Results
HR-ICP-MS results (ppm) revealed concentration of La varies from 1.87–21.47with mean value of 2.75, Ce ranges from 1.16–45.98 with a mean value of 5.086, with Pr(1.07–39.56, av. 0.62), Nd(0.71–26.02 av. 2.36), Sm(0,42- 12.51 av. 0.42), Eu(0.20–4.66 av. 0.07), Gd(0.27–8.79 av. 0.40), Tb (0.29–3.97, av. 0.62), Dy0.28-3.62 (0.34), Ho 0.22–2.76 (0.06), Er0.29–2.64 (0.19), Tm 0.33–3.83(0.033), Yb0.37-3.75 av. 0.23) and Lu (0.29–3.03 av. 0.033) respectively.
The volumes of the REEs in sediments agreed with the arrangement of La > Ce > Pr > Nd > Sm > Gd > Eu > Tb > Tm > Dy > Yb > Lu > Ho > Er which is relatively similar to the previous study in eastern Dahomey basin (Nton, 2001). The range and average values of REE ratios of the sediment under study as calculated revealed as follow: La/Yb (5.1–30.0, av. 12.29), La/Sm (4.3–13. 9, av. 6.79), Gd/Yb (0.8–6.4, av. 1.73), Ce/La (1.1–2.3, av. 1.80), Eu/Eu* (0.1–1.0, av. 0.6) and Ce/((Sm + Gd)/2) (2.1–4.4, av. 3.1) respectively (Table 1).
The summation of rare earth elements show as follows: ∑REE (7497.67), ∑LREE (7810.26), and ∑HREE(131.97) coupled with their ranges and average values. The ∑REE ranges from 2.25 to 6686.75), ∑LREE (42.42–508.56, av. 224.70) and ∑HREE (3.26–40.02, av. 18.85) and the sum ratio of LREE/HREE of 3971.13 respectively. Mostly, samples show similar arrangements to UCC, through strong LREE enrichment (LaN/YbN = 5.1–30.0, (mean = 12.5), moderately flat HREE (GdN/YbN = 0.8–6.4 (mean = 1.7) and Eu anomalies (Eu/Eu*= 0.1–1.0, mean = 0.6).
Table 1
REEabundance in sediments of Ilaje area
S/N | Rare Earth Elements (N = 100) | Range | Mean | UCC (Ronov et al., 1974). |
1 | La | 1.87–21.47 | | 2.75 | 16.0 |
2 | Ce | 1.16–45.98 | | 5.086 | 28.0 |
3 | Pr | 1.07–39.56 | | 0.62 | 4.70 |
4 | Nd | 0.71–26.02 | | 2.36 | 2.10 |
5 | Sm | 0,42- 12.51 | | 0.42 | 5.30 |
6 | Eu | 0.20–4.66 | | 0.07 | 1.40 |
7 | Gd | 0.27–8.79 | | 0.40 | 5,90 |
8 | Tb | 0.29–3.97 | | 0.62 | 0.90 |
9 | Dy | 0.28–3.62 | | 0.34 | 4.90 |
10 | Ho | 0.22–2.76 | | 0.06 | 0.77 |
11 | Er | 0.29–2.64 | | 0.19 | 3.10 |
12 | Tm | 0.33–3.83 | | 0.033 | 0.32 |
13 | Yb | 0.37–3.75 | | 0.23 | 2.70 |
14 | Lu | 0.29–3.03 | | 0.033 | 0.29 |
Correlation analysis
The strength and degree of associations among the REE were assessed with the aid of statistical parameters (Table 2).
