Nuclear Instruments and Methods in Physics Research B 240 (2005) 583–588

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Obsidian provenance studies in archaeology: A comparisonbetween PIXE, ICP-AES and ICP-MS

Ludovic Bellot-Gurlet a,*, Gerard Poupeau b, Joseph Salomon c,Thomas Calligaro c, Brice Moignard c, Jean-Claude Dran c,

Jean-Alix Barrat d, Laurent Pichon c

a Laboratoire de Dynamique Interactions et Reactivite, UMR 7075 CNRS-Universite Paris VI, 94320 Thiais, Franceb Institut de Recherche sur les Archeomateriaux, UMR 5060 CNRS-Universite Bordeaux 3, 33607 Pessac, Francec Centre de Recherche et de Restauration des Musees de France, UMR 171 CNRS-Ministere de la Culture,

Palais du Louvre, 75001 Paris, Franced Domaines Oceaniques, UMR 6538 CNRS-Universite de Bretagne Occidentale, 29280 Plouzane, France

Available online 2 September 2005

Abstract

Elemental composition fingerprinting by PIXE technique is very attractive for obsidian provenance studies as it mayproceed in a non-destructive mode, even if a more complete elemental characterization can be obtained by ICP-MSand/or ICP-AES. Only few studies have compared results obtained by both methods for solid rock samples. In thiswork, elemental compositions were determined by ICP-MS/-AES for international geochemical standards and byICP-MS/-AES and PIXE for inter-laboratory reference obsidians. In addition 49 obsidian source samples and artefactswere analysed by both ICP-MS/-AES and PIXE. Instrumental work and measurement quality control performed forobsidian chemical characterization, underline that PIXE and ICP-MS/-AES provide reproducible, accurate and com-parable measurements. In some volcanic districts the limited number of elements dosed by PIXE is sufficient for thediscrimination of the potential raw sources of obsidians. Therefore, PIXE can be an advantageous substitute toICP-MS/-AES techniques for provenance studies.� 2005 Elsevier B.V. All rights reserved.

PACS: 29.30.Kv; 07.85.Nc; 61.43.Fs

Keywords: PIXE; External beam; ICP-AES; ICP-MS; Obsidian; Cross-measurements

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.nimb.2005.06.216

* Corresponding author. Tel.: +33 1 49 78 11 14; fax: +33 1 49 78 11 18.E-mail address: bellot-gurlet@glvt-cnrs.fr (L. Bellot-Gurlet).

584 L. Bellot-Gurlet et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 583–588

1. Introduction

Obsidian is defined as an aphyric volcanic glasscontaining more than 65% SiO2 which may befound in prehistoric sites up to several thousandkilometres away from its nearest geological‘‘source’’ [1]. Therefore, the determination of thegeographical origin of such obsidians makes asignificant part of stone industry studies. In thecase of obsidian, this goal is by far achieved bygeochemical fingerprinting. The most completeelemental characterizations are obtained by ICP-MS/-AES (inductively coupled plasma-mass spec-trometry/-atomic emission spectrometry) analyses[2,3] while PIXE presents the possibility of non-destructive analysis of large objects. These twomethods are therefore complementary. However,except in Colombia and Ecuador [4,5] they seemto have never been used in combination for obsid-ian provenance studies. In this work based onsome samples originating essentially from Andeansources used by ancient man, we will show thatPIXE and ICP-MS/-AES analyses may possiblybe equally pertinent for obsidian provenancestudies.

2. PIXE experimental procedure

PIXE measurements were performed at theAGLAE facility of the Centre de Recherche et deRestauration des Musees de France using the exter-nal ‘‘milli-beam’’ line of a 2 MV Van de Graafftandem accelerator (National ElectrostaticsPelletron 6SDH-2). During the course of thiswork, several improvements, concerning mainlythe exit nozzle were brought to the experimentalset-up [6–9]. Beam exit windows were either madeof a Kapton foil (13 lm) or more recently of a0.1 lm of Si3N4 foil deposited on a 0.5 mm Siframe [8]. We have used a �3 MeV proton beamimpinging perpendicularly on the sample surface.In order to obtain reliable elemental analyses thebeam diameter was in 100–500 lm range.Two Si(Li) detectors oriented at 45� relative

to the proton beam axis record the low- (0.3–10 keV) and high-energy (5–40 keV) X-ray spec-tra corresponding respectively to the light

