The Influence of Hydrothermal Fluids on the REY-Rich Deep-Sea Sediments in the Yupanqui Basin, Eastern South Pacific Ocean: Constraints from Bulk Sediment Geochemistry and Mineralogical Characteristics

Rare earth elements (REEs) and yttrium (Y), together known as REY, are extremely enriched in deep-sea pelagic sediments, attracting much attention as a promising new REY resource. To understand the influence of hydrothermal processes on the enrichment of REY in deep-sea sediments from the eastern South Pacific Ocean, we conducted detailed lithological, bulk sediment geochemical, and in situ mineral geochemical analyses on gravity core sample S021GC17 from the Yupanqui Basin of eastern South Pacific. The REY-rich muds of S021GC17 are dark-brown to black zeolitic clays with REY contents (ΣREY) ranging from 1057 to 1882 ppm (average 1329 ppm). The REY-rich muds display heavy rare earth elements (HREE) enriched patterns, with obvious depletions in Ce, and positive anomalies of Eu in Post-Archean Australian Shale (PAAS)-normalized REE diagrams. In contrast, the muds of S021GC17 show light rare earth elements (LREE) enriched patterns and positive anomalies of Ce and Eu in the seawater-normalized REE diagrams. Total REY abundances in the core show positive correlations with CaO, P2O5, Fe2O3, and MnO concentrations. In situ analyses of trace element contents by laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP-MS) demonstrate that bioapatite fossils contain high REY concentrations (998 to 22,497 ppm, average 9123 ppm), indicating that they are the primary carriers of REY. The in situ Nd isotope values of bioapatites are higher than the average values of seawater in Pacific Ocean. Fe–Mn micronodules are divided into hydrogenetic and diagenetic types, which have average REY concentrations of 1586 and 567 ppm, respectively. The high contents of Fe-Mn-Ba-Co-Mo, the positive correlations between ΣREY and Fe-Mn, the ratios of Fe/Ti and Al/(Al + Fe + Mn), and the LREE-enriched patterns in the REY-rich muds, combined with high Nd isotope values shown by bioapatite fossils, strongly indicate that the hydrothermal fluids have played an important role in the formation of the REY-rich sediments in the eastern South Pacific Ocean.

The eastern South Pacific Ocean has a relatively complex system of seamounts and ridges, and these divide the area into multiple deep-sea basins [24]. To the east of EPR, the Roggeveen and Chile basins are bounded to the north by the Salay Gomez Ridge (SGR) near 25 °S, which is part of the Nazca Plate that extends towards the coast of Chile [20]. The Nazca Plate is surrounded by the East Pacific Rise to the west, Galapagos rift to the north, and Chile-Peru ridges to the south [25], and the Yupanqui Basin lies in the western part of the Nazca Plate (Figure 1b). The age of the basement in the Yupanqui Basin is <20 Ma [26].
The eastern South Pacific Ocean has rich water masses, which bring in dissolved oxygen and cause sediment decomposition [27]. The North Pacific water masses flow generally towards the Southern Ocean and meet the Antarctic Circumpolar Current (ACC), upstream of Drake Passage. Subantarctic Surface Water (SASW) is found to the north of the Subantarctic Front, and is carried northward in the eastern South Pacific Ocean along the rim of the Subtropical Gyre [27]. Low salinity Antarctic Intermediate Water (AAIW) occurs at depths between 400 and 1000 m [26,28]. The eastern South Pacific Ocean has a relatively complex system of seamounts and ridges, and these divide the area into multiple deep-sea basins [24]. To the east of EPR, the Roggeveen and Chile basins are bounded to the north by the Salay Gomez Ridge (SGR) near 25 • S, which is part of the Nazca Plate that extends towards the coast of Chile [20]. The Nazca Plate is surrounded by the East Pacific Rise to the west, Galapagos rift to the north, and Chile-Peru ridges to the south [25], and the Yupanqui Basin lies in the western part of the Nazca Plate (Figure 1b). The age of the basement in the Yupanqui Basin is <20 Ma [26].
The eastern South Pacific Ocean has rich water masses, which bring in dissolved oxygen and cause sediment decomposition [27]. The North Pacific water masses flow generally towards the Southern Ocean and meet the Antarctic Circumpolar Current (ACC), upstream of Drake Passage. Subantarctic Surface Water (SASW) is found to the north of the Subantarctic Front, and is carried northward in the eastern South Pacific Ocean along the rim of the Subtropical Gyre [27]. Low salinity Antarctic Intermediate Water (AAIW) occurs at depths between 400 and 1000 m [26,28].

