Non-destructive investigation of sandstone blocks used in the Wat Phu temple in Laos and the Banteay Chhmar temple in Cambodia

Sandstone blocks quarried from the late Jurassic to the early Cretaceous Red Terrane Formation were used to construct the Wat Phu temple in Laos and the Banteay Chhmar temple in Cambodia. The sandstone blocks of the Banteay Chhmar temple are gray to yellowish brown in color and their magnetic susceptibilities and Sr contents are relatively high, similar to the sandstone blocks used in the Angkor monument. In contrast, the Wat Phu temple consists of reddish sandstone blocks with significantly lower magnetic susceptibilities and Sr contents than those used in the Banteay Chhmar temple and Angkor monument. The sandstone blocks of the Banteay Chhmar temple were likely supplied from quarries in Ta Phraya, Thailand, and those of the Wat Phu temple are likely to have been supplied from the area near these temples. The Red Terrane Formation is widely distributed throughout Mainland Indochina, and most of these sandstones show low magnetic susceptibilities and low Sr contents, similar to those of the Wat Phu temple. Sandstone with high magnetic susceptibilities and high Sr contents is found in the sandstone quarries in Ta Phraya and the southeastern foothill of Mt. Kulen, which is the supply source of the sandstone blocks used in the Angkor monument, early buildings of the Bakan monument, and Banteay Chhmar temple. The sandstone with high magnetic susceptibility and high Sr content is distributed in limited areas and implies a weak degree of weathering during the sandstone formation process or a difference in the source rocks.


Purpose of the study
Many buildings of the Khmer monuments in Cambodia were constructed using feldspathic arenite (sandstone) quarried from the late Jurassic to early Cretaceous Red Terrane Formation (called the Phu Kradung Formation in Thailand) [1]. The Khmer monuments where such feldspathic arenite is used include the Angkor monument [2,3], the Bakan monument [4], which is also known as the Preah Khan of Kompong Svay, and the Koh Ker monument [5]. In this study, we investigated the sandstone blocks of the Wat Phu temple and its small surrounding temples (Hong Nang Sida, Tao Tao, and Tomo temples) in Laos and the Banteay Chhmar temple in Cambodia, where similar feldspathic arenite blocks are used (Fig. 1).
We also examined the feldspathic arenite in several outcrops of the Red Terrane Formation, which is widely distributed in Mainland Indochina [6,7], and compared it with the sandstone blocks used in the Khmer monuments.
In this study, we report the results of non-destructive magnetic susceptibility measurements and chemical analyses using a portable X-ray fluorescence (pXRF) analyzer on the sandstone blocks of the Wat Phu temple, its surrounding small temples, and the Banteay Chhmar temple. We compare the sandstones (feldspathic arenite) used in these temples with those of other previously investigated monuments (Angkor, Koh Ker, and Bakan monuments) [2,4,5,8,9]. The sandstone (feldspathic arenite) of the Red Terrane Formation was also investigated and its magnetic susceptibility and chemical composition were compared with the sandstone used in the monuments in order to clarify the supply source of the sandstone blocks.
The portable magnetic susceptibility meter is a non-destructive and in-situ investigation method that is extremely useful for clarifying the construction sequence and age of the Angkor monuments built of feldspathic arenite with no differences in chemical composition [8,9]. This method is also useful for identifying the source of supply [10,11]. On the other hand, differences in chemical composition (As, Sr, V, and Y) were found in laterite used in the Angkor monuments and also in the bridges along the East Royal Road. A portable X-ray fluorescence analyzer played an important role in estimating the construction age and the supply range of laterite blocks based on the chemical composition [12,13].

