A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian

Quaternary International xxx (2017) 1e11

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A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian
rka Maro
ti, Ildiko
Harsa
nyi, De
nes Pa
rka
nyi, Veronika Szila
gyi Zsolt Kasztovszky*, Bogla Nuclear Analysis and Radiography Department, Centre for Energy Research, Hungarian Academy of Sciences, Konkoly Thege str. 29-33, H-1121, Budapest, Hungary

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2017 Received in revised form 1 August 2017 Accepted 2 August 2017 Available online xxx

Prompt Gamma Activation Analysis has successfully been applied to provenance research on Carpathian obsidians. The effectiveness of PGAA and a portable XRF device in discriminations of Carpathian, Lipari, Sardinia and Melos origin obsidians was compared on 75 representative geological samples obtained from the Lihotheca Collection of the Hungarian National Museum. Bivariate analyses and Principal Component Analysis have been made based on the individual PGAA and XRF data, as well as on the combination of both data types. Instrumental Neutron Activation Analysis was also applied on a group of 17 samples. The advantages and disadvantages of each method are discussed to determine the best possible way of investigations to fingerprint and characterize long-distance trade items with minimal damage to the samples. © 2017 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Prompt Gamma Activation Analysis Portable XRF Instrumental Neutron Activation Analysis Provenance Obsidian Archaeology Principal Component Analysis

1. Introduction Provenance (place of origin, i.e. the geological sources of the raw material) is a fundamental topic of research in heritage science. Identification of raw material types of the everyday objects (in prehistoric times, mostly various types of rock) provides data about the geographical patterns of distribution of these initial matters. Furthermore, the goods made of the raw materials were transported by prehistoric people so they can be inserted into regional or occasionally long-distance trade networks. The conclusions of provenance studies are mostly based on quantitative elemental compositions. However, since the objects of Cultural Heritage usually have an inestimable value, the characteristic ‘fingerprints’ (isotopic, elemental or mineralogical composition), must be determined without any damage to the object. In this paper, we compare two absolutely non-destructive methods for provenancing archaeological obsidians: Prompt

* Corresponding author. E-mail addresses: [email protected] (Z. Kasztovszky), maroti.
ti), [email protected] (B. Maro [email protected]
nyi), [email protected] (D. Pa
rka
nyi), szilagyi.veronika@ (I. Harsa
gyi). energia.mta.hu (V. Szila

Gamma Activation Analysis and X-ray Fluorescence Analysis, based on measurements carried out at the Centre of Energy Research, Hungary. The performance of Instrumental Neutron Activation
si et al., 2016) Analysis at our upgraded INAA laboratory (Szentmiklo was also investigated on a subset of samples. Obsidian, a homogeneous natural glass of volcanic origin, was a popular raw material during prehistory due to its advantageous mechanical properties (Kasztovszky et al., 2008b). The formation of obsidian requires specific geological conditions (Kasztovszky et al., 2017), so the major obsidian sources are quite well localized and already known all around the world (Pollmann, 1999). The most abundant and historically significant sources in the European and the Near East region can be found in the Mediterranean area (Sardinia, Lipari, Melos and Anatolia) (Williams-Thorpe, 1995). The spread of obsidian artefacts made from raw materials of Lipari and Palmarola origin in Central Italy during the VI millennium B.C. was studied by Radi and Petrinelli Pannocchia (2017). The Central European obsidian sources that have been also known in the prehistory are confined to a very small geological area in the Tokaj-Presov Mountains, at the border of today's Hungary and Slovakia, and in Trans-Carpathian Ukraine (Rokosovi). They are generally mentioned as ‘Carpathian’ obsidian in the archaeological and archaeometry literature (Oddone et al., 1999). The term ‘Carpathian’

http://dx.doi.org/10.1016/j.quaint.2017.08.004 1040-6182/© 2017 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004