Table 2
Correlation for REE in sediments of Ilaje area
| La | Ce | Pr | Nd | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu | Y |
La | 1.000 | | | | | | | | | | | | | | |
Ce | 0.996 | 1.000 | | | | | | | | | | | | | |
Pr | 0.991 | 0.997 | 1.000 | | | | | | | | | | | | |
Nd | 0.977 | 0.987 | 0.991 | 1.000 | | | | | | | | | | | |
Sm | 0.959 | 0.972 | 0.978 | 0.989 | 1.000 | | | | | | | | | | |
Eu | 0.655 | 0.676 | 0.690 | 0.732 | 0.796 | 1.000 | | | | | | | | | |
Gd | 0.960 | 0.969 | 0.974 | 0.982 | 0.986 | 0.793 | 1.000 | | | | | | | | |
Tb | 0.890 | 0.906 | 0.916 | 0.939 | 0.962 | 0.890 | 0.968 | 1.000 | | | | | | | |
Dy | 0.792 | 0.812 | 0.825 | 0.860 | 0.902 | 0.936 | 0.908 | 0.969 | 1.000 | | | | | | |
Ho | 0.679 | 0.698 | 0.713 | 0.755 | 0.811 | 0.958 | 0.822 | 0.918 | 0.970 | 1.000 | | | | | |
Er | 0.675 | 0.697 | 0.714 | 0.758 | 0.806 | 0.923 | 0.825 | 0.913 | 0.967 | 0.980 | 1.000 | | | | |
Tm | 0.598 | 0.613 | 0.628 | 0.670 | 0.725 | 0.919 | 0.749 | 0.851 | 0.915 | 0.965 | 0.946 | 1.000 | | | |
Yb | 0.588 | 0.608 | 0.623 | 0.676 | 0.733 | 0.933 | 0.751 | 0.860 | 0.930 | 0.970 | 0.965 | 0.966 | 1.000 | | |
Lu | 0.565 | 0.580 | 0.595 | 0.635 | 0.699 | 0.917 | 0.717 | 0.829 | 0.891 | 0.952 | 0.923 | 0.974 | 0.965 | 1.000 | |
Y | 0.628 | 0.650 | 0.665 | 0.705 | 0.763 | 0.927 | 0.784 | 0.886 | 0.946 | 0.977 | 0.973 | 0.960 | 0.975 | 0.960 | 1.000 |
Some parameters had very high correlation coefficient although; this does not necessarily signify any relationships among variables, due to interplay of other prevailing factors. Ronov29 explains that REE that exhibit significant association at r > 0.7 are strongly associated, correlation significant relations at r < 0.5–0.7 are mildy correlated, while significance of r ≤ 0.5 are not relatively correlated respectively. The REE were more correlated significantly at p ≤ 0.01 than those at p ≤ 0.05 indicating their high degree of relationship.
Discussion
Provenance studies
REE have been adopted to study some parameters, this includes provenance, grain size distribution, mineralogy, chemical weathering intensity, and diagenesis of siliciclastic sediments (30,31). McLennan [32, 32], reported that REE in terrigenous sediments are mostly unreactive and this has made them very useful for provenance studies. The REE patterns of the adjoined southwestern Nigeria Basement complexes (granite and granodiorite) were correlated with those of the siliciclastic sediments (beach sands) under study. These show overlying patterns with the rocks southwestern Nigeria Basement complexes
The REE concentrations were normalized using chondrite data of [34]. The patterns of chondrite-normalized indicate REE enrichment with depleting HREE and negative Eu anomalies, this suggests fractionation of felsic and intermediate sources composition of the beach and fluvial environment [30], (Fig. 2). The negative Eu anomalies signify plagioclase fractionation in the felsic-intermediate source rocks possibly in the course of evolution or accessory minerals framework mechanism of the sediment deposition [35].
This is similar to the results of [36] granitoids within the southwestern basement complex, where the same chondrite-normalized REE configuration was observed for igneous rocks in southwestern Basement complexes of Nigeria. The similarity in chondrite-normalized REE patterns of sediment under study relative to that of igneous rocks explains further that the mantle-derived material from the southwestern Nigeria basement complexes has considerable influence on the REE patterns of the siliciclastic sediment [37]. This indicates they might have inherited REE arrays of igneous rocks. Values of REE proportions assessed in this study (Table 3) determine the provenance source, geotectonic settings depositional conditions, and environments.
Table 3
REE ratios of the sediment from the Ilaje area
| Rare earth elements ratios | | |
S/N | REES Ratios | Min | Max | Mean | UCC | Remarks (source) |
1 | Ce/La | 1.1 | 2.3 | 1.8 | | Terrigenous source |
2 | La/Sm | 4.3 | 13.9 | 6.8 | 4.20 | Silicic source |
3 | La/Yb | 5.1 | 30.0 | 12.5 | 7.8 | silicic source |
4 | Gd/Yb | 0.8 | 6.4 | 1.7 | 1.0–2.0 | Post Archean sediment |
5 | Eu/Eu* | 0.1 | 1.0 | 0.6 | 0.40–0.94 | Felsic/Quartzose source |
6 | Ce/((Sm + Gd)/2) | 2.1 | 4.4 | 3.1 | 2.1 to 4.4 | Oxidising shallow marine |
Adeigbe [38] explained that (Gd/Yb)N values are adopted to evaluate source rock nature coupled with composition of continental crust. Archean crust commonly has greater (Gd/Yb)N values, above 2.0 in sedimentary rocks, while post-Archean (Table 3) sedimentary rocks have (Gd/Yb)N values commonly range from 1.0 to 2.0. Seventy eight 78 percent of these quaternary sediments have (Gd/Yb)N ratios from 1 to 2.0 indicate source rocks of sediment to be post-Archean rocks, while remaining 22 percent fall within the Archean crust. This has pointed to the fact that the sediment source within this area is from the southwestern basement complexes located within northern part of the study area. The Eu/Eu* average for the sediments under study range from 0.1–1.0; This indicates a felsic source of quartzose sedimentary, intermediate igneous provenances for the deposits [39, 40]. Also, the kin configuration and magnitude of Eu anomaly were used to infer the source rocks nature [13, 14, 16, 42–44].
