Alkali Exchange during Devitrification and Hydration of Glasses in Ignimbrite Cooling Units

Abstract

Alkali-ion exchange between rock and aqueous phases during devitrification and secondary hydration of ignimbrite sheets contributes significantly to observed chemical variations in densely welded glassy and devitrified tuffs. Probably most of the alkali-ion exchange occurs during the cooling period because ion exchange affects interiors of slowly cooling thick (>300 foot) units, whereas rapidly cooling thin (<100 foot) units show no evidence of alkali exchange in their interiors. Leaching by groundwater removes alkalis from partially or nonwelded margins of cooling units. Distinct stratigraphic breaks in trends of alkalis indicate that individual eruptive pulses of ash-flow material, now welded into one cooling unit, had distinctive alkali ratios related to magmatic inhomogeneities.