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Although a variety of physical, optical, petrographic, and chemical attributes are used to characterize volcanic glasses, the use of trace element abundances to "fingerprint" obsidian sources and artifacts has shown the greatest overall success. X-ray fluorescence analytical methods, with their ability to nondestructively and accurately measure trace element concentrations in obsidian, have been widely adopted for this purpose (Harbottle 1982; Rapp 1985; Williams-Thorpe 1995; Glascock et al. 1998; Herz and Garrison 1998; Lambert 1998). Most geologic sources of obsidian are quite homogeneous in their trace element composition, yet demonstrate adequate intersource variability so that individual sources of glass can be distinguished. Because obsidian can be widely dispersed from its primary geologic source due to a variety of geologic and geomorphic processes, specimens of chemically identical glass are sometimes recovered from outcrops spread over large geographic areas (Hughes 1986; Hughes and Smith 1993). These secondary source boundaries are often not as well documented as primary sources but must be carefully considered in obsidian procurement studies (Shackley 1998; Church 2000). Hughes (1986; 1998) points out that these chemically identical obsidian outcrops must be considered as a single chemical group or chemical type and his terminology is followed here. From small scale (household and site) to large scale (regional and interregional) levels of analysis, the spatial source patterning of characterized obsidian artifacts is influenced by many different environmental and cultural factors. Interpretation of these patterns can provide valuable information about the prehistoric behavioral and environmental procurement variables responsible for observed artifact distributions. At the site level of analysis, patterns of source use may suggest the presence of specific activity areas, of single tool manufacturing events, or, in special cases, may point to differential access of goods and the existence of non-egalitarian social structures. At the intersite or regional level of investigation, the geographic patterning of artifacts can provide information about seasonal procurement ranges, territorial and ethnic boundaries, the location of trails and travel routes, the curational value of particular sources or formal artifact types, cultural preferences regarding glass quality and colors, the presence of trade and exchange systems, the existence of intergroup interaction, and the exchange of prestige items between elites of different groups (Ericson 1981; Hughes 1978, 1990; Hughes and Bettinger 1984). The effects of environmental influences such as the distance to source, the location of alternative or competing sources of lithic materials, the distribution of raw materials in secondary deposits, or the presence of potential barriers such as mountain ranges, must all be considered. Bias introduced during sampling by certain recovery methods, artifact size, and the use of small numbers of samples may also affect the reconstruction of the spatial patterning of analyzed artifacts.
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Analysis of samples for different trace element concentrations (Ti, Mn, Fe2O3T, Zn, Ga, Rb, Sr, Y, Zr, Nb, and Ba) is completed using a Spectrace 5000 energy dispersive X-ray fluorescence spectrometer. The system is equipped with a Si(Li) detector with a resolution of 155 eV FHWM for 5.9 keV X-rays (at 1000 counts per second) in an area 30 mm2. Signals from the spectrometer are amplified and filtered by a time variant pulse processor and sent to a 100 MHZ Wilkinson type analog-to-digital converter. The X-ray tube employed is a Bremsstrahlung type, with a rhodium target, and 5 mil Be window. The tube is driven by a 50 kV 1 mA high voltage power supply, providing a voltage range of 4 to 50 kV. The principles of X-ray fluorescence analytical methods are reviewed in detail by Norrish and Chappell (1967), Potts and Webb (1992), and Williams (1987). X-ray fluorescence analytical procedures used in the analysis of all obsidian and basalt samples were originally developed by M. Kathleen Davis (BioSystems Analysis and Northwest Research Obsidian Studies Laboratory).
For analysis of the mid-Z elements zinc (Zn), gallium (Ga), lead (Pb), thorium (Th), rubidium (Rb), strontium (Sr), yttrium (Y), zirconium (Zr), and niobium (Nb), the X-ray tube is operated at 30 kV, 0.30 mA (pulsed), with a 0.127 mm Pd filter. Analytical lines used are Zn (K-alpha), Pb (L-alpha), Th (L-alpha), Rb (K-alpha), Sr (K-alpha), Y (K-alpha), Zr (K-alpha) and Nb (K-alpha). For small specimens, an alternate method using a collimator may be used. For this procedure, the X-ray tube is operated at 45 kV and 0.60 mA. Samples are typically scanned for 200 seconds live-time in an air path.
Peak intensities for the above elements are calculated as ratios to the Compton scatter peak of rhodium, and converted to parts-per-million (ppm) by weight using linear regressions derived from the analysis of twenty rock standards from the U.S. Geological Survey, the Geologic Survey of Japan, and the National Bureau of Standards. The analyte to Compton scatter peak ratio is employed to correct for variation in sample size, surface irregularities, and variation in the sample matrix.