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Z. Kasztovszky et al. / Quaternary International xxx (2017) 1e11

obsidian covers three main types (C1, C2, C3) and some subtypes
, 2014; Kasztovszky et al., 2017 for details). (C2E, C2T, C2Tr) (see Biro The best quality C1 has been transported for hundreds of kilometres; C2 has been circulated in long distance trade, while lower quality C3 have been served as local raw material e as has been shown with the help of analytical results (Williams-Thorpe et al.,
, 2014). An 1984; Oddone et al., 1999; Rosania et al., 2008; Biro overview of Epigravettian (20e17 ka BP) appearances of Carpathian obsidian in Eastern Slovakia is published by Lubomíra Kaminsk
a (2016). For the obsidian, both the major and the trace element concentrations vary only a little within any one location but significantly differ between various sites. Hence, the distances between the groups of objects in the ‘compositional space’ are typically greater than the distances between the objects belonging to one single provenance group (Kilikoglou et al., 1996; Mili
c, 2014). Since the geochemical composition of obsidian is highly characteristic for the geological origin, in many provenance studies precise, as much as possible, non-destructive analytical methods has been applied. Already from the late 1960s, Optical Emission Spectroscopy (OES) (Cann and Renfrew, 1964), Instrumental Neutron Activation Analysis (INAA) (Kilikoglou et al., 1996), X-ray Fluorescence Spectroscopy (XRF) (Tykot, 1997), ion-beam analytical (PIXE, PIGE) methods (Elekes et al., 2000; Le Bourdonnec et al., 2005) and also Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) (Yi and Jwa, 2016) have been applied to determine the geochemical properties of obsidians. Kasztovszky et al. have initiated provenance study of Carpathian obsidians, based on Prompt Gamma Activation Analysis (PGAA) (Kasztovszky et al., 2008a). They have shown that the concentrations of major geochemical components and also traces of B and Cl can contribute to the assignment of different obsidians to their respective geographical sources. All the above methods have already proven their precision and accuracy (Craig et al., 2007; Kasztovszky et al., 2008a; Nazaroff et al., 2009; Jia et al., 2010; Shackley, 2011). Since different methods can detect different set of elements, one has to choose a method that allows the sharpest discrimination. A critical issue for obsidian provenance research is that each instrument should independently and consistently provide the compatible data that will allow coming to the same archaeologically relevant conclusions (Nazaroff et al., 2009; Frahm, 2012; Mili
c, 2014). 2. Materials and methods Since the beginning of the 2000s, Prompt Gamma Activation Analysis has been in use to characterize obsidians. In a co-operation with the Hungarian National Museum (HNM), we have analysed about 140 pieces of geological hand specimens originating from the main Central European and Mediterranean sources (Tokaj Mts., Sardinia, Lipari, Melos, Antiparos, Pantelleria, Palmarola, Yali, Armenia and Anatolia), as well as 34 geological samples from Japan (Kasztovszky et al., 2012). Besides the raw materials from the Comparative Raw Material Collection of the HNM (Lithotheca), archaeological samples from Hungary, Romania, Croatia (Kasztovszky et al., 2009), Bosnia-Herzegovina (Kasztovszky et al., 2009), Serbia, and even from Poland (Sobkowiak-Tabaka et al.,
ski et al., 2015) have been successfully classified 2015; Kabacin into the main obsidian types, based on PGAA. Up to now, around 180 archaeological artefacts have been analysed. It turned out soon that PGAA can well discriminate between obsidian and other silicates (e.g. slags or silex) (Kasztovszky et al., 2008a). Also, we have shown (Kasztovszky et al., 2014) that on the basis of PGAA measurements, it is possible to distinguish between the obsidians originating from the main European-Mediterranean sources. Furthermore, using PGAA results, assignment of archaeological