Lithological Characteristics
The gravity piston core S021GC17 was collected from the Yupanqui Basin in the eastern South Pacific Ocean. The water depth was about 4212 m and the gravity core penetrated 3.04 m below the seafloor. Samples of core (2 cm thick) were taken at intervals of 10 cm from top to bottom, and the samples were numbered sequentially from S021GC17 0-2 to S021GC17 302-304. The REY-rich muds of S021GC17 are homogeneous dark-brown to black zeolitic clays (Figure 2a) characterized by high contents of clay minerals (mainly Kaolinite and illite) (50%), phillipsite (15%), Fe-Mn micronodules (15%), and biogenic apatite (5%), in addition to terrigenous detrital materials (quartz and feldspar) (10%) and carbonate (5%) (Figures 2b,c and 3). The proportions of the different constituents were estimated roughly by microscope observation. The biogenic apatite (fish teeth fossils) and Fe-Mn oxide in the muds showed no peaks on X-ray diffraction analyses (Figure 3), indicating an amorphous or low crystallinity habit. The selected 31 samples were sealed immediately in clean polyethylene bags and placed in 4 • C storage.

Lithological Characteristics
The gravity piston core S021GC17 was collected from the Yupanqui Basin in the eastern South Pacific Ocean. The water depth was about 4212 m and the gravity core penetrated 3.04 m below the seafloor. Samples of core (2 cm thick) were taken at intervals of 10 cm from top to bottom, and the samples were numbered sequentially from S021GC17 0-2 to S021GC17 302-304. The REY-rich muds of S021GC17 are homogeneous dark-brown to black zeolitic clays (Figure 2a) characterized by high contents of clay minerals (mainly Kaolinite and illite) (50%), phillipsite (15%), Fe-Mn micronodules (15%), and biogenic apatite (5%), in addition to terrigenous detrital materials (quartz and feldspar) (10%) and carbonate (5%) (Figure 2b,c, Figure 3). The proportions of the different constituents were estimated roughly by microscope observation. The biogenic apatite (fish teeth fossils) and Fe-Mn oxide in the muds showed no peaks on X-ray diffraction analyses (Figure 3), indicating an amorphous or low crystallinity habit. The selected 31 samples were sealed immediately in clean polyethylene bags and placed in 4 °C storage.

Bulk Sediment X-ray Diffraction (XRD)
X-ray diffraction (XRD) analysis can effectively identify the constituents of the sediments. The bulk mineralogy of the samples was assessed using X-ray diffraction (XRD; InXitu Terra, Mountain View, CA, USA) at the Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources (MNR), Qingdao, China. Five samples of REY-rich muds were prepared by salt washing, drying, crushing, and filling. The accuracy of the goniometer was better than 0.05° (2θ), instrument resolution better than 60%, and overall stability better than ±1%. Whole patterns from 0° to 75° 2θ were scanned at 2°/min for qualitative analysis of major constituent minerals. Mineral identification was performed using MDI Jade 5.0 software.

Bulk Sediment Major-Trace Element Analyses
Bulk-sediment major and trace element analyses were conducted at the Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, MNR, Qingdao, China. The samples were freeze-dried, powdered to 200 mesh. Each 0.05 g sample was weighed, placed in a polytetra fluoroethylene digestion tank, dissolved twice in HF-HNO3 (1:1), and dried again at 190 °C for 48 h. A total of 3 mL of 50% HNO3 was then added to each sample, and the samples were dried at 150 °C for at least 8 h, removed, and analyzed. Element concentrations were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES; for Al, Ca, Fe, K, Mg, Mn, Na, P, Ti) and inductively coupled plasma-mass spectrometry (ICP-MS; for REY, Sc, Cr, Co, Ni, and Cu). Several samples were analyzed in replicate to determine the precision of the measurements, and the elemental composition of the GSD-9 reference standard was measured to confirm the accuracy of the analyses, which was better than 2% for most elements.

