Geochemical Behavior of Different Chemical Elements during Weathering of the Basalts in Changbai Mountain, Northeast China

: Rock weathering is the main source of element geochemical cycle, which has a very important impact on the environment. Three well-developed basalt weathering proﬁles in Changbai Mountain area are selected in this study, and the samples of parent rock, parent material layer, sedimentary layer, and leaching layer are systematically collected. The systematic study showed that the basalt in the study area experienced moderate chemical weathering under warm and humid climate conditions, with leaching of some major elements such as silicon and aluminum. The distribution of the rare earth elements (REE) in the weathering products recalls that of an alkali-basalt, with an evident fractionation between light- and heavy-REE. Such a feature is therefore referable to the parent volcanic rock, although with some degree of leaching.


Introduction
The geochemical behavior of elements during rock weathering has a great impact on the chemical composition, formation, and evolution of soil and sediments. It has become an important link connecting land and ocean, rivers and lakes, and paleoclimate and paleoenvironment. Weathering may be regarded as the premise and foundation of soil occurrence, and soil formation may be regarded as the continuation and deepening of weathering. In the process of weathering and soil formation, the factors affecting the distribution characteristics of soil elements mainly include parent rock and parent material, climatic conditions, hydrochemical properties, and redox conditions [1]. As known, basalt has a higher weathering rate than many other volcanic rocks. At present, basalt weathering research mainly focuses on mineralization, behavior of major and trace elements, and isotopic behavior caused by weathering in tropical climate (Hainan Island) and subtropical climate (Emeishan basalt) [2][3][4][5]. After desilication and aluminum weathering, basalt forms aluminum weathering crust, and gradually develops into iron-rich soil and iron aluminum soil. However, the subsequent enrichment of transition metal elements (such as Cr, Ni, V, Co, Mn, and Zn) may lead to significant environmental risks [6][7][8][9]. Alkali metals and alkaline earth metals with active chemical properties such as Na, Ca, and Mg are easy to lose during rock weathering. However, elements with stable chemical properties such as Th, U, Zr, Nb, Y, and rare earth elements are not easy to migrate [10][11][12][13][14][15][16][17][18][19]. In addition, relative behaviors of immobile elements during weathering are usually stable from rock to its weathered products. The stability of these relative behaviors is similar to the genetic inheritance in biology [20].
The research of Changbai Mountain basalt mainly focuses on the genesis [21], rock geochemistry, age [22,23], and evolution process of basalt [24]. At present, Changbai Mountain has become one of the important natural resource reserves in China. This study comprehensively analyzes a variety of weathering indexes with the help of geochemical methods and systematically discusses the weathering process of basalt in Changbai Mountain area. This study addresses the behavior of elements in the chemical weathering process of basalt in the study area, hence, revealing the impact of traces element enrichment and leaching on the surface soil.

Geological Setting and Location of Samples
Changbai Mountain is located in the east of the Central Asian orogenic belt. The region experienced the evolution of Paleo Asian Ocean tectonic system in Paleozoic, resulting in the final closure of microlandmass (including Erguna block, Xing'an block, Songnen zhangguangcailing block, Jiamusi block, and Xingkai block) and Paleozoic-early Mesozoic Paleoasian Ocean [25,26] (Figure 1). Cenozoic is the main active period of basalt magma. Since Cenozoic, a large number of basaltic volcanic rocks (trachyte basalt, trachyte basaltic andesite, tholeiite, and basaltic andesite) have been exposed in eastern China [27,28].
The study area is located in the east of Jilin Province, covering Baishan and Yanbian ( Figure 1). The ground is high in the southeast and low in the northwest, with an obvious erosion and cutting effect. The study area has a mid-temperate, continental, humid monsoon climate, with an annual average temperature of 3-5 • C, an average temperature of −18 • C in January, and an average temperature of 23 • C in July. The annual precipitation is mainly concentrated from June to August (about 1100 mm), accounting for about 60% of the total annual precipitation. The soil types in the study area mainly include brown coniferous forest soil, dark brown soil, and albic soil. In the vertical zone of Changbai Mountain, the soil of brown coniferous forest is scattered under the coniferous forest with an altitude of 1100-1800 m. Dark brown soil is mainly distributed in the East and southeast of Jilin Province, with an altitude of 500-1200 m. In addition, albic soil is widely distributed in mountainous, semimountainous, and valley terraces in the east and southeast of Jilin Province.
