Dwa Desa Warnana*
| Siti Zulaikah | Amien Widodo | Wien Lestari | Juan Pandu Gya Nur Rochman | Sigit Tri Wicaksono | Fadhila | Hanif Izzudin Zakly | Aditya Pratama | Nordiana Mohd Muztaza | Rodeano Roslee
© 2025 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
OPEN ACCESS
Geochemical characterization has been widely carried out to classify the types of rocks and volcanic materials. Volcanic rocks are a major component of the Earth's crust and have their own unique characteristics. Classification and mapping are particularly beneficial for the exploration of specific minerals. This research focuses on studying and analyzing the geochemistry of volcanic materials in the Bromo-Tengger-Semeru (BTS) area, East Java, Indonesia. Whole-material geochemistry from 55 volcanic product samples from the BTS area was determined by X-ray fluorescence (XRF). The XRF results showed that the dominant compounds were CaO (12.5 Wt%), Fe2O3 (27.5 Wt%), and SiO2 (41.3 Wt%). Results also reveal that mostly the BTS volcanic material was classified in the ultramafic lamprophyre Aillikites series. Meanwhile, the analysis based on the comparison of % K2O vs. % SiO2 values shows that BTS volcanic material is included in the alkaline series absarokite and belongs mainly to the post-caldera stage. These features potentially host rare earth elements (REEs)-bearing minerals.
ultramafic lamprophyre aillikites, alkaline series absarokite, rare earth elements (REEs), Geochemical, Bromo-Tengger-Semeru volcanic material, XRF
Geochemical analysis in several studies has classified the types of volcanic rocks [1-4]. This classification is useful for uncovering the origin of rocks and can even explain the recording of climate change that has occurred in the past [5, 6]. Furthermore, various research fields and applications in the field of volcanology employ geochemical analysis [7, 8]. Classification based on geochemical data has a role that can be used as a proxy indicator for the discovery of critical minerals or important minerals for the industrial world that can be mapped out for further exploration [9-12].
The volcanic material in each location has its characteristics [13, 14]. Mapping and classifying volcanic materials will be very useful to facilitate further exploration of specific minerals [1, 8]. Volcanic material in the form of igneous rocks usually has an abundant content of critical minerals or rare earth elements (REEs), and several previous studies have confirmed this [15-17].
The Bromo-Tengger-Semeru (BTS) in the Eastern part of the Java island, Indonesia, is one of the most visited basaltic andesitic volcanoes in the world [18], and is an ancient volcanic area that is still active [19-21]. The volcanic activity that occurs causes the rock formations in the BTS area to consist of young Quaternary rock formations to old Quaternary rock formations [22]. With such characteristics of volcanic activity and supported by existing rock formations, the BTS area should have the potential mineral abundance stored that can be mapped. However, several geochemical characterization studies have been limited to the active Mount Bromo area with its eruption products [23] and Tengger Caldera [24]. Studies on Mount Semeru are also limited to rock types based on potassium content [25] and focus more on studying disasters resulting from eruptions of Mount Semeru [26-28].
In this paper, we propose an investigation into the chemical content of BTS by X-ray fluorescence (XRF). The use of XRF has been tested effectively and efficiently, widely used to track the presence of minerals [23, 24, 29-32]. Firstly, we conducted a mapping of chemical compounds from volcanic materials, especially dominant compounds based on XRF data. Secondly, we analyse the characteristics of volcanic rocks based on their SiO2 and K2O content [33] and the Si vs. Fe ratio [34]. The results of this analysis are expected to determine the classification of volcanic rock types at the study site. Thirdly, we emphasize the tectonic setting and volcanic environment on the formation of volcanic rocks in the BTS area by using Ti vs V plots [35] and ternary diagrams based on the comparison of the main elements MnO-TiO2-P2O5 [36]. Finally, this study aims to characterize volcanic materials from the BTS area using XRF and classify them based on major element geochemistry to infer their origin and mineralization potential.
2.1 Geology of Bromo-Tengger-Semeru
The Bromo-Tengger-Semeru (BTS) volcanic massif comprises a cluster of calderas and stratocones developed over a deeply eroded Tertiary volcanic arc in South Java [27]. The volcanic arc formed as a consequence of northward subduction along the Java Trench to the south with a convergence rate of about 7 cm per year (see Figure 1). Van Gerven and Pichler [24] explained that the oceanic plate subducts beneath the arc with a dip of + 65°, with the trench reaching a maximum depth of 6000-7000 m. The magmatic belt is superimposed upon old basement rocks, and the late Cenozoic volcanoes are built mostly above Neogene marine strata [24].
