minerals Article Mechanism and Influencing Factors of REY Enrichment in Deep-Sea Sediments Jiangbo Ren 1,2 , Yan Liu 1,3,*, Fenlian Wang 1,2, Gaowen He 1,2,*, Xiguang Deng 1,2, Zhenquan Wei 1,2 and Huiqiang Yao 1,2 1 Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China; [email protected] (J.R.); [email protected] (F.W.); [email protected] (X.D.); [email protected] (Z.W.); [email protected] (H.Y.) 2 Key Laboratory of Marine Mineral Resources, Ministry of Natural Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China 3 Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China * Correspondence: [email protected] (Y.L.); [email protected] (G.H.) Abstract: Deep-sea sediments with high contents of rare-earth elements and yttrium (REY) are expected to serve as a potential resource for REY, which have recently been proved to be mainly contributed by phosphate component. Studies have shown that the carriers of REY in deep-sea sediments include aluminosilicate, Fe-Mn oxyhydroxides, and phosphate components. The ∑REY of the phosphate component is 1–2 orders of magnitude higher than those of the other two carriers, expressed as ∑REY = 0.001 × [Al2O3] − 0.002 × [MnO] + 0.056 × [P2O5] − 32. The sediment P2O5 content of 1.5% explains 89.1% of the total variance of the sediment ∑REY content. According to global data, P has a stronger positive correlation with ∑REY compared with Mn, Fe, Al, etc.; 45.5% of samples have a P2O5 content of less than 0.25%, and ∑REY of not higher than 400 ppm. Citation: Ren, J.; Liu, Y.; Wang, F.; The ∑REY of the phosphate component reaches n × 104 ppm, much higher than that of marine He, G.; Deng, X.; Wei, Z.; Yao, H. phosphorites and lower than that of REY-phosphate minerals, which are called REY-rich phosphates Mechanism and Influencing Factors in this study. The results of microscopic observation and separation by grain size indicate that of REY Enrichment in Deep-Sea Sediments. Minerals 2021, 11, 196. the REY-rich phosphate component is mainly composed of bioapatite. When ∑REY > 2000 ppm, https://doi.org/10.3390/min11020196 the average CaO/P2O5 ratio of the samples is 1.55, indicating that the phosphate composition is be- tween carbonate fluoroapatite and hydroxyfluorapatite. According to a knowledge map of sediment Academic Editor: elements, the phosphate component is mainly composed of P, Ca, Sr, REY, Sc, U, and Th, and its Argyrios Papadopoulos chemical composition is relatively stable. The phosphate component has a negative Ce anomaly and positive Y anomaly, and a REY pattern similar to that of marine phosphorites and seawater. After the Received: 15 December 2020 early diagenesis process (biogeochemistry, adsorption, desorption, transformation, and migration), Accepted: 10 February 2021 the REY enrichment in the phosphate component is completed near the seawater/sediment interface. Published: 13 February 2021 In the process of REY enrichment, the precipitation and enrichment of P is critical. According to current research progress, the REY enrichment is the result of comprehensive factors, including low Publisher’s Note: MDPI stays neutral sedimentation rate, high ∑REY of the bottom seawater, a non-carbonate depositional environment, with regard to jurisdictional claims in oxidation conditions, and certain bottom current conditions. published maps and institutional affil- iations. Keywords: rare earth elements and yttrium; REY-rich phosphate; enrichment mechanism; deep-sea sediments; knowledge map; linear regression Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 1. Introduction This article is an open access article distributed under the terms and Rare earth elements and yttrium (REY) are important strategic resources. With the conditions of the Creative Commons increasing demand of high-tech products and green-tech applications, and gradual deple- Attribution (CC BY) license (https:// tion of easily processed REY resources, diversified REY resources are urgently needed to creativecommons.org/licenses/by/ ensure a stable supply. In particular, heavy REY (HREY) resources appear scarce; 95% of 4.0/). these elements are exclusively produced from ion-absorption-type ore deposits in southern Minerals 2021, 11, 196. https://doi.org/10.3390/min11020196 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 17 Minerals 2021, 11, 196 2 of 16 these elements are exclusively produced from ion‐absorption‐type ore deposits in south‐ ernChina China [1,2 ].