The study samples were taken from three well-developed basalt weathering profiles in the upper reaches of the Songhua River. The location of the soil profile is shown in Table 1. After discarding the humus layer, the weathering profile is divided into four layers from top to bottom, with an average depth of 1.5 m. The different horizons in Figure 2 are called horizons A, B, C, and R. Layer A is the leaching layer: sampling depth is about 5~30 cm; soil in this layer is dark and rich in organic matters. Layer B is the sedimentary layer: sampling depth is about 30~70 cm; soil in this layer is brown and rich in organic matters and microconglomerate. Layer C is the parent material layer: sampling depth is about 70~120 cm; soil in this layer is light brown, containing massive weathering rocks and residual debris. Layer R is parent rock: grey vesiculate basalt of this layer was collected from depths of 120~150 cm.

Materials and Methods
As a first step, soil samples were placed in a glass tray after removing foreign matters such as branches, rods, and leaves, and fully mixed. The sample was crushed to less than 200 mesh by the crusher and sieved. The sieved sample was put into the dryer for cooling. The loss on ignition was determined by the mass loss after calcination at 1000 • C in muffle furnace. According to HJ780-2015 standard, the contents of SiO 2 , Al 2 O 3 , MgO, Fe 2 O 3 , CaO, Na 2 O, K 2 O, P 2 O 5 , TiO 2 , and MnO were determined by powder compression X-ray fluorescence spectrometry (EDX6000b). According to GB/T 14506-2010, the contents of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu were determined by ICP-MS (NEXION350d, PerkinElmer) and sodium peroxide molten samples. In addition, lead, rubidium, strontium, barium, zirconium, niobium, thorium, yttrium, and uranium were determined by closed acid digestion ICP-MS. According to HJ680-2013 standard, arsenic and mercury were determined by potassium borohydride reduction atomic fluorescence spectrometry in aqua regia solution.
Aforementioned analytical methods fulfil the requirements of the Chinese geological and mineral standard DZ/T0011-2015.

Results
The mass fraction and composition of main elements are the most commonly used indicators in the study of weathering. The order of average concentration of the soil profile samples is SiO 2 Table 2). The SiO 2 , Al 2 O 3 , and Fe 2 O 3 are the three elements with the highest content in all samples (the sum exceeds 80%). The total content of rare earth elements (REE) in the samples in the study area is relatively high (Tables 3 and 4). However, Yb and Lu are lower than the background value, the content of rare earth elements in other samples is 1-2.5 times higher than the background value; further, except Ho, Er, Tm, Yb, and Lu, rare earth elements in other samples are lost layer-by-layer from parent rock to leaching layer during weathering, and the loss order is LREE > HREE.
The average value of Sr, Ba, and Nb in the parent rock is higher than that of the setting, and the content of other value is lower than that of the setting ( Table 5). The average value of Ba, Nb, and Y in the parent material layer is higher than that of the setting, and the value of other elements are lower than that of the setting. The average value of Rb, Ba, and Y in the sedimentary layer is higher than that of the setting, and the value of other elements are lower than that of the setting. The average value of Pb and Ba in the leaching layer is higher than that of the setting, and the value of other elements are lower than that of the setting.

Behavior of the Major Elements
With the formation of new mineral assemblage after rock fracture, the main elements of basalt in the study area experienced the process of desalting basement-enrichment of SiO 2 and Al 2 O 3 from parent rock to parent material layer in the early stage of weathering (except silicon and aluminum, other main elements were leached in varying degrees;  Figure 3). In the profiles JLP2, the concentrations of SiO 2 and Al 2 O 3 in parent rocks are 51.77% and 19.20%, while they are 56.12% and 21.24% in the parent material layer. In the profiles JLP3, the concentrations of SiO 2 and Al 2 O 3 in parent rocks are 53.23% and 15.96%, while they are 63.07% and 17.74% in parent material layer. From the parent material layer to the sedimentary layer, weathering experienced silicon enrichment and dealumination (except silicon, other main elements were leached, in which aluminum leaching was strong). In the highest stage of weathering, except manganese, the behavior of other elements from sedimentary layer to leaching layer tends to be stable. Due to acid leaching, Mn is relatively enriched in the leaching layer. Among the three groups of basalt weathering profiles, the change trend of main elements in JLP3 profile is slower than that in the other two groups, suggesting that higher altitude will lead to weaker weathering degree.
