IS OBSIDIAN HYDRATION DATING AFFECTED BY RELATIVE-HUMIDITY

Experiments carried out under temperatures and relative humidities tha t approximate ambient conditions show that the rate of hydration of ob sidian is a function of the relative humidity, as well as of previousl y established variables of temperature and obsidian chemical compositi on. Measurements of the relative humidity of soil at 25 sites and at d epths of between 0.01 and 2 m below ground show that in most soil envi ronments, at depths below about 0.25 m, the relative humidity is const ant at 100%. We have found that the thickness of the hydrated layer de veloped on obsidian outcrops exposed to the sun and to relative humidi ties of 30-90% is similar to that formed on other portions of the outc rop that were shielded from the sun and exposed to a relative humidity of approximately 100%. Surface samples of obsidian exposed to solar h eating should hydrate more rapidly than samples buried in the ground. However, the effect of the lower mean relative humidity experiences by surface samples tends to compensate for the elevated temperature, whi ch may explain why obsidian hydration ages of surface samples usually approximate those derived from buried samples. (C) 1994 University of Washington.