obsidian of unknown provenance to one of the major raw material types can be done with a high confidence (Kasztovszky et al., 2014;
ski et al., 2015). Sobkowiak-Tabaka et al., 2015; Kabacin For this comparative study between PGAA and XRF, we have chosen 75 geological obsidian samples from the Lithotheca of the HNM. The selected pieces represent all the major obsidian types of interest to us, i.e. C1 e 28 pieces, C2E e 13 pieces, C2T e 14 pieces, C3 e 3 pieces, Lipari e 4 pieces, Sardinia e 6 pieces and Melos e 7 pieces. The summarised list of the geological obsidians used in this investigation, can be found in Table 1. The locations of the major European and Mediterranean sources can be seen on Fig. 1. Multiple samples of the same sources have been studied, whenever it was possible in order to verify the variability of the samples from one source. Compositions of further single pieces from Palmarola and Pantelleira and two pieces from Antiparos have been also measured, but, since the low representativeness, the data were not involved into the provenance study. We aimed to answer the following questions: 1, whether for the commonly detectable elements, the concentration values obtained from PGAA and from XRF agree within the uncertainties of the methods. 2, if not, can we find any correlation between the concentration data determined by PGAA and by XRF, respectively. 3, as a practical question, regarding the archaeometric aims, we wished to know whether the same classification of obsidian types can be achieved using portable XRF as it was already demonstrated using PGAA. 4, is it possible to improve the classification by a combination of the results from XRF and PGAA. In addition, a simple provenance case study using Instrumental Neutron Activation Analysis on a selected set of 16 geological samples, representative for each major group has been performed. For INAA, 6 pieces of C1, 3 pieces of C2, 1 piece of C3 types, 2 pieces from Lipari, 2 pieces from Sardinia and 2 from Melos have been chosen. Although INAA is a routine analytical method applied in the provenance research of obsidian since the 1960s (Gordus et al., 1967), the unique situation that both PGAA and INAA are available at the Budapest Neutron Centre, offers the opportunity to apply them as complementary methods, including the portable XRF, as well. 2.1. Prompt gamma activation analysis
vay, The methodology of PGAA has already been published (Re
si et al., 2010). It provides average bulk compo2009; Szentmiklo sitional data of the irradiated volume of the sample and is not affected much by surface contamination or weathering. In principle, all chemical elements can be measured, but with very different sensitivities (Yonezawa et al., 1995). The PGAA analysis of the obsidian samples have been made on an external cold neutron beam at the Budapest Research Reactor between 2003 and 2015. The thermal equivalent intensity of the neutron beam was in the order of 107 s
1cm
2. The irradiation times have been set to 1 h, approximately to achieve acceptable statistical uncertainties in the peaks of the identified elements. The cross section of the beam was adjusted between 24 mm2 and 400 mm2 so as to obtain the optimal count rate for the data acquisition. One must remember that neutrons can penetrate through the whole sickness of the sample, thus the result will be characteristic for the whole irradiated volume (Kasztovszky et al., 2008a). In a typical spectrum of obsidian, the major geochemical components, including H, Na, Al, Si, K, Ca, Ti, Mn, Fe and mostly the traces of B, Cl, Sm, Gd and occasionally Nd can be quantified. PGAA is one of the few analytical methods that can quantitatively measure H, B and Cl e very frequent and important components in geochemistry. Furthermore, as is shown in Kasztovszky et al. (2014), B and Cl are perfect fingerprint elements for provenance of obsidians. In this paper we show that using the Ti contents of the

Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004

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Table 1 Summarised list of the obsidian samples, investigated in this study by PGAA, pXRF and NAA. Number of samples.

Type

Location

PGAA

pXRF

NAA

28 12 15 3 4 6 7 75

C1 C2E C2T C3 Lipari Sardinia Melos

 Cejkov, Vini cky, Kasov, Mala Bara, Velka Bara, Streda nad Bodrogom, Imbreg
d, Erdo } be
nye Ma Tolcsva Rokosovi, Hust Gabeletto, Porticello Sardinia e Monte Arci, Conca Cannas Melos e Adamas, Demenegaki

28 13 14 3 4 6 7 75

28 13 14 3 4 6 7 75

6 1 2 1 2 3 2 17

Fig. 1. Geographical locations of the main European-Mediterranean obsidian sources.

samples, further subgroups of Melian and Sardinian obsidians can be identified. The only major component that is typically found to be below the detection limit of our PGAA system in the studied obsidians is magnesium. All the concentration data are in weight percent (wt%). 2.2. X-ray fluorescence analysis with a portable device The XRF measurements have been made using a handheld Olympus Innov-X Delta XRF device. The model operates at 40 kV

with an energy resolution of 155 eV FWHM for the 5.9 keV Mn Ka line, and is equipped with a rhodium (Rh) anode X-Ray tube and an SDD detector. Although, the model was originally developed mainly for the compositional analysis of industrial metal alloys, other built-in settings open the way for the analysis of natural samples (soils, rocks) and cultural heritage objects (metals, ceramics, stones) as well. Previous studies showed that the device is able to analyse a large number of specimens in a short time with good repeatability (Mili
c, 2014; Lynch et al., 2016). For the purpose of obsidian sourcing, the original built-in ‘Soil’ mode of the

Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004

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instrument was used, which is suitable for the trace element analysis. In ‘Soil’ mode, wide range of elements can be determined including Sr, Rb and Zr, which are important elements in obsidian provenancing. This setting was introduced earlier by Mili
c (2014) as a powerful tool to distinguish different obsidian types from the Carpathians, the Aegean and Central Anatolia in. In our study, the description by Mili
c (2014) and recommendations by Davis et al. (1998) were followed, concerning the geometry. In the 3-beams Soil mode, samples are analysed for 30 s with each of the beams. At the time of this study, we did not have a possibility to calibrate the instrument for obsidian studies, which is planned to perform in the near future. The XRF measurements of the obsidian samples have been done by holding the instrument in hand. The shape, size and surface roughness of the samples were very diverse, the typical dimensions varied between 1 and 10 cm. All the samples were measured under the same conditions using contact geometry on a 7 mm2 spot area chosen to be flat as much as it was possible. Concentrations of Ti, Fe, Mn, Ca, K, V, Cr, Rb, Sr, Y, Zr, S, Cl and Ni have been measured in ‘Soil’ mode and expressed in mg/g. In addition, it was possible to measure the Al and Si contents of the samples using the ‘Two Beam Mining’ mode. This mode operates with two different beams, one for the analysis of elements with Z > 20, and one for to determine additional elements with low Z (12 < Z < 20). Using the ‘Two Beam Mining’ setting, one XRF analysis takes 40 s. The repeatability of the portable XRF measurements has been checked on the selected objects applying exactly the same conditions. The accuracy of both PGAA and portable XRF has been studied through measurements of 2 mm thick powdered geological reference materials JR1 and JR2. 2.3. Neutron activation analysis Seventeen samples already measured with PGAA and XRF were investigated with k0-neutron activation analysis (De Corte, 1987;
si et al., 2016). During the analysis, 30e120 mg samSzentmiklo ples were packed separately in high purity aluminium foils and irradiated for 12 h. In addition, 7e30 mg samples were packed in polyethylene capsules and irradiated for 30 s. The induced radioactivity of the samples has been measured once following the short-term irradiation and two times following the long-term irradiation with a g-ray spectrometry system. The detected elements were the following: Al, Ba, Ca, Cl, Dy, K, Mg, Mn, Na, Ti, V, As, Br, Ce, Co, Cs, Eu, Fe, Gd, Hf, Ir, La, Lu, Nd, Rb, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, U, Yb and Zn. 3. Results and discussion As we have shown earlier (Kasztovszky et al., 2014), based on the chemical elements detected by PGAA, and especially the trace elements B and Cl, one can easily distinguish between the major types of European-Mediterranean obsidians, i.e. between the C1, C2T, C2E, Melos, Sardinia, Armenia and Anatolia types. We have found that boron concentrations are usually around some tens of mg/g, but in the case of Lipari obsidians, it can be as high as 200 mg/g. At the same time, the chlorine contents of Lipari obsidians are also high (3000e3700 mg/g). On the basis of the tested classification of known-source hand-samples, provenance of valuable archaeological object can be later determined without any sampling or destruction of the objects (Kasztovszky et al., 2014). Although, geochemical behaviours of B and Cl in volcanic glass are still the subject of studies (Barnes et al., 2014; Schmitt and Simon, 2004), Brown et al. have successfully used the chlorine concentration data for provenancing of Kenyan obsidian (Brown et al., 2013). As a common process during volcanic degassing, volatiles dissolved in the melt are either released as gases or are trapped in

