Complex phenocryst textures and zoning patterns in andesites and dacites: Evidence of degassing-induced rapid crystallization?

A detailed petrological study is presented of seven phenocryst-poor andesites and dacites erupted from short-lived, monogenetic, peripheral vents in the Mexican volcanic arc. Despite low phenocryst (+ microphenocryst) abundances (55%), these magmas were each multiply saturated with five to seven mineral phases (plagioclase+orthopyroxene+titanomagnetite+apatite+ilmenite ± clinopyroxene ± hornblende ±biotite). Groundmass textures range from nearly complete glass to nearly complete microlite crystallization. Eruptive temperatures, on the basis of FeTi two-oxide thermometry, range from 872 (±13) to 1085 (±16) °C for the andesites and 760 (±18) to 957 (±19)°C for the dacites. Corresponding oxygen fugacities range from -0·9 to +0·6 log units relative to the Ni-NiObuffer. With temperature known, the plagioclase-liquid hygrometer was applied to the sparse plagioclase phenocrysts in each sample; the highest calculated water concentrations range from 6·4 to 2·8wt%H₂O for the andesites and from 8·3 to 4·1wt% H₂O for the dacites. These results require that the magmas were fluid-saturated at depths >9 and >12 km, respectively, for the most hydrous andesite and dacite. The plagioclase population in each sample spans a wide range in composition (≤34 mol % An), which can be attributed to the effects of changing melt water concentration owing to degassing during magma ascent. The orthopyroxene population in each sample also spans a wide range in composition (≤17% Mg-number), and it is proposed on the basis of phase equilibrium experiments from the literature and data from theJANAF thermochemical tables that this also reflects variable melt water concentrations, with higher water leading to more Fe-rich orthopyroxene. Degassing leads to a progressive increase in melt viscosity, and the phenocrysts crystallized over an interval of ~2·4-4·3 log10 Pa s in these samples. The phenocrysts often display diffusion-limited growth textures (e.g. skeletal and hopper crystals, large interior melt hollows) consistent with large undercoolings caused by degassing, which has produced a complicated petrography in these crystal-poor hydrous magmas, including reverse and patchy zoning in plagioclase and pyroxene.