Chemical weathering index is usually used to describe the behavior of elements during weathering. Existing studies have shown that there are more than 30 different chemical weathering indexes, including those for the study of rocks and their weathering products [32][33][34][35]. Generally, chemical weathering index can be divided into two categories: normalized index and non-normalized index. Due to the lack of chemical analyses on bedrock, in the study case, the application of normalization indexes is not possible. The widely used nonstandardized indexes include Parker weathering index (WIP) [36], chemical alteration index (CIA) [37], chemical weathering index (CIW) [38], Coleman weathering index (WIC) [39], and plagioclase alteration index (PIA) [40]. Due to the lack of CO 2 content data in this study, Cao* (CaO in silicate) is calculated in the absence of CO 2 content according to the correction method of McLennan [41]. Parker and CIA in the non-normalized index are used to characterize the element behavior during the weathering of Changbai Mountain basalt.
From the results of major element analysis (Table 1), it can be seen that SiO 2 is enriched in the weathering process, while K 2 O, Na 2 O, and CaO are obviously leached, which is consistent with the characteristics that SiO 2 tends to be active in the rock weathering process. Its total leaching rate is usually very small, and alkali metals and alkaline earth metals will be significantly lost. Therefore, the study area is more in line with the application conditions of Parker index.
Nesbitt and Young proposed the chemical alteration index (CIA) for the first time. According to the early Proterozoic sediments of Huron supergroup in the north of Huron, they reconstructed the paleoclimate [37], which has been widely used at present. According to Nesbitt theory, when the CIA index is between 50-65, it means cold and dry climate conditions, and the weathering in the source area belongs to low chemical weathering degree; when the CIA index is between 65 and 85, the provenance area is in warm and humid climate, and the weathering is of medium chemical weathering degree; when the CIA index is between 85 and 100, the weathering in the provenance area belongs to tropical or subtropical humid and hot climate conditions, and the weathering belongs to strong chemical weathering. The calculation method is as follows: The oxide content is determined by mole fraction. Cao* refers to Cao in silicate-that is, the mole fraction of Cao removed from rock by chemical deposition. The calculation method is as follows: Chemical weathering index is usually used to describe the behavior of elements during weathering. Existing studies have shown that there are more than 30 different chemical weathering indexes, including those for the study of rocks and their weathering products [32][33][34][35]. Generally, chemical weathering index can be divided into two categories normalized index and non-normalized index. Due to the lack of chemical analyses on bedrock, in the study case, the application of normalization indexes is not possible. The widely used nonstandardized indexes include Parker weathering index (WIP) [36], chemical alteration index (CIA) [37], chemical weathering index (CIW) [38], Coleman weathering index (WIC) [39], and plagioclase alteration index (PIA) [40]. Due to the lack of CO2 content data in this study, Cao* (CaO in silicate) is calculated in the absence of CO2 content according to the correction method of McLennan [41]. Parker and CIA in the non-normalized index are used to characterize the element behavior during the weathering of Changbai Mountain basalt.
From the results of major element analysis (Table 1), it can be seen that SiO2 is enriched in the weathering process, while K2O, Na2O, and CaO are obviously leached, which is consistent with the characteristics that SiO2 tends to be active in the rock weathering process. Its total leaching rate is usually very small, and alkali metals and alkaline earth metals will be significantly lost. Therefore, the study area is more in line with the application conditions of Parker index. This is example 1 of an equation: Parker index = (2Na2O/0.35 + MgO/0.9 + 2K2O/0.25 + CaO/0.7) × 100.
Generally, the Parker index reflects the weathering degree, and the corresponding is inversely correlated with the weathering degree. The index of parent material layer is 33. 33 Table 2). Most of the three sections have the characteristics of increasing CIA index from leaching layer to parent material layer. According to the above classification, the CIA index of the parent material layer, sedimentary layer, and leaching layer ranges from 65 to 85, indicating that the climatic conditions in the area are warm and humid, belonging to moderate chemical weathering.
Compared with Changbai Mountain basalt profile, the CIA index of soil samples in Hainan basalt weathering profile is 93.98-97.78, indicating that Hainan basalt is strongly chemically weathered with the enrichment of desilication, aluminum, and iron. Under extreme environmental conditions, the influence of parent rock on the development strength of weathered crust is weakened, and even covered by climatic conditions, resulting in the formation of weathered crust in the stage of medium lateritic or even strong lateritic [2,42].
