Trace Element Analysis by PIYE and ICP-AES of Raw Material and Ancient Serpentine Artifacts from China

DONG Jun-qing, WANG Yong-ya, GAN Fu-xi, 2, LI Qing-hui*

1.Center of Sci-tech Archaeology, Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences, Shanghai 201800, China 2. Fudan University, Shanghai 200433, China

Trace Element Analysis by PIYE and ICP-AES of Raw Material and Ancient Serpentine Artifacts from China

DONG Jun-qing1, WANG Yong-ya1, GAN Fu-xi1, 2, LI Qing-hui1*

1.Center of Sci-tech Archaeology, Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences, Shanghai 201800, China 2. Fudan University, Shanghai 200433, China

This work mainly talks about serpentine mineral with the aim to explore the possible raw materials sources of ancient serpentine artifacts by trace element content analysis. The major and trace elements of serpentine samples from several typical deposits in China were nondestructively determined by external-beam proton-induced X-ray emission (PIXE). For comparison, trace element concentrations were destructively measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results showed the trend of the trace element contents of serpentine jade obtained by the two methods have preferably coherence, which indicate that the nondestructive technique of PIXE can be applied to trace element analysis of serpentine. The relationship between trace element contents and serpentine formation mechanism was discussed. The difference of the trace elements contents in these serpentine minerals is obvious. It can be used to distinguish the different kinds of serpentine formed by different mechanisms. A low amount of Ni and almost no Cr and Co were found in type I serpentine group mineral, whereas significant amounts of Cr, Co and Ni were found in Type II serpentine group mineral. The chemical composition of 18 ancient serpentine artifacts were analyzed by PIXE, they were unearthed from 14 sites and tombs in provinces of Zhejing, Jiangsu, Henan, Anhui and Hubei and dated from Neolithic Age to the Warring States Period (4585 BC—231 BC). By comparing the trace element contents between ancient serpentine artifacts and two kinds of serpentine samples, the provenance of ancient serpentine artifacts were preliminarily inferred. It is beneficial to try to explore the possible raw material of ancient serpentine artifacts based on the relationship between the trace element contents and serpentine formation mechanism in this article.

PIXE; Chinese ancient serpentine artifact; Trace element; Formation mechanism; Original source

Introduction

Jade means beautiful stone in China. The Chinese ancient jade culture has a long history, with a wide distribution, long duration, and numerous varieties. The raw materials of ancient jade artifacts and their original sources are two very important topics in ancient jade artifact research[1-2]. Several studies have focused on determining the sources of nephrite jade artifacts[3-5]. However, little is known regarding the sources of serpentine artifacts[6]. As one of the four most famous jades (nephrite, serpentine, turquoise and Dushan jade) in China, serpentine is one of the most important materials of jade artifacts. Large amounts of serpentine artifacts were unearthed from different archaeological sites, the ages of which range from the middle and late Neolithic Age (4585 BC to 2200 BC) to the Qing Dynasty (1616 AD to 1912 AD). Ascertaining the original sources of serpentine artifacts dating before the West Han Dynasty (206 BC to 9BC), especially from the middle Neolithic Age to the Warring States Period (475 BC to 221 BC ) is of great importance in the research on ancient jade artifacts because of the lack of historical data. The fact that numerous serpentine deposits have been be found in China[7]raises that the problem of provenance of these serpentine artifacts. Previous works[8-11]revealed that the major chemical composition of serpentine formed by different mechanisms is similar. Thus, it is impossible that distinguishing serpentine formed by different mechanisms based on their major chemical composition. Changes in the geological environment are reflected in the trace element content, which is a valuable factor for determining the original source of jade[12-14].

