 104.0 24.0 6.9 27.2 4.6 22.7 5.5 15.2 2.3 13.1 2.2 185.0 587.4 309.6 92.8 3.34 0.53 0.26 1.26

A026-10 135–150

55.6 25.8 108.5 24.5 7.0 28.5 4.8 23.6 5.6 15.5 2.4 13.7 2.3 192.0 607.5

450–465 97.7

297.0 73.7 167.0 97 A026-31

62.33 7.06 1.78 0.79 4.18 0.93

94.2 56.0 24.8 101.5 23.7 6.7 27.0 4.7 22.6 5.5 14.9 2.3 13.3 2.2 185.5 584.9 306.9 92.5 3.32 0.52 0.27 1.23

A026-11 150–165

58.4 27.8 117.0 26.7 7.4 29.4 5.0 25.1 6.1 16.4 2.5 14.2 2.3 202.0 644.8

465–480 104.5

315.0 74.0 166.5 100 A026-32

63.24 7.28 1.77 0.78 4.32 0.96

92.4 57.3 24.5 103.0 23.4 6.7 26.7 4.5 22.6 5.4 15.1 2.4 13.0 2.2 183.5 582.7 307.3 91.9 3.34 0.52 0.28 1.25

A026-12 165–180

73.1 33.9 141.0 32.3 9.1 36.6 6.2 30.0 7.2 19.9 3.0 17.4 2.8 239.0 777.0

480–495 125.5

319.0 72.8 178.0 102 A026-33

62.80 7.34 1.85 0.78 4.43 1.02

88.2 55.7 23.5 99.7 22.1 6.4 25.3 4.3 21.8 5.2 14.5 2.2 12.7 2.1 176.0 559.7 295.6 88.1 3.36 0.51 0.28 1.26

A026-13 180–195

76.5 35.4 148.5 33.7 9.5 37.7 6.4 31.2 7.3 20.4 3.0 17.9 2.9 249.0 810.4

495–510 131.0

297.0 75.6 181.0 98 A026-34

65.25 7.36 1.76 0.79 4.48 1.03

87.9 55.0 23.5 98.2 22.6 6.2 25.2 4.3 21.4 5.2 14.4 2.2 12.4 2.2 175.5 556.2 293.4 87.3 3.36 0.52 0.28 1.21

A026-14 195–210

63.6 28.7 119.0 27.2 7.6 30.2 5.1 25.3 6.1 16.7 2.5 14.3 2.4 205.0 660.2

510–525 106.5

289.0 73.0 178.5 98 A026-35

64.60 7.15 1.72 0.74 4.43 0.99

85.3 54.3 22.5 95.3 21.7 6.1 24.4 4.2 20.6 5.0 13.9 2.1 12.2 2.1 171.0 540.7 285.2 84.5 3.38 0.52 0.29 1.24

A026-15 210–225

54.9 24.5 102.0 23.4 6.6 26.0 4.5 22.2 5.3 14.7 2.2 12.5 2.1 178.0 571.0

525–540 92.1

306.0 77.2 192.5 102 A026-36

64.14 7.38 1.71 0.73 4.53 1.00

87.6 56.1 23.0 97.7 22.3 6.2 24.7 4.2 21.0 5.1 14.2 2.1 12.3 2.0 174.5 553.0 292.9 85.6 3.42 0.53 0.29 1.23

A026-16 225–240

50.0 22.4 97.0 22.1 5.8 24.6 3.7 22.8 4.7 14.2 2.1 11.5 1.8 157.0 525.5

540–555 85.8

284.0 80.7 184.5 95 A026-37

63.32 7.24 1.71 0.76 4.41 1.02

91.0 57.3 24.0 101.5 23.4 6.5 26.2 4.5 22.2 5.4 14.8 2.3 12.8 2.2 183.0 577.1 303.7 90.4 3.36 0.52 0.28 1.22

A026-17 240–255

49.8 22.7 99.8 22.5 5.7 25.4 3.7 23.3 4.9 14.0 2.1 12.1 1.8 161.0 536.3

555–570 87.5

265.0 83.0 225.0 90 A026-38

63.83 7.29 1.72 0.74 4.27 1.06

86.2 52.3 21.7 98.4 22.8 5.6 23.6 3.7 23.4 4.9 13.6 1.9 12.4 1.7 159.0 531.2 287.0 85.2 3.37 0.51 0.28 1.13

A026-18 255–270

585–600 77.3

311.0 106.5 186.5 98 A026-40

62.45 7.73 1.93 0.87 4.40 0.97

97.1 57.5 24.4 110.0 26.2 6.4 28.1 4.3 26.3 5.5 15.6 2.1 14.0 2.0 180.0 599.5 321.6 97.9 3.28 0.51 0.27 1.10