The Bromo–Tengger caldera, with a diameter of about 8 km, formed during the Late Pleistocene (>45 ka and ca. 33 ka); resurgent volcanism has constructed four stratocones, including the active Bromo tuff cone [37]. Bromo volcano is located at 7°56.30’S and 112°57’E in geographical coordinates with an elevation of about 2329 meters above mean sea level (msl). Historically, Bromo volcano has erupted more than 50 times since 1775, and currently, it is the only active cone in Tengger caldera [23].
Mt. Semeru (8°06′05″S, 112°55′E), one of the most active volcanoes on Earth, is the highest mountain in Java (3,676 m) located 25 km to the south of Bromo (Figure 1) [24, 27]. Semeru’s persistent and combined eruptive activity, at least since 1884, is unusual for calc–alkaline composite cones [27]. Semeru is superimposed on and buttressed to the north by the Jambangan complex. To the south and south-east, Semeru overlies weathered tuffs and breccias, and lava flows of the Oligocene–Miocene ‘Tuf and Old Andesite formation [27].
The geological map of Bromo-Tengger-Semeru (BTS) can be seen in Figure 1. Based on geological information, the BTS consists of some sedimentary formations; they are: Tengger Volcanic Rock Formations (Qvt), Bromo Volcanic Rocks (Qvb), Tengger Volcanic Sand Formations (Qvs), and Avalanche Deposits from Nuee Ardente (Qvj) [38-41].
2.2 Sampling and preparation
The samples were obtained from different locations in the Bromo-Tengger-Semeru (BTS) area. In addition to different locations, the samples in this study have various types of samples, including volcanic sand, gravel, volcanic ash, sandy soil, and andesite igneous rock. The difference in sample types is due to different geological formations (see Figure 1). Samples from the Bromo caldera area and the Semeru area are in the form of volcanic sand, sandy soil, and sandstone. Sampling was carried out on the surface at each sampling point. While samples in the form of andesite igneous rock from the Semeru area were sampled by looking at rock outcrops, which were then taken from the surface. Other samples from the Coban Pelangi area, in the form of gravel and volcanic ash, were obtained from outcrops in the form of walls located at the entrance to the Coban Pelangi tourist attraction, with a height of 3 meters. Sampling at this location was carried out on the surface of the walls in each of its layers. A total of 55 samples were taken (see Appendix Table A1). From all the samples obtained, each sample was put into a standing pouch with an amount of ± 1 kg.
Samples were obtained from outcrops that are suspected to be strong as a result of volcanic activity or the former eruption of Mount Bromo-Tengger-Semeru. Therefore, X-ray fluorescence (XRF) tests were performed in order to determine the chemical composition of the volcanic samples. Prior to XRF analysis, the samples were first dried and then smoothed until they became fine granules to eliminate physical effects such as particle size differences and reduce matrix effects. The detection limits for the major elements analyzed by XRF were approximately 0.01% [42].
To ensure data accuracy, standards were run alongside the samples during XRF analysis. Data quality was validated through repeated measurements of international reference materials, with accuracy maintained within ± 5%. Furthermore, the geochemical data obtained were analyzed to classify the type and origin of volcanic material in the BTS area.
3.1 Geochemical analysis
In general, the major element oxides measured from the BTS volcanic material sample consisted of Al2O3, SiO2, K2O, CaO, TiO2, V2O5, Cr2O3, MnO, Fe2O3, CuO, SrO, BaO, Eu2O3, Re2O7, ZnO, P2O5, and Rb2O (see Table 1). Based on the measured oxide compounds, there are dominant compounds from the BTS volcanic rock sample (>10 Wt%), including CaO (average 12.5 Wt%), Fe2O3 (average 27.5 Wt%), and SiO2 (average 41.3 Wt%).
The results obtained from this geochemical test were then mapped with an interpolation technique to determine the distribution of dominant compounds (SiO2, Fe2O3, and CaO) in the BTS area. Figure 2 shows the distribution of the three dominant compounds in the BTS area. Figure 2(a) shows that SiO2 compounds are present in almost all samples of BTS's volcanic material. Spreads with high content are indicated in red. Fe2O3 compounds with the most dominant distribution are located in the Coban Pelangi at the AV10B point (see Figure 2(b)). Figure 2(c) shows the dominant distribution of CaO at four sampling points in the Mount Semeru area. The sampling points are PS 1.1, PS 1.2, PS 1.3, PS 1.5, PS 2.1, PS 2.2, PS 2.3, PS 2.4, PS 3.1, PS 3.2, PS 3.3, PS 4.1, PS 4.2. The range of CaO content in this region is between 13.7-19 wt%, with PS 2.4 having the highest CaO content.
(a)
(b)
(c)
Figure 2. Map of the distribution of dominant compounds in the BTS area (a) SiO2; (b) Fe2O3; (c) CaO
The Fe2O3 content in Bromo-Semeru is a larger amount than that in Arjuno-Welirang. However, the Fe-oxides measured in Arjuno-Welirang are only in samples of volcanic igneous rocks. This result may be because the structure of the BTS rock formation has different chemical content [43]. Furthermore, the dominance of Fe2O3 and differences in CaO and SiO2 distribution in the BTS area reflect changes in magmatic processes or magmatic evolution as a product of volcanic activity in the research area. These results are in accordance with several studies, which show that the composition of Fe, CaO, and SiO2 can reflect the evolution of magmatic processes [44-46].