It is well known that before the West Han Dynasty the ancient Chinese had three jade cultural centers: the West Liaohe River Basin in Northeast China, Yangtze Valley and the Central Plains of China. Most ancient jade artifacts were excavated from the two latter regions. In China, several famous serpentines originate from different deposits, such as Xiuyan serpentine (named from the Xiuyan county, Liaoning province) in northeast China, Xichuan serpentine (from Xichuan county, Henan province) in the Central Plains of China, Lantian serpentine (from Lantian county, Shaanxi province) in northwest China, Luchuan serpentine (from Luchuan county, Guangxi province) in southwest China, and so on. The present work aims to determine the elemental composition of serpentines from different jade deposits (Xiuyan, Xichuan, Lantian and Luchuan) in China and Chinese ancient serpentine artifacts selected from different sites (tombs) to investigate provenance of the raw material of these artifacts. In this work, external-beam proton-induced X-ray emission (PIXE) was used to measure the amount of major and trace elements in the serpentine samples obtained from the four deposits in China. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was also used to determine the trace element concentrations of the samples for comparison. The major and trace element concentrations of the ancient serpentine artifact samples from some representational archaeological sites (tombs) in the Yangtze River Basin and the Central Plains of China were also analyzed by PIXE. By comparing the trace element contents of the Chinese ancient serpentine artifacts with those of the serpentine samples, the types of ancient serpentine artifacts were determined. Trace element concentrations can also be used to infer the original raw material sources of the jade artifacts, providing important insights into the archaeological study of ancient jade artifacts and stone artifacts.

1 Experimental

1.1 Instrumentation

1.1.1 External-beam proton induced X-ray emission spectroscopy

Modified external-beam PIXE experiments were performed at the Institute of Modern Physics, Fudan University. The 9SDH-2 beam line of 3.0 MeV tandem accelerators was used, but the actual energy of protons that reached the sample was 2.8 MeV. This decrease in energy is due to energy loss. The protons reached the sample at a small spot with a diameter of 1 mm. Induced X-ray was collected by a Si (Li) detector with an energy resolution of 165 eV and a full width at half maximum of 5.9 keV. Light elements (Z<11) could not be measured precisely due to the absorption of characteristic X-ray in air. Helium gas was cieculated between the sample and the detector to ensure the measuring accuracy of Na content. The angle between the beam direction and the normal to the sample was 45°. Beam current was set at 0.01 nA during determination of the major elements. The instrument dead time was less than 5%. Chemical composition (Z≥11) of the samples was obtained from the PIXE spectrum using the de-convolution program GUPIX[15]. Details of the external-beam PIXE experiments can be found in the paper by Cheng et al.[16]. The detection limits of PIXE for K and Ca can reach 2 μg·g-1, whereas that for elements with high atomic number was approximately 20 μg·g-1. The measurement error ranged from approximately 2% to 4% for the major elements and from approximately 5% to 15% for the trace elements. H2O content was ignored in the test, and the other element oxide contents were normalized.

1.1.2 Inductively-coupled plasma atom emission spectroscopy

The trace element concentrations of the serpentine samples were also determined by an Iris Intrepid ER/S inductively-coupled plasma-atomic emission spectrometer. Prior to the ICP-AES test, the samples were ground into powder and filtered through a 200 -mesh gauze. The section ground into powder was the same as that selected for the PIXE tests. Approximately 0.5 g of each sample was weighed, put into a platinum crucible, and then decomposed using HF and HClO4. Then, HF was expelled until the HClO4fumes were exhausted. The sample was extracted using HCl and transferred into a measuring flask to determine its volume. High-temperature plasma produced by the interaction of an induced magnetic field and argon gas serves as effective excitation source and charged ions source. The single element standard solutions were supplied at concentrations of 0 (for ultra-pure water), 10, 20, 30, 50, 100, 500, 1 000 and 2 000 μg·L-1(increased to 50 mg·L-1for Fe), To generate calibration curves, the concentration of each element was determined three times. The detection limit of ICP-AES was described in ppb, and the relative error was less than 5%.

1.2 Samples description

1.2.1 Chinese ancient serpentine artifacts

Based on the phase analysis of ancient jade artifact samples unearthed from the Yangtze River Basin and the Central Plains of China, some serpentine artifact samples [from the middle Neolithic age to the Warring States Period (4585 BC to 221 BC)] excavated from the ancient ruins and tombs in the provinces of Zhejiang, Jiangsu province, Henan, Anhui and Hubei (Fig.1), were identified. A total of 18 representative ancient Chinese serpentine artifact samples (Fig.2) with long duration and wide distribution were carefully selected from 14 sites or tombs of the five provinces for PIXE analysis. These artifact samples, commonly ornaments or tools, range from white to yellow and green. All samples were cleaned with anhydrous alcohol before testing. The characters, archaeological cultures, names, dates, locations, and regions of the archaeological samples are shown in Table 1.