rapidly quenched glasses. Volatile element (e.g. Cl, B, F, H2O, and CO2) concentrations of these glasses represent quenched portions of erupting liquid, preserving partly the geochemistry of the parent melt. Chlorine and boron are incompatible, fluid mobile elements. It means that Cl and B remain in the melt phase till the end of the crystallization process and they can be easily mobilized by degassing (Schmitt and Simon, 2004). However, while CO2 and H2O concentrations of obsidian clasts follow the trends of eruptive degassing, Cl concentrations do not track the degassing process (Barnes et al., 2014). In addition, fluid mobile elements partly follow the geochemical behaviour of potassium in the acidic melts (Tonarini et al., 2001, 2003). According to the investigations of Lowenstern et al. (2012) microlite-rich rocks are typically more Cl-depleted than related glassy rocks. In addition, comparison of the Cl composition of microlite-rich and related microlite-free obsidians (Lowenstern et al., 2012) resulted in that Cl concentrations in microlite-rich glasses are lower and it indicated that some Cl is leaving the melt phase. It also implies that microlite-free obsidians which, having more favourable physical properties (e.g. homogeneity, fracture resistance), are commonly used for chipped stone manufacturing generally show higher Cl content. According to the recent research, ‘chlorine can be used as a conservative tracer of fluid source in various geologic settings despite possible H2O and CO2 loss’ (Barnes et al., 2014). This element was successfully applied in differentiating Kenyan obsidian sources (Nash et al., 2011; Brown et al., 2013). Boron concentrations of obsidians are usually around some tens of mg/g, but in the case of Lipari obsidians, it can be as high as 200 mg/g. At the same time, the chlorine content of Lipari obsidian is also high (3000e3700 mg/g). The elevated concentration of B in subductional volcanic rocks originates from the assimilation of volatile-rich materials (e.g. sea-bottom sediments, altered oceanic crust) into the melt (Clift et al., 2001). It implies that the obsidian sources connected to convergent tectonic settings (e.g. Carpathian, Melos, Yali, Lipari) can be more enriched in B than obsidian sources related to divergent plates (e.g. Palmarola, Pantelleria, Sardinia). This is well reflected in our data set where the lowest B concentration values belong to the Sardinian obsidians (7e21 mg/g), while the average B content of obsidians at convergent plate tectonic setting is about 25e68 mg/g, not considering the extremely high values of the Lipari source. Taking into account the low number of the representatives of the Mediterranean obsidian sources, it can be just a hypothesis that the observed relatively low variability of boron content of these obsidian sources can reflect the efficiency of B in the differentiation among the different obsidian sources in Europe. For the investigated set of 75 samples, we have compared the capabilities of PGAA and portable XRF to classify obsidians based on their detected components. When comparing the applicability of XRF, INAA, or PGAA in the characterisation of obsidians, we have to consider that although PGAA detects fewer trace elements, but it has other significant advantages. PGAA gives an average bulk composition for an irradiated volume of a few cm3, depending on the actual beam-size. Since the neutrons can penetrate as deep as a few centimetres into silicate matrix, the result will be fairly representative of the original material, and it is not affected seriously by minor surface contamination. Furthermore, the effect of self-absorption and self-shielding can be adequately taken into account. The PGAA technique has been validated on various standard reference materials, for example on rhyolite which is very similar to the obsidian in composition (Kasztovszky et al., 2008a) and also on glass (Moropoulou et al., 2016). In this paper, we show that without specific calibration for high silica content geological materials, such as obsidian, Olympus Innov-X Delta XRF device

Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004

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produces less accurate data on JR1 standard (Terashima et al., 1994) when using it in ‘Soil’ mode than the PGAA does, for most of the elements that are possible to quantify with both methods, i.e. for Al, Si, Cl, K, Ca, Cl, Ti and Mn. In case of Fe, the accuracies of both methods are similar (see Fig. 2). On the basis of the measurements of JR1 reference material, the concentration data for Si and Al, obtained using ‘Two Beam Mining’ mode of the Olympus Innov-X Delta XRF device have not been used for further statistical analysis. However, for the sake of more reliable XRF results, it is necessary to apply a matrix-specific calibration, which, on the other
vay 2009). hand, is not necessary when applying PGAA (See Re When applying k0-NAA method, the result is also reliable and characteristic for the bulk material. Unfortunately, when the investigated object is bigger than 30 mg, sampling is required, and in that case INAA cannot be considered non-destructive. In other cases, the irradiated samples can be returned to the owner without damage after one year cooling. For the validation of the portable XRF, we have compared concentration data of 75 geological obsidians measured both by PGAA and by XRF. For most of the major components, the XRF data have been determined from ‘Soil’ mode measurements, except for Al and Si, which have been determined from ‘Two Beam Mining’ mode. Correlations between the PGAA and XRF results have been investigated for all the common elements and the correlation coefficients have been calculated. No correlation was found for Al and for Si, poor correlation was found for Ti (R2 ¼ 0.2163) and for K (R2 ¼ 0.2393), while quite good correlation was found for Ca (R2 ¼ 0.7657), for Mn (R2 ¼ 0.8418) and for Fe (R2 ¼ 0.9073). We assume that the poor agreements between PGAA and XRF data, especially for Al and Si, can be explained partly by the absorption of the low energy characteristic X-rays in the air between the sample and the detector. Also, it should be emphasized that PGAA provides compositional data as an average over the whole

