GEOCHEMISTRY OF THE ALCA OBSIDIAN SOURCE, PERU

The Alca source, Peru's largest obsidian deposit, was first used by Paleoindian hunter-gatherers 13,100-11,400 cal yr B.P. at the coastal site Quebrada Jaguay. Alca obsidian was later distributed as an important trade item throughout Peru by a number of prehistoric civilizations including the Wari and Inka. We report on systematic geologic mapping and geochemical characterization of the Alca source region.

Field and laboratory investigations have identified up to six spatially and geochemically distinct Alca obsidian deposits over an area of ~1,000 km² of the southern Peruvian highlands. The largest and most conspicuous of these is a ~412 ha. obsidian deposit at Cerro Condorsayana, located at the east rim of the Cotahuasi Canyon between 4,000 and 4,820 m elevation. We have used a suite of analytical techniques, including Electron Microprobe, Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS), X-Ray Fluorescence (XRF), and Instrumental Neutron Activation Analysis (INAA), to provide the most high-resolution characterization of a Peruvian obsidian source to-date. Alca sub-sources are best discriminated by plotting sample concentrations of Rb vs. Sr, Rb/Sr ratios vs. Zn and Nb, and Mn vs. Ba. A substantial number of other element concentration differences also define these same geochemical groups. Our replicate sample analyses allow inter-technique and inter-laboratory comparisons.

These efforts have determined the spatial extent of Alca obsidian deposits and the patterning of geochemical variability within the Alca source region. We anticipate future non-destructive XRF analysis of Alca obsidian artifacts recovered from the full temporal sequence of prehistoric archaeological sites throughout Peru. Geochemical comparison of these artifacts with the new high-resolution Alca source fingerprint will allow us to better understand the evolution of prehistoric mobility and trade patterns over 13,000 years in the southern Peruvian Andes.