O bsidian compositional analyses have been among the most productive methods for examining ancient exchange networks and culture contacts in Mesoamerica owing to the importance of the material in the pre-Hispanic economy, its widespread regional distribution, and its relatively discrete chemical signature, which allows for highly precise provenience classifica- tions (Clark 2003a; Cobean 2002). Important research topics in obsidian studies include defin- ing the role of obsidian exchange in the develop- ment of Formative period regional polities, the procurement activities of the inhabitants of Clas- sic period Teotihuacan—including the degree to FORMATIVE AND CLASSIC PERIOD OBSIDIAN PROCUREMENT IN CENTRAL MEXICO: A COMPOSITIONAL STUDY USING LASER ABLATION–INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY David M. Carballo, Jennifer Carballo, and Hector Neff This report presents the results of a compositional study using laser ablation–inductively coupled plasma–mass spectrom- etry (LA-ICP-MS) to examine central Mexican obsidian procurement at four sites dating to the Formative and Classic peri- ods. The study demonstrates LA-ICP-MS to be a highly accurate obsidian sourcing technique, with results that are directly comparable to instrumental neutron activation analysis. It documents a shift circa 600 B.C. in which Middle Formative vil- lages in Tlaxcala began to obtain obsidian almost exclusively from sources located in the Mesa Central, when they had pre- viously obtained approximately 50 percent from Mesa Central sources and 50 percent from sources located in the Sierra Madre Oriental. This shift broadly coincided with the development of large regional centers in Tlaxcala-Puebla and sug- gests a linkage between the local political evolution occurring at this time and increased interactions with the Mesa Cen- tral economic sphere,including the Basin of Mexico. Obsidian workshop dump deposits next to Teotihuacan’s Moon Pyramid, dating to a millennium later,demonstrate the continued reliance on predominantly Mesa Central sources but also diversi- fied procurement that drew on several sources. The Teotihuacan deposits exhibit the preferential utilization of particular sources depending on the types of implements being produced. Este informe presenta los resultados de un estudio utilizando ablación por láser y espectrometría de masas con fuente de io- nización de plasma acoplado inductivamente (LA-ICP-MS) para examinar el aprovisionamiento de obsidiana en cuatro sitios del altiplano mexicano, datados entre los períodos Formativo y Clásico. Los análisis con LA-ICP-MS demuestran que es una técnica altamente precisa para este tipo de estudios de obsidianas, cuyos resultados son directamente comparables con los de activación neutrónica. Este estudio indica un cambio en el sistema de obtención de obsidiana ca. 600 a.C., donde las aldeas del Formativo en Tlaxcala comienzan a obtener material casi exclusivamente de fuentes ubicadas en la Mesa Central. Ante- riormente, lo hacían en proporciones similares entre la Mesa Central (50 por ciento) y otras fuentes localizadas en la Sierra Madre Oriental (50 por ciento). Este cambio coincide ampliamente con el desarrollo de los grandes centros regionales en Tlaxcala-Puebla, y sugiere una conexión entre la evolución política local en esos momentos y un incremento en las interac- ciones con la esfera económica de la Mesa Central, incluyendo la Cuenca de México. Los talleres de obsidiana próximos a la Pirámide de la Luna de Teotihuacan, datados en el milenio siguiente, demuestran la continuidad en la dependencia predom- inante de las fuentes de la Mesa Central, pero también el aprovisionamiento diversificado, empleando varias fuentes. Los depósitos de Teotihuacan exhiben la utilización preferencial de fuentes de aprovisionamiento particulares, dependiendo de los tipos de instrumentos que fueron fabricados. David M. Carballo ■ Department of Anthropology, University of California, Los Angeles, 341 Haines Hall, Box 951553, Los Angeles, CA 90095-1553 (cabby@ucla.edu) Jennifer Carballo ■ Department of Anthropology, University of Michigan, Museum of Anthropology, 1109 Geddes, Ann Arbor, MI 48109-1079 (jsmit@umich.edu) Hector Neff ■ Department of Anthropology, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840-1003 (hneff@csulb.edu) Latin American Antiquity, 18(1), 2007, pp. 27–43 Copyright ©2007 by the Society for American Archaeology 27