Obsidian homogeneity study for provenancing using Ion Beam- and Neutron Activation Analysis

F. Eder1, C. Neelmeijer2, M. Bichler1, S. Merchel21Atominstitut, Stadionallee 2, 1020 Vienna, Austria, email: feder@ati.ac.at

2Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf (FZD), P.O. Box 510119, 01314 Dresden, Germany

First IBA results obtained from the obsidian in-house reference sample (Iceland) and from the banded obsidian from Milos (Greece) have shown that further investigations of these speci-mens are essential to define the actual degree of homogeneity [6].

The obsidian in-house reference sample for IBA originates from the highly homogene-ous obsidian source Hrafntinnuhryggur (Iceland) and features three different sur-faces qualities: natural fracture, ground finish (grit 600 diamond lap) and polished (Fig. 1) [7]. The more detailed investigation of at least 3 different spots of each surface should answer the question if the previously revealed deviation of results are actually due to scattering processes as a consequence of sample preparation.

To gain more informations about the differences in the chemical composition be-tween the black and the grey bands of the banded obsidian MLO9 from Demen-egakion (Milos, Greece), a more detailed spatial resolved analysis has been performed by measuring 8 different spots (see Fig. 2).

In order to prove the reliability and complementarity of analytical results the banded specimen mentioned above and two more obsidian samples from Demen-egakion, previously analysed with INAA, have now been investigated with IBA [8].

Ion Beam Analysis: IBA has been carried out at the 5 MV Tandem accelerator of the Ion Beam Center of the FZD. PIXE, PIGE and RBS measure-ments have been performed simultaneously using an external proton beam of 3.85 MeV (at the sample surface) and 0.3 nA beam current (Fig. 3). The combined evaluation of PIXE and PIGE spectra enables the quantitative determination of Na, Al, Si, K, Ca, Ti, Mn, Fe, Zn, Ga, Rb, Sr, Y and Zr.

Instrumental Neutron Activation Analysis:For INAA powdered and homogenized obsidian samples have been irradiated together with international certified reference materials in the 250 kW TRIGA Mk II reactor at the Atominsti-tut in Vienna and at the KFKI Atomic Energy Research Insti-tute in Budapest.After four different decay times γ-ray spectra have been mea-sured with a HPGe detector to obtain the activities of short-, medium- and long-lived activation products (Na, Al, K, Mn, Fe, Sc, Cr, Co, Zn, Rb, Zr, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Hf, Ta, Th and U).

The results obtained by IBA of the cut obsidian sample from Hrafntinnuhryggur revealed no influence of the surface qualities (Table 1). Furthermore, the comparison to literature data proves the reliability of the data obtained [7,9]. The homogeneity of the specimen demonstrates its usefulness as an in-house reference material.

Fig. 4 shows the average of the element concentrations in both the black and the grey bands of the sample MLO9 (Demenegakion, Milos) compared to the overall average. These concentrations were calculated from 8 (3 black, 5 grey) different spots measured with IBA. They lie within the standard deviation and no significant difference in the chemical composition between the black and the grey bands is detecable. The deviation of the results is only due to measurement uncertainties.

The comparison of the chemical fingerprint of three obsidian samples from Demenegakion obtained by INAA and combined external IBA (i.e. simulta-neous PIXE-PIGE) proves the complementarity of these analytical meth-ods (Table 2). Furthermore, a good agreement was found between these ex-perimental results and corresponding PIXE literature data for other samples from the same obsidian source [2].

References: [1] Bugoi R. and Neelmeijer C. NIMB 226 (2004) 136-146. [2] Bellot-Gurlet L. et al. C. R. Palevol 7 (2008) 419-427. [3] Butalag K. et al. NIMB 226 (2008) 2353-2357. [4] Calligaro T. X-Ray Spectrom 189 (2008) 373-377. [5] Hancock R.G.V. and Carter T. J. Archaeol. Sci. 37 (2010) 2436-250.[6] Eder F. et al. Workshop Ion Beam Physics (2010).[7] Tuffen H. and Castro J. M. J. Volcanol. Geoth. Res. 185 (2009) 352–366.[8] Mandl D. diploma thesis, TU Vienna (2001).[9] Jónasson K. Bull. Volcanol. 56 (1994) 561-528.

