ODDSIMS, A New Method of Age Dating Hydrated Obsidian by Secondary Ion Mass Spectrometry

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With National Science Foundation support Dr. Lee Riciputi and his colleagues will conduct a three year study to refine the technique of obsidian hydration dating. Obsidian, also known as `volcanic glass`, was prized by prehistoric peoples because its homogeneous glass-like structure allows it to be flaked with relative ease and a high degree of control. The sharp edges thus produced functioned as knives and other tools. Obsidian was traded widely and has been recovered in many archaeological sites in Middle and South America, Africa and other parts of the Old World. The process of flaking produces fresh external faces and over time these `hydrate` as they absorb water from the atmosphere and soil. The hydration rim visible on the surface thickens over time and this age dependence provides the basis for an absolute dating technique. Scientists can section an obsidian flake, measure the thickness of the rim and then attempt to convert this to a date. Although the technique has been used for many years and holds great potential, the results are often unsatisfactory. Hydration rate appears to be temperature and moisture dependent. It may also be affected by the trace element composition of the obsidian itself. Scientists also disagree on how to model diffusion rates over time. Dr. Riciputi and his colleagues have developed a new approach to obsidian hydration analysis. Rather than use an optical microscope to determine hydration depth, they will employ a more precise secondary ion mass spectrometry technique. They will also address the issue of hydration modeling through a series of controlled experiments in which obsidian from a single source is exposed to environments which vary in temperature and humidity. To apply the results in a real world context, the investigators will analyze Middle American obsidian artifacts whose age has been determined through association with radiocarbon dated materials. These specimens will vary in age as well as the temperature and humidity of their depositional environment. This research is important because, if successful, it will provide archaeologists with a robust dating tool capable of application in many parts of the world.