Bulk Sediment X-ray Diffraction (XRD)
X-ray diffraction (XRD) analysis can effectively identify the constituents of the sediments. The bulk mineralogy of the samples was assessed using X-ray diffraction (XRD; InXitu Terra, Mountain View, CA, USA) at the Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources (MNR), Qingdao, China. Five samples of REY-rich muds were prepared by salt washing, drying, crushing, and filling. The accuracy of the goniometer was better than 0.05 • (2θ), instrument resolution better than 60%, and overall stability better than ±1%. Whole patterns from 0 • to 75 • 2θ were scanned at 2 • /min for qualitative analysis of major constituent minerals. Mineral identification was performed using MDI Jade 5.0 software.

Bulk Sediment Major-Trace Element Analyses
Bulk-sediment major and trace element analyses were conducted at the Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, MNR, Qingdao, China. The samples were freeze-dried, powdered to 200 mesh. Each 0.05 g sample was weighed, placed in a polytetra fluoroethylene digestion tank, dissolved twice in HF-HNO 3 (1:1), and dried again at 190 • C for 48 h. A total of 3 mL of 50% HNO 3 was then added to each sample, and the samples were dried at 150 • C for at least 8 h, removed, and analyzed. Element concentrations were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES; for Al, Ca, Fe, K, Mg, Mn, Na, P, Ti) and inductively coupled plasma-mass spectrometry (ICP-MS; for REY, Sc, Cr, Co, Ni, and Cu). Several samples were analyzed in replicate to determine the precision of the measurements, and the elemental composition of the GSD-9 reference standard was measured to confirm the accuracy of the analyses, which was better than 2% for most elements.

Bulk Sediment Sr-Nd Isotopes
Sr-Nd isotopic signatures can provide important information on the source materials in deep-sea sediments. Bulk sediment Sr-Nd isotope analyses were conducted at the same lab in the First Institute of Oceanography, MNR. Rb, Sr, Sm, and Nd concentrations were measured using a Thermo Fisher Scientific Triton Plus multi-collector thermal ionization mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The isotope ratios were corrected for mass fractionation by normalizing to 88 Sr/ 86 Sr = 8.375209 and 146 Nd/ 144 Nd = 0.7219. The international standards NBS-987 and JNdi-1 were employed to evaluate instrument stability during data collection. The measured international standard NBS-987 yielded 87 Sr/ 86 Sr = 0.710229 ± 0.000012 (2σ) and the JNdi-1 standard yielded 143 Nd/ 144 Nd = 0.512110 ± 0.00012 (2σ) [29].

EMPA Major-Element Analyses
Using the results of the bulk sediment analyses as a guide, typical bioapatite fossils (Figure 4a

LA-ICP-MS REY and Trace Element Analyses
In situ analysis of REY and trace elements in bioapatite fossils, Fe-Mn micronodules, and phillipsites extracted from the REY-rich muds was conducted at Beijing Createch Testing International (Beijing, China). Laser ablation was performed using an ESI NWR 193 nm excimer laser ablation system (Omaha, NE, USA). An Analytik Jena Plasma Quant mass spectrometer (Jena, Germany) was used to acquire ion-signal intensities. The spot diameter of the laser beam was 35 µm and the ablation frequency was 10 Hz. Helium was used as a carrier gas. Each spot analysis consisted of~20 s of background acquisition,~45 s of measurement, and~20 s of cleaning. Elemental abundances were calibrated against multiple synthetic reference glasses (NIST SRM610, NIST SRM612, BHVO-2G, BCR-2G, and BIR-1G). The suitable internal standards were also carried out for data calibration (Ca was used as an internal standard for bioapatites; Mn for Fe-Mn micronodules; and Al for phillipsites), which corresponded to the Ca, Mn, and Al contents measured by EMPA of each spot (refer to Supplementary Tables). The following isotope suite was analyzed: 139 La, 140 59 Co, 60 Ni, 65 Cu. The linear correlation between Ba and Eu contents is weak, suggesting that the mass interference of BaO on the Eu measurement is very minor.

