Further investigation of calibration for obsidian hydration dating using aged high temperature and long-term low temperature hydrated samples

Abstract

Obsidian hydration dating (OHD) has been a concern of research into chronological issues of ancient obsidian artifacts. It is based on the power law equation that hydration rim is proportional to the square-root of time relating hydration depth, diffusion rate (k). Employing the Arrhenius plot experimental aged data at high temperature were used to estimate the k at environmental hydration temperature (T). The present investigation initiated concerns a first approach in using available experimentally aged obsidians (140–180°C) from five geographically world dispersed obsidian sources (California, Mexico, Peru, New Zealand) as well as low temperature (10–40°C) aged data (New Guinea) at different water content (around 0.11% and 0.21%) to explore the calibration equation of k versus T. The low temperature aged data, coupled with the new high temperature experiments produce a family curve of three parameters, Lnk, T (in Kelvin) and %OH (the water content W). From available data for samples with two water values the two new average calibration curves for 0.1 and 0.2%OH are: Lnk = 31.86 – ((12496–16697*W)/T) for low 0.10–0.11 OH% and Lnk = 27.32 –((12496–16697*W)/T) for high 0.21–0.24% OH. The hydration rate k and age results are critically discussed for the respective obsidian source, sensitivity to pristine water content variation, and significant temperature dependence. Emphasis to the activation energy versus water content functional dependence is considered and their linear or power law dependence is elaborated and applied to OHD calculation. Having the equation for low %OH validated for most dated samples encourages its use in dating obsidians that have at present predominantly a range of around 0.10–0.11% structural water value. Several World dating examples of published data (Xaltocan, Papua, Easter Islands, Napa Valley, Japan) have been redated compared to earlier calibration equation with an improved range of satisfactory results.