W I E N

is a natural glass, produced by volcanic eruptions of highly silicic and viscous melt that solidified in amorphous form.

Its characteristic conchoidal fracturing properties enable the production of razor-sharp cutting edges for tools and arms.

Obsidian usually exhibits a very uniform appearance and is gen-erally described as a relatively homogeneous material [5].

Its chemical composition is roughly similar to that of granite.

Homogeneity study

Analytical Results

This investigation is part of a joint project to apply selected analytical methods, in particular IBA, INAA and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), to detect a maximum of compositional diffe-rences between easily available samples of natural obsidian sources in Europe.This knowledge should enable to decide, which least destruc-tive analytical method should be chosen for the analysis of a specific archaeological artefact, on a case by case basis.

The volcanic glass obsidian was one of the most appreciated materials of prehistoric people for cutting tools and has been found at many sites, far away from any natural source. Reliable provenancing can provide evidence of trading routes and contacts and in-formation about exchange patterns and mobility of prehistoric people.The application of analytical methods can solve the problem of obsidian provenancing by means of its highly specific chemical composi-tion: the “chemical fingerprint”. Combined external Ion Beam Analysis (IBA), consisting of Proton Induced X-ray Emission (PIXE), Proton Induced Gamma-ray Emission (PIGE) and Rutherford Backscattering Spectrometry (RBS), are frequently used because of their high sensitivity and the non-destructive beam mode [1-3]. Instrumental Neutron Activation Analysis (INAA) has shown to be the method of choice to obtain additional information because it offers the determination of a complementary set of elements not detectable with IBA [4].

Fig. 1: Obsidian in-house reference sample with three different surface qualities.

INTRODUCTION OBSIDIAN

Fig. 2: Black (B) and grey (G) bands of the obsidian sample MLO9 from Demenegakion (Milos, Greece). ANALYSIS

Table 1: Comparison of microprobe, Inductively Coupled Plasma-Atomic Emission Spec-trometry (ICP-AES), XRF and IBA element concentration data for obsidians from Hrafntin-nuhryggur. Elemental concentrations are given in wt% oxide for major and minor elements and in mg/kg for trace elements. Only those trace elements are listed that have been deter-ined by IBA.

Table 2: Chemical fingerprint of obsidian samples from Demenegakion (Milos). Samples MLO9, MLO10 and MLO13 analysed with both, INAA and IBA, and results compared to PIXE literature data [2]. Elemental concentrations are given in wt% oxide for major and minor elements and in mg/kg for trace elements. The analytical error due to counting statis-tics for INAA is <10% for K, Ca, Sm, Nd, Lu and U and <5% for the other elements.

-15

-10

-5

0

5

10

15

Na Al Si K Ca Ti Mn Fe Zn Rb Sr Zr

devi

atio

n of

mea

n B

& G

[%]

mean B & G σ (mean B & G)mean B(lack)mean G(rey)

Fig. 4: Element distribution in banded obsidian MLO9. Mean values have been obtained from measurements of 8 surface spots (Fig. 2). The deviation of the element concentra-tions in the bands is calculated relatively to those of the overall average composition.

OUTLOOK

5 mm

B1 B3

G1

G4

G2

B2

G5

G3

Fig. 3: External proton beam facility consisting of two Si(Li) detectors, PIXE1 and PIXE2, for the detection of X-rays in two energy ranges, a HPGe detector for analyzing γ-rays (PIGE) and a silicon surface barrier detector for backscattered protons (RBS).

PIXE 1

PIXE 2

RBS

PIGE

Reference [8] this work [8] this work [8] this work [2]

Method INAA IBA INAA IBA INAA IBA PIXE

Sample(No. analyses) MLO9 MLO9 (8) MLO10 MLO10 MLO13 MLO13 (2) P7812a P7827b P78147c M184r