A026-20 285–300

47.0 45.9 21.1 20.6 93.5 89.3 21.5 20.1 5.4 5.3 23.5 22.0 3.6 3.4 21.9 20.7 4.6 4.3 13.1 12.5 1.9 1.8 11.0 10.3 1.8 1.6 147.0 140.0 497.5 475.1 (continued on next page)

570–585 80.6

258.0 90.6 169.5 87 A026-39

63.63 7.44 1.84 0.81 4.24 0.80

91.3 53.5 23.1 103.0 24.3 6.0 25.6 4.0 24.3 5.2 14.0 2.0 13.3 1.9 170.5 562.0 301.2 90.3 3.34 0.51 0.27 1.13

A026-19 270–285

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18

204.0 52.3 90.9 64

0.81

210.0 56.3 96.7 65

MnO Cu

Co Ni Zn

0.74

68.24 5.37 1.47 0.59 3.49

SiO2 Al2O3 CaO P2O5 Fe2O3

68.56 5.25 1.40 0.55 3.26

69.4 43.8 18.5 79.9 18.0 4.6 19.5 3.0 18.4 3.9 10.8 1.6 9.5 1.5 123.5 425.9 234.2 68.2 3.43 0.54 0.28 1.15

346.0 89.1 201.0 108

60.46 8.50 2.05 0.93 4.78 1.21

331.6 102.5 3.24 0.52 0.27 1.11

198.5 49.9 85.8 61

0.72

68.06 4.99 1.43 0.56 3.21

70.1 41.8 18.5 79.4 18.1 4.8 20.0 3.1 18.7 3.9 10.6 1.6 9.5 1.5 126.5 428.1 232.7 68.9 3.38 0.54 0.27 1.18

337.0 92.0 215.0 107

60.52 8.51 2.10 0.95 4.83 1.19

399.7 119.1 3.36 0.53 0.27 1.22

A026-5 60–75

217.0 51.6 94.0 62

0.70

69.38 5.17 1.43 0.60 3.17

75.0 43.1 19.6 85.1 19.8 5.0 21.5 3.3 19.8 4.2 12.0 1.8 10.6 1.6 135.0 457.4 247.6 74.8 3.31 0.52 0.26 1.13

A026-44 645–660

A026-4 45–60

A026-43 630–645

364.0 93.2 207.0 113

61.47 8.80 2.03 0.92 4.91 1.18

347.2 106.0 3.28 0.53 0.27 1.14

A026-3 30–45

A026-42 615–630

358.0 96.1 217.0 114

72.6 44.6 19.4 82.8 19.2 5.0 21.0 3.2 19.4 4.0 11.3 1.7 10.0 1.6 130.0 445.8 243.6 72.2 3.37 0.54 0.27 1.16

371.0 93.8 228.0 110

Cu Co Ni Zn

60.88 8.59 2.06 0.93 4.78 1.17

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ΣREY ΣLREE ΣHREE LREE/HREE (La/Yb)N δCe δEu

61.36 8.69 2.06 0.94 4.71 1.17

SiO2 Al2O3 CaO P2O5 Fe2O3 MnO

338.0 101.0 3.35 0.52 0.27 1.11

A026-41 600–615

337.6 102.0 3.31 0.53 0.27 1.16

ΣLREE ΣHREE LREE/HREE (La/Yb)N δCe δEu

A026-2 15–30

Sample No. Depth/cm

A026-1 0–15

Sample No. Depth/cm

Table 2 (continued)

198.0 51.8 77.9 61

0.72

66.60 5.56 1.54 0.60 3.22

74.0 44.2 19.5 84.5 19.0 4.9 21.1 3.2 19.4 4.2 11.3 1.7 10.4 1.6 132.5 451.5 246.1 72.9 3.38 0.53 0.27 1.14

A026-45 660–675

345.0 90.6 201.0 104

61.04 8.11 2.07 0.95 4.66 1.17

386.2 115.7 3.34 0.53 0.26 1.20

A026-6 75–90

202.0 54.0 105.5 63

0.59

67.27 5.85 1.40 0.55 3.21

75.6 49.2 20.1 85.6 19.4 5.1 21.9 3.2 19.5 4.2 11.5 1.7 10.0 1.6 133.5 462.1 255.0 73.6 3.46 0.56 0.29 1.15

A026-46 675–690

341.0 87.1 199.0 104

62.22 7.79 2.03 0.93 4.53 1.10

377.1 113.5 3.32 0.52 0.26 1.26

A026-7 90–105

183.0 52.1 70.9 58

0.75

69.65 5.07 1.38 0.55 3.22

67.7 42.0 18.2 78.5 18.1 4.6 19.5 2.9 18.0 3.7 10.5 1.5 9.2 1.4 122.0 417.8 229.1 66.7 3.43 0.54 0.28 1.14

