VIRGINIA STEEN-McINTYRE
WEATHERING OF SILICA-RICH VOLCANIC GLASS SHARDS
SHARDof incompletely-hydrated
glass will have
a large refractive index range. The third view (c) occurs at about 12000 years, at least in cool temperate climates (The rate of hydration would be faster in southern
We will turn now to the silicic volcanic glass shards and their uptake of water with time. It occurs in two steps: first hydration, then superhydration. The hydration/superhydration concept can be ilIustrated by the diagram in fig-
Mexieo because the soil temperature is warmer). Here, hydration has proceeded until only a small volume of glass, located in the thickest
ure 2. It follows the history of a small shard of platy, rhyolitie glass as it passes through various stages of water accumulation . In view (a) at the upper left we have our first look at the shard, shortly after birth. It has an enclosed bubble cavity or vesicle, represented
portion of the shard, remains unhydrated. Volume of the low-index, non-hydrated glass becomes smaller as time passes, until at about 15000-20000 years (in a cool temperate climate) none is left. Mter the shard has hydrated, water continues to diffuse through the glass structure and begins to collect as tiny menisci in the tape red ends of closed vesicles (d). This process of superhydration is extremely slow,
by the spindle-shaped form, and sorne thickness, represented by the ridge line running across its upper surface. The shard has not yet begun to hydrate. It has a single refractive index value representing the non-hydrated glass, and the enclosed vesicle is liquid free.
and it may take over ten million years for all the cavities to fill. The remaining views (e-h) show water progressively filling the vesicle until, in the last one, it is full.
View (b) occurs approximately 2 000 years later. Water of hydration has penetrated the glass from all exposed surfaces, raising the refractive index of the glass 0.01. On wedge-
Figure 3a-f shows various the stages of hydration and superhydration as seen through the microscope. AlI fragments are tiny silica-rich volca-
shaped surfaces such as occur around the perimeter of the shard, the hydration rinds
nie glass shards from dated eruptions: (a) Mount Sto Helens, 3000-4000 years; (b) Tlapacoya, ca.
have merged, and appear through the microscope as a high refractive index rim of glass surrounding a low refractive index coreo EACH
10 000 years; (c) Río Frío ashf1ow, > 35000 years; (d) Hueyatlaco ash, 370000 ± 200000 years, 2 sigma (zircon fission track date, Steen-Mclntyre el al., 1981); (e) a Yellowstone tephra, 1200000 years; (f) Bidahochi ash; Pliocene age. A platy glass shard from one of the StoHelens eruptions is seen in figure 3a. The lighter narrow rim in focus at the upper right is the hydration rind (arrow). It concentrates the light because it has a refractive index higher than the non-hydrated glass coreo With the various focal masking methods of microscope ilIumination, the rind shows brilliant colors and is easy to recognize (Steen-Mclntyre, 1977; Wilcox, 1962 and cited references). Now for the bubble cavities or vesicles. See figure 3b for sorne enclosed ones from a glass
Figure 2. Diagram of a tiny volcanic glass shard of rhyolitic composition (high silica) as it passes through various stages of hydration and superhydration. It can take millions ofyears for this double process ofwater absorption to be completed.
shard ofTlapacoya tephra, roughly 10 000 years old. The cavities are outlined in black; a sign that there is essentially no water in them. A
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