3.2 Classification of volcanic materials
Rock classification was performed to study the characteristics of rocks from samples obtained from the field based on the results of geochemical tests. Classification is carried out to determine the type and origin of volcanic samples obtained from the field. Generally, volcanic samples from the BTS area were obtained from three different locations and geological formations.
Classification to determine the type of volcanic sample is done by comparing the SiO2 and K2O content from the results of geochemical tests [33] (Figure 3). The classification of volcanic rocks given by Peccerillo and Taylor [33] has been useful and widely accepted. Based on Figure 3, volcanic samples originating from the Bromo-sand sea caldera are generally classified as alkaline series absarokite. Meanwhile, most samples from the Semeru area were classified in the high-K basalt type, some of which were classified in the alkaline series of the absarokite series. Samples taken from the Coban Pelangi site are almost all undefined due to their low SiO2 content (<45 wt%). Only two samples (AV10A and AV10B) can be classified as alkaline in the abzarokite series.
In Figure 3, we also compare our results with ash and scoria deposit samples from the 2000–2001, 2010–2011, 2015–2016, and 2019 Bromo eruptions [23]. The results from their samples indicate a basaltic-andesite to basalt trachy-andesite melt source beneath the Bromo volcano, with a transition from a medium-K to a high-K composition (2010–2019) [23].
Table 1. X-ray fluorescence (XRF) data of volcanic samples Bromo-Tengger-Semeru (BTS)
|
No. |
Sample ID |
Al2O3 |
SiO2 |
K2O |
CaO |
TiO2 |
V2O5 |
Cr2O3 |
MnO |
Fe2O3 |
CuO |
SrO |
BaO |
Eu2O3 |
Re2O7 |
ZnO |
P2O5 |
Rb2O |
|
1 |
P1.1 |
12 |
44.8 |
3.87 |
11.8 |
2.14 |
0.06 |
0.051 |
0.35 |
23.2 |
0.11 |
0.33 |
0.2 |
0.29 |
0.2 |
0.01 |
0.83 |
0 |
|
2 |
P 1.2 |
12 |
46.5 |
4.26 |
11.3 |
2.04 |
0.05 |
0.047 |
0.29 |
21.3 |
0.099 |
0.3 |
0.23 |
0.33 |
0.2 |
0.006 |
0.89 |
0.11 |
|
3 |
P1.3 |
11 |
40.1 |
4.52 |
12.2 |
2.26 |
0.05 |
0.055 |
0.37 |
26.8 |
0.14 |
0.47 |
0.2 |
0.34 |
0.2 |
0.02 |
1 |
0 |
|
4 |
P 2.1 |
12 |
41.9 |
4.07 |
12.4 |
2.13 |
0.06 |
0.054 |
0.34 |
24.7 |
0.12 |
0.48 |
0.2 |
0.34 |
0.2 |
0.01 |
1.1 |
0 |
|
5 |
P 2.2 |
11 |
39.6 |
3.8 |
13.1 |
2.18 |
0.07 |
0.058 |
0.38 |
27.2 |
0.12 |
0.57 |
0.2 |
0.32 |
0.3 |
0.01 |
1.2 |
0.18 |
|
6 |
P 2.3 |
11 |
45.4 |
4.52 |
11.7 |
2.1 |
0.05 |
0.051 |
0.34 |
23.3 |
0.11 |
0.36 |
0.23 |
0.3 |
0.2 |
0.01 |
0 |
0 |
|
7 |
P2.4 |
0 |
48.7 |
5.04 |
13.3 |
2.38 |
0.05 |
0.058 |
0.4 |
28.2 |
0.14 |
0.44 |
0.26 |
0.38 |
0.2 |
0.01 |
0 |
0.18 |
|
8 |
P 2.7 |
12 |
45 |
4.18 |
12 |
2.13 |
0.05 |
0.047 |
0.34 |
22.8 |
0.11 |
0.35 |
