Application of PIXE to the Characterization of Vitreous Dacites from Archaeolgical Sites in the Atacama Region in Northern Chile
Total Page：16
File Type：pdf, Size：1020Kb
Available online at www.sciencedirect.com NIM B Beam Interactions with Materials & Atoms Nuclear Instruments and Methods in Physics Research B 264 (2007) 333–339 www.elsevier.com/locate/nimb Application of PIXE to the characterization of vitreous dacites from archaeolgical sites in the Atacama region in northern Chile J.R. Morales a, S. Cancino a, P. Miranda a,*, M.I. Dinator a, A. Seelenfreund b a Departamento de Fı´sica, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Nun˜oa, Santiago 1, Chile b Universidad Academia de Humanismo Cristiano, Condell 343, Providencia, Santiago, Chile Received 13 April 2007; received in revised form 17 August 2007 Available online 11 September 2007 Abstract Geochemical characterization studies using PIXE were carried out on 21 vitreous dacite artifacts from early formative archaeological sites in the Atacama region, in northern Chile, and on 13 samples taken from two potential volcanic sources located within the region. Performing statistical analyses it was possible to obtain elemental concentration patterns for the archaeological samples of this material and match some of these artifacts with the geological source samples. Ó 2007 Elsevier B.V. All rights reserved. PACS: 39.10.+j; 29.17.+w; 29.30.Kv Keywords: PIXE; Archaeology; Vitreous dacite; Atacama region; Chile 1. Introduction performed more systematically beginning in the early 1990s. Studies have been centered on obsidian analysis In northern Chile, since the work of Aldunate et al. and using mainly instrumental neutron activation analysis, others [1–4], the upper Salado river Basin, the main tribu- (INAA) or energy dispersive X-ray fluorescence, tary of the Loa River in the Atacama region, has been con- (EDXRF). Most of these studies have been performed sidered a nodal area in the macroregional interaction and for Ecuadorian obsidian by the Berkeley group [5] and exchange networks of the Formative Period (1300–500 for obsidian from the Titicaca region in Bolivia by others BC). During this period, lithic resources were used to man- [6–10]. Later, analyses began at the Missouri Research ufacture tools both from local as from distantly located Reactor facilities for obsidian samples from Bolivia, Peru sources. The local sources, mainly basalts, andesites, vitre- and Argentina [11–23]. In South America, particle induced ous dacites and quartz were used for the manufacture of X-ray emission (PIXE) analysis, to our knowledge, has simple expedient tools. The non-local raw materials used only been performed previously for obsidian materials for the production of stone tools, are mainly of two types: from Ecuador and Colombia, and in central Chile [24–27] obsidian and siliceous sedimentary rocks. Studies in volca- using the facilities of the Centre de Recherche et de Resta- nic glass geochemistry are of great importance for under- uration des Muse´es de France, the Australian Nuclear standing the sources of raw materials, as well as for Science and Technology Organization, ANSTO, and the inferring exchanges and group interactions in past socie- University of Chile, respectively. So far, other types of vit- ties. In South America this type of analysis started to be reous rocks from archaeological contexts in South America have never been analyzed by PIXE. Also, it looks like PIXE has been rarely used as a routine way of analysis * Corresponding author. Tel.: +56 2 9787281. of large numbers of archaeological materials, for which E-mail address: [email protected] (P. Miranda). it is ideally suited due to its non-destructive analytical 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.084 334 J.R. Morales et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 333–339 capabilities [25,27]. For the current research we used PIXE the vitreous dacite are mostly large bifacial tools and dart to characterize vitreous dacite samples both from archaeo- points. logical and geological contexts, in order to ascribe archae- For this pilot study, in total 34 samples were chosen for ological artifacts to their geological source of origin. This analysis. Of these, 21 correspond to archaeological artifacts information in conjunction with the analysis performed (denoted by Axx), and the remainder are samples from the on other artifacts (ceramic and obsidian) should eventually geological sources, five from the Linzor source, three from allow us to confront Rees and De Souza hypothesis [4] rel- the ‘‘Paniri upper source’’ and five from the ‘‘Paniri lower ative to the use of local vs. non-local raw materials in the source’’ (all source samples labeled as Zxx). study area, during the Formative Period. On the basis of macroscopic characteristics such as