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Geochemistry of reversible hydratable tephra from the Trans Mexican Volcanic Belt

Liberto De Pablo^1,*, Mercedes Doval^2, and Angel La Iglesia^3,

¹ Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., México
² Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid, 28040 Madrid, Spain
³ Departamento de Petrología y Geoquímica, Instituto de Geología Económica CSIC-UCM, Facultad de Ciencias Geológicas, Universidad Complutense, 28040 Madrid, Spain

Correspondence: ^* E-mail: liberto{at}servidor.unam.mx

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 °C, composed of sodic and non-sodic glasses of surface activity caused by ^IVAl substituting in Q⁴(1Al) and Q⁴(2Al) positions, minor ^VAl, tetrahedra terminating in NBOs, and insufficient Na and Ca to charge balance Al in the glass network. Adsorption is of molecular water H₂O_m in interstitial sites, H-bonded to silanol groups, to the silica network, and to other H₂O_m molecules. Sodic glasses contain 71.80–77.77 wt% SiO₂, are partially devitrified to crystallites (~5 nm size) of Na-plagioclase and clinopyroxene, and exhibit minor low-grade metamorphism to <1 vol% crystals of mazzite (~10 µm size). Sodium-free glasses are more siliceous, with 74.84–83.88 wt% SiO₂, show partial devitrification to crystallites (~5 nm size) of Ca-plagioclase and clinopyroxene, with minor low-grade metamorphism of glass and plagioclase to <1 vol% crystals of laumontite (~10 µm size).

Key Words: Hydratable glass • hydratable rhyolite glass • hydratable tephra • Malinche tephra • Mexican Volcanic Belt