0.2 |
0.31 |
0.2 |
0 |
0 |
0.11 |
|
9 |
P 2.8 |
12 |
43.1 |
4.27 |
12.6 |
2.11 |
0.04 |
0.047 |
0.35 |
24.2 |
0.12 |
0.4 |
0.2 |
0.29 |
0.2 |
0.01 |
0 |
0.13 |
|
10 |
P 3.1 |
13 |
46.5 |
4.1 |
12.4 |
1.85 |
0.04 |
0.045 |
0.31 |
21.2 |
0.11 |
0.35 |
0.2 |
0.3 |
0.2 |
0.01 |
0 |
0 |
|
11 |
P 4.1 |
12 |
44.8 |
4.02 |
12.4 |
2.02 |
0.05 |
0.049 |
0.31 |
22.3 |
0.11 |
0.38 |
0.2 |
0.3 |
0.2 |
0 |
0.99 |
0.12 |
|
12 |
P4.3 |
8.9 |
27.3 |
3.39 |
8 |
2.7 |
0.073 |
0.073 |
0.28 |
41.27 |
0.15 |
0.41 |
0.2 |
0.39 |
0.2 |
0.02 |
1.5 |
0 |
|
13 |
P 5.1 |
12 |
44.4 |
3.47 |
13.3 |
2.22 |
0.071 |
0.053 |
0.33 |
22.6 |
0.11 |
0.44 |
0.2 |
0.28 |
0.2 |
0.008 |
0 |
0 |
|
14 |
P 5.2 |
10 |
39.4 |
4.24 |
12.2 |
2.39 |
0.07 |
0.058 |
0.39 |
28.9 |
0.14 |
0.45 |
0.2 |
0.46 |
0.2 |
0.02 |
1 |
0 |
|
15 |
P 5.3 |
10 |
38.5 |
4.21 |
13.1 |
2.23 |
0.06 |
0.063 |
0.39 |
28.7 |
0.15 |
0.57 |
0.3 |
0.39 |
0.2 |
0.02 |
1.2 |
0 |
|
16 |
P 6.1 |
11 |
43.3 |
4.27 |
12.6 |
2.11 |
0.05 |
0.054 |
0.34 |
24.8 |
0.12 |
0.4 |
0.2 |
0.32 |
0 |
0 |
0 |
0 |
|
17 |
L2 |
9.4 |
38.6 |
3.04 |
11 |
3.28 |
0.1 |
0.051 |
0.39 |
31.9 |
0.14 |
0.3 |
0 |
0.45 |
0.2 |
0.03 |
0.93 |
0.14 |
|
18 |
L3 |
9.1 |
38 |
2.98 |
11.1 |
3.26 |
0.098 |
0.054 |
0.42 |
33 |
0.13 |
0.32 |
0 |
0.41 |
0.2 |
0.04 |
0.93 |
0 |
|
19 |
L4 |
9.1 |
37.3 |
2.92 |
10.9 |
3.17 |
0.083 |
0.061 |
0.44 |
34.2 |
0.14 |
0.4 |
0.2 |
0.4 |
0.3 |
0.03 |
0 |
0 |
|
20 |
L5 |
0 |
40.2 |
3.22 |
12.2 |
3.58 |
0.12 |
0.067 |
0.47 |
37.5 |
0.15 |
0.42 |
0 |
0.56 |
0.32 |
0.02 |
0.96 |
0.18 |
|
21 |
L6 |
9.3 |
38.3 |
2.91 |
11 |
3.08 |
0.086 |
0.062 |
0.42 |
32.3 |
0.14 |
0.37 |
0.2 |
0.37 |
0.2 |
0.03 |
1 |
0.15 |
|
22 |
L7 |
9.6 |
37.9 |
3 |
11 |
3.2 |
0.091 |
0.061 |
0.41 |
32.6 |
0.14 |
0.33 |
0 |
0.41 |
0.2 |
0.03 |
1 |
0 |
|
23 |
L8 |
9.1 |
38.5 |
2.68 |
10.7 |
3.25 |
0.11 |
0.063 |
0.42 |
34 |
0.15 |
0.33 |
0 |
0.42 |
0.2 |
0.03 |
0 |
0 |
|
24 |
L9 |
8.7 |
37.4 |
2.46 |
10.3 |
3.21 |
0.11 |
0.061 |
0.44 |
34.9 |
0.16 |
0.37 |
0 |
0.44 |
0.2 |
0.03 |
0.99 |
0.15 |
|
25 |
L10 |
8.5 |
36.3 |
2.71 |
10.8 |
3.18 |
0.11 |
0.056 |
0.77 |
35.1 |
0.17 |
0.44 |
0 |
0.48 |
0.2 |
0.04 |
1.2 |
0 |
|
26 |
AV1 |
8.9 |
36.7 |
3.28 |
11.4 |
3.09 |
0.088 |
0.061 |
0.45 |
33.4 |
0.13 |
0.38 |
0.2 |
0.43 |
0.27 |
0.02 |
1.2 |
0 |
|
27 |
AV2 |
8.7 |
37.5 |
3.34 |
11 |
3.14 |
0.08 |
0.057 |
0.46 |
33 |
0.14 |
0.38 |
0.2 |
0.41 |
0.2 |
0.04 |
1 |
0.17 |
|
28 |
AV3 |
9.2 |
38.9 |
3.44 |
11.6 |
3.08 |
0.087 |
0.055 |
0.44 |
31 |
0.13 |
0.36 |
0 |
0.4 |
0.2 |
0.03 |
1.2 |
0 |
|
29 |
AV4 |
9.8 |
40.1 |
3.4 |
10.8 |
3.01 |
0.085 |
0.047 |
0.41 |
30 |
0.13 |
0.35 |
0.2 |
0.36 |
0.2 |
0.03 |
0.9 |
0.15 |
|
30 |
AV5 |
9.9 |
40 |
2.93 |
10.6 |
3.1 |
0.078 |
0.054 |
0.42 |
31.67 |
0.14 |
0.3 |
