Obsidian homogeneity study for provenancing using Ion Beam-and Neutron Activation Analysis, F Eder, C Neelmeijer, M Bichler, S Merchel

Tags: homogeneity, Ion Beam Analysis, Neutron Activation Analysis, Ion Beam, Rutherford Backscattering Spectrometry, Forschungszentrum Dresden-Rossendorf, Vienna, Austria, volcanic glass, sample preparation, PIXE, obsidian source, chemical composition, Ion Beam Physics and Materials Research, RBS, KFKI Atomic Energy Research Institute, Ion Beam Center, surface barrier detector, B1 B3 G1 G4 G5 G2 B2 G3 Fig, PIGE, literature data, proton beam
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Obsidian homogeneity study for provenancing using Ion Beam- and Neutron Activation Analysis F. Eder1, C. Neelmeijer2, M. Bichler1, S. Merchel2 1Atominstitut, Stadionallee 2, 1020 Vienna, Austria, email: [email protected] 2Institute of Ion Beam Physics and materials research, Forschungszentrum Dresden-Rossendorf (FZD), P.O. Box 510119, 01314 Dresden, Germany
INTRODUCTION 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 information 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 Composition: 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]. Homogeneity study
OBSIDIAN is a natural glass, produced by Volcanic Eruptions of highly silicic and viscous melt that solidified in amorphous form. Its chemical composition is roughly similar to that of granite. 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 generally described as a relatively homogeneous material [5].
5 mm Fig. 1: Obsidian in-house reference sample with three different surface qualities. ANALYSIS
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 specimens are essential to define the actual degree of homogeneity [6]. The obsidian in-house reference sample for IBA originates from the highly homogeneous obsidian source Hrafntinnuhryggur (Iceland) and features three different surfaces 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 between the black and the grey bands of the banded obsidian MLO9 from Demenegakion (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 Demenegakion, previously analysed with INAA, have now been investigated with IBA [8].
B1 B3
G1
G4
G5 G2
B2
G3
Fig. 2: Black (B) and grey (G) bands of the obsidian sample MLO9 from Demenegakion (Milos, Greece).
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 measurements 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 Atominstitut in Vienna and at the KFKI Atomic Energy ReSearch Institute in Budapest. After four different decay times -ray spectra have been measured 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).
PIXE 2
PIXE 1 RBS
PIGE
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 -rays (PIGE) and a silicon surface barrier detector for backscattered protons (RBS).
Analytical Results 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. simultaneous PIXE-PIGE) proves the complementarity of these analytical methods (Table 2). Furthermore, a good agreement was found between these experimental results and corresponding PIXE literature data for other samples from the same obsidian source [2].
Reference
[7]
Method Sample (No. analyses) SiO2 TiO2 Al2O3 FeO MnO CaO Na2O K2O Zn Ga Sr Y Zr
microprobe
S11b (110) N9a (136) S37c (100)
75.23 0.23 12.00 3.28 0.11 1.66 4.15 2.75
75.01 0.22 12.01 3.23 0.11 1.68 4.19 2.75
75.17 0.22 12.02 3.13 0.11 1.66 4.58 2.88
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
[9] ICP-AES
KR 42 A-THO
75.28 0.35 12.36 2.84 0.09 1.94 3.92 2.83
74.38 0.29 11.98 3.59 0.10 1.80 4.60 2.96
162 142 n.d. n.d. 137 112 130 125 434 512
XRF AKP 74.72 0.26 12.28 3.78 0.11 1.64 3.82 2.63 136 n.d. 93 93 433
this work IBA
AKP nat (5) AKP pol (3) AKP gr (3)
74.87 0.28 11.50 3.47 0.11 1.78 4.99 2.91
74.88 0.27 11.81 3.34 0.11 1.71 5.01 2.78
74.93 0.27 11.71 3.36 0.11 1.71 4.99 2.83
128
120
123
19
19
18
86
83
87
79
83
79
439
418
427
Table 1: Comparison of microprobe, Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), XRF and IBA element concentration data for obsidians from Hrafntinnuhryggur. 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 deterined by IBA.
OUTLOOK 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 differences between easily available samples of natural obsidian sources in Europe. This knowledge should enable to decide, which least destructive analytical method should be chosen for the analysis of a specific archaeological artefact, on a case by case basis.
15
mean B & G
(mean B & G)
mean B(lack)
10
mean G(rey)
deviation of mean B & G [%]
5
0
-5
-10
-15
Na Al
Si
K
Ca Ti Mn Fe Zn Rb Sr
Zr
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 concentrations in the bands is calculated relatively to those of the overall averAge Composition.
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) 352­366. [8] Mandl D. diploma thesis, TU Vienna (2001). [9] Jуnasson K. Bull. Volcanol. 56 (1994) 561-528.
Reference Method Sample (No. analyses) Na2O Al2O3 Si2O K2O CaO TiO2 MnO FeO Sc Cr Co Zn Rb Sr Zr Sb Cs Ba La Ce Nd Sm Eu Tb Yb Lu Hf Ta Th U
[8] this work
INAA
IBA
MLO9 MLO9 (8)
4.11 13.51 n.d. 2.73 n.d. n.d. 0.06 1.27
4.54 12.94 75.75 3.37 1.71 0.22 0.07 1.35
2.10
n.d.
0.74
n.d.
1.1
n.d.
35
27
114
92
n.d.
92
119
94
0.18
n.d.
3.3
n.d.
472
n.d.
26
n.d.
47
n.d.
15
n.d.
2.9
n.d.
0.51
n.d.
0.38
n.d.
2.0
n.d.
0.34
n.d.
3.28
n.d.
0.76
n.d.
12.47 n.d.
3.4
n.d.
[8] this work INAA IBA
MLO10 MLO10
3.92 10.92 n.d. 2.24 n.d. n.d. 0.05 1.25
4.54 12.80 75.57 3.44 1.81 0.24 0.07 1.49
2.05
n.d.
1.31
n.d.
1.0
n.d.
34
30
114
112
n.d.
101
1206
99
0.19
n.d.
3.33
n.d.
485
n.d.
25
n.d.
47
n.d.
15
n.d.
2.9
n.d.
0.50
n.d.
0.40
n.d.
2.0
n.d.
0.34
n.d.
3.25
n.d.
0.77
n.d.
12.68 n.d.
3.4
n.d.
[8] this work
INAA
IBA
MLO13 MLO13 (2)
4.26 13.55 n.d. 2.82 n.d. n.d. 0.06 1.33
4.47 12.74 75.84 3.46 1.75 0.23 0.07 1.40
2.21
n.d.
1.09
n.d.
1.1
n.d.
37
27
121
90
n.d.
102
128
92
0.18
n.d.
3.49
n.d.
501
n.d.
27
n.d.
52
n.d.
17
n.d.
2.9
n.d.
0.53
n.d.
0.41
n.d.
2.1
n.d.
0.36
n.d.
3.40
n.d.
0.78
n.d.
13.28
n.d.
3.7
n.d.
[2] PIXE
P7812a P7827b P78147c M184r
3.63 13.36 76.25 2.94 1.46 0.19 0.06 1.21
3.85 13.49 75.97 2.94 1.44 0.18 0.05 1.20
3.32 13.71 75.83 3.06 1.41 0.21 0.05 1.23
3.75 13.40 76.25 2.94 1.41 0.18 0.05 1.16
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n.d. n.d.
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32
31
33
30
128
118
122
118
136
125
119
124
135
121
128
126
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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 statistics for INAA is <10% for K, Ca, Sm, Nd, Lu and U and <5% for the other elements.

F Eder, C Neelmeijer, M Bichler, S Merchel

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