Overview of investigated temples
The Wat Phu temple is located in the Champasak region in southern Laos (Fig. 2), 260 km northeast of the Angkor monument. The Wat Phu temple was built on the southeastern foothill of Mt. Phu Kao. The sandstone and laterite buildings of the Wat Phu temple are believed to have been built primarily between the 11th and 13th centuries (Fig. 3a). Part of the top sanctuary is made of brick and its construction is thought to date back to the 10th century or earlier [14]. We also investigated small temples in the vicinity including the  Hong Nang Sida (Fig. 3b), Tao Tao, and Tomo temples. These temples were mainly constructed using gray or reddish feldspathic arenite. We therefore also investigated the sandstones in quarries along the southeastern slopes of Mt. Phu Kao (Fig. 3c) to the west of the Wat Phu and Hong Nang Sida temples and on the riverbed near the Tomo temple.
The Banteay Chhmar temple is located 110 km northwest of the Angkor monument and was built in the late Bayon period (late 12th to early 13th centuries) [15]. The Banteay Chhmar temple consists of a central temple (Fig. 3d) surrounded by an outer gallery with dimensions of 220 m in the east-west direction and 180 m in the north-south direction, satellite temples located to the north, south, east, and west, and a fire shrine. Gray to yellowish brown sandstone and laterite were used to build this temple. The sandstone quarry is located in Ta Phraya situated on the southern foothill of the Dangrek Range in Thailand, 25 km west-northwest of the temple, with several stepped quarrying traces at a height of approximately 5-6 m (Fig. 3e). [16]. An approximately 12-km-long canal from the Banteay Chhmar temple toward the sandstone quarry can be seen on Google Earth®, and it is speculated that this canal was used to transport the sandstone blocks.

Methods
The following non-destructive investigations were carried out on the sandstone (feldspathic arenite) used in the Wat Phu temple, its surrounding temples, and Banteay Chhmar temple, and in outcrops of the Red Terrane Formation.

Chemical composition measurements using a portable XRF
Chemical composition analysis for stone cultural properties by a non-destructive method using a pXRF analyzer is a very useful method for identifying the types of stone materials and for understanding differences in chemical composition [5,13,[17][18][19][20].
Non-destructive chemical analyses of the sandstone blocks used in the Banteay Chhmar, Wat Phu, Hong Nang Sida, Tao Tao, and Tomo temples were conducted using a pXRF analyzer (Delta Premium; Innov-X Systems Inc., Waltham, MA, USA) in a soil mode (Fig. 4). XRF analysis can determine the presence and concentration of a range of metallic and non-metallic elements by measuring the secondary X-rays emitted from a sample when it is excited by X-rays. A calibration of the pXRF analyzer was performed prior to analysis using Japanese standard rock samples (JA-1, JA-2, JB-1b, JB-2, JB-3, JG-1a, JG-2, JGb-1, JR-1, and JR-2) [21]. The accuracy of measurement by the portable XRF analyses is shown in Tables 1 and 2. The total measurement time was set to 60 s. Measurements were conducted on the surfaces of 10 sandstone blocks that were not covered with soil, lichen, or algae from each building, and an average value was calculated. In the sandstone outcrops, the analysis was conducted on flat rock surfaces broken by a hammer. In the sandstone quarries, the analysis was conducted on the flat broken surfaces of fallen sandstone fragments.

Magnetic susceptibility measurement
Stones with the same chemical composition can often have different magnetic susceptibilities. Magnetic susceptibility measurements can be performed quickly and non-destructively, and can be a very effective technique for clarifying differences in stone materials used in cultural properties [2,8,9,17,18].
Non-destructive magnetic susceptibility measurements were conducted on the sandstone blocks used in the Banteay Chhmar, Wat Phu, Hong Nang Sida, Tao Tao, and Tomo temples using a portable magnetic susceptibility meter (SM30; ZH Instruments, Brno, Czech Republic) (Fig. 4). The magnetic susceptibility meter contains an oscillator with a pickup coil. The change in frequency is proportional to the amount of magnetic susceptibility of the rock. In order to find out about the change, it is necessary to measure the oscillator frequency twice. The first measurement is performed on the surface of the rock. The second measurement is carried out when the meter is away from the rock. After the second step is finished, both values are subtracted and displayed (mode 1) (the user's manual of the magnetic susceptibility meter SM30). We measured 5 × 5 cm sandstone block surfaces. The measurement time was approximately 2 s and an accuracy of 1 × 10 − 6 SI units was obtained. Magnetic susceptibility measurements were made on 50 sandstone blocks at each building and an average value was calculated. Measurements were carried out on the flat surfaces of sandstone blocks not covered with soil, lichen, or algae. Magnetite is considered to be the main reason for the magnetic susceptibility of the sandstone.

