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Keywords: obsidian studiessourcingobsidian hydrationarchaeologyarchaeometryX-ray fluorescencechemical characterizationobsidianobsidian hydration dating.

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Color, in some cases, was an important factor in the selection ethio use of particular obsidian sources in prehistory for an example, see Hughesyet color, translucency, and sheen—especially when assessed visually—can vary within a single obsidian source or flow and are therefore unreliable markers of geologic provenance. In other words, are the of an analysis accurate and repeatable on another instrument?

Unfortunately, the sourcing and dating methods used for obsidian remained problematic for at least the first two decades, and we are still refining our instrumentation, analysis methods, and data today. Finally, the ability to analyze artifacts in restricted regions or collections has resulted in both the identification of additional geologic sources and the clarification of ly unknown patterns of lithic resource use that are changing the Fremont in which we understand the prehistoric past. Some archaeologists and archaeometrists use the term to deate a single locus on the landscape, in other words, where obsidian was likely procured or where geologic samples for source libraries were taken.

However, temperature and pressure are continuously changing, and as a result the magma will continue to evolve until the temperature reaches the point at social the magma begins to solidify or the magma erupts, effectively halting the system.

In the s that follow, I will provide a summary of the current status of the field, including techniques used in characterization and sourcing studies, obsidian hydration, and regional syntheses, as dating as some comments on new directions in obsidian studies moving forward.

Obsidian forms in a variety of different colors, with black and red as the most frequently observed colors, but gray, green, lavender, and brown are also found. It begins with an overview of obsidian and the unique formation processes that create it before turning to a discussion of the ificance of dating and sourcing techniques for understanding prehistoric obsidian trade and exchange. Rhyolitic melts that erupt and social before crystals have time to form result in a glass Carmichael et al. When the magma begins to rise within the crust and slowly starts decreasing in pressure and losing heat to the surrounding matrix, the minerals that were formerly in solution are no longer in equilibrium and begin to crystallize from the liquid.

The utility of Fremont characterization studies in sourcing archaeological obsidian samples has been proven many times over, and submitting obsidian artifacts for X-ray fluorescence analyses or other characterization methods has become standard practice among archaeologists in areas where obsidian is commonly recovered.

One step toward solving this problem is through the calibration of these machines using a suite of source standards, such as those compiled by the Missouri University Research Reactor and used in the factory calibration of all pXRF instruments sold by Bruker AXS, one of the larger pXRF manufacturers that markets to archaeologists. Rhyolitic magma forms as a liquid deep within the earth at high temperature ethio pressure.

Today, archaeometric obsidian studies continue to grow, particularly through the refinement of sourcing and dating methodologies, and the addition of new geochemical sources to databases in understudied areas such as East Africa.

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Obsidian is a volcanic glass that forms during rapid cooling of high-silica, rhyolitic lavas Blatt and Tracy : Rhyolitic lava is extremely viscous, and this viscosity contributes to its ability to form a glass. During this liquid stage, elements circulate throughout the melt, which contributes to the homogeneity of the Fremont obsidian flow. However, performing analyses in this way, without repeatable analyses of published obsidian standards and without empirical calibrations, has been severely criticized for a lack of accuracy Shackley ; Speakman and Shackley Specifically, the social is that the characterization data may be revealing elemental concentrations that are not correct—either higher or lower than the actual concentration—and more dangerously, that they are not skewed directionally.

But obsidian studies are not limited to laboratory analyses. In high-viscosity magmas, diffusion rates are low, so crystals will grow more slowly. This is a statistical exercise in that the archaeometrist is determining the most likely fit between artifact and obsidian flow in order to assess the provenance of an obsidian sample.

Provenance is the term used to deate the geologic origin of a sample, while provenience is used to indicate the place of archaeological recovery of an artifact. Obsidian hydration dating is performed by cutting a small sample from an artifact and then measuring the thin hydration rind that has formed on the surface of archaeological obsidian. Specifically, can source data generated from one instrument or analysis be used as a geologic source library for comparison dating characterization data obtained from another instrument or analysis?

Ethio of characterization methodologies have demonstrated good comparative outcomes when data are generated by experienced archaeometrists using instruments calibrated to published geologic standards Shackley a. This article discusses the current status of archaeological obsidian studies, including techniques used in characterization and sourcing studies, obsidian hydration, and regional syntheses. Finally, it addresses some important questions relating to obsidian research and suggests new directions in the field. Inthe first comprehensive volume on obsidian studies was published R.

Shackley, Archaeological Obsidian Studies: Method and Theoryproviding a summary of new directions in obsidian study.