The pattern and trending structure of the plot coupled with the negative anomaly, all proved the quaternary sediments are dominantly from felsic source area. Roser [42–45] used the trends and patterns of the REE coupled with the Eu anomaly to infer sediment provenance. Cerium anomaly (Ce/Ce*) values in the sediments under study ranges from 2.1 to 4.4 indicate an oxidized, and shallow marine environment [38]. The REE arrangement in this study aligns with Upper Continental Crust (UCC), they are enriched in LREE and depleted in HREE with low negative Eu anomalies, suggest derivation of sediments of quartzose provenance source rock.
Discriminant plot of CeN against (La/Yb)N ratio indicates the sediment to be dominated by quartzose provenance source17 (Fig. 3). Ce concentration was been adopted as a sensitive indicator to access the redox nature of these sediment under study, the Ce anomalies reflect an oxidizing environment, an indication that feldspar had been removed from the melts through fractional crystallization. About 80 percent of Ce anomaly (Ce/Ce* ≥ 1) in the sediments suggest a depositional process in an oxic condition, whereas the other 20 percent falls within anoxic condition [46]. Therefore, the depletion or enrichment of this REE reflect change in oxygenation conditions of depositional sediment [47].
Normalized/Chondrites Plot of REES and Geotectonic Settings
REE Chondrite-normalized pattern of siliciclastic sediments publicised fairly greater LREE enrichment comparing with HREE, with depletion in Eu anomaly; these were influenced due to the availability of the hornblende mineral in the parent source rock [48, 49, 50] principally in the siliciclastic sediment under study. These trends indicate acidic source rock for the siliciclastic deposits under study. Additional factors applied in determining the REE quantity in rocks involve using (La/Yb)cn fractions to distinguish the enrichment of the light Rare Earth Elements (LREE) over heavy Rare Earth Elements (HREE) with source rock areas [4, 51].
The normalized Chondrite trends of the samples under study revealed a characteristic nature of continental margin settings with enrichment of LREE, negative Eu anomalies, and flat HREE nature. These sediments' potential source rocks were furthermore matched with chondrite-normalized REE patterns of rocks from the southwestern basement complex, this demonstrated that quartzose provenance rocks are possible source rocks under studied. REE arrangement also indicates the enrichment of LREE compared to HREE as displayed in Figs. 2a-d, these suggest the removal of feldspar during magmatic crystallization of source rock. This evidence indicates that enrichment in LREE compare with HREE is measured through hornblende in parent source rock as suggested by [50], especially in siliciclastic sediments.It was observed that the patterns of REE distribution in the sediment and the adjoined rock show a close correlation. This indicated the coherent link between the siliciclastic sediments and quartzose sandstone host rock of the coastal plain sand [52]. However, the process of intense lateritization of the quartzose rocks during scarce rainfall coupled with more precipitation led to the leaching and washing of all the impurities during sluggish erosional effects, resulting from progradation activities in this study area.
Economic Potentials of the REE
REE are elements that contain distinctive properties, among which are; excessive heat resistance, sturdy magnetism, great electrical conductivity, and high luster. These properties make them useful in the production of materials including cell phones, lights, magnets, batteries, loudspeakers, and wind turbines. Other areas include the production of parts used in common items; light-emitting diodes (LEDs), fiber optics, and compact fluorescent lights., they are used as catalysts, phosphors, and polishing compounds and for controlling air pollution, It can also be used as illuminated screens on electronic gadgets, and the polishing of optical-quality glass. The demand for Rare earths has increased over time, this has increased their uses. Items including cellular telephone parts and accessories are used worldwide.
REE frequently applied to produce electronics which include: lanthanum, cerium, neodymium, samarium, europium, terbium, and dysprosium. They are used to monitor earthquakes and explosions effects on the ground, in medicine and metrology, helped to produce lasers, computer hard disks, help to treat and diagnose cancer. Although, REE have been found useful in producing microphones, a car headlight, a catalyst for self-cleaning ovens, ceramics, polishing powder, camera lenses, spark plugs, energy fluorescent bulbs, microwave filters, aerospace components, additives in metal–halide lamps, mercury vapor lamps, as well as radioactive tracing agents in the oil refinery [53], this sediments based on the REE present with the geochemical standard available is found to be capable of serving a good raw material as earlier mentioned.