Magnetic Susceptibility as a Geophysical Proxy for Rare Earth Elements (REEs) in Volcanic Deposits of Semeru Volcano, Indonesia

The sample collection location is situated in the Mount Semeru area. Mount Semeru is the highest mountain on the island of Java, located in the same area as Mount Bromo, forming the Bromo-Tengger-Semeru mountain range (BTS). Mount Semeru is one of the Quaternary volcanoes, with volcanic activity still active on Mount Semeru [27, 28]. The selection of Mount Semeru as the sampling site is based on the numerous studies that have been conducted in this location, but mapping of rare earth elements (REEs) is still very rarely done. One of the geochemistry studies conducted on Mount Semeru was the investigation of Europium (Eu) and the comparison of the Arjuno-Welirang elements with Bromo-Semeru [29, 30].

The samples in this study were volcanic material, including andesite, sand, sandy soils, and volcanic ash taken from the Semeru area, with four sampling points, with a total of 13 samples used in this research. The details of the sampling points are as follows: P1 (consisting of samples PS 1.2, PS 1.3, and PS 1.5), P2 (consisting of samples PS 2.1, PS 2.2, PS 2.3, PS 2.4, and PS 2.5), P3 (consisting of samples PS 3.1, PS 3.2, and PS 3.3), and P4 (consisting of samples PS 4.1 and PS 4.2) (see Figure 1). Geologically, Mount Semeru is dominated by pyroclastic, igneous rock and andesitic lava [31]. The sample collection locations on Mount Semeru are situated within two main lithologies that make up Mount Semeru. Samples PS 1.2–PS 1.5 and PS 2.1–PS 2.5 are located in the geological formation of the Volcanic Deposits of Semeru (Qvs). Samples PS 1.2–PS 1.5 are associated with the Wuni Formation (Tmw). This sample is located on the southern slope of Mount Semeru, 11 km from the summit of Mount Semeru. Generally, the sampling location is a river that becomes a flow of lahar and lava when Mount Semeru erupts. While samples PS 2.1–PS 2.5 are associated with the Mandalika Formation (Tomm), this sample location is close to the P1 sampling site, specifically on the southeast side of the summit of Mount Semeru. Similar to the P1 sampling site, this location is a river area that becomes a flow path for lahar and lava when Mount Semeru erupts. This condition allows for the mixing of sedimentation from various geological formation due to geological formations due to erosion during eruptions, carried by the lahar [32]. Therefore, it is very possible that the samples taken not only represent the local geology but are also representative of the volcanic material from Mount Semeru. Two other sampling locations, namely PS 3.1–PS 3.3 and PS 4.1–PS 4.2, are located in the Volcano Deposits of the Jembangan formation (Qvj), with samples PS 4.1–PS 4.2 being associated with the Volcanic Deposits of the Tengger formation (Qvt) [31]. Sampling at points P3 and P4 was taken from rock outcrops that are products of volcanic activity in the Mount Semeru area (see Figure 2(a)). There are outcrops of andesite rock, which is an extrusive igneous rock. Unlike the P1 and P2 samples, which tend to be transported sedimentary material, the samples from points P3 and P4 are bedrock. Sampling at points P3 and P4 makes the collected samples representative because the rock outcrops found are the original magma products of long-standing volcanic processes. Additionally, samples from this rock outcrop are free from mixing or alteration processes caused by transported sediments from lahar or river flows.