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However, echoing Shackleyit is important for pXRF practitioners to make sure they understand the instrumentation, geology, and data output in order to make valid source asments based on characterization data see Fauman-Fichman for a cautionary tale about erroneous source asments. The biggest advantage of pXRF technology is that they can be used on objects that cannot be moved to a laboratory, either due to size, transport restrictions i.

Eventually, the magma will again reach equilibrium for a given temperature and pressure.

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To determine these geochemical data, chemical characterization techniques such as X-ray fluorescence XRF Shackley a, instrumental neutron activation analysis NAA Glascock et al. Hydration dates are affected by factors such as environmental conditions and chemical compositions that were not anticipated when the technique was first described and published. Some elements are particularly variable in the liquid melt due to their inability to combine into solid forms.

Some geologic loci may contain multiple discrete flows with different chemical atures Tykot ; Ambroz et al. X-ray fluorescence XRF is one of the most commonly employed chemical characterization methods used on lithic artifacts made from volcanic materials Freund ; Hughes and Smith ; Glascock et al.

Examples of these incompatible elements routinely used in dating sourcing studies include rubidium Rbstrontium Srbarium Bayttrium Yand zirconium Zr. Physically, obsidian is a glass with the properties of a liquid in all respects except for the ability to flow easily Cann : ethio As a result, obsidian fractures conchoidally, making it an ideal material for stone tool manufacture.

New portable X-ray fluorescence pXRF technology is further ideal in archaeological contexts because it enables analysis of obsidian specimens in situ in the field or museum to obtain geochemical data Sr, Y, Zr, Ti, Nb, and Rb social necessitating transport of artifacts to a separate laboratory facility and allowing for large-scale geologic characterization to take place directly on Fremont quarry site, natural outcrop, or obsidian flow.

Differences in obsidian appearance can be attributed to diverse factors such as microlites, gas bubbles, chemical variation, oxidation, or incorporation of foreign material into still liquid lava.

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Manufacturers and archaeometry practitioners are trying to devise ways to make their data comparable between instruments. Research investigating the chemical homogeneity of obsidian sources has revealed that individual flows within rhyolite domes sometimes possess trace element chemical differences vast enough to warrant false asment to distant sources Hughes and Smith ; Hughes ; Shackley a ; Tykot Further complications for sourcing can include secondary sources of obsidian that were often used prehistorically but did not necessitate travel to or quarrying at the original geologic source, such as nodules that have been transported through erosion in river systems and drainages over hundreds of miles Shackleya, Because methods for chemical characterization of obsidian have been discussed in detail elsewhere Taylor ; Davis et al.

This also permits researchers to reexamine and retest specimens on site as needed and without additional cost.

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Most obsidian, however, is neither uniform nor high quality, and therefore obsidian flows that could have been used for prehistoric tool production are rare. At the same time, minerals also crystallize and subtract other elements from solution.

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However, sometimes errors can also arise unexpectedly. Reliable source asments are generally only possible through geochemical techniques. Rapid pXRF analysis, in which source assessments can be revealed in mere seconds, can further revolutionize the way in which obsidian is integrated in research de through the analysis of much larger s of samples Frahm et al. These are called incompatible elements, and they can be uniquely diagnostic in characterization and sourcing of obsidian.

Most of the time, problems with the accuracy of characterization data can be recognized and corrected, particularly when an experienced archaeometrist examines the data for an unusual example of this, see Shackley and Dillian However, this lack of accuracy is most concerning when the instrument and data are in the hands of an inexperienced archaeometrist who might not realize the errors appearing in the instrument output. Archaeologists remained skeptical of the utility of these new techniques at first, relying more consistently on visual sourcing of obsidian and typologies or radiometric dating of associated artifacts for chronological information.

In characterization and sourcing studies, early geologic source libraries were woefully inadequate, and data were hard to manipulate and interpret prior to the development of more robust computing platforms.

The unique formation processes that create obsidian are important to understand because they contribute to its chemical makeup, and subsequently its characterization and provenance assessment. Thorough sampling is necessary to determine the homogeneity of specific geologic sources prior to source asments of archaeological specimens. Approximately fifty years ago, a great leap forward occurred in obsidian studies, marking the beginning of obsidian research as it is conducted today.

In some cases, the data, in the form of an assessment of the elemental composition of an obsidian object, are not accurately reproduced from instrument to instrument, or they do not conform to the published and accepted elemental composition of geologic obsidian standards.

Now pXRF analyses are conducted by archaeologists instead of in specialized archaeometry laboratories, and even sometimes on site, where the data can be used to help drive ongoing research methodologies in real time. New geologic source databases are continually being generated and published for ly understudied areas through ongoing geologic and archaeological survey Kuzmin et al.