A026-47 690–705

311.0 80.0 191.0 97

63.68 7.44 2.00 0.92 4.39 1.08

351.9 105.9 3.32 0.54 0.26 1.22

A026-8 105–120

197.0 55.1 77.1 61

0.80

68.89 5.06 1.41 0.58 3.28

70.8 41.7 19.3 83.9 18.7 4.9 20.7 3.2 18.7 3.9 11.0 1.6 9.6 1.5 127.0 436.5 239.3 70.2 3.41 0.54 0.26 1.16

A026-48 705–720

236.0 71.8 167.0 81

64.80 6.47 1.88 0.83 3.84 0.94

333.5 99.9 3.34 0.54 0.25 1.24

A026-9 120–135

203.0 58.6 78.0 65

0.84

69.14 5.11 1.41 0.57 3.42

72.4 43.1 19.8 84.6 19.5 5.3 21.6 3.1 19.7 4.0 11.5 1.6 9.7 1.6 130.0 447.5 244.7 72.8 3.36 0.55 0.26 1.20

260.0 73.4 170.5 81

65.27 7.01 1.86 0.84 4.16 1.05

341.8 101.0 3.38 0.54 0.25 1.23

213.0 61.2 90.2 64

0.89

68.60 5.57 1.57 0.65 3.63

82.9 47.1 22.3 97.9 22.0 5.7 24.6 3.7 22.2 4.6 13.4 1.8 11.0 1.8 149.0 510.0 277.9 83.1 3.34 0.56 0.25 1.14

306.0 95.7 211.0 98

59.85 9.24 2.36 1.11 5.41 1.40

414.9 123.1 3.37 0.53 0.26 1.23

218.0 61.4 118.0 67

0.67

67.75 5.62 1.65 0.70 3.63

91.2 48.8 24.5 104.5 24.6 6.3 25.9 4.0 24.6 5.2 14.9 2.1 12.8 1.8 164.5 555.7 299.9 91.3 3.28 0.53 0.24 1.17

397.0 99.6 224.0 114

60.6 8.83 2.24 1.03 5.40 1.38

434.6 126.8 3.43 0.54 0.26 1.24

A026-14 195–210

227.0 64.7 107.5 66

0.86

68.14 5.72 1.60 0.67 3.72

87.0 50.4 23.3 101.5 22.5 6.0 26.0 3.8 23.6 5.0 13.8 2.0 12.1 1.9 158.5 537.4 290.7 88.2 3.30 0.53 0.26 1.15

A026-52 765–780

A026-13 180–195

A026-51 750–765

A026-12 165–180

A026-50 735–750

222.0 72.4 161.0 80

66.06 6.62 1.81 0.80 3.94 0.96

319.1 96.4 3.31 0.53 0.25 1.23

A026-11 150–165

A026-49 720–735

217.0 68.7 172.0 79

66.39 6.44 1.79 0.79 3.71 1.01

317.0 94.3 3.36 0.54 0.26 1.23

A026-10 135–150

217.0 61.9 98.6 63

0.87

68.12 5.48 1.52 0.61 3.56

82.5 47.1 21.9 95.2 21.5 5.7 23.6 3.6 21.9 4.6 13.2 1.9 11.5 1.8 149.0 505.0 273.9 82.1 3.34 0.53 0.26 1.18

A026-53 780–795

349.0 81.2 187.5 93

64.01 7.28 1.88 0.84 4.71 1.16

352.6 102.6 3.44 0.55 0.26 1.24

A026-15 210–225

215.0 59.2 95.5 63

0.83

68.64 5.39 1.46 0.58 3.47

82.9 46.7 21.9 95.6 21.9 5.6 23.5 3.6 22.2 4.7 13.0 1.9 10.9 1.8 149.0 505.2 274.6 81.6 3.37 0.56 0.25 1.15

A026-54 795–810

327.0 76.2 177.5 88

65.24 6.53 1.68 0.72 4.18 0.97

303.5 89.5 3.39 0.54 0.27 1.25

A026-16 225–240

217.0 62.8 81.0 65

0.84

67.47 5.56 1.46 0.60 3.53

77.2 44.4 20.8 88.5 19.9 5.4 22.0 3.4 20.9 4.3 11.9 1.7 10.6 1.6 139.0 471.6 256.2 76.4 3.35 0.54 0.25 1.20

