Source Area Weathering , PaleoEnvironment and Paleo-Climatic Conditions of Soils from Bitumen Rich Ode Irele Area of Ondo State , Nigeria

The study is aimed at determining the sourceweathering, paleo-redox and paleoclimatic conditions of soil from Ode Irele area of Ondo State, Nigeria. X-Ray Fluorescence (XRF) and the Laser Ablation Inductively Coupled Plasma-Mass Spectrometry ICP-MS) analyses wereusd to determine the major, trace and rare earth element compositios used for the study. The Chemical index of alteration (CIA), Chemical index of weathering (CIW), Plagioclase index of alteration (PIA), Mineralogical index of alteration (MIA), Th/U nd a plot of CIA against Al2O3 as well as the A-CN-K Ternary diagram suggest a high degree of weathering of the source rocks. The positive correlation between Al2O3 with TiO2, K2O, Fe2O3 and MgO implies that they occur in clay minerals formed from weathering. The average K2O/Al2O3 ratios of the investigated samples is ≈0.006, which suggests prevalence of clay minerals relative to other minerals. Low U/Th ratio ranging between 0.14 and 0.50 (average 0.18) relative to PAAS (0.21) and UCC (0.26) suggests deposition under oxic conditions. V/Vi + Ni ratios indicate suboxic to anoxic environment of deposition. However, plots of V/V+ Ni vs Ni/Co and V/Cr vs Ni/Co indicate oxic, dysoxic and suboxic to anoxic environments all suggesting deposition under oxic and suboxic conditions. The Ode Irele sediments was deposited in low saline water with a strong continental rather than marine influence in an arid and semi-arid climatic condition; the low average ratio of P2O5/Al2O3 (0.014) is in unison with freshwater environments.


I. INTRODUCTION
The use of major, trace and rare earth elements of siliciclastic sedimentary rocks is an important tool as they are sensitive indicators of source rocks [1], weathering erosion dynamics [2], tectonic settingpaleoweatherng conditions, paleo climate and post depositional changes [3]- [11]. Rare earth elements Hf and Zr have been used to reflect the characteristics of their parent rocks. Authors such as [12]- [17] have used other geochemical parameters to infer the paleo-redox conditions of ancient sediments. References [18] and 19] used the elemental ratios of Ni/Co and V/Cr to deduce the redox conditions during the deposition of weathered materials such as shale, because Ni/Co and V/Cr ratios are related to low oxygen levels during the deposition. The ratio of the most immobile elements to the most mobile elements increases towards passive margins due to the relative tectonic stability [20]- [22] and thus prolonged weathering. The present study aims at identifying the weathering characteristics, paleo-redox and paleoclimati conditions of the soil from Ode Irele through the use of major, trace and rare earth elements.

II. MATERIALS AND METHODS
Fifteen soil samples were analysed by Laser ablation microprobe Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) method [23] at the Central laboratory of the Stellenbosch University, South Africa. The ICP-MS instrument is a Perkin-Elmer Sciex ELAN 5100 coupled with a UV (266 nm) laser. The laser was operated with 1 mJ/pulse energy and 4 Hz frequency for silicate minerals, and 2 -Hz frequency with the laser beam focused above the sample surface for carbonates and silicate glass. Spot diameter for these analyses is 30-50µm. NIST 610 glass was used as a calibration standard for all samples, with 44 Ca as an internal standard. Analytical precision is 5% at the ppm level. Details of ICP-MS and laser operating conditions have been published by [24] and [25]. The results of the analyses are reported in trace and rare earth elements. The post-Archean Australian Shale (PAAS) values were used for comparison while the REE data were normalized to the chondrite values of [26]. The normalized Eu* anomaly Eu/Eu* is calculated by using the equation Eu/Eu* = EuN/(SmN x GdN ) 0.5 where N the subscript represents the chondrite normalized values [26]. The X-RayFluorescence Spectrometry (XRF) method was used to analyse for the major element concentrations at the Central laboratory of the Stellenbosch University, South Africa. The results show the oxides percent by weight: SIO2, AL2O3, Fe2O3, CaO, MgO, K2O, MgO, MgO, MnO, Na2O, TiO2 and LOI. Fig. 1 [28], [29].

