The Coso volcanic field reexamined: Implications for obsidian sourcing and hydration dating research

Obsidian from the Coso locality, Inyo County, California, has long been regarded by archaeologists as a single “source.” However, studies by the U.S. Geological Survey have provided evidence of geochemical variability among flows within the volcanic field. to determine whether these geochemical distinctions could be applied productively to archaeological research, nondestructive x-ray fluorescence analyses were conducted on samples recently collected from 15 obsidian-bearing loci. the results of this research show that geochemically distinct varieties of artifact-quality obsidian can be recognized within the Coso volcanic field, and subsequent nondestructive x-ray fluorescence analyses of artifacts from two nearby archaeological sites document that different geochemical varieties of obsidian were used prehistorically to manufacture tools. Implications of these results for studies of prehistoric exchange and obsidian hydration dating are discussed.     

Chemical characterization and hydration rate development for New Mexican obsidian sources

The obsidian hydration dating of prehistoric sites requires that each type of natural glass be identified on the basis of its elemental constituents in order that the appropriate hydration rate constants may be applied. In New Mexico, obsidian occurs in the form of in situ flows and as secondarily deposited detrital material in the sediments of ancestral watercourses. the chemical analysis of 153 geological and archaeological samples from both contexts has resulted in the identification of nine major obsidian sources. the hydration rates for New Mexican obsidians developed by different researchers are compared and evaluated in light of current knowledge on glass-water interaction. It is argued that a hydration rate developed in silica saturated solution or at 100% relative humidity are the preferred conditions for laboratory hydration rate development.     

Obsidian provenancing and magmatic fractionation in central oregon

In many instances, geologically distinct obsidian flows located within even a relatively small geographic area can be uniquely identified by their chemical composition. This happens to be true for several obsidian sources from central Oregon. Internally each obsidian locality is chemically homogeneous, but the obsidian rocks from different collection sites exhibit chemical differences. Based on the geochemical variations and on K/ Ar dating of the end members of the chemical differentiation trend, these differences are related to the fractionation of a single Late Miocene magma chamber, dated at 6.5 Ma. By understanding the underlying causes of the chemical differences, constraints are disclosed that will govern the possible chemical variations of other, as yet unidentified but related obsidian flows. These can be useful for identifying the possible natural sources of obsidian artifacts which do not match known obsidian sources, and for suggesting possible geographic areas where these as yet undiscovered obsidian flows may be found. © 1993 John Wiley & Sons, Inc.     

Low-temperature isotopic exchange in obsidian: Implications for diffusive mechanisms

While a great deal is known about the interaction between water and rhyolitic glasses and melts at temperatures above the glass transition, the nature of this interaction at lower temperatures is much more poorly understood. This paper presents the results of a series of isotopic exchange experiments aimed at further elucidating this process and determining the extent to which a point-by-point analysis of the D/H or ¹⁸O/¹⁸O isotopic composition across the hydrated rim on a geological or archaeological obsidian sample can be used as a paleoclimatic monitor. Experiments were performed by first hydrating the glass for 5 days in water of one isotopic composition, followed by 5 days in water of a second composition. Because waters of near end-member compositions were used (nearly pure ¹H₂¹⁶O, ¹H₂¹⁸O, and D₂¹⁶O), the relative migration of each species could be ascertained easily by depth-profiling using secondary ion mass spectrometry (SIMS). Results suggest that, during hydration, both the isotopic composition of the waters of hydration, as well as that of intrinsic water remaining from the initial formation of the glass vary dramatically, and a point-by-point analysis leading to paleoclimatic reconstruction is not feasible.     