Fig.1 Distribution of serpentine jade samples and ancient serpentine jade artifact samples from China

Fig.2 Chinese ancient serpentine jade artifact samples (A1 to A18)

1.2.2 Serpentine mineral samples

Serpentine minerals can be divided into two types based on their formation mechanism. Type Ⅰ serpentine is primarily formed by contact metasomatism between Mg-rich carbonates and intermediate acidic intrusive rocks, whereas Type Ⅱ serpentine is mainly formed by metasomatism of ultramafic rocks (mainly olivine) and low-medium temperature[17-18]. The serpentine samples (Fig.3; Table 2) were collected from four well-known jade deposits in China, the locations of which are shown in Fig.1. To distinguish these serpentines, they were named according to their geographic location. Based on previous studies, Xiuyan jade[19], Lantian jade[20]and Luchuan jade[21]are Type Ⅰ serpentines, whereas the Xichuan jade is a Type Ⅱ serpentine[22]. The geological context and tectonic setting can be found in these papers. Given that the shapes of the samples were formed by processing, the shapes indicate no special meaning. The samples were polished using cerium oxide powder (300 meshes, approximately 50 μm in diameter), immersed in pure alcohol, and then ultrasonically cleaned for 10 min. The section of each sample displaying a uniform color was selected for testing, so a single analysis could be representative.

Table 1 Description of Chinese ancient serpentine artifacts

*cited from reference [6]

Fig.3 Serpentine jade samples (S1 to S10)

Table 2 Description of serpentine samples from China

No.SampleColorDeposit(Province)latitude/longitudeTypeS110LN-2GreenXiuyanLiaoningProvince40.39N,123.41EtypeⅠS210LN-3Yellowish-greenXiuyanLiaoningProvince40.39N,123.41EtypeⅠS310LN-6LightGreenXiuyanLiaoningProvince40.39N,123.41EtypeⅠS4LT2DarkGreenLantianShaanxiProvince34.15N,109.15EtypeⅠS509LT2Greenish-blackLantianShaanxiProvince34.15N,109.15EtypeⅠS610LC-1Yellowish-greenLuchuanGuangxiProvince22.33N,110.25EtypeⅠS710LC-2Yellowish-greenLuchuanGuangxiProvince22.33N,110.25EtypeⅠS809XC-6Greenish-blackXichuanHenanProvince33.23N,111.53EtypeⅡS910XC-1Greenish-blackXichuanHenanProvince33.23N,111.53EtypeⅡS1010XC-2Greenish-blackXichuanHenanProvince33.23N,111.53EtypeⅡ

2 Results and Discussion

2.1 Chemical composition analysis of the serpentine samples

The chemical compositions of the serpentine samples determined by PIXE are listed in Table 3. The major chemical components of the samples are MgO and SiO2. The MgO content ranges from 39.45% to 46.96%, whereas the SiO2content ranges from 43.54% to 50.94%. The Al2O3content of the samples is low and less than 2.5%. The Fe2O3content in the deeply-colored samples, such as S1, S4, S5, S8, S9 and S10, is above 2%. As for the trace elements, the Cr2O3and NiO contents of S8 to S10 range from 0.07% to 0.53% and from 0.17% to 0.37%, respectively. Almost no Cr2O3and NiO are present in S1 to S7.

The trace element contents，as determined by ICP-AES, are given in Table 4. The Cr, Co and Ni contents of the Type Ⅱ serpentine samples (S8 to S10) range from 962 to 998 μg·g-1, from 37 to 74 μg·g-1and from 958 to 1 350 μg·g-1, respectively. These values are much higher than those of the Type I serpentine samples (S1 to S7), which have a very low amount of Ni and no Cr or Co. These results are consistent with the trace element concentrations determined by PIXE. The three-dimensional distribution of the Cr, Co and Ni contents of the serpentine samples is shown in Fig.4. The two types of serpentine are obviously different in terms of their trace element content, which can be used to distinguish them from each other.