LA-ICP-MS REY and Trace Element Analyses
In situ analysis of REY and trace elements in bioapatite fossils, Fe-Mn micronodules, and phillipsites extracted from the REY-rich muds was conducted at Beijing Createch Testing International (Beijing, China). Laser ablation was performed using an ESI NWR 193 nm excimer laser ablation system (Omaha, NE, USA). An Analytik Jena Plasma Quant mass spectrometer (Jena, Germany) was used to acquire ion-signal intensities. The spot diameter of the laser beam was 35 µm and the ablation frequency was 10 Hz. Helium was used as a carrier gas. Each spot analysis consisted of ~20 s of background acquisition, ~45 s of measurement, and ~20 s of cleaning. Elemental abundances were calibrated against multiple synthetic reference glasses (NIST SRM610, NIST SRM612, BHVO-2G, BCR-2G, and BIR-1G). The suitable internal standards were also carried out for data calibration (Ca was used as an internal standard for bioapatites; Mn for Fe-Mn micronodules; and Al for phillipsites), which corresponded to the Ca, Mn, and Al contents measured by EMPA of each spot (refer to Supplementary Tables). The following isotope suite was analyzed: 139 La, 140 59 Co, 60 Ni, 65 Cu. The linear correlation between Ba and Eu contents is weak, suggesting that the mass interference of BaO on the Eu measurement is very minor.

LA-MC-ICP-MS Sr-Nd Isotope Analyses
In situ Sr-Nd isotope analyses of bioapatite in the REY-rich muds were conducted at Beijing Createch Testing International (Beijing, China). Laser sampling was performed using a Resolution SE 193 nm excimer laser ablation system. The analytical system involved a Neptune MC-ICPMS. The spot diameter of the laser beam ranged between 35 and 50 µm and the ablation frequency was 10 Hz. The aerosol ablated by the laser was transported to the mass spectrometer using helium as a carrier gas. The isotope ratios were corrected for mass fractionation by normalizing to 88 Sr/ 86 Sr = 8.375209 and 147 Sm/ 149 Sm = 1.0868. The apatite standard sample Durango was employed to evaluate the MC-ICP-MS instrument stability during data collection, yielding 87 Sr/ 86 Sr = 0.70599 ± 0.00045 (2σ) and 143 Nd/ 144 Nd = 0.512478 ± 0.000029 (2σ) [30].

Bulk Sediment Major and Trace Element Analyses
Results of the bulk sediment major and trace element analyses are presented in Table 1a. The REY-rich muds are characterized by high contents of    Ce, and positive anomalies in Eu and Y (Figure 6a). In contrast, on seawater-normalized REE diagrams the samples show light rare earth elements (LREE) enriched patterns and positive anomalies in Ce and Eu (Figure 7a).

Bulk Sediment Sr-Nd Isotope Compositions
Considering the compositional regularity of sediments, we selected five samples for Sr-Nd isotope analyses at equal intervals. The results of Sr-Nd isotope analyses of the REY-rich muds (at depths of 0-2, 80-82, 160-162, 240-242, and 302-304 cm) are presented in Table 2.

Bulk Sediment Sr-Nd Isotope Compositions
Considering the compositional regularity of sediments, we selected five samples for Sr-Nd isotope analyses at equal intervals. The results of Sr-Nd isotope analyses of the REY-rich muds (at depths of 0-2, 80-82, 160-162, 240-242, and 302-304 cm) are presented in Table 2. The initial 87 Sr/ 86 Sr ratios for the REY-rich muds at different depths vary from 0.70919 to 0.71084, and their initial 143 Nd/ 144 Nd values range from 0.512450 to 0.512487. The calculated εNd(0) values range from -3.667305 to -2.945548. The εNd(0) values of sediments increase gradually with depth, which is consistent with the trend displayed by REY abundances.

EMPA and LA-ICP-MS Major and Trace Element Analyses
Major element, trace element, and REY compositions of the bioapatite fossils, Fe-Mn micronodules, and phillipsite extracted from the REY-rich muds at depths of 0-2, 80-82, 160-162, 240-242, and 302-304 cm in the core, were measured using EMPA and LA-ICP-MS, and the data are listed in Supplementary Tables S1-S3.
The Fe-Mn micronodules in the REY-rich sediments from the Yupanqui Basin can be divided into two types according to their composition: type I Fe-Mn micronodules (Figure 3d (Figures 6c and 7c) (Figures 6d and 7d).