Concentration dependence of water diffusion in obsidian and dacitic melts at high-pressures

The diffusion of water in natural obsidian and model dacitic melts (Ab90Di8Wo2, mol %) has been studied at water vapor pressure up to 170 MPa, temperatures of 1200°C, H₂O contents in melts up to ～6 wt % using a high gas pressure apparatus equipped with a unique internal device. The experiments were carried out by a new low-gradient technique with application of diffusion hydration of a melt from fluid phase. The water solubility in the melts and its concentration along $C_{H_2 O} $ diffusion profiles were determined using IR microspectrometry by application of the modified Bouguer-Beer-Lambert equation. A structural-chemical model was proposed to calculate and predict the concentration dependence of molar absorption coefficients of the hydroxyl groups (OH^?) and water molecules (H₂O) in acid polymerized glasses (quenched melts) in the obsidian-dacite series. The water diffusion coefficients $D_{H_2 O} $ were obtained by the mathematical analysis of concentration profiles and the analytical solution of the second Fick diffusion law using the Boltzman-Matano method. It was shown experimentally that $D_{H_2 O} $ exponentially increases with a growth of water concentration in the obsidian and dacitic melts within the entire range of diffusion profiles. Based on the established quantitative correlation between $D_{H_2 O} $ and viscosity of such melts, a new method was developed to predict and calculate the concentration, temperature, and pressure dependences of $D_{H_2 O} $ in acid magmatic melts (obsidian, rhyolite, albite, granite, dacite) at crustal T, P parameters and water concentrations up to 6 wt %.     

The Pachuca obsidian source,Hidalgo, Mexico: A geoarchaeological perspective

Sierra Las Navajas, known to archaeologists as “the Pachuca obsidian source,” has been a major source of obsidian to Mesoamerican societies for more than 3000 years, producing a fine green obsidian unique in Middle America. It was the primary source of the obsidian that formed the economic backbone of the major sociopolitical centers of Classic period Teotihuacán, epi‐Classic Toltec Tula, and Aztec Tenochtitlán. In this paper, the obsidian of Sierra Las Navajas is discussed in the following contexts: (1) geologically, because the extraordinary quality of the Pachuca obsidian, its ease of extraction, and its distinctive color and chemistry are a direct result of its geologic emplacement; (2) locally, as the different mining localities within Sierra Las Navajas reflect the varying needs of the cultures working them; and (3) globally, as the obsidians of Las Navajas were used in concert with obsidians from other sources, and were traded great distances across Mesoamerica. © 2004 Wiley Periodicals, Inc.     

Structural evolutions of an obsidian and its fused glass by shock-wave compression

Shock-recovery experiments for obsidian and its fused glass have been carried out with pressure up to 35 GPa. Structural evolution accompanying the shock compression was investigated using X-ray diffraction technique, Raman and infrared spectroscopy. The densities of obsidian and its fused glass increased with applied shock pressure up to 25 GPa. Densification reached a maximum of 4.7 and 3.6% for obsidian and its fused glass, respectively. The densification mechanism is attributed to reduction of the T–O–T angle, and changes in ring statistics in the structure. Density reduction observed at greater than 25 GPa of applied shock pressure is due to partial annealing of the high-density glass structures brought by high post-shock residual temperature. The density of fused glass is almost equal to its original value at 35 GPa while the shocked obsidian has a slightly lower value than its original value. Amorphization of crystallites present in the obsidian due to shock compression is probably the cause of the density decrease. The structural evolution observed in shock-compressed obsidian and its fused glass can be explained by densification resulting from average T–O–T angle reduction and increase of small rings, and subsequent structural relaxation by high post-shock temperature at applied shock compression above 25 GPa.     

Distribution and sources of obsidian in the Rio Grande gravels of New Mexico

The obsidian in the gravels deposited by the Rio Grande in New Mexico has interested archaeologists of the region, particularly the use of these gravels by prehistoric populations and the implications for obsidian sourcing studies. Previous investigations of Rio Grande gravel obsidian have focused on obsidian in the archaeological record. This study focuses on the natural occurrence and distribution of obsidian in the gravels and the implications for archaeological investigations. Spatial sampling of the gravels clearly indicate that obsidian, as well as other chipped stone material, is not uniformly distributed across the landscape. Geochemical analysis of the obsidian in the gravels establishes the true source constituents for the obsidian present in the gravels. The main source area for obsidian in the Rio Grande gravels is the Jemez Mountains, although some obsidian comes from Grant's Ridge, Polvadera, and No Aqua sources. Sources south of Mount Taylor, such as Red Hill and Mule Creek, do not occur in the Rio Grande gravels of southern New Mexico. © 2000 John Wiley & Sons, Inc.     