Na2O 4.11 4.54 3.92 4.54 4.26 4.47 3.63 3.85 3.32 3.75Al2O3 13.51 12.94 10.92 12.80 13.55 12.74 13.36 13.49 13.71 13.40Si2O n.d. 75.75 n.d. 75.57 n.d. 75.84 76.25 75.97 75.83 76.25K2O 2.73 3.37 2.24 3.44 2.82 3.46 2.94 2.94 3.06 2.94CaO n.d. 1.71 n.d. 1.81 n.d. 1.75 1.46 1.44 1.41 1.41TiO2 n.d. 0.22 n.d. 0.24 n.d. 0.23 0.19 0.18 0.21 0.18MnO 0.06 0.07 0.05 0.07 0.06 0.07 0.06 0.05 0.05 0.05FeO 1.27 1.35 1.25 1.49 1.33 1.40 1.21 1.20 1.23 1.16

Sc 2.10 n.d. 2.05 n.d. 2.21 n.d. n.d. n.d. n.d. n.d.Cr 0.74 n.d. 1.31 n.d. 1.09 n.d. n.d. n.d. n.d. n.d.Co 1.1 n.d. 1.0 n.d. 1.1 n.d. n.d. n.d. n.d. n.d.Zn 35 27 34 30 37 27 32 31 33 30Rb 114 92 114 112 121 90 128 118 122 118Sr n.d. 92 n.d. 101 n.d. 102 136 125 119 124Zr 119 94 1206 99 128 92 135 121 128 126Sb 0.18 n.d. 0.19 n.d. 0.18 n.d. n.d. n.d. n.d. n.d.Cs 3.3 n.d. 3.33 n.d. 3.49 n.d. n.d. n.d. n.d. n.d.Ba 472 n.d. 485 n.d. 501 n.d. n.d. n.d. n.d. n.d.La 26 n.d. 25 n.d. 27 n.d. n.d. n.d. n.d. n.d.Ce 47 n.d. 47 n.d. 52 n.d. n.d. n.d. n.d. n.d.Nd 15 n.d. 15 n.d. 17 n.d. n.d. n.d. n.d. n.d.Sm 2.9 n.d. 2.9 n.d. 2.9 n.d. n.d. n.d. n.d. n.d.Eu 0.51 n.d. 0.50 n.d. 0.53 n.d. n.d. n.d. n.d. n.d.Tb 0.38 n.d. 0.40 n.d. 0.41 n.d. n.d. n.d. n.d. n.d.Yb 2.0 n.d. 2.0 n.d. 2.1 n.d. n.d. n.d. n.d. n.d.Lu 0.34 n.d. 0.34 n.d. 0.36 n.d. n.d. n.d. n.d. n.d.Hf 3.28 n.d. 3.25 n.d. 3.40 n.d. n.d. n.d. n.d. n.d.Ta 0.76 n.d. 0.77 n.d. 0.78 n.d. n.d. n.d. n.d. n.d.Th 12.47 n.d. 12.68 n.d. 13.28 n.d. n.d. n.d. n.d. n.d.U 3.4 n.d. 3.4 n.d. 3.7 n.d. n.d. n.d. n.d. n.d.

Reference [7] [9] this work

Method microprobe ICP-AES XRF IBA

Sample(No. analyses) S11b (110) N9a (136) S37c (100) KR 42 A-THO AKP AKP nat (5) AKP pol (3) AKP gr (3)

SiO2 75.23 75.01 75.17 75.28 74.38 74.72 74.87 74.88 74.93TiO2 0.23 0.22 0.22 0.35 0.29 0.26 0.28 0.27 0.27Al2O3 12.00 12.01 12.02 12.36 11.98 12.28 11.50 11.81 11.71FeO 3.28 3.23 3.13 2.84 3.59 3.78 3.47 3.34 3.36MnO 0.11 0.11 0.11 0.09 0.10 0.11 0.11 0.11 0.11CaO 1.66 1.68 1.66 1.94 1.80 1.64 1.78 1.71 1.71Na2O 4.15 4.19 4.58 3.92 4.60 3.82 4.99 5.01 4.99K2O 2.75 2.75 2.88 2.83 2.96 2.63 2.91 2.78 2.83

Zn n.d. n.d. n.d. 162 142 136 128 120 123Ga n.d. n.d. n.d. n.d. n.d. n.d. 19 19 18Sr n.d. n.d. n.d. 137 112 93 86 83 87Y n.d. n.d. n.d. 130 125 93 79 83 79Zr n.d. n.d. n.d. 434 512 433 439 418 427