[1,2]. The The mining mining of ion-adsorption-type of ion‐adsorption‐type ore ore causes causes environmental environmental damage, damage, and and the thesupply supply of HREY of HREY may may be at be risk. at risk. However, However, in recent in recent years, years, a certain a certain type type of deep-sea of deep sedi-‐sea sedimentment enriched enriched in REY in andREY HREY, and HREY, readily readily recovered recovered from samples from samples by simple by acid simple leaching, acid leaching,is expected is expected to serve as to a serve potential as a sourcepotential of source REY [3 –of5 ].REY [3–5]. REYREY-rich‐rich deep-seadeep‐sea sediments were first first discovered in the central North PacificPacific and southwestern Pacific Pacific [4], [4], and and subsequently subsequently reported reported across across most most of the of Pacific, the Pacific, Indian, Indian, and Atlanticand Atlantic oceans oceans (Figure (Figure 1) [3,6–9].1)[ 3,6– 9At]. Atpresent, present, deep deep-sea‐sea sediments sediments in in the the western western Pacific Pacific have the greatest resource potential, with the REY content ( ΣSREY) in the enrichment layer as high as 7974 ppm off Minamitorishima Island [[1,3,6,9].1,3,6,9]. Figure 1. Contents of REY ( ΣSREY) in surface and surface layer within 3 m samples of deep deep-sea‐sea sediments from the PacificPacific and Indian oceans. Data are from [[1,3,4,6–8,10–15].1,3,4,6–8,10–15]. TheThe motionmotion tracktrack isis 55 Ma/point,Ma/point, and the data are from [[16].16]. The data of Chl-aChl‐a (chlorophyll content) in surface seawater areare fromfrom [[17].17]. Studies reveal that phillipsite, clay minerals, Fe-MnFe‐Mn oxyhydroxides, and phosphate components are are all all REY REY carrier carrier phases phases [10,18]. [10,18 These]. These phases phases prefer prefer to accumulate to accumulate in slow in sedimentaryslow sedimentary environments, environments, and so and it is so difficult it is difficult to define to define the contribution the contribution of various of various com‐ ponents.components. Recent Recent studies studies have have further further confirmed confirmed that that the the ∑∑REYREY of of clay clay and and phillipsite themselvesthemselves are not high, with ∑∑REY generally not exceeding 256 ppm [[19,20].19,20]. Under the weak influence influence of of phosphate phosphate components components (P (P2O25O <5 0.25%),< 0.25%), ∑REY∑REY has hasa certain a certain positive positive cor‐ relationcorrelation with with Al2 AlO32 Ocontent.3 content. However, However, in inREY REY-rich‐rich deep deep-sea‐sea sediments, sediments, the the contribution contribution of phillipsite phillipsite or or clay clay minerals minerals to toREY REY enrichment enrichment is easily is easily masked masked by the by phosphate the phosphate com‐ ponent.component. FeFe-Mn‐Mn oxyhydroxides have strong adsorption capacity, and hydrogenetic Fe-MnFe‐Mn nodules can greatly enrich REY ( ∑∑REY,REY, excepting Ce, generally generally reaches reaches 2000 2000 ppm) ppm) [21]. [21]. The REY enrichment of metalliferous sediment in the Southeastern Pacific Pacific was initially thoughtthought to to be be related related to to hydrothermal hydrothermal processes processes [4]. [4 However,]. However, there there is no is significant no significant cor‐ relationcorrelation between between ∑REY∑REY and and the theFe and Fe and Mn Mncontents contents in deep in deep-sea‐sea sediments. sediments. Moreover, Moreover, the ∑theREY∑REY of Fe of‐Mn Fe-Mn micronodules micronodules in deep in deep-sea‐sea sediments sediments is isrelatively relatively low low (246–333 (246–333 ppm) ppm) [20]. [20]. The ∑∑REYREY of of hydrothermal hydrothermal Fe Fe-Mn‐Mn deposits deposits is is 1–2 1–2 orders orders of of magnitude magnitude lower lower than than that that of hydrogeneticof hydrogenetic Fe‐ Fe-MnMn deposits; deposits; rapid rapid hydrothermal hydrothermal processes processes have have an anobvious obvious dilution dilution ef‐ effect on REY [22]. Recent comparison of fine-grained analyses of samples from the South- Minerals 2021, 11, 196 3 of 16 east Pacific and central North Pacific show that apatite is the main host of REY, even in metalliferous deep-sea sediments near the East Pacific Rise [23]. The results of in situ analyses of bioapatite and chemical leaching of phosphate component from deep-sea sediments showed that the ∑REY of the phosphate component was 1–2 orders of magnitude higher than that of other REY carriers [3,12,19,24]. Among the major oxides of deep-sea sediment samples, ∑REY and P2O5 always maintain a strong positive correlation. The results show that the REY patterns of deep-sea sediments are similar to those of marine phosphates. The higher the content of SREY, the closer their REY patterns are [6,12,25]. The P in deep-sea sediments mainly exists in the form of REY-rich phosphate, and changes in the P content have a significant influence on the content and patterns of REY [12]. In summary, these studies have indicated that the high SREY of deep-sea sediments is closely related to the phosphate component. However, not all P forms REY-rich phosphates. In addition to the above studies, lots of studies have discussed the mechanism of REY enrichment in deep-sea sediments from the perspectives of the marine P cycle, sedimentation rate, sediment types, redox conditions, water ∑REY, and bottom ocean current [4,6,26–28].