Morphological and geochemical analysis of the Laguna Blanca/Zapaleri obsidian source in the Atacama Puna

This paper describes a large obsidian deposit located along the southern banks of Laguna Blanca, on the eastern slope of the Jarellón volcanic caldera near the Chilean–Bolivian border. The obsidian at this site occurs in flows or sheets of deflated black or reddish to brown pebbles, redeposited on the shores of a lake. Blocks of obsidian are only found around the caldera rim. Here we analyze the shape of the obsidian pebbles and their geochemistry, comparing them with previously published data. The results indicate that the geochemical composition of the samples strongly matches previous analyses of obsidian from cultural contexts. This obsidian source was one of the most important sites of obsidian procurement since at least the Formative Period in the Atacama Puna region. © 2010 Wiley Periodicals, Inc.     

Las Cargas: Characterization and Prehistoric Use of a Southern Andean Obsidian Source

The importance of obsidian from the northern Patagonian source at Las Cargas is reflected in its early use (∼8000 years B.P.) and extensive geographic diffusion but is nonetheless surprising in light of the source's high altitude (located in the Andes Cordillera), which makes it both difficult to access under ideal conditions and inaccessible for much of the year. Prehistoric use of the Las Cargas source can inform us about mobility, subsistence choices, economics of stone consumption, trade, and territoriality. Here we present the results of various lines of evidence (surface survey, X‐ray fluorescence and instrumental neutron activation analyses, artifact morphometry, and obsidian hydration dating) used to characterize obsidian from Las Cargas and its prehistoric use during the Holocene. Results indicate that Las Cargas obsidian occurs at the source as blocks and nodules, which are chemically homogeneous and of variable quality. Use of the source was nearly continuous through time, and the primary knapping activities performed there were the production of blanks and preparation of cores.     

The Topaz Basin archaeological obsidian source in the transition zone of central Arizona

The Classic Period Migration Project involves the analysis of archaeological sites at Perry Mesa in central Arizona and resulted in the discovery of several small marekanite1 obsidian artifacts that signaled a previously unlocated source. The source was eventually located in the Topaz Basin area of the upper Cienega Creek stream basin, southwest of Camp Verde, Arizona.² While this locality solves the “unknown” sources in the Perry Mesa archaeological assemblage, it has not appeared in the archaeological record of Arizona with any frequency. The glass itself is an excellent medium for tool production, so its near absence in the archaeological record is likely attributable to social/territorial causes as well as limited secondary deposition, and along with other “minor” sources points to the archaeological utility of understanding these smaller sources. © 2009 Wiley Periodicals, Inc.     

Microlite orientation in obsidian flow measured by synchrotron X-ray diffraction

Clinopyroxene and plagioclase (andesine) microlites in an obsidian flow from Glass Mountain (NE California, USA) display strong alignment. Synchrotron X-ray diffraction, coupled with Rietveld analysis, was used to quantify crystallographic-preferred orientation (CPO). Clinopyroxene, with a rod-shaped morphology, shows a strong alignment of [001] in the flow direction and (010) aligned parallel to the inferred flow plane. Andesine, with a platy morphology, displays an alignment of (010) platelets in the flow plane. Some pole densities exceed 90 multiples of random distribution. Applying a model of rigid ellipsoidal inclusions in a viscous matrix, the local pure shear strains are between 2 and 3.     

Subsource characterization: Obsidian utilization of subsources of the Coso volcanic field,Coso Junction,California, USA

Six chemical subsource groups were identified in the analysis of 84 obsidian samples collected from subsource locations at Coso volcanic field, California. In prehistoric times, Coso provided obsidian for artifacts found from San Francisco Bay to San Diego to Death Valley to the eastern Mojave Desert. Subsource groups were defined by instrumental neutron activation analysis (INAA) of 29 elements followed by cluster analysis, principal component analysis, and bivariate plotting. The new data are compared to previously published INAA and X‐ray fluorescence data. Characterization of 55 obsidian artifacts from archaeological sites located approximately 100 miles from Coso suggests preferential usage of specific subsources as a function of the directionality of travel. The results are consistent with a bimodal (resident and itinerant) model of procurement. This research illustrates the importance of accurate sourcing of obsidian artifacts when attempting to define subsource usage. © 2004 Wiley Periodicals, Inc.     