Table 3 Chemical compositions of serpentine samples determined by PIXE (Wt. %)

The quantitative values of each element concentration, as determined by PIXE and ICP-AES, are not identical. This finding may be attributed to the fact that the quantitative element content determined by ICP-AES is the average concentration of the powdered samples, whereas PIXE determines the concentrations of points on the surface of block samples. In addition, the measurement errors of the two methods are different. The measurement errors would increase with decreasing element concentration. Thus, these methods can only provide a qualitative comparison. These reasons could explain the significant differences between the values obtained from PIXE and ICP-AES for Al2O3, CaO and Fe2O3. Overall, ICP-AES is a destructive method, because the samples were ground into powder and cannot be applied to ancient jade artifacts. PIXE does not have such requirement; thus, it is suitable for intact archaeological artifacts.

Table 4 Minor and trace elements of serpentine samples determined by ICP-AES (μg·g-1)

Fig.4 Three-dimensional distribution of the Cr, Co, and Ni contents of serpentine jade samples

2.2 Chemical composition analysis of the ancient serpentine artifact samples

The major and trace elements in the ancient serpentine artifact samples, as determined by PIXE, are listed in Table 5. The major chemical components of the samples are MgO and SiO2. The MgO content ranges from 33.51% to 44.66%, whereas the SiO2content ranges from 44.17% to 53.43%. The MgO content is lower, and the SiO2content is higher in the jade artifacts than in the serpentine samples. This finding may be attributed to the leaching process on the surface, during which some MgO is lost. However, the MgO and SiO2contents of A12 are both lower than those of the other samples because of its high Fe2O3content. The Al2O3content of most of the samples is high, which may have resulted from weathering mechanisms.

Table 5 Chemical compositions of ancient serpentine artifacts determined by PIXE (Wt. %)

*cited from reference [6]

With regard to the trace element contents of the ancient serpentine artifact samples, the Cr2O3contents of A1, A7, A9, A11, A12, A13, A14, and A17 range from 0.06% to 0.8%. The CoO content ranges from 0% to 0.07%, and the NiO content ranges from 0.18% to 0.32%. In all other samples, Cr2O3, CoO and NiO are nearly absent.

2.3 Relation between the formation mechanisms and trace element concentrations of the serpentine samples

The results demonstrate that the trace element contents of the Type Ⅰ and Type Ⅱ serpentines are significantly different. The Type Ⅰ serpentine samples including S1 to S7, also called Mg-rich carbonate type serpentine, formed by contact metasomatism between Mg-rich carbonates and intermediate acidic intrusive rocks. The bedrocks of Xiuyan jade, Lantian jade, and Luchuan jade mainly consist of magnesite and dolomite. Their iron content is also mainly from magnesite and dolomite, both of which are trigonal carbonate minerals. Given that magnesite has low iron content and often coexists with dolomite and calcite, a high amount of Ca and a low amount of iron are available for the formation of Type Ⅰ serpentine. The type Ⅱ serpentine samples including S8 to S10, also called ultramafic rock type serpentines, formed by metasomatism of ultramafic rocks (mainly olivine) and low-medium temperature hydrotherms. The bedrocks of Xichuan jade mainly consist of olivine. Their iron content is mainly derived from olivine, which belongs to the orthorhombic island silicate minerals. The ionic radii of Fe2+(0.74), Cr3+(0.63), Co2+(0.72) and Ni2+(0.69) are similar to that of Mg2+(0.66). Thus, Fe2+, Co2+, Ni2+, Cr3+and Mg2+can act as absolute isomorphic substitutes for one another in olivine. Furthermore, olivine is often associated with chromite containing a high content of Cr3+. Hence, Cr, Co, and Ni are present in high amounts in olivine, which accounts for their high amounts in Type Ⅱ serpentines. In addition, Cr, Co and Ni are also siderophile. Thus, higher amounts of Cr, Co and Ni are present in Fe-rich ultra-basic rocks than in Mg-rich carbonate rocks. Overall, the Type Ⅱ serpentine samples have significantly higher amounts of Cr, Co, and Ni than Type I serpentine samples. This difference can be used to distinguish between the two kinds of serpentine.