0.2 |
0.38 |
0.2 |
0.04 |
0 |
0 |
|
31 |
AV6 |
9.8 |
37.7 |
2.68 |
11 |
3.14 |
0.1 |
0.059 |
0.51 |
33.8 |
0.16 |
0.41 |
0 |
0.45 |
0.2 |
0.03 |
0 |
0 |
|
32 |
AV7 |
11 |
41.9 |
3.11 |
10.8 |
2.9 |
0.092 |
0.054 |
0.38 |
29 |
0.12 |
0.29 |
0.2 |
0.32 |
0.2 |
0.02 |
0 |
0 |
|
33 |
AV8A |
9.1 |
39 |
2.74 |
9.8 |
3.13 |
0.076 |
0.066 |
0.4 |
33.5 |
0.15 |
0.42 |
0 |
0.48 |
0.2 |
0.03 |
0.96 |
0 |
|
34 |
AV8B |
9.3 |
39.7 |
2.98 |
10.5 |
3.02 |
0.076 |
0.062 |
0.41 |
31.5 |
0.13 |
0.38 |
0.2 |
0.41 |
0.2 |
0.03 |
1.1 |
0 |
|
35 |
AV9A1 |
0 |
39.7 |
3.48 |
11.1 |
3.33 |
0.08 |
0.066 |
0.53 |
38.7 |
0.23 |
0.5 |
0.2 |
0.48 |
0.2 |
0.05 |
1.2 |
0 |
|
36 |
AV9A2 |
9.3 |
39.7 |
3.44 |
10.1 |
2.98 |
0.06 |
0.055 |
0.41 |
31.3 |
0.14 |
0.38 |
0.2 |
0.39 |
0.2 |
0.05 |
1.1 |
0.16 |
|
37 |
AV9B |
9.6 |
41.1 |
3.38 |
10.1 |
3.02 |
0.06 |
0.058 |
0.42 |
30.8 |
0.13 |
0.33 |
0.2 |
0.4 |
0.2 |
0.03 |
0 |
0 |
|
38 |
AV10A |
0 |
45.3 |
3.54 |
11.6 |
3.29 |
0.09 |
0.059 |
0.46 |
34.3 |
0.12 |
0.37 |
0.2 |
0.42 |
0.3 |
0.03 |
0 |
0 |
|
39 |
AV10TA |
11 |
43.6 |
3.28 |
9.19 |
2.8 |
0.075 |
0.055 |
0.36 |
28.1 |
0.11 |
0.31 |
0 |
0.36 |
0.2 |
0.03 |
1 |
0 |
|
40 |
AV10B |
0 |
47 |
3.84 |
10.4 |
3.01 |
0.07 |
0.062 |
0.48 |
32.6 |
0.14 |
0.43 |
0.3 |
0.41 |
0.2 |
0.05 |
1.1 |
0 |
|
41 |
AV11 |
9.1 |
38.2 |
2.67 |
10.2 |
3.03 |
0.092 |
0.06 |
0.4 |
33.6 |
0.14 |
0.39 |
0.2 |
0.37 |
0.2 |
0.03 |
1 |
0.18 |
|
42 |
PS 1.1 |
15 |
42.6 |
1.39 |
16.3 |
1.77 |
0.097 |
0.05 |
0.34 |
21.4 |
0.085 |
0.29 |
0.07 |
0.31 |
0.17 |
0 |
0 |
0 |
|
43 |
PS 1.2 |
15 |
43.8 |
1.49 |
16.2 |
1.57 |
0.068 |
0.057 |
0.37 |
20 |
0.078 |
0.37 |
0.1 |
0.26 |
0.2 |
0 |
0 |
0 |
|
44 |
PS 1.3 |
15 |
44.1 |
1.52 |
16 |
1.57 |
0.072 |
0.048 |
0.37 |
20.1 |
0.079 |
0.35 |
0.1 |
0.25 |
0.18 |
0 |
0 |
0 |
|
45 |
PS 1.5 |
16 |
46.7 |
1.24 |
17.3 |
1.12 |
0.03 |
0.04 |
0.39 |
16.6 |
0.065 |
0.38 |
0.1 |
0.24 |
0.2 |
0 |
0 |
0 |
|
46 |
PS 2.1 |
16 |
47.6 |
1.59 |
16.7 |
1.26 |
0.05 |
0.044 |
0.31 |
15.3 |
0.069 |
0.32 |
0.1 |
0.22 |
0.17 |
0 |
0 |
0 |
|
47 |
PS 2.2 |
16 |
48.5 |
1.49 |
16.4 |
1.19 |
0.05 |
0.045 |
0.29 |
14.7 |
0.084 |
0.31 |
0.09 |
0.21 |
0.16 |
0 |
0 |
0 |
|
48 |
PS 2.3 |
16 |
46.9 |
1.61 |
16.7 |
1.28 |
0.05 |
0.044 |
0.33 |
16.1 |
0.071 |
0.31 |
0.08 |
0.22 |
0.18 |
0 |
0 |
0 |
|
49 |
PS 2.4 |
17 |
46 |
1.4 |
19 |
1.18 |
0.057 |
0.038 |
0.27 |
14.2 |
0.066 |
0.35 |
0.07 |
0.24 |
0.17 |
0 |
0 |
0 |
|
50 |
PS 2.5 |
17 |
49.9 |
1.7 |
16.7 |
0.975 |
0.03 |
0.036 |
0.26 |
12.8 |
0.082 |
0.33 |
0.2 |
0.19 |
0.15 |
0 |
0 |
0 |
|
51 |
PS 3.1 |
16 |
31.4 |
0.37 |
13.7 |
2.17 |
0.13 |
0.076 |
0.54 |
34.02 |
0.14 |
0.37 |
0 |
0.41 |
0.27 |
0 |
0.81 |
0 |
|
52 |
PS 3.2 |
17 |
33.9 |
0.45 |
13.7 |
2.06 |
0.12 |
0.065 |
0.45 |
30.8 |