A. Geochemistry
The chemical composition of the Ode Irele samples are presented in Tables I, II and III. The results of the major oxides (Table I) shows a relatively enriched SiO2 (59.44-86.65) Wt. %, Al2O3 (5.86-21.14) and Fe2O3 (2.93-7.29) with low contents of TIO2 (0.98-1.79) and strongly depleted Na2O (0-0.01) and K2O (0.10-0.09). Other oxides such as MnO, MgO CaO and Na2O have low concentrations that are less than unity (1). The low contents of these oxides may be attributed to chemical destruction under oxidizing condition during weathering. The ratio of SIO2/Al2O3 (2.81-14.78) shows moderate silica to alumina content. K2O/AL2O3 ratio is low (0.09-0.51), an indication of low K-bearing mineral contents in relation to alumina. Chemical ratios of SiO2/Al2O3 and Na2O/K2O in the samples may be the result of degree of weathering of initially Na2O rich (plagioclase) rock either at the source and/or in transit to the depositional basin. Soluble oxides such as Na2O and CaO showed no correlation with suggesting non-association with illitic clay minerals. Fig. 3 shows that Al2O3 correlates positively with TiO2 indicating that TiO2 is an essential major constituent of illitic clay mineral. Al2O3 correlates positively with K2O, suggesting that potassium is an important trace constituent of clay mineral. These correlations suggest that potassiumbearing minerals exert substantial control over the Al2O3 distribution, which infers that the contents of the clay minerals primarily control the contents of these elements [30]. The positive correlation between Al2O3 with Fe2O3 and MgO implies that they occur in clay minerals formed from weathering. The negative trend between Al2O3 and SiO2 suggests that the major element composition of the samples is controlled largely by the relative amount of quartz and feldspar versus clay minerals. The positive correlation of Al2O3 with P2O5 likely is related to the presence of nonaluminous phase(s) which might bear some P in these rocks [31]. Complex and other geological units, after [30]. The low average ratio of P2O5/Al2O3 (0.014) is in unison with freshwater environments [32]. The ratio of K2O/Al2O3 can also be used as an indicator of the original composition of ancient mud rocks [33]. The average K2O/Al2O3 ratios of the investigated samples is ≈0.006, which suggests prevalence of clay minerals relative to other minerals [34]. The trace elements composition is presented in Table II and they show variations in their compositions. Ba (46.13) is highly depleted relative to UCC (550) and PAAS (650) just as Sc (4.68) is enriched relative to UCC (11) and impoverished in relation to PAAS (16). Sr (30.38) is highly depleted in the soil of the present study in relation to UCC (350) and PAAS (200). The strong depletion of Sr in all the samples can be linked to scanty carbonate content [35]. Zn (40.4) is highly impoverished in the study area relative to PAAS (85) and UCC (71).
Ba and Sr are associated and are highly depleted in the soil of the present study. Ba, Sr, Cs, Th and U are large ion lithophile elements (LIlEs) which are regarded as mobile elements during post magmatic alterations. U (3.34) is enriched relative to UC (2.80) and PAAS (3.10). These elements are bound in clay minerals and are concentrated during weathering [36]. [37]. However, U and Th behave differently during weathering though both of them are chemically mobile. U mobility leads to decrease in the ratio of U/Th. In the present study, the average U/Th ratio is 0.18 (Table IV)

B. Source-area weathering
Weathering indices are useful to measure the degree of weathering of the terrestrial rocks from which sediment grains are derived. The properties of the parent rocks such as mineralogy, chemical, structural and factors of climate affect the susceptibility of such rocks to weathering. Chemical weathering indices are commonly used in recent and old profile studies [38], [39]. Authors such as [40], [41] suggested that the chemical composition depends on the composition and weathering conditions at the source rock area. An approach towards assessing detrital mineralogy is to use the Index of Compositional variation (ICV); defined as: [(CaO + K2O+Na2O+Fe2O3 (t) + MgO + MnO + TiO2) / Al2O3)] [34]. The average ICV values for the studied samples is ≈0.49, which is significantly lower than the average values for both UCC (1.27) and PAAS (0.89), indicating intense