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irradiated volume, while XRF show the composition of the upper few-ten-micron layer of the sample. It has been shown earlier, that among the components detected by PGAA, B, Cl and Ti can determine an important ‘fingerprint-like’ pattern. It can be seen on Fig. 3a and b that according to the Ti content, not only the major groups of C1, C2, C3, Lipari, Melos and Sardinia, but also subtypes within the Melos (Demenegaki and Adamas) and Sardinia (A, B and C) can be separated well (i.e. the distances between the groups are typically greater than the scatter of the groups, although the number of Sardinian samples are too low). Nevertheless, despite the fact that without the calibration for obsidian measurements, the XRF concentration data are not exactly representative for the bulk, the XRF data can be used for a fast but powerful characterisation of the obsidian objects and to differentiate between major groups even using the general factory-set calibration. In Fig. 3c and d, we show the classification of 75 geological obsidians, based on their Rb, Sr and Zr contents e the most frequently used components measured by XRF method. When comparing Fig. 3aeb and ced, it can be seen that based on the measured PGAA and XRF data, almost equally powerful classification can be done, but based on PGAA data, additional separation of a C2E subgroup, as well as distinction between the Melos Adamas and Melos Demenegaki. Besides the bivariate plots, various kinds of multivariate analysis can be used to identify groups of the samples based on the chemical composition. We have applied Principal Component Analysis (PCA), where the “Euclidean distance” in the multidimensional compositional space is the measure of the dissimilarity between the objects. For PCA we used XLSTAT 2016 software by Addinsoft, (2016). PCA was applied to a filtered and normalized compositional data set of both the PGAA and the pXRF, and in addition to a combined data set of the two methods. The PGAA data set was analysed in its complete form which involves 13 variables (Si, Ti, Al, Fe, Mn, Ca, Na, K, H, Cl, B, Sm, and Gd). For the multivariate analysis of pXRF data, a selection

Fig. 2. Certified values vs. data measured by PGAA, XRF and INAA on JR1 geological reference material.

Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004

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Z. Kasztovszky et al. / Quaternary International xxx (2017) 1e11

of 8 variables (Ti, Fe, Mn, Ca, K, Rb, Sr, Y, and Zr) was used. Elements that were measured with greater uncertainty in such experimental conditions, namely Si, Al, V, and Cr were excluded from the analysis. Finally, the combined PGAA and pXRF data set with 17 variables was a result of a compilation using Si, Ti, Al, Fe, Mn, Ca, Na, K, H, Cl, B, Sm and Gd concentrations measured by PGAA, and also Rb, Sr, Y and Zr concentrations measured by Olympus Innov-X Delta handheld XRF. The potentials in separation of groups can be compared on different diagrams of first and second Principal Components; applied on PGAA data alone (Fig. 4), on XRF data alone (Fig. 5) and on the combined PGAA and XRF data (Fig. 6). According to Fig. 4b, the main discriminative parameters proved to be the Si content, which allowed separating the obsidian variety according to the rate of acidity (the higher the silica content, the higher rate of acidity). Further distinctive parameters are the Ca and Cl contents, as well as K, or equivalently, Sm and Gd. Based on Fig. 4a, samples from Lipari, Melos and the C3 type form well defined groups, while C1 type ones are only partly separated from the merged C2E and C2T types. Samples from Sardinia do not form a definite group. In the XRF data based factor loading diagram (Fig. 5b), Fe, Ca (or equivalently Sr) and Rb were found to be the main discriminative parameters. According to Fig. 5a, using the concentration data measured in ‘Soil’ mode of the Olympus Innov-X Delta XRF device, one can easily separate the Lipari-, Melos- and C3-type obsidians, however, C2E, C2T, Sardinia and partly C1 merge together. Finally, we have applied Principal Component Analysis on a combined data set of PGAA and XRF measurements. Concentration data of Si, Ti, Al, Fe, Mn, Ca, Na, K, H, Cl, B, Sm and Gd are taken from PGAA and Rb, Sr, Y and Zr are taken from XRF measurements. Even more enhanced separation of Lipari, Melos and the C3 type samples can be observed. The C1 group is only moderately separates from the C2E and C2T groups. The samples from Sardinia again, do not seem to form a definite group. From the applicability study of k0-Neutron Activation Analysis in provenance of obsidians, hierarchical analysis (HCA, Murtagh, 1984) was applied on the concentration data measured by NAA. We found that even with a small number of samples, HCA is able to reproduce the expected main clusters of the samples (Fig. 7). To summarise, as provenance study is an analytical research tool to be utilized in archaeology, one has to make a decision about the best methods to apply, based on previous research and the available technical capabilities. Based on the literature and on our experiments, we recommend considering the following points when provenance study of obsidians is performed. Identification of obsidian sources can be done at different levels. At the first level, one has to separate obsidian from other possibly occurring vitrified or sub-microscopically crystalline materials (e.g. intermediate to basic volcanic glasses, nano-microcrystalline rocks, artificial glasses or slags). This can be done by the determination of the major element composition. For obsidians, the high silica (SiO2>70 wt%) and total alkaline oxide content (Na2O þ K2O~4e12 wt%) are very characteristic, together with relatively low calcium oxide (CaO<2.5 wt%) and low mafic oxides content (FeO þ MnO þ TiO2<3.5 wt%). The destructive (lab-based) XRF (Craig et al., 2007; Phillips and Speakman, 2009; Millhauser et al., 2011), (LA-)ICP-MS/AES/OES (Barca et al., 2007; Craig et al.,