Bioapatites
Previous studies concluded that biogenic Ca phosphates, such as bioapatite, are the main carriers of REY in deep-sea sediments and that biogenic phosphate occurs mainly in the form of fish teeth [33,34]. Bioapatite has a strong adsorption capacity for REY complexes, and is the main carrier of REYs in the REY-rich muds of the Indian Ocean and the western and central North Pacific [7,8,14,16]. Kashiwabara et al. [17] reported XAFS evidence that apatite is the main host phase of La in the Pacific REY-rich mud. Kon et al. [14] conducted in situ geochemical analyses of the bioapatite fossils from the Minami-Torishima area in the western Pacific, and reported that bioapatite fossils are the main REY-hosting phase with the REY range from 9300 to 32,000 ppm. Liao et al. [16] reported the average contents of 6182 ppm of REY in bioapatites from muds of the central North Pacific.
The CaO and P 2 O 5 contents of REY-rich muds in the Yupanqui Basin show positive correlations with REY ( Figure 5), which suggests that a Ca-phosphate phase is the dominant carrier of REY. The average REY content of bioapatites fossils from the Yupanqui Basin is 9123 ppm (with 8921 ppm exclusion of Ce), which confirms that bioapatite is the main REY carrier. The bioapatites are typically fish teeth, and their REY concentrations decrease significantly from the tooth roots to the tips, suggesting that the REYs enter through the roots and diffuse to the tips (Figure 8a,c). This occurs because the dense enamel that coats the bioapatite hinders REY incorporation. REY can only diffuse through the breach at the root, leading to the observed REY gradients [16].
We proposed the assumption that the content of P in the sediments was entirely supplied by bioapatites. Thus, it can be calculated that the percentage of bioapatite in the sediments is 5.1%. Considering that the average REY content of the bioapatites is~9000 ppm and REY in the bulk sediment is~1000 ppm, it can be roughly estimated that the 5% bioapatite accounts for~45% of the REY budget in the bulk sediments. Our results show that bioapatite is the main carrier of REY, even in the eastern South Pacific where significant REY accumulation is related to hydrothermal Fe-Mn (oxyhydr) oxides from the EPR.
average REY content of bioapatites fossils from the Yupanqui Basin is 9123 ppm (with 8921 ppm exclusion of Ce), which confirms that bioapatite is the main REY carrier. The bioapatites are typically fish teeth, and their REY concentrations decrease significantly from the tooth roots to the tips, suggesting that the REYs enter through the roots and diffuse to the tips (Figure 8a,c). This occurs because the dense enamel that coats the bioapatite hinders REY incorporation. REY can only diffuse through the breach at the root, leading to the observed REY gradients [16]. We proposed the assumption that the content of P in the sediments was entirely supplied by bioapatites. Thus, it can be calculated that the percentage of bioapatite in the sediments is 5.1%. Considering that the average REY content of the bioapatites is ~9000 ppm and ∑REY in the bulk sediment is ~1000 ppm, it can be roughly estimated that the 5% bioapatite accounts for ~45% of the REY budget in the bulk sediments. Our results show that bioapatite is the main carrier of REY, even in the eastern South Pacific where significant REY accumulation is related to hydrothermal Fe-Mn (oxyhydr) oxides from the EPR.

Fe-Mn Micronodules
The ∑REY contents of our REY-rich samples not only correlate with CaO and P2O5, but also other elements that are not present in bioapatite (e.g., Fe2O3 and MnO) ( Figure 5), which suggests that Feand Mn-bearing phases also contribute to the formation of the REY-rich muds. The Fe-Mn