Magnetic susceptibility
The magnetic susceptibility of the sandstone blocks used in the Wat Phu (Fig. 5), Hong Nang Sida, Tao Tao, and Tomo temples ranges from 0.14 to 0.42 × 10 − 3 SI units and mainly between 0.20 and 0.32 × 10 − 3 SI units (Table 1) (Supplementary Material 1). However, the magnetic susceptibilities of the balusters in the northern and southern palaces of the Wat Phu temple are slightly higher (0.36× 10 − 3 and 0.42 × 10 − 3 SI units, respectively) than the sandstone blocks used for buildings in the investigated temples (Table 1). This suggests that the sandstone blocks used for the balusters in the palaces of the Wat Phu temple were supplied from different quarries from those of the buildings in the Wat Phu temple.

Chemical compositions
The following elements were detected in all of the sandstone blocks: K, Ca, Ti, Cr, Mn, Fe, Cu, Zn, Rb, Sr, Y, Zr, and Pb (Table 1) (Supplementary Material 2). Systematic differences were identified in the Sr and Rb contents. The sandstone blocks in the following locations are relatively rich in Sr and Rb: balusters in the northern (176 and 57 ppm, respectively) and southern (188 and 57 ppm) palaces of the Wat Phu temple, the sanctuary of the Tao Tao temple (146 and 44 ppm), and the Hong Nang Sida temple (129 and 59 ppm). In the other locations, the Sr and Rb contents are low and the average values are 110 and 35 ppm or less, respectively. The K content also tends to be low (1700-3800 ppm).

Magnetic susceptibility
Magnetic susceptibility measurements were performed at a total of 141 locations. Fig. 6 shows the measurement locations and average magnetic susceptibility at each location. The full measurement results are shown in Supplementary Material 3. The measurements were made on 50 sandstone blocks at each location. The minimum and maximum average values were 0.94 × 10 − 3 and 2.36 × 10 − 3 SI units, respectively, and the overall average was 1.68 × 10 − 3 SI units (Table 2). At the Bayon, Ta Prohm, Preah Khan, and Banteay Kdei temples in the Angkor monument, which were built during the same Bayon period, the magnetic susceptibility of the   sandstone blocks changes systematically over time and it was therefore possible to estimate the order of construction based on the magnetic susceptibility [8]. However, in the case of the Banteay Chhmar temple, no systematic change in magnetic susceptibility was observed and thus the order of construction could not be estimated. Magnetic susceptibility was also measured at quarries in Ta Phraya, Thailand (Fig. 3f), which is considered to be the supply source for the Banteay Chhmar temple sandstone. Large quarrying traces were found in four locations with average magnetic susceptibilities of 1.42, 1.20, 2.05, and 1.76 × 10 − 3 SI units, respectively, and 1.70 × 10 − 3 SI units in total (Supplementary Material 4). This value is nearly the same as the average magnetic susceptibility for the sandstone blocks of the Banteay Chhmar temple (1.68× 10 − 3 SI units). Fig. 7 shows a frequency diagram of the magnetic susceptibility. Although the number of measurements at the Ta Phraya quarries is small, the sandstone at the Banteay Chhmar temple and Ta Phraya quarries show similar shaped frequency diagrams. This indicates that the Ta Phraya quarries are likely to be the host quarries of the Banteay Chhmar temple sandstone.