Fig. 3. a. Bivariate plots showing the precision of discrimination between 75 geological obsidians using Ti and B data, measured by PGAA. b. Bivariate plots showing the

precision of discrimination between 75 geological obsidians using Ti and Cl data, measured by PGAA. c. Bivariate plots showing the precision of discrimination between 75 geological obsidians using Sr and Zr data measured by portable XRF. d. Bivariate plots showing the precision of discrimination between 75 geological obsidians using Sr and Rb data measured by portable XRF.

Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004

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Fig. 4. a. Principal Component Analysis on PGAA data of 75 geological obsidians. PGAA data: Si Ti, Al, Fe, Mn, Ca, Na, K, H, Cl, B, Sm and Gd. b. Factor loadings for the Principal Component Analysis of the PGAA data.

2007; Phillips and Speakman, 2009) and SEM-EDS (Acquafredda et al., 1999; Le Bourdonnec et al., 2010) and non-destructive PGAA (Kasztovszky et al., 2008a), PIXE (Elekes et al., 2000; Constantinescu et al., 2002; Butalag et al., 2008) and SEM-EDS } et al., 2014) can developed for large-sample investigation (Bendo be applied to obtain this kind of information. The latter two can be reliably used when no cortex-layer can be found on the object. At the second level, one can try to differentiate between various

obsidian formations. Usually, in archaeometry we do not aim to identify the formation process but only to find distinguishing criteria which lead to an identification of the geographical source of the archaeological obsidians (Hughes and Smith, 1993; Chataigner et al., 1998). Genesis of acidic lavas is a complex process that is controlled by many factors, i.e. source area composition, partial melting, magma mixing, assimilation of country rock, volatile components, crystal

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Fig. 5. a. Principal Component Analysis on XRF data of 7 geological obsidians. XRF data: Ti, Fe, Mn, Ca, K, Rb, Sr, Y and Zr. b. Factor loadings for the Principal Component Analysis of the XRF data.

settling (Hildreth, 1981; Best, 1982; Wilson, 1989). Enrichment or depletion of given elements reflects what happened to the parent melt of obsidian before the eruption. Measurements of every major, minor and trace element can be successfully applied in obsidian provenance studies, because measured data relate to a sub-process which affects the concentrations of the given element in the parent melt of obsidian. Fractional crystallization of different elements determined the chemical composition in the obsidian study of

Fralick et al. (1998). Ca, Sr and Ba can partition into feldspars in a felsic melt, and the melt itself (and the deriving obsidian) will be depleted in alkaline earth elements (Fralick et al., 1998). Inversely, Na, Fe, Ti, Mn, Zr, Y and Zn can substitute into amphiboles (or minor mineral phases), while these elements are not incorporated e.g. in pyroxene (Cameron et al., 1980; Gunderson et al., 1986; Fralick et al., 1998). Melts which contain crystallized amphibole will produce obsidian depleted in Na, transition metals, Zr and Y, while

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Z. Kasztovszky et al. / Quaternary International xxx (2017) 1e11

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Fig. 6. Principal Component Analysis on combined PGAA and XRF data of 75 geological obsidians. PGAA data: Si, Ti, Al, Fe, Mn, Ca, Na, K, H, Cl, B, Sm and Gd; XRF data: Rb, Sr, Y and Zr.