A026-55 810–825

303.0 65.8 145.5 80

66.62 6.18 1.63 0.70 3.99 0.97

283.1 85.4 3.31 0.55 0.26 1.16

A026-17 240–255

203.0 60.9 72.1 62

0.86

68.49 5.65 1.44 0.58 3.55

79.3 45.5 20.9 91.4 21.2 5.6 23.1 3.5 21.3 4.4 12.5 1.8 10.9 1.7 142.5 485.6 263.9 79.2 3.33 0.54 0.26 1.18

211.0 56.5 98.5 65

69.15 5.21 1.48 0.59 3.46 0.81

258.5 76.6 3.37 0.55 0.26 1.18

A026-20 285–300

267.8 72.5 152.7 85

0.94

65.36 6.75 1.71 0.74 4.04

90.1 54.3 23.8 101.7 23.4 6.3 25.9 4.2 22.8 5.2 14.4 2.1 12.4 2.0 171.1 531.8 299.6 89.0 3.37 0.54 0.27 1.19

Average Value

221.0 58.9 110.0 66

69.73 5.47 1.50 0.63 3.68 0.86

269.1 81.4 3.31 0.54 0.26 1.12

A026-19 270–285

A026-56 825–847

285.0 63.5 139.5 76

71.49 6.34 1.71 0.76 4.22 1.01

288.0 87.3 3.30 0.53 0.26 1.11

A026-18 255–270

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Fig. 8. Profiles of REY content, its characteristic parameters and mean grain size (Mz) of bulk-sediment samples from the core MG026. Fig. 9. Post-Archean Australian shale normalized REE patterns of the MG026 core sediments, average continental crust, marine sediments from the Yellow Sea and Central Indian Ocean Basin, hydrogenetic nodules from the eastern Pacific and surface/deep seawater from central North Pacific. Data sources: a- Taylor and McLennan (1981); b- Liu (2004); c- Pattan and Parthiban (2011); d- He et al. (2011); e- Fröllje et al. (2016). The PAAS values are given by McLennan (1989). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

REYs, δEu and δCe, highly variable LREE/HREE and largest Mz of 8.46 μm.

MgO, CaO, K2O, P2O5, MnO and TiO2 are, respectively, 4.28%, 4.04%, 2.01%, 1.71%, 1.51%, 0.74%, 0.94% and 0.29%. Most major elements have similar distribution trends with depth below seafloor except for SiO2 and Na2O. The total REY contents (ΣREY) are between 417.8 and 810.4 ppm (average: 531.8 ppm) (Fig. 8). The ΣLREE/ΣHREE ratio (Σ (La, Ce, Pr, Nd, Sm, Eu)/Σ (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)) is between 3.2 and 3.6. The average (La/Yb) N ratio (post-Archean Australian shale normalized) is 0.54, indicating LREE depletions (Figs. 8 and 9). Europium anomalies (δEu) and Cerium anomalies (δCe) were calculated using the following formulae:

δEu =

EuN δCe = SmN ·GdN

5. Discussion 5.1. Carriers of REY Pearson correlation matrix performed on REYs and some major and trace elements (ΣREY, ΣLREE, ΣHREE, La, Ce, SiO2, Al2O3, Na2O, Fe2O3, MgO, CaO, K2O, MnO, P2O5, TiO2, Ba, Co, Cu, Ni and Sr) is shown in Table 3. The variables in this matrix correlate very strongly positive with ΣREY (r = 0.95): ΣLREE, ΣHREE, La, CaO and P2O5; variables are strongly positive correlated with ΣREY (r = 0.85): Fe2O3 and MnO; variables are well positive correlated with ΣREY (r = 0.80): Al2O3, Co and Ni. The SiO2 shows distinct negative correlation with ΣREY (r = −0.795). In addition, the correlation coefficient between CaO and P2O5 reaches 0.993, and the correlation coefficients between all pairs of variables Fe2O3, MnO, Ni, Cu, Co and Zn vary from 0.763 to 0.925. The very strong positive correlation between CaO and P2O5 suggests that most of the phosphorus and calcium are hosted in the same mineral, i.e., apatite. Apatite is known to contain variable amount of rare earth elements (Puchelt and Emmermann, 1976; Nathan, 1984). Since extensive formation of authigenic apatite in deep sea (> 5000 m depth)

Ce N LaN ·PrN

The samples are also characterized by having significant negative δCe (0.23–0.31) and slightly positive δEu (1.10–1.28). Core MG026 can be divided into four layers according to the ΣREY and δEu values (Fig. 8): (1) Layer A (0–255 cm): Characterized by low REYs, high δEu, δCe and LREE/HREE and Mz of 6.02 μm; (2) Layer B (255–360 cm): Characterized by medium REYs, low δEu, LREE/HREE and (La/Yb)N and the smallest Mz of 5.50 μm; (3) Layer C (360–540 cm, REY-rich layer): Characterized by large REY variation, high δEu, low δCe, medium LREE/HREE and highly variable Mz (average: 6.08 μm); (4) Layer D (540–847 cm, REY-depleted layer): Characterized by low 19