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Figure 1. Sampling location of Semeru volcanic materials

The samples taken from the field were obtained from outcrops found in the field. Then, samples were taken by 1000 g and placed in a plastic bag for transport to the laboratory (see Figure 2(a)). Each sample is then prepared in the laboratory for magnetic susceptibility measurements and sample preparation for REE determination. In the laboratory, the sample was dried at room temperature, ground into powder, and inserted into a standard cylindrical plastic holder for magnetic measurement (see Figure 2(b)). For optimal sample measurement, the grain size of the powdered sample is ensured to reach 200 mesh. Samples were weighed using a digital weight balance. The magnetic susceptibility of all samples was measured at two frequencies using the Bartington Magnetic Susceptibility Meter MS2B with frequencies of 470 Hz and 4700 Hz (see Figure 2(c)). Frequency-dependent susceptibility $\left(\chi_{f d}\right)$ is defined using the following Eq. (1):

$\chi_{f d}(\%)=\left[\frac{\chi_{470-} \chi_{4700}}{\chi_{470}}\right] \times 100 \%$             (1)

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Several representative samples were prepared from the remaining powder to analyze REE elements using inductively coupled plasma-optical emission spectrometry (ICP OES). The presence of REE can be determined using Geochemical methods such as ICP OES and ICP MS [1]. REE analysis in rocks and minerals with ICP MS was also carried out by Sindern [33]. Measurement using ICP-OES has the advantage of high analytical accuracy and relatively short processing time, enabling the analysis of all elements quickly. The measurement of REE elements with ICP-OES requires the decomposition of solid samples to dissolve minerals into a solution containing metals. This process generally involves a mixture of HF, HNO₃, and HClO₄ acids [34]. REE measurements using ICP OES were conducted at the Laboratory for Testing of Nuclear Mineral Resources of the National Research and Innovation Agency (BRIN). Trace element analysis was conducted using a PerkinElmer ICP-OES 8300 system. The instrument features an argon plasma source, a peristaltic pump for sample introduction, and an optical emission spectrometer. Argon and Nitrogen gases were supplied via regulators, and a chiller maintained optimal operating temperatures. The instrument detection limits (IDL) for Tb are 0.9, for Pr, Gd, Nd, Ce, Sm, and Dy are 0.10, for La and Eu are 0.12, and for Y are 0.13, with a measurement uncertainty of 0.20.

For analysis, a new method was established within the Winlab 32 software. This involved selecting the target rare earth elements (REEs) and their corresponding wavelengths. Instrumental parameters were configured with a delay time of 20 seconds and a replicate count of 3 or 7. A flush time of 15 seconds was applied for the sampler and peristaltic pump. Calibration standards were prepared and introduced sequentially after blank analysis. Sample concentrations were determined by analyzing samples against the established calibration curve. All data acquisition and processing were performed using the Winlab 32 software.

Both magnetic susceptibility and geochemistry REE data were then analyzed using Pearson's correlation. This analysis is used to evaluate the linear relationship between two variables, thereby providing information on how strongly the physical and chemical parameters of volcanic material are correlated. It also serves to identify and understand correlation patterns to be used as proxy indicators. In this study, Pearson correlation analysis is applied to examine the possibility of using magnetic susceptibility data as a proxy indicator for the presence of REE in volcanic materials.

Based on the test results for the REE content in the Semeru volcanic material, it can be determined that the light REE (LREE) content of Semeru includes La, Ce, Pr, Nd, Sm, and Eu. Meanwhile, Semeru's heavy REE (HREE) includes Gd, Tb, Dy, and Y. From the entire Semeru REE data, the order of the average REE content is from largest to smallest (see Figure 3).

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Figure 3. The trend of Semeru REE content from largest to smallest

Figure 3 shows that the largest element content is La, followed by Ce, Gd, Pr, Nd, Tb, Sm, Eu, and the smallest is Dy. This data provides information that LREE (average 70.12 ppm) in Semeru is more dominant than HREE in Semeru (average 27.38 ppm). This pattern is generally shared by samples originating from the Earth's mantle [38]. The same analysis of oceanic or continental arc samples, which include basalt, basaltic andesite, dacite, and rhyolite, also shows that the largest REE content is La and the smallest is Dy [39].