A tritium-exchange method for obsidian hydration shell measurement

A new radiochemical method for measuring the amount of water in the hydrated layer on the surface of obsidians exchanges tritiated water with the water in the layer (20 μl of 5 Ci ml^?1 at 90°C for 10 days) and then back-exchanges it (in 150 ml of water at 35°C for ～ 200 hr.). The activity of the back-exchange water (F) is monitored by liquid scintillation counting of aliquots extracted at known time intervals (t). The activity so measured is then related to the thickness of the hydration rim. A sheet diffusion model shows that the thickness of the hydration shell (l) is inversely proportional to the slope of the F vs.$\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ plot. Comparison of l-values so obtained between obsidians, whose age (x) is inferred from archaeological occupation layers containing radiocarbon-dated wood and charcoal, suggests a relationship between l and x. Implications for New Zealand prehistory are briefly considered. The technique, which is non-destructive, appears particularly applicable to young glasses where the development of hydrated layers may be inadequate for accurate optical measurement.     

Obsidian from the northern sector of the Main Ethiopian Rift: implications for archeology

Obsidian is abundant in the Main Ethiopian Rift (MER). Petrological and geochemical features of obsidian from four volcanic centers in the MER, namely Birenti, Dofen, Fentale and Kone, are presented. Compositional and petrological variability is noted among the Dofen and Fentale obsidian, but not in those from Kone and Birenti where each have separate but uniform elemental composition. The Fentale and Kone obsidian were source materials for the artifacts of a number of Middle Stone Age and Later Stone Age/Neolithic sites in the region. We have yet to determine whether Dofen and Birenti were sources for archeological artifacts. The study also shows that volcanic episodes from a single center do not necessarily result in compositional variability.     

Siltstone from Southern Patagonia: Its Source and Archaeological Artifact Distribution in Santa Cruz Province,Argentina

Hunter‐gatherer mobility and spheres of interaction are important characteristics worthy of investigation in Patagonian archaeology. One way to approach these is by studying the distribution of lithic archaeological materials. Siltstone (limolite) artifacts are found along the western strip of southwestern Patagonia, Argentina. Based on geomorphological studies and the high density of archaeological material, a source was located along the western margin of Cardiel Lake. Neutron activation analysis of samples from the source and archaeological sites in several neighboring basins allowed us to model its circulation. Siltstone's archaeological distribution indicates that its regional circulation had a southerly direction dating from the early Holocene. This southern vector shows an important difference when compared to the distribution of obsidian from Pampa del Asador, which has a broader circulation pattern. This could be related to a greater availability of high‐quality lithic materials north of the siltstone source. This work also contributes to the construction of a lithic source database for southern Patagonia.     

Diffusion-controlled spherulite growth in obsidian inferred from H<Subscript>2</Subscript>O concentration profiles

Spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian. The growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass, and rejection of non-crystal forming components from the crystallizing region. Water concentration profiles measured by synchrotron-source Fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth, and that it diffuses outward ahead of the advancing crystalline front. We compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth. Using a diffusion-controlled growth law, we find that spherulites can grow on the order of days to months at temperatures above the glass transition. The diffusion-controlled growth law also accounts for spherulite size distribution, spherulite growth below the glass transition, and why spherulitic glasses are not completely devitrified. An erratum to this article can be found at     