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1 0.714 0.330 0.664 0.718 0.922 0.080 0.825 0.843 0.875 0.291 0.965

1 0.778 0.899 0.993 0.847 −0.434 0.884 0.836 0.852 0.664 0.817

1 0.682 0.740 0.525 −0.431 0.627 0.519 0.530 0.779 0.469

1 0.895 0.828 −0.452 0.815 0.842 0.798 0.674 0.786

1 0.853 −0.412 0.887 0.834 0.866 0.643 0.817

1 −0.087 0.931 0.884 0.941 0.518 0.962

1 −0.252 −0.291 −0.126 −0.213 −0.066

1 0.856 0.898 0.585 0.892

1 0.863 0.501 0.923

1 0.474 0.932

1 0.447

1

sediments is rare due to the lack of calcium carbonate, the main phosphorus-bearing substances in the samples are fish teeth and bones, the main inorganic component of which is biological apatite (Narasaraju and Phebe, 1996; Shen et al., 2009). The crystal structure of biological apatite is consistent with that of synthetic hydroxyapatite (HAP), but its physical and chemical characteristics are more complicated. The correlation coefficient of ΣREY with CaO and P2O5 is 0.950, indicating that fish teeth is the main REY carrier. In fact, previous research shown that about 70% REY of the whole REY budget concentrates in biological apatite, or fish teeth (Wang et al., 2016). In marine sediments, the major elements Fe, Mn and trace elements Cu, Co, Ni and Zn are the main metallic components of micro-nodules and ferromanganese oxyhydroxides (Bao, 1993). The correlations between Fe2O3, MnO, Co, Ni, Zn and ΣREY indicate that there is correlations (in the Fe-Mn content) also exist between the micro-nodules and rare earth elements. It is known that Mn oxides and Fe hydroxides contain adsorption capacity (García-Sánchez et al., 1999; Post, 1999), and therefore micro-nodules could be a minor REY carrier. The Fe2O3, MnO, Co, Ni, Zn contents also correlated with CaO and P2O5 (r ≥ 0.80), which may be attributed to the phosphatization of micro-nodules (Fig. 7c). Aluminum is mainly hosted in phillipsite and clay minerals (e.g., montmorillonite and illite) in deep sea sediments (Pattan and Parthiban, 2011). The strong correlation between Al2O3 and ΣREY (r = 0.840) indicates that the amount of phillipsite and/or clay minerals is related to REE enrichment. However, Dubinin (2000) showed that phillipsite accretions are either REE-depleted or marked by a positive Ce anomaly, inconsistent with the REE patterns of the bulk sediments. Therefore, REEs are unlikely to be accumulated in phillipsite. The contents of phillipsite and REY are statistically correlated probably due to sedimentation rate (described in Section 5.3). Fig. 10 shows that the percentage of clay-size (< 4 μm) fraction is positively correlated with ΣREY, whereas that of sand-/silt-size fraction is negatively correlated with ΣREY. As the clay-size fraction also contains components other than clay minerals, further work is needed to constrain the actual contribution of clay minerals to REY enrichments. Most silica in the samples is hosted in siliceous biological skeletons. The negative SiO2 vs. ΣREY correlation shows that the biogenic silica may have significantly diluted the REE content, consistent with what was previous suggested (Liu, 2004).

1 0.836 0.911 0.648 0.922 0.907 0.940 −0.301 0.911 0.925 0.897 0.620 0.918

5.2. Sources of REYs Compared with the REE patterns of the average continental crust, many marine sediments (Yellow Sea and Central Indian Ocean Basin), hydrogenetic nodules (eastern Pacific) and surface/deep seawater (central North Pacific) (Fig. 9), the CCFZ sediments contain the following characteristics: (1) The REE contents (except for Ce) are higher than those of the sediments from the Yellow Sea and Central India Ocean Basin; (2) The REE patterns show distinct negative δCe and slightly positive δEu, similar to the central North Pacific water, probably reflecting seawater inheritance. The small amount of micro-nodules (with positive δCe) in the samples may have reduced the negative δCe inherited from the seawater. Deep seawater has one order of magnitude higher REE contents than surface seawater, which accounts for the relatively high REE contents of the CCFZ (> 5000 m deep) sediments; (3) Relatively flat HREE patterns similar to those of the average continental crust and marginal sea, which suggests certain terrigenous REE inputs for the CCFZ sediments. We already know that the REE content in seawater increases with depth (Fröllje et al., 2016). Because particle settling speed is generally related to particle length (Shanks and Trent, 1980; Alldredge and Gotschalk, 1988, 1989; Syvitski et al., 1995; Stemmann et al., 2004), the setting speed of fish teeth and bones is almost constant. If we divide the water column into only two parts, the surface seawater and the deep seawater, the time it takes for the fish teeth and bones to pass through

Bold figures: some noteworthy data which are especially explained in the text.