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Figure 4. Plot Sr/Y-SiO₂ shows that the two samples containing Y are in the range below the barren intrusions [40]

The sample category in the oceanic or continental arc rock type is also confirmed by the Sr/Y-SiO₂ plot analysis (shown in Figure 4). There are two samples of volcanic material containing Yttrium. Figure 4 shows that two materials contain Yttrium, showing that this material is still under barren intrusion. According to Loucks [40], this volcanic material probably came from oceanic or continental arc basalt, basaltic andesite, dacite, and rhyolite rocks. It also confirmed Semeru's location, that is, in the ring of fire where subduction between the Eurasia and Indo-Australia plates exists [40].

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Based on the relation between Europium and Silicon oxide content in the Eu-SiO₂ graph (see Figure 5), it can be seen that there are two types of distribution of Semeru volcanic material based on Eu(n)/Eu*. Europium (Eu) can generally effectively determine the possibility of sedimentary rocks acquiring REE from their parent rocks [42]. Through the anomaly of Eu normalization values, both positive and negative anomalies are produced. Positive anomalies are almost shown by most of the Semeru samples. This provides information that the Semeru samples are an accumulation of feldspar (Eu(n)/Eu*>1) [43]. Meanwhile, negative anomalies are shown by samples PS 1.5 (andesite), PS 2.5 (sandy soils), and PS 4.2 (sandy soils), indicating the presence of plagioclase/K-feldspar fractionation in the magma [44]. In general, the pattern shown in the Semeru samples on the graph (Figure 5) forms a vertical pattern with a SiO₂ range of 40-50 wt%. Although based on the Eu content, this material does not show a unity, based on SiO₂, this material confirms the existence of a unity.

Rocks with Eu(n)/Eu* > 1 are likely to contain a significant accumulation of feldspar or plagioclase [45]. This impacts the main factors used to identify mineral phases, whether crystallization processes or accumulation, to determine REE characteristics (both HREE and LREE) [46, 47]. Henderson [48] provided distribution coefficients indicating that clinopyroxene generally tends to accumulate HREE. Meanwhile, plagioclase tends to accumulate LREE more than HREE [48]. Additionally, plagioclase naturally and significantly fractionates Eu [49]. This phenomenon occurs because Eu easily binds as Eu²⁺ in feldspar [43]. Therefore, rocks enriched in plagioclase tend to show a positive Eu anomaly when accumulation occurs [43, 49]. If this is related to what happens in the Semeru sample, there is an enrichment of LREE caused by the accumulation of feldspar with a positive Eu anomaly.

Table 3. Pearson correlations between magnetic susceptibility and rare earth elements

Pearson's correlation analysis, shown in Table 3, explores the relation between REE and magnetic susceptibility. The relations between magnetic susceptibility and REE content in Semeru volcanic material can be used as a proxy indicator for the presence of REE. The magnetic susceptibility range of the Semeru volcanic material shows similarities with other volcanic materials, such as Bromo [50]. This result reinforces the data, as the samples used are still similar since both Bromo and Semeru are located in the same area.

Pearson correlation analysis between REE and magnetic susceptibility shows that $\chi_{ {lf}}$ is significantly correlated with several elements such as Pr, La, Gd, Nd, Ce, and Sm. $\chi_{ {lf}}$ is also positively correlated with Tb with R 0.906 with a significance of 0.01 (See details in Table 3). Several elements are significantly correlated, for example, Ce vs Pr, La vs Pr, and Pr vs Nd. Low-frequency magnetic susceptibility $\chi_{ {lf}}$ also significantly correlates with total light REE (LREE). The correlation between $\chi_{ {lf}}$ and LREE shows a significant inverse correlation.