Fe-Mn Micronodules
The REY contents of our REY-rich samples not only correlate with CaO and P 2 O 5 , but also other elements that are not present in bioapatite (e.g., Fe 2 O 3 and MnO) ( Figure 5), which suggests that Fe-and Mn-bearing phases also contribute to the formation of the REY-rich muds. The Fe-Mn micronodules in sediments formed as a result of hydrogenetic, diagenetic, and hydrothermal plume processes, and can also act as REY carriers into the sediments [1,11,13,35,36]. Fe-Mn micronodules have a low degree of crystallinity and are active scavengers of REYs [1,[37][38][39]. Menendez et al. [40] reported an average REY content of hydrogenetic Fe-Mn micronodules in deep-sea sediments of the Atlantic Ocean of 3620 ppm. Fe-Mn micronodules in REY-rich deep-sea muds recovered from the central North Pacific Ocean contain REY concentrations ranging from 439 to 1654 ppm [16].
To clarify the genetic type of micronodules, their elemental contents of Mn, Fe, Co, Ni, and Cu were plotted on a ternary discrimination diagram (Figure 9a) [41]. Most of the Fe-Mn micronodules fall into the hydrogenetic and mixed-type fields, whereas the Mn micronodules plot in the diagenetic field, which indicates the co-existence of different genetic mechanisms. Most of the micronodules plot in the transition area between the hydrogenetic and diagenetic fields, and the diagenetic-type (Fe)-Mn micronodules cluster mainly in the oxic diagenesis area on the 100 × (Zr + Y + Ce) vs. 15 × (Cu + Ni) vs. (Fe + Mn)/4 diagram proposed by Josso et al. [42] (Figure 9b). The average REY content of Fe-Mn micronodules is 1586 ppm (with 490 ppm exclusion of Ce), and the average REY content of Mn micronodules is 567 ppm (with 222 ppm exclusion of Ce), which indicates that hydrogenetic micronodules can release REY during the early stages of diagenesis. Given the high proportion of Fe-Mn micronodules in the sediments (15%), they contribute as much as 22.5% of the REY content of the sediments. Considering Ce is the least valuable element in REY, the economical values of micronodules in the sediments represent a significant discount.
were plotted on a ternary discrimination diagram (Figure 9a) [41]. Most of the Fe-Mn micronodules fall into the hydrogenetic and mixed-type fields, whereas the Mn micronodules plot in the diagenetic field, which indicates the co-existence of different genetic mechanisms. Most of the micronodules plot in the transition area between the hydrogenetic and diagenetic fields, and the diagenetic-type (Fe)-Mn micronodules cluster mainly in the oxic diagenesis area on the 100 × (Zr + Y + Ce) vs. 15 × (Cu + Ni) vs. (Fe + Mn)/4 diagram proposed by Josso et al. [42] (Figure 9b). The average REY content of Fe-Mn micronodules is 1586 ppm (with 490 ppm exclusion of Ce), and the average REY content of Mn micronodules is 567 ppm (with 222 ppm exclusion of Ce), which indicates that hydrogenetic micronodules can release REY during the early stages of diagenesis. Given the high proportion of Fe-Mn micronodules in the sediments (15%), they contribute as much as 22.5% of the REY content of the sediments. Considering Ce is the least valuable element in REY, the economical values of micronodules in the sediments represent a significant discount. Diagenesis can lead to the remobilization and inter-elemental fractionation of REYs, and these effects often occur in conjunction with redox reactions in the pore waters of sediments [32]. Previous researchers have suggested that REEs were associated primarily with Fe-Mn oxides in the sediments, then subsequently released to pore water during early diagenesis, and ultimately incorporated by bioapatite [43][44][45]. Variations in the Ce anomalies can be induced by the transfer of REEs from Mn-Fe (oxyhydr)oxides to bioapatite during early diagenesis [45]. Hence, it can be inferred that the REE patterns with positive Ce anomalies in the Fe-Mn micronodules could overprint the primary composition of the bioapatite. The Ce anomalies in our samples of bioapatite show a general increase with depth of burial (Figure 8d), which implies that the uptake of REEs was due to their release from the Fe-Mn micronodules. During early diagenesis, hydrogenetic Fe-Mn micronodules released REYs and associated metals into the pore water, forming the Mn micronodules (Figure 4f, g). Although the complete process remains an open question, the role of pore water in the formation of the REY-rich muds of the eastern South Pacific cannot be neglected. Diagenesis can lead to the remobilization and inter-elemental fractionation of REYs, and these effects often occur in conjunction with redox reactions in the pore waters of sediments [32]. Previous researchers have suggested that REEs were associated primarily with Fe-Mn oxides in the sediments, then subsequently released to pore water during early diagenesis, and ultimately incorporated by bioapatite [43][44][45]. Variations in the Ce anomalies can be induced by the transfer of REEs from Mn-Fe (oxyhydr)oxides to bioapatite during early diagenesis [45]. Hence, it can be inferred that the REE patterns with positive Ce anomalies in the Fe-Mn micronodules could overprint the primary composition of the bioapatite. The Ce anomalies in our samples of bioapatite show a general increase with depth of burial (Figure 8d), which implies that the uptake of REEs was due to their release from the Fe-Mn micronodules. During early diagenesis, hydrogenetic Fe-Mn micronodules released REYs and associated metals into the pore water, forming the Mn micronodules (Figure 4f,g). Although the complete process remains an open question, the role of pore water in the formation of the REY-rich muds of the eastern South Pacific cannot be neglected.