0.11 |
0.32 |
0 |
0.39 |
0.07 |
0.03 |
0.6 |
0 |
|
53 |
PS 3.3 |
16 |
34.2 |
0.45 |
15.3 |
2.08 |
0.14 |
0.069 |
0.43 |
30.8 |
0.11 |
0.31 |
0 |
0.39 |
0 |
0 |
0 |
0 |
|
54 |
PS 4.1 |
16 |
47.8 |
1.74 |
16.3 |
1.13 |
0.05 |
0.035 |
0.38 |
15.7 |
0.056 |
0.38 |
0.2 |
0.23 |
0.16 |
0 |
0 |
0 |
|
55 |
PS 4.2 |
16 |
45.4 |
2.42 |
15.3 |
1.2 |
0.03 |
0.043 |
0.43 |
17.9 |
0.083 |
0.47 |
0.2 |
0.28 |
0.2 |
0.007 |
0 |
0 |
Several researchers [24, 47] in the study of rock composition evolution have reported that the high-K affinity was considered to be induced by younger and post-caldera magma, while, in contrast, the medium-K rocks were attributed to pre-caldera. This means that all samples we took came from magma activities after the caldera in BTS was formed.
Basalt rocks with High-K can be estimated to have rare earth elements (REE) in the range of 90-610 ppm [48]. The REE ratios measured include La, estimated to have a ratio of 3.6-34 ppm, and Eu with a ratio of 0.24-0.40 ppm [49, 50]. This result can certainly be justified with further research so that the content of other critical minerals may also be mapped in the BTS area. Understanding the distribution and potential of REE in volcanic rock is crucial for resource exploration and environmental management.
Further classification uses the Si vs Fe ratio relation [34]. The Si vs Fe ratio is a key factor in the geochemical classification of sediments and minerals. It helps in distinguishing between different types of sediments and minerals based on their composition. Based on the results of geochemical tests carried out by volcanic samples in the BTS area, there is a lot of Fe, so the classification using this data is considered necessary to look at the type of rock based on the content of silica (Si) and iron (Fe) from the BTS volcanic sample.
The classification in Figure 4 shows that the BTS volcanic samples are aillikite. Aillikites are part of the broader group of ultramafic lamprophyres. In this case, these types of rocks have a small number of outcrops, especially those exposed on the surface. The classification results of the rocks at BTS, which are classified as ultramafic lamprophyre rock types, are apparently identical to the volcanic rocks in the Arjuno-Welirang Mountains [43]. Mount Arjuno-Welirang is located 75 km northwest of Mount Bromo. This suggests that these mountains had a similar tectonic setting during their formation.
Aillikites are enriched in REEs. Studies show that trace elements, especially REEs, show a general rise in levels from aillikites to other minerals, indicating their enrichment. Additionally, aillikites exhibit very REE-enriched patterns, particularly in LREEs (light rare earth elements) [51, 52].