Chemical composition
There are no notable differences between the sandstone of the Banteay Chhmar temple and that of the Angkor, Bakan, and Koh Ker monuments, with Sr and Rb contents in the range of approximately 173-220 and 50-82 ppm, respectively. (Table 2

) (Supplementary Material 5).
The surface of the sandstone at the quarries in Ta Phraya is severely altered. Sandstone fragments that had fallen around the quarries were thus broken with a hammer and cross sections were analyzed using a pXRF analyzer ( Table 2) (Supplementary Material 4). The measured Sr and Rb contents were in the range of approximately 154-195 and 49-86 ppm, respectively. The Rb content was similar to that of the Banteay Chhmar temple sandstone, but the Sr content was approximately 25 ppm lower on average. Considering that Sr tends to be leached more rapidly by weathering than Rb, the chemical composition of the Ta Phraya sandstone can be considered to be nearly the same as that of the Banteay Chhmar temple sandstone.

Sandstone outcrops and quarries of the Red Terrane Formation in Laos
Magnetic susceptibility measurements and pXRF analyses were conducted in the following sites ( Fig. 2): Quarry 1: ancient quarry behind the sanctuary of the Wat Phu temple, Quarry 2: ancient quarry behind the Hong Nang Sida temple (Fig. 3c) Material 2). Aside from Quarry 4 and Outcrop 1, the K contents tend to be low (2400-5200 ppm).

Sandstone outcrops of the Red Terrane Formation in Cambodia
Measurements of sandstone in the Red Terrane Formation were conducted at one modern quarry in Preah Vihear province, three outcrops in Kratie province, three outcrops in Mondulkiri province, and one outcrop in Ratanakiri province in Cambodia (Table 2) (Supplementary Material 5). The modern quarry in Preah Vihear province is located 38 km northeast of the Bakan monument and contains sandstone with mostly low average magnetic susceptibility (0.51× 10 − 3 SI units), although some areas with high magnetic susceptibility were also noted. These average values are low compared with the sandstone blocks of the Angkor monument. However, the average Sr and Rb contents are 218 and 66 ppm, respectively, which are similar to those of the Angkor monument sandstone. The average magnetic susceptibility of the sandstone at the outcrops in Kratie, Mondulkiri and Ratanakiri provinces were as low as 0.24, 0.19, and 0.41 × 10 − 3 SI units, respectively, the average Sr contents were 201, 162, and 133 ppm, and the average Rb contents were 68, 53, and 72 ppm. The magnetic susceptibility in all places was lower than that of the sandstone in the Angkor monument. The outcropped sandstones in Kratie province showed similar Sr and Rb values to the Angkor monument sandstone. The Rb contents in the sandstones of Mondulkiri and Ratanakiri provinces were similar to those of the Angkor monument, whereas the Sr contents were slightly lower.

Sandstone in the My Son monument and sandstone outcrops in Vietnam
Measurements were made on the sandstone blocks used in the B1 building, which is the only sandstone structure in the My Son monument, and those in the openings of other buildings. Measurements were also made on the sandstone outcrops of the Red Terrane Formation approximately 44 km west of Nha Trang City and 75 km northeast of Ho Chi Minh City (Table 2)   approximately the same as the Angkor monument sandstone. The Sr contents of the My Son monument sandstone were essentially the same as the Angkor monument sandstone, whereas the values at the outcrops were lower.

Sandstone outcrops of the Red Terrane Formation in Thailand
Measurements were made at three locations (southwest, west, and northwest) on the sandstone of the Red Terrane Formation (Phu Kradung Formation) distributed in the western areas of the Khorat Plateau [22] (Table 2)   and 75 ppm, and average Rb contents of 50, 42, and 49 ppm. The sandstone in the southeastern area showed nearly the same composition as the Angkor monument sandstone, but the Sr contents were low in the other areas.

