Abstract
While a great deal is known about the interaction between water and rhyolitic glasses and melts at temperatures above the
glass transition, the nature of this interaction at lower temperatures is much more poorly understood. This paper presents the
results of a series of isotopic exchange experiments aimed at further elucidating this process and determining the extent to
which a point-by-point analysis of the D/H or 18O/18O isotopic composition across the hydrated rim on a geological or
archaeological obsidian sample can be used as a paleoclimatic monitor. Experiments were performed by first hydrating the
glass for 5 days in water of one isotopic composition, followed by 5 days in water of a second composition. Because waters
of near end-member compositions were used (nearly pure 1H2
16O, 1H2
18O, and D2
16O), the relative migration of each species
could be ascertained easily by depth-profiling using secondary ion mass spectrometry (SIMS). Results suggest that, during
hydration, both the isotopic composition of the waters of hydration, as well as that of intrinsic water remaining from the initial
formation of the glass vary dramatically, and a point-by-point analysis leading to paleoclimatic reconstruction is not
feasible.
