Geochemistry of reversible hydratable tephra from the Trans Mexican Volcanic Belt

Abstract

Rhyolitic glass of high, reversible adsorption water (to 12.63 wt%) occurs in pyroclastic rocks from the La Malinche stratovolcano in the Mexican Volcanic Belt. The glass constitutes 98 vol% of the pyroclastics. It is a heterogeneous glass that dehydrates reversibly at 72 degrees C, composed of sodic and non-sodic glasses of surface activity caused by Al-IV substituting in Q(4)(1Al) and Q(4)(2Al) positions, minor Al-V, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H2Om in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H2Om molecules. Sodic glasses contain 71.80-77.77 wt% SiO2, are partially devitrified to crystallites (similar to 5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (similar to 10 mu m size). Sodium-free glasses are more siliceous, with 74.84-83.88 wt% SiO2, show partial devitrification to crystallites (similar to 5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (similar to 10 mu m size).