T216 Obsidian from Magma to Artifact: Geological and Archaeological Perspectives

Tuesday, 20 October 2009
1:30 PM - 5:30 PM
E146

WATER IN ANATOLIAN OBSIDIAN: FACTORS INFLUENCING HYDROUS SPECIES CONCENTRATIONS WITH IMPLICATIONS FOR SOURCING AND DATING OF ARTIFACTS

Paper number: T216-16
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

Abundant rhyolitic volcanism in southwest Asia provided numerous sources of natural glass for exploitation by successive civilizations in the Fertile Crescent. The unique geochemical composition of each lava flow provides a recognizable ‘fingerprint’ for obsidian artifacts recovered in the region. The presence of water in natural glasses has proven invaluable to both geological and archaeological studies of obsidian. The initial composition of the magma, its cooling rate, and subsequent low temperature hydration control the relative abundances of two distinct hydrous species (OH hydroxyl groups and molecular H2O). The strong temperature dependence of the equilibrium between these species has allowed geologists to distinguish samples that retain their initial magmatic water contents versus those that have experienced low temperature hydration after eruption. Thus, the relative abundance of the two hydrous species in obsidian has been used to establish cooling rates of glasses, and the diffusion front into exposed surfaces has been used to establish a relative dating technique for the manufacture of obsidian artifacts. It is the concentration of OH (and not molecular H2O) that controls the growth rate of the hydration rim and is essential for quantifying the time elapsed since manufacture.

We have measured hydrous species concentrations in glasses sampled from a suite of felsic lava flows in Turkey, Azerbaijan, Armenia, Georgia, and Iran. The relative abundance of the two hydrous species distinguish samples that experienced significant water addition shortly after their eruption from those that retained their initial magmatic water contents. Samples with excessively high water contents (> 3 wt%) are associated with low equilibration temperatures (< 300 ºC) and show depletions in Na and Fe relative to non-hydrated samples (with water contents < 1 wt% and equilibration temperatures ~500 ºC). Some relatively water-rich samples (> 1 but < 3 wt%) show high equilibration temperatures (~500 ºC) and no observable depletion in Na and Fe (reflecting no post-eruption hydration), whereas some low water contents show low equilibration temperatures with corresponding depletions in Na and Fe (reflecting some degree of post-eruption hydration). We aim to extend our study to obsidian artifacts from the region.