Sourcing

Geo-referenced sampling: three scales

Within the Göllüdağ Volcanic Complex obsidians are found in the form of outcrops, blocks (bombs) and secondary sources. Almost all of these outcrop types have been sampled  to illustrate the variability in their geochemical and geomagnetic characteristics throughout the Göllüdağ complex (1). It is thus possible to work at a very small scale; for example, we can focus on a dome or even more closely on a particular outcrop. This methodology is implemented to define what is an obsidian source in this specific geological complex, one of the most complex of Anatolia.

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Laboratory analysis using the LA-ICP-MS and magnetic properties

The geological corpus is being studied by B. Gratuze (IRAMAT, Orléans) and K. Erturaç (IFEA) to complete the database (Gratuze 1999; Gratuze et al. 2005; Binder et al. 2011). The data from this new set of chemical characterization are now in course of being analysed in conjunction with the geological data. Once the sample have been analysed by LA-ICP-MS, the same samples are characterized using magnetic properties (Mc-Dougall et al. 1983). Obsidian studies have reached a stage when there are many methods and laboratories carrying out chemical characterization, and there is much more widespread access to PXRF devices, and, not surprisingly, more critical attention to the reliability and comparability of different techniques (Frahm 2010; Hancock – Carter 2010; Poupeau et al. 2010; Shackley 2010). We hope that the multi-facetted, collaborative efforts of this project will lead to a solid and widely applicable set of descriptions of the characteristics of the obsidians in this Cappadocian volcanic complex (4).

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Magnetic properties have recently been proposed as a complementary approach to track the source of archaeological materials (Feinberg et al. 2009; Weaver et al. 2009). Unfortunately, despite recent efforts information on the magnetic properties of obsidian is still limited because of the number of samples or sources analyzed worldwide is small which limits the statistical rigor of correlations. In order to systematically use this new method we first need to increase the available database and to understand, the origin of the variations within a source and between sources. About 700 pieces of obsidian have been used to determine the magnetic properties of this geological material at Göllüdag. The magnetite grain size entrapped within the glass was investigated using micromagnetic measurements (4) as well as scanning electron microscopy. Several obsidian outcrops were studied and we found variable quantities of magnetite as well as highly heterogeneous grain sizes. In order to illustrate the variability in grain size we have chosen an example from the Kabaktepe south area. Two obsidian outcrops (Kaletepe-Kömürcü and Eriklidere) both located south of the Kabaktepe dome belong to the same obsidian chemical group (5). The first outcrop is a low obsidian fan of 1.4 m thick and 11 m long. Samples were taken both vertically and horizontally to cover the entire outcrop.  The second site is a sub-vertical 187 m thick obsidian dyke that outcrops along the Eriklidere river. A sample was taken every 10 m.

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