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Although their geochemical range of variability may be greater than obsidian, artifacts composed of fine-grained volcanic material (FGV's; i.e., basalts, andesites, rhyolites) can often be similarly successfully characterized. The nondestructive analysis of FGV's is limited to dense fine-grained samples free of phenocrysts or other inclusions. Outcrops of FGV's, however, are generally more common and more geographically widespread than those of obsidian and it is likely that in most geographic areas we will not be able to assign specific geologic sources to artifacts. For more information about FGV characterization methods and our ongoing Tahoe National Forest region project, click HERE.
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For best results, samples selected for XRF analysis should be no less than 10 mm in diameter and a minimum of 1.5 mm in thickness. Slightly smaller samples (7-10 mm in diameter and 0.5-1.5 mm thick) will show some distortion in element values but may still be reliably characterized (using a collimator) in many cases. In any case, the use of small specimens is not recommended in complex source areas or regions where the source universe is poorly understood. The surface of the items to be analyzed should be clean and preferably free from labels or residues - a simple wash with tap water and a toothbrush will usually suffice. However, if artifacts already have painted sample numbers, the numbers may be left intact - even when paint is removed, some residue is left behind and it's better if the location of the number is obvious. Interference to the analysis by paint, when it occurs, is usually reflected in elevated levels of titanium, zinc, or lead.
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In traditional X-ray fluorescence trace element studies, samples are powdered and pelletized before analysis (Norrish and Chappell 1967; Potts and Webb 1992). In theory, the irregular surfaces of most obsidian artifacts should induce measurement problems related to shifts in artifact-to-detector reflection geometry (Hughes 1986:35). Early experiments with intact obsidian flakes by Robert N. Jack, and later by Richard Hughes, however, indicate that analytical results from lenticular or biconvex obsidian surfaces are comparable to those from flat surfaces and pressed powder pellets, paving the way for the nondestructive analysis characterization of glass artifacts (Hughes 1986:35-37; Jack 1976). The minimum optimal sample size for analysis has been found to be approximately 10 mm in diameter and 1.5-2.0 mm thick. Later experimental studies conducted by Shackley and Hampel (1993) using samples with flat and slightly irregular surface geometries have corroborated Hughes' initial observations. In a similar experiment, Jackson and Hampel (1993) determined that for accurate results the minimum size of an artifact should be about 10 mm in diameter and 1.5 mm thick. Details about the effects of sample size and surface geometry are discussed in detail by Davis et al. (1998). Agreement between the U. S Geological Survey standard RGM-1 (Glass Mountain obsidian) values and obsidian test samples was good at 1 mm thickness and improved markedly to a thickness of 3 mm.
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The diagnostic trace element values and ratios (these may typically include Ti, Mn, Fe2O3T, Zn, Rb, Sr, Y, Zr, Nb, and Ba) used to characterize the samples are compared directly to those for known obsidian sources such as those reported in the literature and with unpublished trace element data collected through analysis of geologic source samples . Artifacts are correlated to a parent obsidian source or chemical source group if diagnostic trace element values fall within about two standard deviations of the analytical uncertainty of the known upper and lower limits of chemical variability recorded for the source. Occasionally, visual attributes are used to corroborate the source assignments although sources are never assigned on the basis of only megascopic characteristics.
Diagnostic trace elements, as the term is used here, refer to trace element abundances that show low intrasource variation and uncertainty along with distinguishable intersource variability. In addition, this refers to elements measured by X-ray fluorescence analysis with high precision and low analytical uncertainty. In short, diagnostic elements are those that allow the clearest geochemical distinction between sources. Trace elements generally refer to those elements that occur in abundances of less than about 1000 ppm in a sample. For simplicity in this report, we use the term synonymously with major and minor elements such as iron, titanium, and manganese, which may be present in somewhat larger quantities.