The significant inverse correlation between$\chi_{ {lf}}$ and LREE can be explained as follows. Based on rock classification from SiO₂ content, the average SiO₂ content of Semeru volcanic samples shows an average of <45 wt%, thus classified as an ultramafic rock type [29, 30]. According to Henderson [51], ultramafic rocks are not the type of rocks that carry large amounts of REE. The REE content in this type of rock naturally ranges only from 69–1455 ppm [51]. Naturally, one of the main constituents of ultramafic rocks is the mineral olivine [52]. Olivine ((Mg,Fe)₂SiO₄) is generally enriched in Mg and Fe as its main components, with Si <45 wt%. The REE content in olivine is very small; however, as a silicate, olivine may still be associated with REE. Some silicates associated with REE include zircon (ZrSiO₄) which contains many HREE (Y, Er, Yb, Lu), titanite (CaTiSiO₅) which stores large amounts of LREE (La, Ce dan Nd), allanite which is very rich in REE, and monazite ((Ce, La, Pr, Nd, Th, Y) PO₄) which is not technically a silicate but in nature often associates with silicate rocks and serves as a major carrier of REE [2]. Furthermore, monazite and zircon are the main REE minerals found in Indonesia [53]. Meanwhile, those four silicates are complex silicates capable of storing large amounts of REE [54, 55]. whereas olivine is the simplest silicate, so its REE content is only small or limited [56].

The finding of an inverse correlation between $\chi_{ {lf}}$ and LREE, as well as total REE, can be interpreted as the influence of the dominance of olivine, one of the constituent minerals of ultramafic rocks. The Mg-Fe content in olivine is associated with high magnetic susceptibility values but suppresses REE and LREE concentrations, making them lower. Thus, the higher the $\chi_{ {lf}}$, the lower the total REE and LREE content. The role of olivine in the Semeru volcanic samples is supported by sampling points P3 and P4, where both points are geologically composed of pyroxene-olivine basaltic lava from the Qvj formation (see Figure 1). In addition, this is possible because the volcanic material underwent alteration, leading to a higher concentration of LREE elements due to the mobilization of hot fluids [57, 58]. This process causes the magnetic minerals to experience a dilution effect [59]. Additionally, in general, the LREE elements have paramagnetic properties, and some of them have diamagnetic properties, such as La [60, 61]. This result certainly strengthens the analysis of the Eu anomaly in the volcanic material sample from Mount Semeru, which has accumulated feldspar or plagioclase, thus showing a positive anomaly [45]. This phenomenon provides additional information: although the correlation between $\chi_{ {lf}}$ and total REE and LREE are inverse, the presence of a positive Eu anomaly indicates that plagioclase fractionation has not been significant [62].

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The plot of the relation between several rare earth elements and magnetic susceptibility can be seen in Figure 6. Magnetic susceptibility is not significantly correlated with heavy REE (HREE), where the greater the susceptibility, the total HREE almost does not change. Plot $\chi_{ {lf}}$ vs LREE, HREE, and the TotREE are shown in Figure 6. Based on Figures 6, which show the relationship between $\chi_{ {lf}}$ and total REE and LREE, the quadratic regression pattern reveals a consistent downward trend. This pattern reinforces the previous finding that when $\chi_{ {lf}}$ is high, the total REE and LREE concentrations decrease or remain low.

This study has limitations due to the number of sampling points obtained in the field. This results in spatial grid coverage that is still insufficiently dense for such a wide research area. However, the findings of this study can serve as an initial insight into the REE characteristics of volcanic materials in the Semeru area. Further research can be developed by increasing the number of systematically distributed sampling grid points. This is necessary to validate the REE characteristics of the Semeru area to develop a comprehensive distribution map.

This work was supported by Universitas Negeri Malang through Indonesian Research Collaboration (RKI) 2023 funding with the grant number 10.5.43/UN32.20.1/LT/2023. Some samples were taken with funding supported through Hibah Kerjasama Internasional INTERNAL 2022 with the grant No. 9.5.993/UN32.20.1/LT/2022, where the first author is the chief researcher. Thanks to Laboratory for Testing of Nuclear Mineral Resources – BRIN for measuring REE data, and we also gratefully acknowledge the Indonesian Collaboration Research Consortium.