Use of uranium decay series for dating an archaeological smelting site

Abstract

Through the identification of phases and their isotopic composition and variability, an assessment of the applicability of uranium decay series dating to El Manchon slags was made. El Manchon is the only Mesoamerican site to exhibit smelting technology. Uranium series dating is typically used on geologically old natural material, but the El Manchon slags were not suitable for other dating techniques. There are four requirements of uranium series dating: measurable presence of appropriate isotopes, cogenetic phases within the material, isotopic fractionation between phases, and the ability to physically separate the phases. This is the first attempt to date archaeological material with the uranium series dating method. Petrographic reflected light microscopy was used to identify the phases in the slags. Electron beam microanalysis was used to identify the chemical composition of the identified phases. Ion beam microanalysis was used to assess the isotopic fractionation between the phases. Electron pulse disaggregation, hand-sorting, and magnetic separations were performed to separate the phases. The slags are composed of four different phases: a silica-melt phase, a quartz-like phase, a copper phase, and a copper-iron-sulfide phase. These four phases are in abundant presence with sufficient isotopic fractionation to make the El Manchon slags suitable for uranium series dating.