1 −0.246 0.020 −0.172 0.035 −0.301 −0.244 −0.195 0.251 −0.197 −0.196 −0.164 −0.194 −0.078 1 −0.148 0.947 0.898 0.900 0.660 0.841 0.903 0.973 −0.134 0.940 0.888 0.939 0.584 0.950 ΣREY ΣLREE ΣHREE La Ce SiO2 Al2O3 Na2O Fe2O3 MgO CaO K2 O MnO P2O5 TiO2 Ba Co Cu Ni Sr Zn

1 0.998 0.993 0.997 0.941 −0.795 0.840 −0.223 0.868 0.651 0.950 0.743 0.870 0.950 0.776 −0.447 0.832 0.793 0.812 0.696 0.763

1 0.993 0.997 0.943 −0.792 0.839 −0.225 0.869 0.635 0.947 0.768 0.872 0.944 0.771 −0.451 0.835 0.791 0.800 0.727 0.753

1 0.996 0.912 −0.777 0.824 −0.242 0.859 0.617 0.956 0.759 0.872 0.953 0.759 −0.505 0.830 0.786 0.784 0.696 0.736

1 0.921 −0.773 0.820 −0.237 0.852 0.609 0.949 0.764 0.865 0.949 0.751 −0.481 0.821 0.778 0.791 0.714 0.731

1 −0.869 0.910 −0.129 0.900 0.788 0.889 0.693 0.832 0.887 0.859 −0.202 0.864 0.831 0.871 0.687 0.865

1 −0.926 −0.123 −0.863 −0.876 −0.850 −0.654 −0.758 −0.830 −0.890 0.133 −0.889 −0.828 −0.864 −0.539 −0.911

Na2O Al2O3 SiO2 Ce La ΣHREE ΣLREE ΣREY

Table 3 Correlation matrix of the elements analyzed in the bulk-sediment samples from the core MG026.

Fe2O3

MgO

CaO

K2O

MnO

P2O5

TiO2

Ba

Co

Cu

Ni

Sr

Zn

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Fig. 10. Correlation diagrams of ΣREY vs. percentage of sand, silt and clay contents.

the surface seawater is as much as the time to go through the deep seawater. Then, the amount of REE absorbed by the fish teeth & bones should be related to the concentration of REE in seawater. The REE concentration in deep seawater is ∼10 times that of surface seawater, so the REE in fish teeth & bones from deep seawater is ∼10 times greater than that from surface seawater. In addition, the biological apatite is the main REE carrier in deep sea mud, and the fish teeth & bones will continue to contact deep seawater before they are buried by new sediments. Therefore, the contribution of the deep seawater to the REEs of the deep-sea mud is more than 10 times that of the surface seawater. Comparing the (La/Yb)N, (La/Sm)N and (Gd/Yb)N ratios of the surface/deep seawater, average continental crust and the CCFZ sediments of this study (Table 4), it is found that the (La/Yb)N ratios of the CCFZ sediments range between those of the seawater and average continental crust, and the LREE and HREE characteristics resemble more the seawater and average continental crust, respectively. It has been suggested that LREEs tend to adsorb onto particle surfaces and HREE tend to remain in seawater (Koeppenkastrop et al., 1991; Koeppenkastrop and De Carlo, 1992, 1993; Sholkovitz, 1992), which may explain the observed LREE and HREE features in the CCFZ sediments.

Table 4 (La/Yb)N, (La/Sm)N and (Gd/Yb)N ratios of the average continental crust, central North Pacific surface/deep water and central North Pacific sediments. Geological bodies

(La/Yb)N

(La/Sm)N

(Gd/Yb)N

Data source

Central North Pacific surface water Central North Pacific deep water Central North Pacific sediments (n = 56) Average continental crust

0.38

0.68

1.06

0.35

0.89

0.64

0.54

0.56

1.26

Fröllje et al. (2016) Fröllje et al. (2016) This study

1.01

0.97

1.05

Taylor and McLennan (1981)

5.3. Controlling factors for REY enrichment The vertical δCe profile of the core MG026 shows δCe < 1 for all layers, indicating a consistent oxidizing environment (Fig. 8). The δCe values contain very small variation but have no correlation with the ΣREY changes. Other sedimentary redox indices (e.g., U/Th, V/Cr and Ni/Co; Jones and Manning, 1994) have no correlation with the ΣREY changes either, suggesting that redox changes have no major effect on REY enrichments. The mean grain size (Mz) is negatively correlated with ΣREY in all the sediment layers (Figs. 8 and 11), suggesting that the smaller the sediment grain size, the higher the REY content, consistent with the previously reported ‘granularity effects’(Qin et al., 1987; Zhao and Yan, 1994). However, the influence of Mz on ΣREY is different in the four

Fig. 11. Bivariate diagram of ΣREE and Mz in different layers of the core MG026.

