Volatile Evolution in Silicic Magmas of Torfajökull Volcano, as Determined by FTIR Micro-Spectroscopy

FTIR micro-spectroscopy of crystals, their melt inclusions, and glassy matrix from silicic products of the Torfajökull central volcano, south central Iceland characterize volatile concentrations in the magma chamber during melt evolution, phenocryst formation, and prior to eruption. Water contents that are atypically low for silicic magmas may reflect the dry nature of parental mafic magmas produced at the mid-Atlantic Ridge. Water content in matrix glass, crystals and their melt inclusions varies between eruptive sites and locations spanning much of the volcano's lifetime, from ~80,000 yrs BP to the most recent eruption at 1490 AD, and seem to reflect melt composition and/or evolution. This possible composition- dependent volatile variation may be due to inherent differences in chemical composition between individual, relatively small, independently evolving silicic magma bodies, the contamination of and volatile dilution by injections of mafic magma from the nearby Veidvötn fissure swarm, or both. Differences in volatile content between melt inclusions and quenched matrix glass reflect characteristics of degassing prior to and during eruption, and similar differences between different eruptive sites reflect differences in eruption style and timing. Along with volatile content, calculations of magma temperatures and gas saturation pressures using feldspars and their melt inclusions, may aid in the determination of the residence depths of Torfajökull's silicic magmas and the depths of injection of Veidvötn mafic magmas, having implications for the mechanisms of the injection-triggered eruptions prevalent at Torfajökull.