Infra-red Photoacoustic and Secondary Ion Mass Spectrometry Measurements of Obsidian Hydration Rims

A procedure has been developed for measuring obsidian hydration rim thicknesses on archaeological artefacts using infra-red photoacoustic spectroscopy (IR-PAS). Calibration of the IR-PAS values with depth was completed using secondary ion mass spectrometry (SIMS) and optical microscopy to relate the quantity of diffused water to the measured thickness of the hydration layer. By monitoring the absorbance intensity at 1630cm−1wavenumbers, hydration rims between approximately 1μm and 12μm may be accurately measured and used for chronometric age estimates. SIMS depth profiling has the ability to measure layers less than 1μm thick, extending the age calibration to samples of less than 100 years in age.