rock type, color and shine and applying standard morphological lithic tool anal- 3. Experimental ysis, the authors have classified the raw materials used for tool manufacture at the sites. Based on this analysis, sev- The PIXE analytical technique has been implemented at eral rock types, vitreous dacite being one of them, were the Nuclear Physics Laboratory of the University of Chile classed as being local. The present study is the first exper- in Santiago, Chile. Proton beams are provided by the Van iment-based attempt to indicate the provenance of the de Graaff accelerator, KN3750 built by High Voltage raw materials used, hoping that in the long run an extended Engineering. Sample irradiation takes place in a vacuum study will provide important information to evaluate the measuring chamber mounted on a dedicated beam line. A percentage of local vs non-local resources in the study area. Canberra Si(Li) cryogenic detector Model 7300 having 220 eV FWHM resolution at 5.9 keV collects the induced 2. Samples X-rays. At the beam spot the sample surface makes an angle of 45° with respect to the incident beam. X rays from By comparison with obsidian, other types of lithic mate- the sample pass to the detector which is located in the open rial like dacites are far less well studied specially in relation air at 1 mm distance from the window chamber made of with source characterization. Vitreous dacite, in accor- 6.3 lm thick Mylar film. The detector encompasses a solid dance with the Peccerillo and Taylor diagram [29] is a vol- angle DX/4p equal to (2.8 ± 0.3) · 10À4 and is located canic glassy rock, high in SiO2 and relatively low in its 68 mm from the sample. Standard electronic circuitry was K2O contents. As source material, this rock has been used for pulse shaping optimization and collection by an detected in our study area on the slopes of two volcanoes, ORTEC PC MCA Model Trump-8k. The spectroscopic Toconce and Paniri (Fig. 1). system was energy calibrated with a 241Am source. For On the slope of the Toconce volcano at 4300 m altitude, the present work, 2.2 MeV proton beams were used with there is a small distinctive outcrop, which we called the typical beam currents in the range of 0.1–0.5 nA up to a Linzor source. It shows extensive quarrying and mainly total charge of 0.1 lC. Additional information on the lab- primary reduction of boulders at the quarry for the manu- oratory facilities is given elsewhere [32]. facture of bifacial tool blanks, with little evidence of sec- The special characteristics of the materials analyzed in ondary reduction of the tool preforms [30]. At this this work, such as irregular shapes, and thickness of a quarry site, the dacite rock contains small plagioclase nee- few millimeters, forced for manual positioning of each sam- dles in a glassy matrix with fine magnetite disseminations ple in the chamber. Since the sample material is non-con- [31]. ductive, the use of an indirect method to measure the The Paniri source rock is found at two places on the vol- electric charge is used. The total charge deposited by the canic slope. At the so called¨Paniri upper source at 4500 m beam on the samples, was determined by the simultaneous altitude on the eastern slope of Paniri volcano, the raw irradiation of the sample and a Mylar film coated with material appears in the form of small boulders (of about cobalt ordered from Micromatter Inc. located at 2 mm in 50–80 cm in diameter) on a shoulder of the volcanic slope. front of the sample. In an X-ray spectrum the net number Some primary flaking and initial reduction stages for tool of counts of the Co characteristic K lines was associated to manufacture can be observed around the boulders, but the charge deposited on the sample during the irradiation since there are no final tools or other evidence of their man- time. The total charge is determined from a calibration ufacture at this place, it seems that most of the later reduc- function obtained previously, between the total number tion was performed at somewhat lower altitudes. Samples of cobalt counts in the film, and the total charge measured for analysis were taken here from several different boul- by a Faraday cup. Details of this method are given else- ders. Vitreous dacite is also found on the southern side of where [33]. the volcano at the base of the cone, and was denoted as Natural cobalt in these vitreous dacite samples was not ‘‘Paniri lower source’’. Archaeological samples analyzed detected by PIXE in irradiations performed in similar con- come from five archaeological sites located no more than ditions, but without the Micromatter film. In a previous 20 km from the Paniri volcano and about 25 km in direct analytical study using PIXE/PIGME on Chilean obsidian line from the Linzor source. One site (Alero Derrumbado) by one the authors, [27] no Co was detected.