Fig. 12. Correlation diagrams of Mz vs. contents of biological SiO2, P2O5 and MnO.

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layers. The ΣREY vs. Mz correlation in the REY-richer and Mz-smaller layers (R2 = 0.9365 in layer b; R2 = 0.7481 in layer c) is higher than that in the REY-poorer and Mz-larger layer (R2 = 0.4041 in layer d). This suggests that the granularity is more effective in controlling the REY contents if the Mz is sufficiently small. The biological SiO2 content of the CCFZ samples can be calculated by SiO2 bio = SiO2 total −3.3 Al2O3, with SiO2/Al2O3 = 3.3 for the Pacific pelagic clay (Bischoff and Piper, 1979). Correlations between biological SiO2 content and Mz (Fig. 12) show that the presence of large siliceous skeletons can significantly increase the sediment Mz. The biological SiO2 content reflects the upper primary productivity (Dugdale et al., 1995), with which the sedimentation rate and the sediment Mz decrease. Although the fish teeth and the Fe-Mn micro-nodules also have large particle sizes, their content are too low to affect the sediment Mz. Both P2O5 (substitutes the content of fish teeth and bones) and MnO (substitutes the content of Fe-Mn micro-nodules) are negatively correlated with Mz (Fig. 12), indicating that the contents of fish teeth and micronodules are higher with smaller Mz. Contents of the REY carriers (esp. fish teeth) in deep sea sediments directly control the degree of REY enrichments. Fish teeth are nanocrystalline bone minerals with fibrous and uneven surface (Fig. 7d), and the ‘tubes’ along the hexagonal axis allow filling and/or ionic substitution in the apatite crystal defects. Generally, the finer the crystals, the more intense the biological activities are (Cai et al., 2007). Rare earth elements and yttrium can replace the Ca2+ ions in the apatite lattice (Nathan, 1984; Zhu et al., 2004), and hence apatite can accommodate all the trivalent REY ions. The two main minerals in Fe-Mn micro-nodules are goethite and todorokite (Fig. 7a and b): Goethite (α-FeOOH) has stable chemical properties, a high specific surface area and absorptivity, and has an important influence on ion migration and precipitation (García-Sánchez et al., 1999); Todorokite is widely distributed in marine Mn-nodules, and tends to form 3 × 3 type tunnel structures in its crystals (Post, 1999). Hence todorokite can absorb much heavy metals and REYs (Balakhonov et al., 2008; Cui et al., 2009). Fe-Mn micro-nodules can take up ions and complexes from ambient seawater, which is widely accepted to be a scavenging mechanism for REY in submarine environment (German et al., 1990). The XRD patterns of the samples (Fig. 4) show that the diffraction peak intensity of phillipsite and merlinoite increases, and the diffraction peak intensity of clinoptilolite gradually appears with increasing ΣREY, while the quartz diffraction peak has no such trend, which suggests that the crystallinity and species number of the zeolite minerals increase with the REY contents (or vice versa). With the supply of volcanic materials, a slow sedimentation favors the zeolitization of volcanic glass and/or feldspars and reduces the dilution of phillipsite content by excessive siliceous biological skeletons sediments (Cronan, 1980). Hence, the amount of phillipsite reflects to some extent the sedimentation rate. Under low sedimentation rate, the time of exposure to seawater for the surface sediments increases, which gives more time for the seawater REYs to replace Ca2+ in the biological apatite in sediments. Besides, the low sedimentation rate allows time for phillipsite to grow and incorporate Ca2+ ions (released from fish teeth) into its crystal lattice by replacing K+ and Na+ ions (Petzing and Chester, 1979). This may have generated the statistically positive correlation between the phillipsite content and ΣREY described in Section 5.1, both of which may be controlled by the sedimentation rate. Therefore, although phillipsite in deep-sea sediments is unlikely to be a REY carrier, its content reflects the sedimentation rate and can provide important indications to the sediment REY contents.