Phillipsite
In a pelagic environment, phillipsite that forms by the alteration of volcanic ash is the one of the most important constituents in the REY-rich muds. Bernat [33] reported that REE contents in fish teeth fossils are two to three orders of magnitude higher than those in phillipsite. Dubinin [46] measured the REE concentrations in phillipsite samples from the Southern Basin of the Pacific, and suggested that phillipsite does not accumulate REY. Kon et al. [14] reported REY contents of 60-170 ppm in phillipsites by in situ mineral analyses.
The REY contents of phillipsites in our samples, measured by in situ LA-ICP-MS spot analyses, range from 35 to 127 ppm (average 80 ppm), showing that phillipsite is not the main carrier. The requisite for the formation of phillipsite in a marine sediment is an environment with a sufficiently low sedimentation rate, which is similar to the formation of pelagic clays with high REY contents [47].
The growth of large phillipsite grains may reflect a low sedimentation rate and long-term exposure of crystals on the seafloor [48]. Hence, phillipsite in the REY-rich muds of the eastern South Pacific is an indicator of low sedimentation rates.

The Influence of Hydrothermal Fluids
Previous researchers suggested that seawater was the main source of the REY in REY-rich sediments, based on the similar REE patterns of seawater and REY-rich muds [7,34,49]. The REY patterns in Figure 6a,b for the muds and the bioapatites do indeed show characteristics similar to those of seawater with their obvious depletions in Ce and positive anomalies of Y, and this indicates the REYs were mainly derived mainly from seawater. This is confirmed by the Sr-Nd isotopic compositions of the bulk muds, which are consistent with the isotopic characteristics of seawater in the Pacific ( Figure 10). However, the high contents of Fe, Mn, Ba, Co, and Mo in the REY-rich muds from the Yupanqui basin conceivably imply the influence of other fluids (Table 1a).
It is well known that the East Pacific Rise (EPR) is a fast-spreading mid-ocean ridge [50] that is accompanied by intense hydrothermal activity due to high rates of magma production [11,51]. The dominance of Ce (IV) in the eastern South Pacific could reflect the larger contribution of hydrothermal Fe-Mn oxyhydroxides in this area compared with the central North Pacific [52]. The 3 He anomalies in seawater (Figure 1b) show that hydrothermal activity can spread far from a mid-ocean ridge, which is consistent with the distribution of REY-rich muds in the Pacific Ocean [1,[11][12][13]. Hydrothermal fluids are enriched in iron (Fe) and manganese (Mn) by more than 10 6 relative to ambient deep ocean concentrations. Recent studies have also confirmed that Fe-Mn derived from hydrothermal fluid vents can be transported over distances of hundreds or thousands of kilometers [23,53,54]. The modern marine environment is generally in an oxic state, as seen in the pronounced negative Ce anomaly of seawater and bioapatite. It is reasonable to suggest that hydrothermal fluids containing considerable amounts of Fe-Mn oxyhydroxide particulates were spread eastwards from the EPR. The oxidized state of the deep-sea environment may contribute Fe-Mn metal ions to the formation of Fe-Mn micronodules, which would promote the enrichment of REY in marine sediments [55,56].
Most of the other REY-rich muds reported in the central North Pacific, the western Pacific, and the Indian Ocean appear to be unrelated to hydrothermal fluids [7][8][9]. However, the influence of hydrothermal fluids on the sediments in the Yupanqui Basin from the eastern South Pacific Ocean is significant.
The formation of the REY-rich muds in the eastern South Pacific Ocean appears to be associated strongly with hydrothermal activity, as suggested by the positive correlations between REY and both Fe 2 O 3 and MnO ( Figure 5). The REY-rich sediments from the eastern South Pacific Ocean are enriched in Fe, Mn, Ba, Co, and Mo, which are much higher than the REY-rich sediments from the Indian Ocean, and the western and central North Pacific Ocean [7,8,14,57]. The clear enrichment in these elements is a typical characteristic of hydrothermal fluids and associated precipitates occurring at EPR [37]. The bulk sediments show LREE-enriched patterns and clear Eu positive anomalies in the seawater-normalized REE diagram, and the ratios of Fe/Ti and Al/(Al+Fe+Mn) range from 26.4-39.8 (greater than 20) and 0.24-0.33 (less than 0.35), which all strongly support the involvement of hydrothermal input during the formation of the REY-rich sediments [37,58]. Moreover, the 143 Nd/ 144 Nd values of the bioapatites are higher than the average values for Pacific seawater, which confirms the overprint of hydrothermal fluids ( Figure 10).
Thus, we can draw a conclusion that hydrothermal activity plays an important role in the formation of REY-rich muds in the eastern South Pacific. Hydrothermal fluids from the EPR containing considerable amounts of Fe-Mn metal ions have spread thousands of kilometers, precipitating Fe-Mn micronodules in the Yupanqui Basin, where significant quantities of REY are absorbed by the micronodules. Plot of ( 87 Sr/ 86 Sr) vs ( 143 Nd/ 144 Nd) for the REY-rich muds and bioapatites. Natural variability of the Nd isotope composition in different rocks, sediments, and waters, given εNd(0) values relative to CHUR, modified after [59]. Mid-oceanic ridge basalts proposed by [60]. River water and river sediment εNd(0) values after [61,62]; seawater εNd(0)values for the different ocean basins after [63] and [64]. CHUR = chondritic uniform reservoir; CFB = continental flood basalt; OIB = ocean island basalt; IAB = island arc basalt; MORB = mid-oceanic ridge basalt. EPR/Chile Ridge field proposed by [65].