Aside from their formation mechanisms, the serpentine samples in this study can also be divided into two kinds based on their trace elements. S1 to S7 belong to Type Ⅰ, whereas S8 to S10 belong to Type Ⅱ. The ancient serpentine artifact samples can be divided into two kinds as well. A1, A7, A9, A11, A12, A13, A14 and A17 belong to Type Ⅱ, whereas A2, A3, A4, A5, A6, A8, A10, A15, A16 and A18 belong to Type Ⅰ.

2.4 Provenance of the Chinese ancient serpentine artifact samples

Among the ancient serpentine artifact samples from the Central Plains region, the jade artifact sample A11 and the serpentine samples S8 to S10 collected from Xichuan, are Type II serpentines. The major and trace element contents of these samples are similar. Given that the age of A11 dated to Yangshao Culture was approximately 4585 BC, and that the scope of activities of the Chinese ancestors was relatively small at that time, the raw material of A11 originated from the local Xichuan region based on the raw materials of the jade artifact samples from local or nearby resources. By the same token, the raw materials of A12 and A13 from Luoyang, A14 from Anyang and A17 from Bengbu should also be from Xichuan. All serpentine samples from Luchuan Guangxi, Lantian Shaanxi and Xiuyan Liaoning are Type I serpentines. However, Luchuan is far from the Central Plains region, which is more than 1000 km, and the transportation capacity is very low in the ancient era. Thus, long-distance material exchange was not popular at that time. Luchuan is not likely to be the source of the raw materials of the ancient jade artifact samples from Central Plains region. The raw materials of A15, A16, and A18 may be from Lantian Shaanxi or Xiuyan Liaoning. However, these artifact samples could also have been produced in the local region. This indicates that the activities range of people is very small during Yangshao Culture, and the jade material can only be collected from within the vicinity of their residence. Later, the increased scope of activities also expands the exchange between the people, allowing them to obtain jade material from distant places. This finding is of great significance in the study of economic and cultural exchanges during that time.

The Xiawanggang site is located in Xiawanggang Village, Xichuan County, the time of the site range from Yangshao Culture to Western Zhou Dynasty. Aside from serpentine, various jade materials such as tremolite, turquoise, chlorite, muscovite and calcite are used. The sources of these raw materials remain unknown[23]. Based on the comparison of the trace elements of serpentine samples, A11 from the second period of the Yangshao Culture was preliminarily confirmed form the local region. This result provides a basis for the exploration of the use of the jade resources of Xiawanggang ancestors.

For the ancient serpentine artifact samples from the lower Yangtze River Basin, the jade axe A1 excavated in the Jiangjiashan site in Zhejiang Province and samples A7 and A9 of the Liangzhu Culture, unearthed in Jiangsu Province, are Type Ⅱ serpentines. Other samples (A2, A3, A4, A5, A6, A8 and A10) of the Liangzhu Culture are Type Ⅰ serpentines. The result suggests that two kinds of serpentine were used during the Liangzhu Culture. The Type Ⅰ serpentine artifact samples of the Liangzhu Culture, unearthed from various sites in Zhejiang Province, are quite similar to the tremolite jade artifact samples unearthed from the Zhejiang Liangzhu site[3]. To date, serpentine mines have not been found in the province of Jiangsu and Zhejiang. Thus, the provenances of the ancient serpentine artifact samples of the Liangzhu Culture require further study. The viewpoint proposed by Liu et al.[6]regarding A5 from Xiuyan in Liaoning Province deserves further discussion. Nevertheless, the identification of the original raw material sources of the ancient serpentine artifact samples is a long-term project that requires further investigation using different methods.