weathering in the source area. Reference [42] evaluated the degree of chemical weathering/alteration sediments source rocks by calculating the chemical index of alteration (CIA), where CIA molar (Al2O3/[Al2O3+CaO+Na2+K2O]). This index works well when Ca, Na and K decrease as the intensity of weathering increases [42]. Reference [43] proposed the chemical index of weathering (CIW) similar to the CIA except for K2O which is not in the equation. CIW= molar (Al2O3/[Al2O3+CaO+Na2O]). The CIA and CIW are interpreted in similar way with values of 50 for unweathered upper continental crust and roughly 100 for highly weathered materials with complete removal of alkali and alkaline-earth elements [44], [45]. Low CIA values (i.e. 50 or less) reflects cool and / or arid conditions [46]. The CIA (96.83-99.36) Av=98.65%, and CIW (97.34-99.87) Av= 99.25% contents for the Ode Irele soil samples yielded almost the same value indicating a high degree of weathering of the source materials. The results of the major elements geochemistry of the study area showed moderate values of SiO2 and relatively low quantity of K2O (K-feldspar) and Na2O (Na-feldspar) suggesting that the source rocks may have been exposed to weathering. However, low K2O in the soil may be ascribed to intense chemical weathering [47] which enhanced leaching of the soil and removal of alkalis in solution. Low K2O/Al2O3 ratio (0.09-0.5) caused by the presence of high proportion of kaolinite possibly led to the moderate to intense chemical weathering of the source rocks [48]. Fig. 4 shows the samples plotting at the Al2O3 (A) apex indicative of intense weathering.  from [49] A plot of CIA against Al2O3 in Fig. 5 indicates a high intensity of weathering in the area.
The intensity of chemical weathering can also be estimated using the plagioclase index of alteration (PIA) given in molecular proportion as PIA= (Al2O3-K2O) /[Al2O3+CaO+Na2-K2O]) x 100 where CaO* is the CaO residing in the silicate fraction [50]. In this study the PIA values range from 97.71 to 99.87% indicating high degree of weathering. Also, the high PIA values indicate that the plagioclase in the possible parent rock displayed high weathering condition which has resulted in low CaO content especially with increasing PIA values. The increased chemical weathering implies that the parent rocks are steadily depleted in plagioclase and enriched in aluminous clay minerals [51]. The mineralogical index of alteration (MIA) is another weathering parameter proposed by [52] and it is calculated as: MIA= 2*(MIA -50). MIA values between 0 and 20 % are designated as incipient, i.e just starting; 20-40 % (weak); 40-60 % (moderate), and 60-100 % intense to extreme degree of weathering. The extreme value of 100% indicates complete weathering of the primary material. The MIA values for the study area ranges from 93.66 to 98.72 % indicating that the source material has been subjected to intensive weathering thus confirming the earlier results of CIW, CIA and PIA The Th/U ratio can also be used to trace the weathering history because of oxidation and loss of U during the weathering process. The Th/U ratio (6.75) in the study area is strongly enriched relative to PAAS (4.76) implying a high degree of weathering of the source material, just as the ratio increases with kaolinite content [53]. However, Th/U ratio may not necessarily reflect source area weathering condition since the ratio can change due to different oxidation state [54], [55]. The high degree of weathering condition is affirmed by the plot of Th/U versus Th where the samples trend above the upper crust value (Fig. 6). Fig. 6. Th/U vs. Th plot for studied samples. Fields and trends from [50] C. Paleoenvironment and paleoclimate Many scholars have used certain trace elements in soils/sediments as powerful tools directly or indirectly to evaluate the paleo-redox conditions. Elements such as Cu, Zn, Ni, Cr, V, and U have been severally used by authors such as [53]- [59] and [60] to evaluate the paleoredox conditions. The ratio of U/Th is another redox indicator that is higher in organic -rich mudstone compared to sandstone [56]. U/Th ratio below 1.25 infers oxic conditions of deposition; values above 1.25 indicate suboxic and anoxic depositional conditions [56], [61]. The ratio of U/Th in the present study ranges between 0.14 and 0.50 with an average of 0.18 relative to PAAS (0.21) and