Conclusions
1. Hydrothermal fluids from the EPR may contribute a large amount of Fe-Mn metal ions, leading to the formation of Fe-Mn micronodules that enable the enrichment of REY in the REY-rich deep-sea sediments.
2. Bioapatite fossils are the main carrier for REYs in the REY-rich deep-sea sediments. REYs enter through the roots of the bioapatites and diffuse to the tips. It can be estimated roughly that the 5% bioapatites present in the samples account for ~45% of the REY budget in the analyzed sediments.
3. Ferromanganese micronodules contribute as much as 22.5% of the REY contents of the sediments, thus also making them an important REY-hosting phase in the eastern South Pacific. During early diagenesis, hydrogenetic Fe-Mn micronodules released REYs into the pore water, which are ultimately absorbed by bioapatites. 4. Phillipsite is not a main host of REY in REY-rich muds from the eastern South Pacific Ocean. This indicates a low sedimentation rate.
Thus, we can draw a conclusion that hydrothermal activity plays an important role in the formation of REY-rich muds in the eastern South Pacific. Hydrothermal fluids from the EPR containing considerable amounts of Fe-Mn metal ions have spread thousands of kilometers, precipitating Fe-Mn micronodules in the Yupanqui Basin, where significant quantities of REY are absorbed by the micronodules.

Conclusions
1. Hydrothermal fluids from the EPR may contribute a large amount of Fe-Mn metal ions, leading to the formation of Fe-Mn micronodules that enable the enrichment of REY in the REY-rich deep-sea sediments.
2. Bioapatite fossils are the main carrier for REYs in the REY-rich deep-sea sediments. REYs enter through the roots of the bioapatites and diffuse to the tips. It can be estimated roughly that the 5% bioapatites present in the samples account for~45% of the REY budget in the analyzed sediments.
3. Ferromanganese micronodules contribute as much as 22.5% of the REY contents of the sediments, thus also making them an important REY-hosting phase in the eastern South Pacific. During early diagenesis, hydrogenetic Fe-Mn micronodules released REYs into the pore water, which are ultimately absorbed by bioapatites. 4. Phillipsite is not a main host of REY in REY-rich muds from the eastern South Pacific Ocean. This indicates a low sedimentation rate.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/12/1141/s1, Table S1: The major elements content (%) and trace elements abundance (ppm) of bioapatite from the REY-rich muds in the S021GC17; Table S2: The major elements content (%) and trace elements abundance (ppm) of micronodules from the REY-rich muds in the S021GC17; Table S3: The major elements content (%) and trace elements abundance (ppm) of phillipsite from the REY-rich muds in the S021GC17