According to the above weathering index calculation, the parent rock weathering degree of the three sections is low. Therefore, the element mobility M = 100 can be determined × (The enrichment or leaching rates of elements in different profiles are calculated by RI-RP/RP) formula. In the same section, RI represents the X element content in weathered samples, and RP represents the X element content of parent rock samples. In the parent material layer, sedimentary layer, and leaching layer of the three sections, the cumulative mobility of SiO 2 is the highest (Figure 4), indicating its enrichment characteristics in the weathering process, while other elements are leached in the weathering process.
The average values of Cao element mobility in layers A, B, and C are −0.79, −0.83, and −0.74, respectively, indicating that Cao is most seriously lost during weathering (Table 6; Figure 3). K 2 O mobility fluctuates the least, and the average value of the three profiles is 0, indicating that its mobility is the smallest and most stable in the weathering process, and the stability of Al 2 O 3 and TiO 2 is second only to K 2 O. As the weathering resistance of minerals is opposite to the Bowen reaction order, the higher the crystallization temperature of minerals, the lower the stability of minerals. Nepheline and zeolite are sometimes formed in the matrix together with alkali feldspar and/or potassium feldspar. Potassium in basalt is mainly enriched in medium feldspar, potassium feldspar, or biotite. Due to its strong stability, K element fluctuations are relatively low and stable in the weathering process.
Through the calculation of main element mobility, it can be found that K2O is the most stable in the study area. Therefore, mass balance calculation can be used in this study.
τi,j = (Cj,w/Cj,p)/( Ci,w/Ci,p) − 1 (4) Formula (4) is widely used to evaluate the migration and redistribution of elements during the formation of chemical weathering profiles [43][44][45]. The element gain and loss in the profile can be described by the change of the ratio of elements to inactive elements relative to the ratio in the parent rock. Cj,w and Cj,p are the concentrations of a certain element J in soil and fresh parent rock, respectively. Ci,w and Ci,p are the most stable concentrations of element I in soil and fresh parent rock (K2O is selected as element I in this study). If τi,j < 0, it reflects the migration or net loss of elements in the profile relative to the parent rock; if τi,j > 0, the elements are relatively enriched or net obtained in the profile; if τi,j= 0, the behavior of elements in the profile is relatively stable.
The enriched elements (average enrichment rate) in the parent material layer of the three sections are SiO2 > Al2O3 > Fe2O3, while the lost elements (average loss rate) are Cao > Na2O > MgO > MnO > P2O5 > TiO2 ( Table 6). The enriched sedimentary layer is SiO2, and the lost elements are Cao > P2O5 > MgO = Na2O > TiO2 > Fe2O3 > MnO > Al2O3. In the leaching layer, SiO2 is enriched, and the lost elements are Cao > P2O5 > MgO > Na2O > Fe2O3 > TiO2 > MnO > Al2O3.   As the weathering resistance of minerals is opposite to the Bowen reaction order, the higher the crystallization temperature of minerals, the lower the stability of minerals. Nepheline and zeolite are sometimes formed in the matrix together with alkali feldspar and/or potassium feldspar. Potassium in basalt is mainly enriched in medium feldspar, potassium feldspar, or biotite. Due to its strong stability, K element fluctuations are relatively low and stable in the weathering process.
Through the calculation of main element mobility, it can be found that K 2 O is the most stable in the study area. Therefore, mass balance calculation can be used in this study.
Formula (4) is widely used to evaluate the migration and redistribution of elements during the formation of chemical weathering profiles [43][44][45]. The element gain and loss in the profile can be described by the change of the ratio of elements to inactive elements relative to the ratio in the parent rock. C j,w and C j,p are the concentrations of a certain element J in soil and fresh parent rock, respectively. C i,w and C i,p are the most stable concentrations of element I in soil and fresh parent rock (K 2 O is selected as element I in this study). If τ i,j < 0, it reflects the migration or net loss of elements in the profile relative to the parent rock; if τ i,j > 0, the elements are relatively enriched or net obtained in the profile; if τ i,j = 0, the behavior of elements in the profile is relatively stable.
