EXPLORING GLASS AND MINERAL COMPONENTS OF OBSIDIAN WITH ELECTRON MICROPROBE ANALYSIS AND THE IMPLICATIONS FOR ARCHAEOLOGICAL SOURCING AND DATING (OR: TREATING OBSIDIAN LIKE THE MIXTURE IT IS)

The macroscopic characteristics of obsidian are misleading. This apparently homogeneous volcanic “glass” is actually a mixture of both glass and mineral components. Most obsidians contain microlites (fine-grained mineral inclusions) and phenocrysts (possibly as large as several millimeters). Others have used these inclusions for a wide range of geological studies; however, there are also implications for archaeological sourcing and dating.

Bulk analytical techniques like NAA and XRF do not permit separate elemental analyses of the glass and inclusions. Electron microprobe analysis (EMPA) is a spot technique and, therefore, is well suited to analyzing mixtures like obsidian and other rocks. EMPA incorporates both X-ray microanalysis and scanning electron microscopy, allowing elemental fingerprinting of obsidian and petrographic observations, respectively.

EMPA has greater accuracy and precision than SEM-EDS and also has fewer element overlaps and lower minimum detection limits. Though EMPA is often regarded as only a major-element technique, modern instruments, software modifications, and careful analytical procedures can reduce the detection limits for some trace elements into the double- and even single-digit-ppm range. Comparisons of major- and trace-element obsidian data from EMPA and other techniques show favorable agreement.

Mineral inclusions in obsidian can be distributed heterogeneously, most notably in visible flow bands, potentially affecting elemental fingerprints if abundances change from sample to sample. For example, the presence of monazites in obsidian has implications for using REEs (rare earth elements) in sourcing, and the abundance of zircons can affect the measured concentrations of Zr. Both minerals can also be used for dating, so recognizing their presence is advantageous. Sanidine is useful for K/Ar dating, so EMPA can establish which obsidians are well suited to such dating.

I will discuss obsidians (and the mineral inclusions within them) from three main regions: (i) Mexico and the American Southwest, (ii) Anatolia and the Transcaucasus, and (iii) the East African Rift. I will also discuss obsidian artifacts, sourced non-destructively using EMPA, from the Bronze-Age archaeological site of Tell Mozan (ancient Urkesh) in northeastern Syria.