(11 6 Z 6 26) and heavy (Z P 26) constituentelements. A 49 lm Al window was placed infront of the high-energy detector to attenuatethe low-energy X-rays from the matrix elementsand the low-energy detector is protected frombackscattered particles by a permanent magnet.A helium gas was blown between the sampleand this detector to improve the low-energyX-ray transmission by the surrounding atmo-sphere [6]. A beam current of �1 nA was usedto avoid sample alteration and elemental migra-tion. With this intensity the spectrum acquisitiontime could be limited to 7–10 min, correspondingto a predetermined proton dose, high enough, toachieve elemental analysis with the necessaryprecision.X-ray spectra were all processed with the ‘‘95’’

version of the GUPIX software [10] for internalconsistency of a databank initiated in 1996 [11].Major element contents were calculated with theirsum as oxides normalized to 100%. Trace elementconcentrations were calculated following normal-ization between low- and high-energy spectrabased on the Fe-oxide content. The reproducibil-ity of measurements was controlled using aninternal (one Andean obsidian) standard. Underthese conditions the content of 15 major andtrace elements: Na, Al, Si, Cl, K, Ca, Ti, Mn,Fe, Zn, Ga, Rb, Sr, Zr and Ba was determined.Two elements, Cl and Ba, sometimes present neartheir detection limit, could only be determinedonly with large (10–15%) uncertainties, and werethus discarded for some obsidian source identifi-cation. In peralkaline obsidians the Y and Nbcontents were sometimes high enough to bedetermined.All obsidians samples from geological outcrops

and a fraction of the obsidian artefacts collected inarchaeological sites were analysed under the formof polished sections. Other artefacts needed to benon-destructively treated and ‘‘natural’’ surfaceshad to be used. Control experiments showed thatin the archaeological materials studied [12,13]meteoric alteration did not affect the artefacts sur-face composition measured. In order to take intoaccount possible local heterogeneities, measure-ments were systematically made on three spotsper sample.

Table 1Illustration of obsidian composition range with four samplesderived from PIXE measurements

Source Quiscatola Sierra de Pachuca Glass Buttes

Country Ecuador Mexico Oregon, USAReference QSC E PCC GTTNa (%) 2.82 3.83 2.85Al (%) 6.82 6.06 6.98Si (%) 36.38 35.69 36.04Cl (ppm) 1647K (%) 3.46 3.39 3.24Ca (%) 0.38 0.08 0.56Ti (ppm) 643 1028 788Mn (ppm) 309 1008 291Fe (%) 0.37 1.56 0.61Zn (ppm) 23 221 27Ga (ppm) 15 29 15Rb (ppm) 177 211 105Sr (ppm) 84 73Y (ppm) 109Zr (ppm) 60 1008 105Nb (ppm) 97

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3. ICP-AES and ICP-MS experimental procedures

For ICP-AES and ICP-MS measurementsobsidians were crushed to �10–30 mg fragments,of which �150 mg weight were dissolved by acidetching. In each analytical batch, internationalrock standards (as BHVO-1, BIR-1, RGM-1,SRM 278, UBN, WS-E) and blank solutions wereprepared in the same way. Major elements: Al, Fe,M, Mg, Ca, Na, K, Ti and P were determined byICP-AES technique using a Jobin Yvon JY24apparatus. Trace elements: Co, Rb, Sr, Y, Zr,Nb, Cs, Ba, Tm, Hf, Ta, Pb, La, Ce, Pr, Nd,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu Thand U were measured by ICP-MS using a FisonsPlasma Quad turbo 2+ system applied to diluteddaughter-solutions. Quantitative determination ofthese trace elements was deduced from a Tm-spik-ing prepared for ICP-MS analyses [14,15].

Fig. 1. Relative dispersion on duplicated PIXE measurementsfor three analysed samples (in brackets, the number ofmeasurements for each sample).

4. Sampling and results

PIXE and ICP-MS/-AES measurements werecarried on 49 samples, 34 of which were collectedfrom volcanic formations and 11 from archaeo-logical artefacts from Colombian and Ecuado-rian settlements. Two other geological samplesare samples from Glass Buttes (Western UnitedStates, GTT) and Sierra de Pachuca (Mexico,PCC) obsidians sources, which have been used ina previous intercomparison laboratory elementalanalyses [16]. The last two samples, provided byV. Francaviglia (CNR, Roma, Italy) originatedfrom the Western Mediterranean islands of Pan-telleria (PAN-BTO) and Sardinia (ARC-URS).In general, the three PIXE spot measurements

performed on each sample gave concordant resultswithin the statistical uncertainties. The reproduc-ibility of PIXE results was checked by repeatedmeasurements over several years on three obsidianpolished sections. The compositions given byPIXE of these obsidians are listed in Table 1. Asillustrated in Fig. 1, the standard deviations ofthe element contents are essentially <10%. As ex-pected for an obsidian matrix, the relative disper-sion of the major elements, especially Na, Al, Si,K and Fe is <5%. Some major to minor elements

may present dispersion from <5% to �10%, whilefor trace elements it is most often >5%. It was alsoobserved, that for some elements such as Ca andZr, the dispersion tends to vary inversely with con-tent. This can essentially be attributed to the statis-tics/peak fitting errors in relation with obsidianmatrix elemental composition and also itshomogeneity.

