WATER IN GLASS AND FELDSPAR IN ICELANDIC RHYOLITES

Water concentrations in melt inclusions, in feldspar crystals, and in matrix glass in the postglacial (<10,000 years) Laugahraun and Namshraun silicic flows of the Torfajökull  silicic complex were determined by Fourier transform infrared spectroscopy (FTIR).   Torfajökull is located at the boundary between the Veidvötn fissure system in the Eastern Rift Zone, and the South Eastern Zone, an area of non-rifting, older crust (MacDonald et al., 1990). Laugahraum and Namshraun are two of the small silicic complexes located at the northeast corner of Törfajokull, along the northeast-southwest trending Veidivötn fissures. Although the 250 km³ (MacDonald et al., 1990) Törfajokull plateau is dominated by nearly anhydrous rhyolites, these centers host rock compositions ranging from basalt to rhyolite (Mørk, 1984; McGarvie et al., 1990).  Water is present in the Laugahraun rhyolite flow as both molecular water and hydroxide in melt inclusions (average 0.3 +/- 0.05 wt.%) and in matrix glass (average 0.6 +/- 0.09 wt%).  Water in the vesicular Namshraun flow is present as molecular water in matrix glass (average 0.13 +/- 0.02 wt.%) and in feldspar phenocrysts (average 0.09 +/- 0.01 wt.%). Low water concentrations in these rhyolites reflect the low water concentrations in rocks generated at mid-ocean ridges.  The larger water concentration in matrix glass than in melt inclusions in the Laugahraun flow may be a result of water partitioning as a trace component into crystallizing phases, depleting the host melt as crystallization proceeds.  Differences in water concentration between the Namshraun and Laugahraun flows may reflect different extents of degassing, and/or may be a consequence of the mingling of the Namshraun rhyolite with a mafic component from the adjacent Veidvötn basaltic fissure.