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Additional summary volumes have been published more recently that provide an expanded overview of the field Glascock et al. Some pXRF users have stated that internal consistency is sufficient and accuracy might not matter, but this has been criticized for veering widely from best practices in archaeometry Shackley Ellery Frahm ab has argued that the accuracy of the pXRF instrument, as measured by how well the resulting data compare to published obsidian standards, might not matter as long as it is possible to use these data to answer archaeological questions.

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Further limiting its availability, natural glass decomposes into perlite within a few million years of its formation, so only relatively recent obsidian flows usually contain glassy nodules large enough to have been used for stone tools. Translucency may vary from clear to opaque, and banded or mossy color arrangements are common. It is often the most appropriate method for archaeological specimens because samples do not need to be damaged or destroyed for analysis.

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Obsidian hydration practitioners soon discovered that environmental context, including temperature, humidity, soil acidity, fire history, and even microenvironments, as well as obsidian chemistry, played a ificant role in the hydration rate of individual obsidian sources. Additionally, there has been a demonstrated advantage to on-site analyses that can help inform the direction and scope of ongoing research Frahm et al.

Yet despite potential problems with the use of pXRF in obsidian analyses, the introduction of this new instrumentation has resulted in a massive democratization of the field. Instead, it is what we do with these obsidian data that is important in advancing knowledge about our past. The limitations of this are twofold: first, in the accuracy of the analytical method; and second, in the comprehensiveness of the geologic database. Particularly in regions where the export of artifacts to an archaeological laboratory is limited, rapid pXRF analysis can yield ificantly more information, including the identification of rare sources in an archaeological assemblage Frahm et al.

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To grow crystals, mineral particles must diffuse through the magma to crystal surfaces. It also explores obsidian hydration dating methods and equations, factors that can affect the date asments for hydration specimens, and the various uses of obsidian in prehistoric times. In theory, calibrating instruments to known standards should make all instrument data comparable, and therefore, source samples run on one instrument can produce data that may be used by others in determining provenance.

However, this does not mean that all pXRF data are bad, but instead, that best practices, including analysis of well-documented obsidian standards concurrently with each analysis run, are important in maintaining rigor Ethio and Shackley Some demonstrated variability exists in the sensitivity of different characterization methods and between individual instruments Glascock ; Shackley ; Goodale et al. This shift has ificant pros and cons, which I will discuss in more detail next. Yet there is an underlying prerequisite for adequate geologic analysis to occur prior to and concurrently with archaeological sourcing studies.

Suddenly, obsidian as a social material became both sourceable and dateable, allowing for archaeological research pertaining to trade and exchange, population movements, procurement, and territory; and subsequently innovative new questions on gender, labor, status, and symbol, all with chronological context. These errors can occur through operator error, such as by analyzing a dating that is too small, or if Fremont sample is not making good contact with the analysis window, the sample has some kind of unrecognized surface contamination, or other factors.

A of archaeometrists have recognized that there are potential problems with accuracy and interpretation of data when pXRF instruments are used, particularly by inexperienced analysts Shackleya ; Goodale et al. The advent of obsidian hydration dating in the s also marked a watershed in archaeological obsidian studies, yet problems quickly became apparent when dates obtained through obsidian hydration were compared with those achieved through other methods.

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Liquid magma is at equilibrium such that for a given temperature and pressure, Fremont are dissolved within the melt and the liquid is saturated with these minerals. This was a watershed moment in archaeological obsidian studies. Through chemical characterization, it is possible to generate elemental data that are then matched with the best fit geologic point of origin, or provenance.

Portable X-ray fluorescence instruments have resulted in a dating in who performs and interprets characterization data, specifically shifting from obsidian analyses being conducted by professional archaeometrists at a handful of commercial or academic laboratories, to many obsidian analyses being conducted in the field laboratory or even directly on site by the archaeologists themselves. Calendar dates are based on rind thickness calibrated to additional factors such as obsidian chemistry, relative humidity, and effective hydration temperature EHT.

InJ. Cann and Colin Renfrew published a research article in the Proceedings of the Prehistoric Society New Series outlining the importance of new characterization and sourcing techniques for social prehistoric obsidian trade and exchange. Many flows contain phenocrysts, inclusions, or vesicles, which hinder conchoidal fracture. Because each magma pool is the product of a unique combination of pressure, temperature, contributing wall rocks, and crystallization processes, the ethio composition of the products formed from that magma pool, such as obsidian, will be unique, and they will even be distinctive for different eruptive events of a single volcano as the magma continues to evolve over time.