Magnetic susceptibility
The magnetic susceptibilities of the sandstone blocks used in the Banteay Chhmar temple as well as the Angkor monuments [2,8,9] are higher than those used in the Wat Phu, Hong Nang Sida, Tao Tao and Tomo temples. The magnetic susceptibility of the sandstone blocks in the Banteay Chhmar temple is relatively high and ranges from 0.9 to 2.4 × 10 − 3 SI units (Fig. 6). These values are similar to Fig. 8. Sr versus magnetic susceptibility diagram for the sandstones in the studied monuments, quarries, and outcrops. those of the sandstone blocks in the Angkor monuments (0.7 to 8.8 × 10 − 3 SI units) and the early-stage buildings of the Bakan monument (1.4 to 2.0 × 10 − 3 SI units). The sandstone blocks with relatively high magnetic susceptibilities ranging from 0.7 to 1.3 × 10 − 3 SI units are also used in the Koh Ker monument [5]. The sandstone blocks used in the Angkor monument and early-stage buildings of the Bakan monument are deduced to have been supplied from the southeastern foothill of Mt. Kulen [4], where many traces of ancient sandstone quarries remain [10,11,[23][24][25][26][27][28]. The magnetic susceptibilities of the sandstone blocks in the Wat Phu temple and its surrounding temples (0.14 to 0.42 × 10 − 3 SI units) are remarkably lower than those used in the Angkor monuments, the Banteay Chhmar temple, and the early-stage buildings of the Bakan monument. The Red Terrane Formation is widely distributed in Cambodia, Thailand, Laos, and Vietnam. Except for the southeastern foothill of Mt. Kulen, and the southern foothill of the Dangrek Range where the Ta Phraya sandstone quarry is situated, the sandstone of the Red Terrane Formation shows low magnetic susceptibility, ranging from 0.2 to 0.5 × 10 − 3 SI units. Fig. 9. Sr versus Rb diagram for the sandstones in the studied monuments, quarries, and outcrops.

Chemical composition
The Sr and Rb contents of the sandstone blocks in the Banteay Chhmar temple are 173-220 and 50-82 ppm, respectively, which are similar to those in the Angkor, Koh Ker, and Bakan monuments (Supplementary Material 5). However, the Sr (72-189 ppm) and Rb (23-60 ppm) contents of the sandstones used in the Wat Phu temple and its surrounding temples are lower than the above-mentioned monuments and temples (Table 2).
Judging from these trends, the higher magnetic susceptibility and higher Sr and Rb contents of the sandstone used in the Angkor monument, early-stage buildings of the Bakan monument, supplied from the southeastern foothill of Mt. Kulen, and Banteay Chhmar temple, supplied from Ta Phraya in the southern foothill of the Dangrek Range, seem to be rather exceptional.

Relationship between magnetic susceptibility and chemical composition
The sandstone of the Red Terrane Formation of the western (Thailand) and eastern areas (Laos) of the Khorat Plateau, Kratie, Mondulkiri and Ratanakiri provinces in Cambodia, and Nha Trang and Dinh Quan areas in Vietnam shows low magnetic susceptibility and low Sr contents (Figs. 8-10). The magnetic susceptibility and Sr contents of the sandstone of the Red Terrane Formation in these locations show similar trends to the sandstones of the Wat Phu, Hong Nang Sida, Tao Tao and Tomo temples. In contrast, only the sandstone quarries in Ta Phraya and the southeastern foothill of Mt. Kulen show high magnetic susceptibility and high Sr contents. Sandstone with high Sr but with low magnetic susceptibility is observed, for example, in the modern quarry in Preah Vihear province in Cambodia, some outcrops in Kratie province in Cambodia, the My Son monument in Vietnam, the balusters of the palaces of the Wat Phu temple in Laos, and the outcrops in the southwestern part of the Khorat Plateau in Thailand.
As described above, the sandstone of the Red Terrane Formation ranges from high to low magnetic susceptibility and Sr contents, but the Sr content tends to decrease later than the magnetic susceptibility (Fig. 8). That is, the sandstone of the Red Terrane Formation with high Sr contents of 170-250 ppm is observed even though the magnetic susceptibility shows a low value of 0.2 to 0.5 × 10 − 3 SI units. The magnetic susceptibility of the sandstone is considered to be mainly proportional to the amount of magnetite contained therein, whereas Sr is presumed to be mainly contained in plagioclase. Sandstone forms when source rocks are weathered and sand grains are transported by water currents, deposited, and lithified. The above-mentioned measurement results for the sandstone of the Red Terrane Formation seem to indicate that magnetite weathering is more likely to proceed than plagioclase weathering. Conversely, the sandstone with high magnetic susceptibility shows high Sr contents (170-250 ppm). A similar tendency is seen for Rb ( Fig. 9) but is slightly less clear. Rb is considered to be mainly contained in K-feldspar, which is considered to be likely more slowly weathered than plagioclase. From this assessment, it can be concluded that the sandstone of the Red Terrane Formation with high magnetic susceptibility and high Sr contents was weakly weathered during its formation process and conversely, the sandstone with low magnetic susceptibility and Sr contents likely formed under the strong weathering process.

