Obsidian hydration dating
Obsidian is a volcanic glass that was sometimes used as a raw material in the manufacture of stone tools such as projectile points, knives, or other cutting tools through the process of flintknapping. Obsidian obeys the property of mineral hydration, and absorbs water, when exposed to air, at well defined rate. When an unworked nodule of obsidian is initially fractured, there is typically less than 1% water present. Over time, water slowly diffuses into the artifact forming a narrow "band," "rim," or "rind" that can be seen and measured with many different techniques such as a high-power microscope with 40-80 power magnification b) depth profiling with SIMS secondary ion mass spectrometry. In order to use obsidian hydration for absolute dating, the conditions that the sample has been exposed to and its origin must be understood or compared to samples of a known age (e.g. as a result of radiocarbon dating of associated materials.)
Obsidian hydration dating was introduced in 1960 by Irving Friedman and Robert Smith of the U.S. Geological Survey. Their initial work focused on obsidians from archaeological sites in western North America.
Today the technique is applied extensively by archaeologists to date prehistoric sites and sites from protohistory in California and the Great Basin of North America. It has also been applied in South America, the Middle East, the Pacific Islands, including New Zealand and Mediterranean Basin.
To measure the hydration band, a small slice of material is typically cut from an artifact. This sample is ground down to about 30 micrometers thick and mounted on a petrographic slide. The hydration rind is then measured under a high-power microscope outfitted with some method for measuring distance, typically in tenths of micrometers. The technician measures the microscopic amount of water absorbed on freshly broken surfaces. The principle behind obsidian hydration dating is simple–the longer the artifact surface has been exposed, the thicker the hydration band will be.
Secondary Ion Mass Spectrometry (SIMS) procedure
In case of measuring the hydration rim using the depth profiling ability of the secondary ion mass spectrometry technique, the sample is mounted on a holder without any preparation or cutting. This method of measurement is non-destructive.
Several factors complicate simple correlation of obsidian hydration band thickness with absolute age. Temperature is known to speed up the hydration process. Thus, artifacts exposed to higher temperatures, for example by being at lower elevation, seem to hydrate faster. As well, obsidian chemistry, including the intrinsic water content, seems to affect the rate of hydration. Once an archeologist can control for the geochemical signature of the obsidian (e.g., the "source") and temperature (usually approximated using an "effective hydration temperature" or EHT coefficient), he or she may be able to date the artifact using the obsidian hydration technique. Water vapor pressure may also affect the rate of obsidian hydration.
The reliability of the method based on Friedman’s empirical age equation (x²=kt, where x is the thickness of the hydration rim, k is the diffusion coefficient, and t is the time) is questioned from several grounds regarding temperature dependence, square root of time and determination of diffusion rate per sample and per site, apart of some successful attempts on the procedure and applications.
Several commercial companies and university laboratories provide obsidian hydration services.
- Meighan, Clement (1976). R.E. Taylor, ed. Advances in Obsidian Glass Studies. pp. 106–119. ISBN 978-0-8155-5050-1.
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- Meighan, Clement (1983). "Obsidian Dating in California". American Antiquity 48 (3): 600–609.
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- Ambrose, W., Novak, S.W., Abdelrehim, I., 2004. "Powdered obsidian for determining hydration rates and site thermometry". Mediterranean Archaeology and Archaeometry 4, 2, 17-31.
- Liritzis.I (2006) "SIMS-SS A new obsidian hydration dating method: analysis and theoretical principles". Archaeometry, 48 (3), 533-547.
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- Eerkens, J.W; Vaughn, K.J; Carpenter, T.R; Conlee, C.A; Moises Linares Grados; Schreiber, K (2008) "Obsidian hydration dating on the South Coast of Peru" Journal of Archaeological Science 35, 2231-2239
- Liritzis, I and Laskaris, N (2009) "Advances in obsidian hydration dating by secondary ion mass spectrometry: World examples. Nucl. Instr. Meth. In Physics Research B, 267, 144-150.