3 Conclusion

The significant differences in the trace element contents of serpentine are attributed to different formation mechanisms. The Cr, Co, and Ni contents of the Type II serpentine samples are much higher than those of the Type I serpentine samples. This observation can be used to distinguish the two kinds of serpentine. Ancient serpentine artifacts are found in many ancient ruins and tombs in China. Based on the results of trace element analysis by PIXE, the raw material of the ancient serpentine artifact A11 originated from Xichuan, Henan Province. PIXE was found to be an effective non-destructive analysis method for studying ancient serpentine artifacts. The raw materials sources of the other samples were also preliminarily inferred. PIXE was found to be an effective non-destructive analysis method for studying ancient serpentine artifacts. These results shed light on the original raw material sources of Chinese ancient serpentine artifacts and provide a useful archaeological reference for serpentine in other countries. For this reason, it can be applied to more serpentine ancient serpentine artifacts excavated from archaeological sites or tombs and museum objects in China the provenance of which has to be determined.

Acknowledgments： We extend our gratitude to the Zhejiang Province Institute of Archaeology, Henan Province Institute of Archaeology, Nanyang City Institute of Archaeology, Anyang City Institute of Archaeology, Luoyang City Cultural Relics Work, Anhui Province Institute of Archaeology, Museum of Bengbu City, Museum of Nanjing, Museum of Jiangyin City, Cultural Relics Bureau of Kunshan City, Hubei Province Institute of Archaeology, Xiangfan City Institute of Archaeology and other collaborators for their support. The authors are also grateful to Professor Huangsheng Cheng of Fudan University for his help during the PIXE test.

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*通讯联系人

TL99

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利用PIXE和ICP-AES 对蛇纹石原料及中国古代蛇纹石玉器的微量元素分析

董俊卿1, 王永亚1, 干福熹1,2, 李青会1*

1. 中国科学院上海光学精密机械研究所科技考古中心，上海 201800 2. 复旦大学，上海 200433

以蛇纹石这一中国传统的玉材为对象，旨在通过微量元素含量分析来探索古代蛇纹石玉器原料的可能来源。采用无损的外束质子激发X射线荧光技术(PIXE)对来自中国几个典型矿区的蛇纹石样品主量元素和微量元素进行了分析。同时也采用有损的电感耦合等离子体原子发射光谱技术(ICP-AES)对这些蛇纹石样品的微量元素含量进行了比较分析。结果表明，两种分析方法所获取的蛇纹石微量元素含量趋势具有一致性，说明PIXE无损分析技术可以应用于蛇纹石的微量元素分析研究。讨论了蛇纹石的微量元素含量与地质成因之间的关系，这些蛇纹石的微量元素含量存在明显的差异，这种差异可以用来区分不同地质成因的蛇纹石。Ⅰ型地质成因的蛇纹石中微量元素中Ni含量较低，且几乎不含Cr和Co，而Ⅱ型地质成因的蛇纹石中则含有较高微量元素的Cr，Co和Ni。采用PIXE技术分析了来自浙江、江苏、河南、安徽和湖北等省出土的新石器时代至战国时期(4585 BC—221 BC)14个遗址或墓葬出土的18件蛇纹石玉器的化学成分，通过与两种地质成因类型蛇纹石的微量元素比较分析，初步推测了这些蛇纹石玉器原料的可能来源。以蛇纹石微量元素含量与地质成因类型之间的关系来探索古代蛇纹石玉器玉料可能的来源是一种有益的尝试。

PIXE；中国古代蛇纹石玉器；微量元素；地质成因；玉料来源

2015-05-22，

2015-10-11)

Foundation item： The 973 Program (2012CB720906), the Shanghai R&D Public Service Platform Construction Project (13DZ2295800), and the National Key Technology Support Program(2013BAK08B08)

10.3964/j.issn.1000-0593(2016)11-3780-09

Received： 2015-05-22; accepted： 2015-10-11

Biography： DONG Jun-qing, (1980—), Assistant Researcher, main research: scientific and technological research of ancient jade artifact, glass and ceramic e-mail: djqing_2005@126.com *Corresponding author e-mail: qinghuil@sina.com