UCC (0.26) implying a lower U.Th ratio that indicates deposition under oxic conditions. Several authors have used the Ni/Co ratios as a redox indicator [62]- [66]. The Ni/Co ratios below 5 indicate oxic environments, values above 5 indicate suboxic and anoxic environments [56]. The Ni/Co ratios for the soil under study vary between 3.80 and 9.02 (average = 6.91) showing deposition in reducing environments. However, only one sample with 3.80 as Ni/Co ratio indicates deposition in oxygenated environments. V/Cr ratio has been used as an index of paleo-oxygenation [67], [66] due to the incorporation of Cr in the detrital fraction of sediments and its possible substitution for Al in the clay structure [67], [68]. V/Cr ratios above 2 that is from 2 to 4.25 indicate dyoxic conditions and greater than 4.25 infer suboxic to anoxic conditions whereas values below 2 suggest oxidizing environments [56]. The V/Cr ratio of the present work ranges from 0.6 to 1.34 (average=1. 19) and below that of PAAS (1.36) indicating an oxic depositional environments. V/Sc ratio below 9.1 according to [69] indicate oxic depositional environment. V/Sc ratio for the studied soil is between 4.06 and 7.04 (average=6.10) indicating that the soil sediments were deposited under oxic condition. Furthermore, the Cu/Zn ratio is also used as a redox parameter. High Cu/Zn ratios imply reducing depositional conditions, whereas low Cu/Zn ratios suggest oxidizing conditions [70]. Cu/Zn ratio in this study varies between 0.25 to 0.65 with an average of 0.50 in relation to 0.59 of PAAS indicating that the soil of Ode Irele was deposited under oxidizing environments. The ratio (Cu+Mo)/Zn is shown to be a relevant paleoredox indicator for bottom water and sediment surface. For the stream sediments, (Cu+Mo)/Zn ratios is between 0.3 and 0.74, which suggests oxic paleoenvironment. Plots of V/V+ Ni versus Ni/Co and V/Cr versus Ni/Co can also be used to interpret the redox conditions. V/V+ Ni ratio greater than 0.84 suggests euxinic conditions, from 0.54 -0.82 represents anoxic conditions and 0.46-0.60 represents dysoxic conditions [71]. The V/Vi + Ni ratios for the soil range from 0.72 to 0.87 (average = 0.78) which indicate suboxic to anoxic environment of deposition. However, the plot of V/V+ Ni versus Ni/Co (Fig. 7) shows the samples falling into the oxic, dysoxic and suboxic to anoxic redox columns. V/Cr versus Ni/Co plot (Fig. 8) also shows the samples appearing in various fields (oxic to dysoxic and suboxic to anoxic environments), an indication that the sediments may have been deposited under oxic and suboxic conditions. Furtherance, the value of the ratio of V/Vi + Ni (0.72-0.83) presents a scenario in which the euxinic sediments was deposited in anaerobic -reducing environment (anoxic) where the euxinic bodies of water are strongly stratified having an oxic highly productive thin surface layer and anoxic sulfidic bottom water [72]. Fig. 9 shows the samples plotting in the marine-terrestrial oxic to dysoxic zone. The applicability of the ratios of SiO2/(Al2O3+K2O+Na2O) for paleoclimatic condition [73] during deposition of sediments in the basin is well recognized by many workers. The samples plotting mainly in the arid and semi-arid climatic conditions, which reflects paleoclimatic condition during the deposition of Ode Irele samples (Fig. 10). Fig. 11 indicates the samples plotting in the continental zone. According to [65], Sr and Ba are regarded as indicators of paleosalinity. A high Sr/Ba ratio reflects high salinity, and a low Sr/Ba ratio indicates low salinity [73]. The Ode Irele samples have a Sr/Ba ratio between 0.53 and 0.73 (average 0.65), indicating low saline water during deposition and a strong continental rather than marine influence.

IV. CONCLUSION
Weathering indices CIW, CIA, PIA, MIA, Th/U, the average ICV value (≈0.49), a plot of CIA against Al2O3 as well as the A-CN-K Ternary diagram indicate a high degree of weathering of the source rocks of the Ode Irele sediments. K2O/Al2O3 ratios implies prevalence of clay minerals relative to other minerals. Paleo-redox parameters points to deposition under oxic to suboxic conditions. The Ode Irele sediments was deposited in low saline water with a strong continental rather than marine influence in an arid and semi-arid climatic condition; the low average ratio of P2O5/Al2O3 (0.014) alludes to a possible freshwater environment.