The enriched elements (average enrichment rate) in the parent material layer of the three sections are SiO 2 > Al 2 O 3 > Fe 2 O 3 , while the lost elements (average loss rate) are Cao > Na 2 O > MgO > MnO > P 2 O 5 > TiO 2 ( Table 6). The enriched sedimentary layer is SiO 2 , and the lost elements are Cao > P 2 O 5 > MgO = Na 2 O > TiO 2 > Fe 2 O 3 > MnO > Al 2 O 3 . In the leaching layer, SiO 2 is enriched, and the lost elements are Cao > P 2 O 5 > MgO > Na 2 O > Fe 2 O 3 > TiO 2 > MnO > Al 2 O 3 .

Behavior of the Trace Elements
Rare earth elements (REE) have special geochemical properties and are generally considered inactive in supergene geological processes. The information of source rocks carried by REE will not be lost, so it is widely used in sediment source tracking. Nevertheless, we believe that rare earth elements are active in the process of chemical weathering and they may be enriched, migrated, and differentiated in the weathering profile. The basic rocks contain a large amount of feldspar and pyroxene, LREE tends to be enriched in feldspar and biotite, while HREE tends to be enriched in amphibole, pyroxene, and garnet.
The content of rare earth elements in basalt weathering products in the study area has obvious inheritance with the parent rock, but the total amount of rare earth elements in weathering products has leaching tendency. The REE pattern of the samples normalized to chondrite values displays a downslope trend (the differentiation of light and heavy REE is obvious; Figure 5), similar to the characteristics of ocean island basalt (OIB). The content of rare earth elements in parent rock samples is (241~271) × 10 −6 , with an average of 257 × 10 −6 ; ΣLREE is (223~248) × 10 −6 , average 237 × 10 −6 ; ΣHREE is (17.6~22.5) × 10 −6 , with an average of 20.5 × 10 −6 . The content of rare earth elements in weathering products (parent material layer, sedimentary layer and leaching layer) is (169~303) × 10 −6 , with an average of 230 × 10 −6 ; ΣLREE is (153~280) × 10 −6 , with an average of 211 × 10 −6 ; ΣHREE value is (15.89~23.4) × 10 −6 , with an average of 19.4 × 10 −6 (Tables 3 and 4). The REE characteristics of weathering products are compared with those of parent rocks, suggesting that they have inheritance properties. LREE/HREE of parent rock is between 11.0 and 12.7, with an average of 11.6; the range of (La/Sm) N is between 3.27 and 4.65, and the range of (La/Yb) N is between 15.2 and 26.6. LREE/HREE of weathering products is between 8.43 and 14.3, with an average value of 10.9; the range of (La/Sm) N is between 8.30 and 17.5. The fractionation of LREE and HREE in the parent rock is obvious, but the (La/Yb) N gradually decreases with the strengthening of weathering, and the fractionation degree of LREE and HREE decreases. The δEu of the parent rock is between 0.93 and 1.05, with an average value of 1.0. There is no Eu anomaly, indicating that plagioclase did not undergo obvious fractional crystallization during magmatic evolution. The δEu of Weathering products is between 0.56 and 0.93, with an average value of 0.80. There is an obvious negative anomaly in Eu. Under reduction conditions, Eu mainly exists in the form of Eu 2+ , which leads to Eu showing different geochemical behavior from other trivalent rare earth elements. In the geological and geochemical process, uranium is separated from other rare earth elements to form positive or negative uranium anomalies. The formation of partially reduced acidic environment at the redox interface after basalt weathering may be the main cause of Eu negative anomaly [46].