Based on the environment in which they are formed, volcanic materials can be classified according to several studies that have been conducted. Among the classifications carried out is research [35, 36]. Shervais [35] classifies volcanic rocks using the plot Ti vs. V. The Ti-V diagram is generally used for determining tectonic settings of mafic and ultramafic rocks, especially basalts, because it directly reflects mantle source characteristics and degree of partial melting. This Ti vs. V plot distinguishes the environment from which the rock comes from into four different arcs. Among the environments classified by the Ti vs V plot, namely, arc tholeiites, Mid Ocean Ridge Basalt (MORB), and Back-Arc Basalts (BAB), continental flood basalts, ocean-island, and alkali basalts (Figure 5).
Ultramafic rock are characterized by low TiO2 contents (>2 wt%) and BTS volcanic samples (aillikite) have a range 1% - 3.58% TiO2 (see Table 1) and a Ti/V ratio (Figure 5) showing a very large variation range from values roughly similar to those of typical arc tholeiites (Ti/V <20 for some Semeru samples) to values between 20 to 50 for most samples. They are similar to typical MORB (or BAB) and continental flood basalts.
To further study the magmatic evolution of the formation of volcanic samples in the BTS area, a ternary diagram analysis was carried out based on the comparison of the main elements MnO-TiO2-P2O5 [36]. Based on the ternary diagram, the classification is divided into five different environments, including Mid-Ocean Ridge Basalt (MORB), Island Arc Tholeiite (IAT), Calc-Alkaline Basalt (CAB), Seamount Tholeiite (OIT), and Seamount Alkalic (OIA). Figure 6 shows that the volcanic material of the BTS area represents the most similarity with the Calc-Alkaline Basalt (CAB) and Island Arc Tholeiite (IAT) environments.
Figure 6. Ternary diagram of TiO2-MnO×10-P2O5×10 for volcanic material in the BTS area
Boundary lines and corresponding rock classification are based on study [36].
The Calc-Alkaline Basalt (CAB) and Island Arc Tholeiite (IAT) magma series are the two most important igneous differentiation trends and are broadly expressed by the crustal dichotomy of andesitic continental crust and basaltic oceanic crust (MORB) [53]. Tholeiitic magmas may occur in all tectonic settings, while calc-alkaline magmas appear to be uniquely associated with the convergent margin setting [54], such as those at our study site. Furthermore, the Bromo-Tengger-Semeru (BTS) volcano indicates that the calc-alkaline rocks would not be expected as a function of the depth of the Wadati-Benioff zone, whereas the Wadati-Benioff zone reaches a depth of about 100-150 km beneath the south coast of Java and approximately 600 km beneath the Java Sea [24].
Bromo-Tengger-Semeru (BTS) volcanic material geochemically contains natural compounds including Al2O3, SiO2, K2O, CaO, TiO2, V2O5, Cr2O3, MnO, Fe2O3, CuO, SrO, BaO, Eu2O3, Re2O7, ZnO, P2O5, and Rb2O. The dominant compounds from the geochemical test results include CaO (12.5 Wt%), Fe2O3 (27.5 Wt%), and SiO2 (41.3 Wt%). The mapping carried out shows that SiO2 content tends to be found in all areas of Bromo Semeru. Meanwhile, Fe2O3 is mostly found in outcrops in Coban Pelangi, and CaO is mostly found in the Semeru area. The dominance of Fe2O3 and differences in CaO and SiO2 distribution in the BTS area reflect changes in magmatic processes or magmatic evolution as a product of volcanic activity in the research area.
Based on the K2O vs SiO2 classification, the rock types of the BTS volcanic material are mostly alkaline series absarokite, and some others are classified as high-K basalt types. The rocks with high K affinity are thought to be induced by the post-caldera stage, so it can be concluded that all samples originate from magma activity after the caldera at BTS was formed. Based on the Si vs Fe classification, BTS volcanic materials are classified in the ultramafic lamprophyre series as ailikites. The ailikite is thought to be enriched in REE, particularly in LREEs (light rare earth elements). They are also similar to typical MORB (or BAB) and continental flood basalts with a Ti/V ratio range of 20-50. Meanwhile, according to the classification of the main elements of MnO-TiO2-P2O5, BTS volcanic materials represent the most similarity with the Calc-Alkaline Basalt (CAB) and Island Arc Tholeiite (IAT) environments. The CAB appears to be uniquely associated with the convergent margin setting; however, it is located in an area where talc-alkaline rocks would not be expected as a function of the depth of the Wadati-Benioff zone.
Finally, although this study provides information about the geochemical content of materials and infers their origin and mineralization potential at the study site, we also acknowledge the limitations of this study, especially in detailing the rare earth elements (REE) content of high K basalt-type materials or rocks. Our focus in the future is on efforts to trace REE sources, determine their potential, predict REE volumes, and determine the priority scale of mining in a study area. Further exploration activities, such as Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) measurements, are needed to confirm REE concentrations.