Fig. 7. Hierarchical cluster analysis on compositional data of 16 geological obsidians, measured by INAA.

crystallization of pyroxene will modify the obsidian composition to enrich it in the same elements (Cameron et al., 1980; Fralick et al., 1998). Volatile constituents (e.g. B, Cl and F), being fluid mobile elements, are related to the volcanic systems, and partly behave similarly to potassium in the acidic melts (Tonarini et al., 2001, 2003). The elevated concentration of boron in subductional volcanic rocks originates from the assimilation of volatile-rich materials (e.g. sea-bottom sediments, altered oceanic crust) into the melt

(Clift et al., 2001). The elements with the highest variability can be used to differentiate between samples of different provenance (Shackley, 1995; Fralick et al., 1998). Of these, the alkaline earth elements can be successfully detected by INAA or ICP-MS/AES/OES, while the transition metals can routinely be measured by XRF (pXRF) or ICPMS/AES/OES, which are destructive methods (except for pXRF). A complete spectrum of rare earth elements is perfectly measurable by INAA, while important volatiles (B and Cl) can be sensitively detected by the non-destructive PGAA and partly PIXE. Although many of the above mentioned methods proved to be successful in the determination of fingerprint-like trace elements, like Rb, Sr, Nb, Y, Zr and Ba, only a very few of the methods can be performed without sampling of the archaeological objects, which is impermissible in the case of valuable objects of cultural heritage. The portable XRF instruments have very attractive features, such as being fast, user-friendly, easy to use and also they are relatively cheap. Apparently, precautions are required during the interpretation of the analytical results. Using the Olympus Innov X Delta type portable XRF, it is possible to measure elements heavier than magnesium, while with He flushing, sodium is also detectable. Because of the limited penetration depth of the X-rays, XRF provides information only from the upper few tens of microns of the sample. The quantitative results are strongly affected by the sample to detector geometry. Only the result obtained from a smooth and flat surface of the sample is reliable, though Davis et al. (1998) have found that this error is nearly always negligible when compared to counting uncertainty. Sample size and its relation to the size of the detector window also have influence on the measurement (Davis et al., 1998). However, recent tests (Oddone et al., 1999; Millhauser et al., 2011) on the potentially disturbing factors (e.g. measurement time, presence of surface residue, incomplete

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detector window coverage due to small sample size, concave surfaces) showed that these factors did not significantly affect the ability to attribute sources to artefacts. Finally, due to the overlapping of certain Ka and La lines, a mistaken identification of X-ray lines and consequent erroneous quantification of the composition can happen. These systematic errors, however, do not necessarily show up when a large set of samples is measured with the same equipment, but it makes it difficult to compare the data measured and published by different laboratories. Keeping in mind the above mentioned sources of the uncertainties, we conclude that application a specific calibration set to our Olympus Innov-X Delta instrument would be necessary to make it more accurate for obsidian measurements. On the other hand, when it is requested to determine the absolute elemental composition of bulky geological or archaeological objects made of obsidian, PGAA proved to be the best choice, because the probing thermal or cold neutrons can penetrate many centimetres into the object, and thus a characteristic average composition can be obtained without sampling. Furthermore, PGAA is sensitive for most of the major geochemical components e except Mg which is under the quantification limit in all of the investigated obsidians. 4. Conclusions We compared the applicability of PGAA, portable XRF and instrumental INAA to provenance archaeological obsidians. We have demonstrated that although XRF and INAA measure more trace elements in obsidian than PGAA, PGAA is unique in the ability to determine boron, chlorine and hydrogen, and also measures well most of the major components. As it was demonstrated, PGAA gives reliable composition data representative for a few cm3 bulk sample, whereas XRF provides information only on the near-surface composition, and the result is somewhat influenced by sample geometry. Composition data provided by INAA is also representative of the bulk, as is PGAA, but unfortunately INAA is destructive in most instances. We have shown that grouping on the basis of B, Cl and Ti content measured by PGAA is in a few points more detailed than on the basis of Rb, Sr and Zr measured by portable XRF using the built-in ‘Soil’ mode. When exact and absolute compositional data are requested, for instance in order to build a database for characterisation in archaeometry or geochemistry, we recommend PGAA or, when sampling is allowed, INAA. PGAA is especially useful for items far from their place of origin where the physical integrity of the sample is crucial. A portable XRF is useful when fast grouping into known categories is requested for a large set of objects and absolute concentration data are not of highest priority. Based on our experiences, we would like to extend the utilization of our portable XRF instrument to study silica-based geological material in the future. Acknowledgements This research was funded by the Hungarian Scientific Research Fund (OTKA), under the contract No. K100385. The authors are
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s Marko
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Please cite this article in press as: Kasztovszky, Z., et al., A comparative study of PGAA and portable XRF used for non-destructive provenancing archaeological obsidian, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.004