Large ion lithophile elements (LILE, Ba, Sr, Rb, K) can form independent minerals and are soluble in water. Their chemical properties are active and easy to move after weathering. At the initial stage of weathering, Ba and Sr in the study area are largely lost from the parent rock to the parent metal layer. With the deepening of weathering, their contents are gradually stable. The content of Ba in the parent rock is 2.44 times the background value; the content of leaching layer is 1.52 times of the background value; and Rb is enriched with weathering ( Figure 6). K is most stable during weathering (Figure 3). Large ion lithophile elements (LILE, Ba, Sr, Rb, K) can form independent minerals and are soluble in water. Their chemical properties are active and easy to move after weathering. At the initial stage of weathering, Ba and Sr in the study area are largely lost from the parent rock to the parent metal layer. With the deepening of weathering, their contents are gradually stable. The content of Ba in the parent rock is 2.44 times the background value; the content of leaching layer is 1.52 times of the background value; and Rb is enriched with weathering ( Figure 6). K is most stable during weathering (Figure 3). High field strength elements (HFSE, Zr, Nb, Th, Y, U) generally have stable chemical properties, high electricity price, ion field strength, and small radius and are generally immobile during geological processes such as metamorphism and different types of alteration, including weathering. Therefore, they are usually used to restore the original properties of rocks after later changes. During weathering, five high field strength elements in the study area show different behaviors. Zr and Nb are leached; the average movement of Zr is the smallest; all tested high field strength elements are relatively stable, while Th, Y, and U are slightly enriched, among which the average movements of Th and U are the largest and most unstable among all tested high field strength elements (Figure 7). High field strength elements (HFSE, Zr, Nb, Th, Y, U) generally have stable chemical properties, high electricity price, ion field strength, and small radius and are generally immobile during geological processes such as metamorphism and different types of alteration, including weathering. Therefore, they are usually used to restore the original properties of rocks after later changes. During weathering, five high field strength elements in the study area show different behaviors. Zr and Nb are leached; the average movement of Zr is the smallest; all tested high field strength elements are relatively stable, while Th, Y, and U are slightly enriched, among which the average movements of Th and U are the largest and most unstable among all tested high field strength elements (Figure 7). High field strength elements (HFSE, Zr, Nb, Th, Y, U) generally have stable chemical properties, high electricity price, ion field strength, and small radius and are generally immobile during geological processes such as metamorphism and different types of alteration, including weathering. Therefore, they are usually used to restore the original properties of rocks after later changes. During weathering, five high field strength elements in the study area show different behaviors. Zr and Nb are leached; the average movement of Zr is the smallest; all tested high field strength elements are relatively stable, while Th, Y, and U are slightly enriched, among which the average movements of Th and U are the largest and most unstable among all tested high field strength elements (Figure 7).  In this study, there are differences in the concentrations of SiO 2 between the parent rocks of these three profiles and basalt. However, the characteristics of trace elements of parent rocks from these profiles and different generations of basalts from the Changbai mountain area are comparable on the spider map ( Figure 8). In terms of LILE, both of them are enriched in Ba and Sr, but depleted in Rb and K. With regard to HFSE, they are enriched in Nb, while depleted in U and Y. The characteristics of trace elements support that the parent rocks belong to basalt. In this study, there are differences in the concentrations of SiO2 between the parent rocks of these three profiles and basalt. However, the characteristics of trace elements of parent rocks from these profiles and different generations of basalts from the Changbai mountain area are comparable on the spider map ( Figure 8). In terms of LILE, both of them are enriched in Ba and Sr, but depleted in Rb and K. With regard to HFSE, they are enriched in Nb, while depleted in U and Y. The characteristics of trace elements support that the parent rocks belong to basalt.  [48]. XNT11-002, JJXT12-01, WCH003, and SW-02 reference values after [48].

Conclusions
The weathering of basalt in Changbai Mountain belongs to moderate weathering under warm and humid climate and forming silica rich soil in the leaching layer. In the weathering process, the enrichment rate of SiO2 is the highest, the leaching loss of CaO is the most serious, and K2O is the most stable.
The distribution of the rare earth elements (REE) in the weathering products, normalized to chondrite values, closely recalls that of an OIB-type basalt, with an evident Figure 8. Trace element spider diagram. Chondrite and OIB reference values after [48]. XNT11-002, JJXT12-01, WCH003, and SW-02 reference values after [48].

Conclusions
The weathering of basalt in Changbai Mountain belongs to moderate weathering under warm and humid climate and forming silica rich soil in the leaching layer. In the weathering process, the enrichment rate of SiO 2 is the highest, the leaching loss of CaO is the most serious, and K 2 O is the most stable.
The distribution of the rare earth elements (REE) in the weathering products, normalized to chondrite values, closely recalls that of an OIB-type basalt, with an evident fractionation of LREE with respect to HREE. Such a feature is therefore referable to the fresh parent volcanic rock, which is thought to be an alkaline basalt. Nevertheless, some degree of leaching has occurred during weathering processes in the order of LREE > HREE. Therefore, LREE and HREE fractionation decreased with the increase in weathering degree.
Large ion lithophile elements (LILE), such as Ba, Sr, and Rb, are very mobile in the weathering process, having been either enriched or leached with respect to the original volcanic rock distribution.
High field strength elements (HFSE), such as Zr, Nb, Th, Y, and U, although much less mobile than LILE, display a different mobility degree during the weathering processes, with Zr and Nb being generally more stable than Th and U.

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Data Availability Statement:
The data presented in this study are available on request from the corresponding author.