吉林东部含细石器遗存的初步研究

文章对近年来发现于吉林东部即长白山山地地区的12处旧石器遗址中9处含有细石器的遗存予以关注,并将它们界定为"含细石器遗存"。由于延边大洞遗址的材料整理工作尚未结束,只对其中的8处进行了实际分析和研究。主要从细石核型式和细石器工艺技术两个方面对这些遗存中发现的细石器进行了分析,同时把与细石器伴生的非细石器制品作为石制品组合从原料、技术、器物大小和类型等方面给予探讨。基于上述几方面的分析研究,提出该地区含细石器遗存包括了以细石器为主体、以小石器为主体和以大石器为主体的3种类型,并初步认为该地区细石器制作技术来源于华北地区。遗存年代为旧石器时代晚期或偏晚。     

Obsidian Subsource Identification in the Sierra de Pachuca and Otumba Volcanic Regions,Central Mexico,by ICP‐MS and DBSCAN Statistical Analysis

Otumba and Sierra de Pachuca obsidian deposits in Central Mexico have been important sources of raw material since pre‐Hispanic times. Numerous archaeological investigations have suggested that the economical and political expansion of major Mesoamerican societies were linked to the control of obsidian sources and distribution of quarried material. Sierra de Pachuca contains several obsidian flows and numerous quarries throughout the region that were preferentially exploited by different cultures. The Otumba Volcanic Complex has four important obsidian domes, but three of them have not been studied in detail. A geochemical characterization of subsources from the Sierra de Pachuca and Otumba Volcanic Complex is an important step toward future sourcing of obsidian artifacts that would help provide insight into spheres of influence and trade by past cultures in Central Mexico. Having this purpose in mind, inductively coupled plasma mass spectrometry (ICP‐MS) was used to analyze obsidian samples collected from five separated locations at Sierra de Pachuca and four at Otumba, followed by statistical analysis (density‐based spatial clustering of applications with noise, DBSCAN). We were able to distinguish three chemically distinctive subsources in Sierra de Pachuca and three in Otumba. This study illustrates the importance of accurate characterization of obsidian raw material when attempting to define subsource usage.     

DIffusion of Eu and Gd in basalt and obsidian

Tracer diffusion coefficients of ¹⁵³Gd and ¹⁵²Eu in olivine tholeiite have been determined at temperatures between 1150 and 1440°C. The results are identical for both tracers within experimental error. Between 1440 and 1320°C the diffusion coefficients are given by D(Eu, Gd) = 0.058 exp(?40,600/ RT). Between 1320 and 1210°C, the diffusion coefficients are constant at D = (1.4 ± 0.4) × 10^?7 cm²s^?1 and between 1210 and 1150°C, the D values drop irregularly to 4 × 10^?9 cm²s^?1. The liquidus temperature (1270°C) lies within the region of constant D. Such anomalous behavior has not been encountered in previous studies of Ca, Sr, Ba and Co diffusion in basalt. To explain the constant D value near the liquidus, we speculate that the structure of the melt changes as a function of temperature in such a way that the normal temperature dependence of the diffusivity is compensated. For example, the rare earth ions may be displaced from their (high temperature) octahedral coordination sites to other sites where they are more readily dissociated and therefore become progressively more mobile. The behavior below 1210°C may be the result of relatively stable complexes or molecules in the melt or of the formation of a REE bearing crystalline phase that has so far escaped detection. Preliminary results for Eu diffusion in obsidian are D (Eu, 800°C) = 5 × 10^?13 cm² s^?1 and D (Eu, 950°C) = 1.5 × 10^?11 cm² s^?1. These data are consistent with an activation energy of 59 Kcal mole^?1. These low diffusivities indicate that the partitioning of REE in crystallizing intermediate and acidic melts may be controlled by diffusion in the melt rather than equilibrium between the crystal surface and the bulk melt.The diffusion data are applied to partial melting in the mantle, in an attempt to explain how LREE enriched tholeiites may be derived from a LREE depleted mantle source. In this model LREE diffuse from garnet bearing regions that have small melt fractions into garnet free regions that have relatively large melt fractions. REE diffusion is so slow that this process is quantitatively significant only in small partially molten bodies (diameter ～1 km or less) or in larger, but strongly flattened bodies. Internal convective motion during diapiric rise would also increase the efficiency of the process.