Formation condition and sediment source of the sandstone
As mentioned above, stronger weathering during the sandstone formation process is considered to have resulted in the lower magnetic susceptibility and Sr contents of the sandstone of the Red Terrane Formation. Most of the sandstone is therefore considered to have been strongly weathered during its formation. Such sandstone is commonly observed in wide areas of Thailand, Cambodia, Laos, and Vietnam. In contrast, the sandstones at quarries on the southeastern foothill of Mt. Kulen in Cambodia and those in Ta Phraya on the southern foothill of the Dangrek Range in Thailand have exceptionally high magnetic susceptibility and Sr contents compared with other regions. This may indicate that weathering during the sandstone formation process was relatively weak in these areas. In addition, because the Red Terrane Formation is distributed over a very wide area of 1200 km north-south and 800 km east-west in Thailand, Cambodia, Laos, and Vietnam, it is likely that the sediment source also differed from place to place. It is therefore possible that magnetite was abundant in the source rocks of the sandstone as one factor of high magnetic susceptibility in Mt. Kulen and Ta Phraya areas. The results presented here clearly indicate the differences of these particular areas compared with other areas of the study location.

Conclusions
Non-destructive magnetic susceptibility measurements and chemical analyses using a pXRF analyzer were conducted on the sandstone blocks of the Wat Phu temple, its surrounding small temples, and the Banteay Chhmar temple. The sandstone blocks of the above-mentioned temples were supplied from the Red Terrane Formation. The sandstone of this Formation, distributed throughout the Mainland Indochina, was also investigated, and its magnetic susceptibility and chemical composition were compared with the sandstone blocks used in the temples. We elucidated that most of these sandstones show low magnetic susceptibilities and low Sr contents, similar to those of the Wat Phu temple and its surrounding temples. Most of the sandstone of the Red Terrane Formation is therefore considered to have been strongly weathered during its formation process. On the contrary, sandstone with high magnetic susceptibilities and high Sr contents is found in the sandstone quarries of Ta Phraya and the southeastern foothill of Mt. Kulen, which are the supply source of the sandstone blocks used in Banteay Chhmar temple and the Angkor monument, respectively. The sandstone with high magnetic susceptibility and high Sr content is distributed in limited areas and implies a weak degree of weathering during the sandstone formation process or a difference in the source rocks.

Author contribution statement
Etsuo Uchida: Conceived and designed the experiments, Performed the experiments, Analyzed and interpreted the data, Wrote the paper.
Yu Sulu & Rui Du: Performed the experiments, Analyzed and interpreted the data.

Data availability statement
Data included in article/supp. material/referenced in article.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Science (Uchida: nos. 23401001 and 19KK0016) and Waseda University Grants for Special Research Projects (Uchida: no. 2016B-137). The field investigation in the Wat Phu, Hong Nang, Tao Tao, and Tomo temples was conducted with the permission of the Ministry of Information, Culture and Tourism of Laos. We would like to express our sincere thanks to Soukahn Chithpanya and Phisith Sihalarth of National University of Laos and Oudomsy Keosaksith and Amphol Sengphachanh of the Wat Phu World Heritage Site Office for their kind help. The investigation in the Banteay Chhmar was conducted with the permission of the Cambodian Ministry of Culture and Fine Arts. The sandstone investigation in Thailand was conducted with the permission of the National Council of Thailand. The authors are grateful to two anonymous reviewers for their insightful reviews and valuable comments to improve the quality of the manuscript. We thank Edanz Group for editing a draft of this manuscript.