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Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts

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Abstract

A detailed petrological study is presented for six phenocryst-poor obsidian samples (73–75 wt% SiO2) erupted as small volume, monogenetic domes in the Mexican and Cascade arcs. Despite low phenocryst (+microphenocryst) abundances (2–6 %), these rhyolites are each multiply saturated with five to eight mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± zircon ± hornblende ± clinopyroxene ± sanidine ± pyrrhotite). Plagioclase and orthopyroxene phenocrysts (identified using phase-equilibrium constraints) span ≤30 mol % An and ≤15 % Mg#, respectively. Eruptive temperatures (±1σ), on the basis of Fe–Ti two oxide thermometry, range from 779 (±25) to 940 (±18) °C. Oxygen fugacities (±1σ) range from −0.4 to 1.4 (±0.1) log units relative to those along the Ni–NiO buffer. With temperature known, the plagioclase-liquid hygrometer was applied; maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2.6 to 6.5 wt%. This requires that the rhyolites were fluid-saturated at depths ≥2–7 km. It is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts, despite their low abundance, can be attributed to changing melt water concentrations owing to degassing during magma ascent. Phase-equilibrium experiments from the literature show that higher dissolved water concentrations lead to more Fe-rich orthopyroxene, as well as more calcic plagioclase. Loss of dissolved water leads to a progressive increase in melt viscosity, and phenocrysts often display diffusion-limited growth textures (e.g., dendritic and vermiform), consistent with large undercoolings caused by degassing. A kinetic barrier to microlite crystallization occurred at viscosities from 4.5 to 5.0 log10 Pa s for these rhyolites, presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent.

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Acknowledgments

This paper is dedicated to the memory of Ian Carmichael, who had an abiding interest in obsidians that began when he was an undergraduate and first introduced to the “pitchstones” in Scotland. As far back as the 1960s, Ian recognized the opportunity that glassy rhyolites provided to extract information on pre-eruptive temperatures, oxidation states and melt water concentrations, which eventually led to his life-long pursuit of the application of thermodynamics to magmatic systems. We were privileged to talk over the early stages of this study with Ian, who—as always—gave valued advice and encouragement. Ian’s thermodynamic “intuition” led him to long argue (at least to his graduate students) that dissolved water should favor Mg(OH)2 over Fe(OH)2 species in the melt, and he expected this to affect ferromagnesian phases in hydrous magmas. These conversations planted the seed for us to think about progressive loss of dissolved water as an explanation for the wide range in orthopyroxene compositions in the rhyolites from this study. This work was funded by the National Science Foundation: EAR-12503685 and EAR-9911352 (equipment grant for electron microprobe at the University of Michigan), along with discretionary funds from the University of Michigan.

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    Laura E. Waters & Rebecca A. Lange

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Waters, L.E., Lange, R.A. Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts. Contrib Mineral Petrol 166, 731–754 (2013). https://doi.org/10.1007/s00410-013-0919-9

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