4. Geoarchaeological Techniques

For a precise characterisation of both rock from the quarries and archaeological artefacts, we used XRD and petrography. As serpentinite (antigorite) and gneiss suffered a process of diagenesis, we used a hollow drill in order to take archaeological samples that had been buried in clayed soils for centuries. We took samples from the core of the artefacts.

After we had characterised the archaeological materials, we noticed that there was a cultural and also a systematic pattern in the choice of serpentinite (antigorite) for the manufacture of precious goods by the inhabitants of the Olmec sites. All the examples pertained to the same kind of rock.

Later, we sampled the geological quarries that showed the same mineralogical characteristics. At Cuicatlán (Oaxaca) and Tehuitzingo (Puebla), we applied both techniques – petrography and XRD – in order to characterise the samples.

NAA

Neutron Activation Analysis was used on the samples taken from the Olmec sites in order to determine their provenance.

Methodology

The samples were brushed, then washed by ultrasonic agitation using a 10% solution EXTRAN (Merck Co.) in distilled water.
The samples were powdered with an agate mortar.
Samples of 200mg were used for long irradiation, and 20mg for short irradiation.
The reference material was an equal quantity of the SRM-278 obsidian rock from the National Institute of Standards and Technology. Each sample was analysed at least twice to assess statistics.
Irradiation of greenstone samples was performed in a TRIGA MARK III nuclear reactor at a neutron flux of 1 x 10¹³ cm-² s-¹. Samples of 200mg were neutron irradiated for 2 hours. These samples were allowed to decay 12-14 days, and then gamma-ray spectra were recorded for 1 hour. The relevant trace elements were La, Ce, Cs.

Figure 19: NAA analysis of Olmec archaeological sites and quarries

By using Gauss 5.0, we examined the distribution of the ellipse of one of the quarries to an interval of 95 per cent. We can see the diference of the provenience of the rocks between Tehuitzingo and Cuicatlán (Fig. 19).

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