Fig. 2. Relative dispersion of ICP-AES and ICP-MS measure-ments on three duplicated preparations for six samples. Fig. 3. Relative deviations of ICP-AES and ICP-MS measure-

ments from certified values for four geostandards (see text formore details).

Fig. 4. (a) Relative variations of PIXE versus ICP-AES or ICP-MS for 49 obsidian samples. (b) Mean relative variations overmeasurements presented in (a).

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ICP-MS/-AES reproducibility was controlledby three duplicate preparations and measurementsof aliquots from six obsidian samples representingvarious geochemical types (Fig. 2). Mother samplesolutions were obtained from the total dissolutionof �100 mg of bulk rock. Thirty six elements persample were determined, of which nine: Al, Fe,Mn, Mg, Ca, Na, K, Ti and P were measured byICP-AES and the remaining ones by ICP-MS.For ICP-AES appropriately diluted daughter

solutions allowed us to reach standard deviationson element contents <5%. A single dilution factor(about 3000) was applied for ICP-MS in order toobtain a reproducibility better than 3%. As a con-sequence, some elements whose concentrationsvary widely from sample to sample (as Ba, from<10 to >100 ppm), may exhibit large dispersionvalues when they are present near their detectionlimits. This indicates that in such specific cases asecond and more adapted dilution factor is neededin order to obtain all element concentrations withthe desired precision. In any case, dispersion val-ues are within the range expected for counting sta-tistics, indicating that the adopted chemicalprocedure for sample preparation is reliable.ICP-MS/-AES accuracy was checked using

international rock standards. A good agreementfor most elements was obtained between our re-sults and the certified values for four geostandards(obsidian SMR278, rhyolite RGM-1 and basaltsBHVO-1 and BIR-1 [17]), as shown in Fig. 3.Large differences only occur for elements whosecontents, in some standards, are far below that

of obsidians and thus for which our dilutionfactors were not adapted (i.e. K in BHVO-1 andRb in BIR-1). As already observed in obsidianSRM278 many elements, such as Ba, given as

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‘‘information values’’, seem to be far from reality[18,19].PIXE measurements were performed on solid

rock samples. Under these conditions their accu-racy could not be directly checked from aliquotsof geological standard samples as these materialsare only available as powders. However, amongthe 36 elements determined by ICP-MS/-AES, 11could also be measured by PIXE. The results ofa coupled PIXE and ICP-MS/-AES analysis of49 samples are shown in Fig. 4, displaying sampleper sample, PIXE and ICP-MS/-AES data (upperdiagram) and their mean relative variations (lowerdiagram). A general agreement between thesetwo techniques is observed, although some slightsystematic differences appear for Ca, Ti, Mn, Feand possibly Zr. The large differences which ap-pear in some samples in the top diagram ofFig. 4 are due either to the low counting statisticswhen an element content is close to the detectionlimit (as for Sr or Zr in PIXE) or sometimes to aheterogeneous distribution (as for Fe). The lattercause is especially important for PIXE analyses,given the limited size of the analysed volume.

5. Final comments and conclusion

For obsidian provenance studies, the PIXEtechnique presents the advantage of being totallynon-destructive. However, the number of ele-ments attainable by this method is not always suf-ficient for source discrimination. In such casesPIXE analyses can be completed with anotherexperimental approach such as ICP-MS/-AES.Bugoi et al. [20] have suggested, from twoobsidian sample analysis, that PIXE and LaserAblation-ICP-MS may concurrently be used inprovenance studies. We had previously shownfrom aliquots of two distributed samples [16] thata good agreement could be reached betweenPIXE and ICP/AES-MS. We extended here ourcoupled PIXE and ICP-MS/-AES measurementsto 49 samples. Over several years, we haveobserved for measured elements a good reproduc-ibility within the range of <5 to �10% and <3–5%for PIXE and ICP, respectively. On the average,the elemental determinations with these two meth-

ods were in agreement to better than 15%. Suchan agreement is in general good enough for prov-enance studies.

Acknowledgements

We are indebted to the colleagues, archaeome-trists and archaeologists, who provided the studiedsamples: G. Bigazzi (Pisa, Italy), V. Francaviglia(Roma, Italy), M.D. Glascock (Columbia, USA),N. Guillaume-Gentil (Neuchatel, Switzerland), C.Gnecco, D. Patino (Popayan, Colombia), M. Vill-alba, A. Yepez (Quito, Ecuador), M. Guinea(Madrid, Spain), J.-F. Bouchard (Paris, France).This project was partly supported by GDR 2114of CNRS ‘‘Physico-Chimie des Materiaux du Pat-rimoine Culturel’’ (CHIMART2).

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