This work is supported by Institut Teknologi Sepuluh Nopember under the Capacity Development Program of Higher Education for Technology and Innovation Project Asian Development Bank Loan No 4110-INO, and Indonesian Collaborative Research (Contract number 26/IT2/T/HK.00.01/V/2024), Universitas Negeri Malang (Contract number 5.4.38/UN32.14.1/LT/2024) and Universitas Negeri Padang (Contract number 219/UN35/KU/2024).
Table A1. Bromo-Tengger Semeru (BTS) volcanic sample data
|
No. |
Sample ID |
UTM Zone 49S |
Sample Origin |
Sample Type |
|
|
Easting |
Northing |
||||
|
1 |
P1.1 |
715753.03 |
9122552.17 |
Bromo |
Volcanic Sand |
|
2 |
P 1.2 |
715753.03 |
9122552.17 |
Bromo |
Volcanic Sand |
|
3 |
P1.3 |
715756.1 |
9122549.47 |
Bromo |
Volcanic Sand |
|
4 |
P 2.1 |
715618.61 |
9122765.85 |
Bromo |
Volcanic Sand |
|
5 |
P 2.2 |
715620.1 |
9122762.69 |
Bromo |
Volcanic Sand |
|
6 |
P 2.3 |
715639.66 |
9122740.5 |
Bromo |
Volcanic Sand |
|
7 |
P2.4 |
715679.31 |
9122683.01 |
Bromo |
Volcanic Sand |
|
8 |
P 2.7 |
715696.53 |
9122710.03 |
Bromo |
Volcanic Sand |
|
9 |
P 2.8 |
715623.57 |
9122802.09 |
Bromo |
Volcanic Sand |
|
10 |
P 3.1 |
716610.82 |
9121968.2 |
Bromo |
Volcanic Sand |
|
11 |
P 4.1 |
717215.49 |
9121974.32 |
Bromo |
Volcanic Sand |
|
12 |
P4.3 |
717215.64 |
9121983.61 |
Bromo |
Volcanic Sand |
|
13 |
P 5.1 |
717730.93 |
9121993.66 |
Bromo |
Volcanic Sand |
|
14 |
P 5.2 |
717697.19 |
9122019.26 |
Bromo |
Volcanic Sand |
|
15 |
P 5.3 |
717692.7 |
9122026.14 |
Bromo |
Volcanic Sand |
|
16 |
P 6.1 |
717578.16 |
9121918.61 |
Bromo |
Volcanic Sand |
|
17 |
L2 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
18 |
L3 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
19 |
L4 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
20 |
L5 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
21 |
L6 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
22 |
L7 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
23 |
L8 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
24 |
L9 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
25 |
L10 |
705581.53 |
9113976.58 |
Coban Pelangi |
Gravel |
|
26 |
AV1 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
27 |
AV2 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
28 |
AV3 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
29 |
AV4 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
30 |
AV5 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
31 |
AV6 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
32 |
AV7 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
33 |
AV8A |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
34 |
AV8B |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
35 |
AV9A1 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
36 |
AV9A2 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
37 |
AV9B |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
38 |
AV10A |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
39 |
AV10TA |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
40 |
AV10B |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
41 |
AV11 |
705581.53 |
9113976.58 |
Coban Pelangi |
Volcanic Ash |
|
42 |
PS 1.1 |
715071.86 |
9092007.6 |
Semeru |
Volcanic Sand |
|
43 |
PS 1.2 |
715071.86 |
9092007.6 |
Semeru |
Volcanic Sand |
|
44 |
PS 1.3 |
715071.86 |
9092007.6 |
Semeru |
Volcanic Sand |
|
45 |
PS 1.5 |
715071.86 |
9092007.6 |
Semeru |
Andesite |
|
46 |
PS 2.1 |
720638.17 |
9096060.39 |
Semeru |
Volcanic Sand |
|
47 |
PS 2.2 |
720638.17 |
9096060.39 |
Semeru |
Volcanic Sand |
|
48 |
PS 2.3 |
720638.17 |
9096060.39 |
Semeru |
Volcanic Sand |
|
49 |
PS 2.4 |
720638.17 |
9096060.39 |
Semeru |
Andesite |
|
50 |
PS 2.5 |
720638.17 |
9096060.39 |
Semeru |
Sandy soils |
|
51 |
PS 3.1 |
724894.84 |
9108533.88 |
Semeru |
Sandy soils |
|
52 |
PS 3.2 |
724894.84 |
9108533.88 |
Semeru |
Sandy soils |
|
53 |
PS 3.3 |
724894.84 |
9108533.88 |
Semeru |
Volcanic Sand |
|
54 |
PS 4.1 |
719160.06 |
9111497.43 |
Semeru |
Andesite |
|
55 |
PS 4.2 |
719160.06 |
9111497.43 |
Semeru |
Sandstone |
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