Michael D. Glascock

Fracturing of the Panamanian Isthmus during initial collision with South America

Tectonic collision between South America and Panama began at 23–25 Ma. The collision is significant because it ultimately led to development of the Panamanian Isthmus, which in turn had wide-ranging oceanic, climatic, biologic, and tectonic implications. Within the Panama Canal Zone, volcanic activity transitioned from hydrous mantle-wedge−derived arc magmatism to localized extensional arc magmatism at 24 Ma, and overall marks a permanent change in arc evolution. We interpret the arc geochemical change to result from fracturing of the Panama block during initial collision with South America. Fracturing of the Panama block led to localized crustal extension, normal faulting, sedimentary basin formation, and extensional magmatism in the Canal Basin and Bocas del Toro. Synchronous with this change, both Panama and inboard South America experienced a broad episode of exhumation indicated by (U-Th)/He and fission-track thermochronology coupled with changing geographic patterns of sedimentary deposition in the Colombian Eastern Cordillera and Llanos Basin. Such observations allow for construction of a new tectonic model of the South America–Panama collision, northern Andes uplift and Panama orocline formation. Finally, synchroneity of Panama arc chemical changes and linked uplift indicates that onset of collision and Isthmus formation began earlier than commonly assumed.

A Systematic Approach to Obsidian Source Characterization

A systematic approach to sample collection, chemical analysis, and statistical evaluation of obsidian sources is recommended before significant numbers of artifacts should be analyzed. Multivariate statistical methods based on Mahalanobis D 2 enable robust rejection of erroneous source assignments and lead to sourcing of artifacts with a very high degree of confidence. These multivariate procedures can also assist in identifying and evaluating abbreviated analytical methods that are more rapid and less expensive. Test cases based on obsidian data from sources in Mesoamerica are presented.

Neutron activation analysis and provenance research in archaeology

Neutron activation analysis is a powerful quantitative analytical technique with application in a broad range of disciplines such as agriculture, archaeology, geochemistry, health and human nutrition, environmental monitoring and semiconductor technology. Due to its excellent sensitivity, great accuracy and precision, and versatility, the technique is a suitable method for analysing many different types of samples. Archaeologists, in particular, have made extensive use of neutron activation analysis for the purpose of characterizing archaeological materials and determining their provenance. This paper presents a brief history of the technique and its application to archaeology, describes the physics behind the analytical method, and explains how the method is generally employed to determine the sources of archaeological materials.

High-Precision Trace-Element Characterization of Major Mesoamerican Obsidian Sources and Further Analyses of Artifacts from San Lorenzo Tenochtitlan, Mexico

High-precision trace-element analyses for 208 geological samples representing 25 mesoamerican obsidian sources were obtained using instrumental neutron-activation analysis to measure a total of 28 elements persample. These are the first detailed chemical studies ever published for many of the source areas. Especially intensive analyses were made for six sources in the states of Veracruz and Puebla in Mexico from the region of Pico de Orizaba volcano. In addition, source determinations are provided for 65 artifacts from the Olmec site of San Lorenzo Tenochtitlan, Veracruz. The investigations presented here constitute an important basis for associating obsidian artifacts with specific sources, therety making possible the reconstruction of Prehispanic trade systems.

Gold solubility, speciation, and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H2O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as ≈1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, CHClb, (3 × 103 to 1.1 × 104 ppm) is ≈40 ppm (by weight) and is independent of the HCl concentration under those conditions. For CHClb of 1.1 × 104 to 4.0 × 104 ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log CAub = [2.2 · log CHClb] − 7.2(1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations ( 1.1 × 104 ppm). The calculated Nernst partition coefficient (DAub/m) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3 × 103 to 1.1 × 104 ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents.

Instrumental Neutron Activation Analysis and Multivariate Statistics for Pottery Provenance

The application of instrumental neutron activation analysis and multivariate statistics to archaeological studies of ceramics and clays is described. A small pottery data set from the Nasca culture in southern Peru is presented for illustration.

A review of the origins of metal-rich Pennsylvanian black shales, central U.S.A., with an inferred role for basinal brines

Abstract Compared to most shales, even most black shales, numerous thin Pennsylvanian marine black shales of the U.S. Midwest are very enriched in organic C (5–40wt%) and heavy elements. For example, instrumental neutron activation analysis of 74 samples of mostly black and dark gray Pennsylvanian shales of the U.S. Midwest average 1300 ppm Zn, 85 ppm U, 655 ppm Mo, 130 ppm Se and 55 ppm Cd—amounts sufficient to raise concerns about heavy element pollution. Direct precipitation of sulfides and fixation by abundant organic matter during sedimentation and early diagenesis may account for heavy metals contained by shales in most cases. Metal supply calculations, however, indicate that a special source would be needed to supply metals for a strictly syngenetic origin. Possible sources for such metals include submarine hot springs and Pennsylvanian seas that may have been more enriched in metals than modern ocean water. At least at some localities, additional metal values may have been added later from basinal brines (e.g. Zn) or from modern groundwaters (e.g. Mo, Se, U). The actions of modern supergene processes may fix metals, mitigating detrimental effects from black shales.

Reduction strategies and geochemical characterization of lithic assemblages : A comparison of three case studies from western North America

Based on a simple model of lithic procurement, reduction, and use, we generate predictions for patterns in source diversity and average distance-to-source measurements for flaked stone assemblages left behind by small-scale and residentially mobile populations. We apply this model to geochemical data from obsidian artifacts from three regions in western North America. As predicted, results show markedly different patterns in the geochemical composition of small flakes, large flakes, and formal tools. While small flakes and tools tend to have greater source diversity and are on average farther from their original source, the large flake assemblage is composed of fewer and closer sources. These results suggest that a failure to include very late stage reduction (e.g., pressure flakes) and microdebitage in characterization studies may bias interpretations about the extent of residential mobility and/or trade patterns because more distant sources will be underrepresented.

Determining the Geological Provenance of Obsidian Artifacts from the Maya Region: A Test of the Efficacy of Visual Sourcing

During the last four decades, mesoamerican archaeologists regularly have employed various chemical assay techniques to determine the geological sources of obsidian artifacts. In recent years, the reliability of these analytical procedures has increased and their costs have declined, encouraging the assay of ever larger samples. Nonetheless, several constraints make it unlikely that compositional data will be used routinely to attribute entire collections to their geological sources. This report describes a test of visual sourcing, a technique that for many sites in the Maya region is only slightly less accurate than compositional assay. We also propose sampling strategies that combine visual and compositional sourcing in ways that allow large collections to be accurately sourced at low costs. Finally, we suggest ways to develop the technique for use throughout Latin America.

Gold and platinum in shales with evidence against extraterrestrial sources of metals

Abstract Few black shales contain concentrations of precious metals higher than average continental crust (i.e. ∼5 ppb Au). Yet Au and Pt alloys have been reported from the Kupferschiefer in Poland. Moreover, thin sulfide beds in certain Chinese and Canadian shales contain several hundred ppb Au, Pd and Pt and average ∼4% Mo and ∼2.5% Ni in an association that is difficult to explain. Volcanic and non-volcanic exhalations, hydrothermal epigenesis involving either igneous or sedex fluids, biogenic processes and low-temperature secondary enrichment are among the possible factors involved in deriving Ni, PGE and Au for black shales and sulfide beds in black shales. Extraterrestrial sources have been invoked in some cases (e.g., the Cambrian of China). However, available data on abundances of PGE indicate relatively low values for Ir (