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 five days in water of one isotopic composition, followed by five 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.