apatite, ferromanganese oxyhydroxides, albite and barite. The total REY contents of the sediments vary from 417.8 ppm to 810.4 ppm, and are strongly correlated with the P2O5 and CaO contents and moderately correlated with the Fe2O3 and MnO contents, suggesting that the fish teeth and Fe-Mn micro-nodules are the two major REY carriers. The negative REY vs. SiO2 correlation suggests that the siliceous biological debris may have diluted the REY contents. (2) The bulk-sediment REE patterns are featured by distinct negative Ce anomalies and slight positive Eu anomalies, similar to those of the seawater. The bulk-sediment HREE patterns mimic those of the average continental crust. We suggest that the REYs in the sediments were mainly seawater derived. (3) The phillipsite analyzed is unlikely to be a REY carrier, but its content reflects the sedimentation rate, and thus sheds light on the REY content in deep-sea sediments. The three controlling factors for the REY enrichments are namely the mean grain size, the amount of fish teeth and bones and the sedimentation rate. Under low sedimentation rate, the mean grain size decreases and the fish teeth content increases, leading to the REY enrichments. Acknowledgements This study was jointly funded by the National Natural Science Foundation of China (40473024, 40343019), the 13th Five Year Plan Project (DY135-R2-1-01, DY135-R2-1-05), the 12th Five Year Plan Project (DY125-13-R-05), the project of China Geological Survey (GZH201100303-05), the Higher Education Research Fund (20040558049, 20120171130005), the Colleges of Guangdong Province (2011) and the Fundamental Research Funds for Central Universities (12lgjc05, 09lgpy09). Staffs of the Instrumental Analysis Research Center (Sun Yat-Sen University) are thanked for their assistance in the electron microscopic and geochemical analyses. Prof. Franco Pirajno, the editor in chief of OGR, and two anonymous reviewers are thanked for their critical comments and constructive suggestions which are very helpful for improvement of this manuscript. References Alldredge, A.L., Gotschalk, C., 1988. In situ settling behavior of marine snow. Limnol. Oceanogr. 33, 339–351. Alldredge, A.L., Gotschalk, C.C., 1989. Direct observations of the flocculation of diatoms blooms: characteristics, settling velocity and formation of diatoms aggregates. Deep Sea Res. I 36, 159–171. Balakhonov, S.V., Churagulov, B.R., Gudilin, E.A., 2008. Selective cleaning of ions of heavy metals from water solutions using the H-form of todorokite synthesized by the hydrothermal method. J. Surf. Invest. 2 (1), 152–155. Bao, C.W., 1993. Polymetallic nodules and manganese crust as related to landforms and water-depth in the central Pacific. Geol. Res. South China Sea 5, 95–105. Baturin, G.N., Yushina, I.G., 2007. Rare earth elements in phosphate-ferromanganese crusts on pacific seamounts. Lithol. Min. Resour. 42 (2), 101–117. Bergersen, D.D., 1995. Cretaceous hotspot tracks through the Marshall Islands. Proc. ODP Sci. Results 144, 605–613. Bischoff, J.L., Piper, D.Z., 1979. Marine Geology and Oceanography of the Pacific Manganese Nodule Province. Plenum Press, pp. 397–436. Cai, Y., Liu, Y., Yan, W., Hu, Q., Tao, J., Zhang, M., Shi, Z., Tang, R., 2007. Role of hydroxyapatite nanoparticle size in bone cell proliferation. J. Mater. Chem. 17 (36), 3780–3787. Cronan, D.S., 1980. Underwater Minerals. Academic Press, pp. 362. Cui, H.J., Feng, X.H., Liu, F., Tan, W.F., Qiu, G.H., Chen, X.H., 2009. Progress in the study of todorokite. Adv. Earth Sci. 10, 1084–1093. Dubinin, A.V., 2000. Geochemistry of rare earth elements in oceanic phillipsites. Lithol. Min. Resour. 35 (2), 101–108. Dugdale, R.C., Wilkerson, F.P., Minas, H.J., 1995. The role of a silicate pump in driving new production, In: Belkin, I.M., Priede, I.G. (Eds.), Deep Sea Research Part I: Oceanographic Research Papers 42(5), 697–719. Edmond, J.M., Chung, Y., Sclater, J.G., 1971. Pacific bottom water: Penetration east around Hawaii. J. Geophys. Res. 76, 8089–8097. Eittreim, S.L., Ragozin, N., Gnibidenko, H.S., Helsley, C.E., 1992. Crustal age between the Clipperton and Clarion fracture zones. Geophys. Res. Lett. 19 (24), 2365–2368. Folk, R.L., 1980. Petrology of Sedimentary Rocks. Hemphill Publishing Company, pp. 26. Folk, R.L., Ward, W.C., 1957. Brazos River bar: A study in the significance of grain size parameters. J. Sediment. Petrol. 27 (1), 3–26. Folk, R.L., Andrews, P.B., Lewis, D.W., 1957. Detrital sedimentary rock classification and

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