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Church, Tim. 2000. Distribtion and Sources of Obsidian in the Rio Grande Gravels of New Mexico. Geoarchaeology 15:649-678. Davis, M. Kathleen, Thomas L. Jackson, M. Steven Shackley, Timothy Teague, and Joachim H. Hampel. 1998. Factors Affecting the X-Ray Fluorescence (EDXRF) Analysis of Archaeological Obsidian. In Archaeological Obsidian Studies: Method and Theory, edited by M. Steven Shackley, pp. 159-180. Advances in Archaeological and Museum Science Series. Plenum Publishing Co., New York, New York. Ericson, Jonathon E. 1981. Exchange and Production Systems in Californian Prehistory: The Results of Hydration Dating and Chemical Characterization of Obsidian Sources. BAR International Series 110, Oxford, England. Glascock, Michael D., Geoffrey E. Brasswell, and Robert H. Cobean. 1998. A Systematic Approach to Obsidian Source Characterization. In Archaeological Obsidian Studies: Method and Theory, edited by M. Steven Shackley, pp. 15-65. Advances in Archaeological and Museum Science Series. Plenum Publishing Co., New York, New York. Harbottle, Garman. 1982. Chemical Characterization in Archaeology. In Contexts for Prehistoric Exchange, edited by Jonathon E. Ericson and Timothy K. Earle, pp. 13-51. Academic Press, New York, New York. Herz, Norman and Ervan G. Garrison. 1998. Geological Methods for Archaeology. Oxford University Press, New York, New York. Hughes, Richard E. 1978. Aspects of Prehistoric Wiyot Exchange and Social Ranking. Journal of California Anthropology 5(1):53-66. Hughes, Richard E. 1986. Diachronic Variability in Obsidian Procurement Patterns in Northeastern California and Southcentral Oregon. University of California Publications in Anthropology 17, Berkeley, California. Hughes, Richard E. 1990. The Gold Hill Site: Evidence for a Prehistoric Socioceremonial System in Southwestern Oregon. In Living With the Land: The Indians of Southwest Oregon, edited by Nan Hannon and Richard K. Olmo, pp. 48-55. Southern Oregon Historical Society, Medford. Hughes, Richard E. 1998. On Reliability, Validity, and Scale on Obsidian Sourcing Research. In Unit Issues in Archaeology: Measuring Time, Space, and Material, edited by Ann F. Ramenofsky and Anastasia Steffen, pp. 103-114. University of Utah Press, Salt Lake City, Utah. Hughes, Richard E. and R. L. Bettinger. 1984. Obsidian and Prehistoric Cultural Systems in California. In Exploring the Limits: Frontiers and Boundaries in Prehistory, edited by Suzanne P. DeAtley and Frank J. Findlow, pp. 153-172. BAR International Series 223, Oxford, England. Hughes, Richard E. and Robert L. Smith. 1993. Archaeology, Geology, and Geochemistry in Obsidian Provenance Studies, in Effects of Scale on Archaeological and Geoscientific Perspectives, edited by J. K. Stein and A. R. Linse, pp. 79-91. Geological Society of America Special Paper 283, Boulder, Colorado. Jack, Robert N. 1976. Prehistoric Obsidian in California I: Geochemical Aspects. In Advances in Obsidian Glass Studies: Archaeological and Geochemical Perspectives, edited by R. E. Taylor, pp. 183-217. Noyes Press, Park Ridge, New Jersey. Jackson, Thomas L. and Joachim Hampel. 1993. Size Effects in the Energy-Dispersive X-ray Fluorescence (EDXRF) Analysis of Archaeological Obsidian (Abstract). International Association for Obsidian Studies Bulletin 9:8. Norrish, K. and B. W. Chappell. 1967. X-Ray Fluorescence Spectrography. In Physical Methods in Determinative Mineralogy, edited by J. Zussman, pp. 161-214. Academic Press, New York, New York. Potts, Philip J. and Peter C. Webb. 1992. X-Ray Fluorescence Spectrometry. Journal of Geochemical Exploration 44:251-296. Rapp, George, Jr. 1985. The Provenience of Artifactual Raw Materials. In Archaeological Geology, edited by George Rapp, Jr. and John A. Gifford, pp. 353-375. Yale University Press, New Haven, Connecticut. Shackley, M. Steven and Joachim Hampel. 1993. Surface Effects in the Energy-Dispersive X-ray Fluorescence (EDXRF) Analysis of Archaeological Obsidian (Abstract). International Association for Obsidian Studies Bulletin 9:10. Skinner, Craig E. 1983. Obsidian Studies in Oregon: An Introduction to Obsidian and An Investigation of Selected Methods of Obsidian Characterization Utilizing Obsidian Collected at Prehistoric Quarry Sites in Oregon. Unpublished Master's Terminal Project, Interdisciplinary Studies, University of Oregon, Eugene, Oregon. Skinner, Craig E. 1995. Obsidian Characterization Studies. In Archaeological Investigations, PGT-PG&E Pipeline Expansion Project, Idaho, Washington, Oregon, and California, Volume V: Technical Studies, by Robert U. Bryson, Craig E. Skinner, and Richard M. Pettigrew, pp. 4.1-4.54. Report prepared for Pacific Gas Transmission Company, Portland, Oregon, by INFOTEC Research, Inc., Fresno, California, and Far Western Anthropological Research Group, Davis, California. Williams, K. L. 1987. An Introduction to X-Ray Spectrometry: X-Ray Fluorescence and Electron Microprobe Analysis. Allen & Unwin, Boston, Massachusetts. Williams-Thorpe, O. 1995. Obsidian in the Mediterranean and the Near East: A Provenancing Success Story. Archaeometry 37:217-248.
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