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Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at ∼ 700 K, at a rate of between 7 and 80 K.min−1. We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved H 2 O concentration of 2.12 wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved H 2 O concentration of 0.12 wt%. Our results show that at a storage temperature of 1123–1193 K, the viscosity of the stored magma is ∼ 3×105 Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to ∼ 2×109 Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface. • A model is presented for the Krafla rhyolite H2O- and temperature-dependent rheology. • The Krafla rhyolites are viscoelastic and will respond in a brittle manner at high strain rates. • Magma rheology models are crucial for future drilling goals at Krafla volcano. [ABSTRACT FROM AUTHOR]

The magmatic contribution into geothermal fluids in the central Taupo Volcanic Zone (TVZ), New Zealand, has been attributed to either andesitic, ‘arc-type’ fluids, or rhyolitic, ‘rift-type’ fluids to explain the compositional diversity of discharge waters. However, this model relies on outdated assumptions related to geochemical trends associated with the magma at depth of typical arc to back-arc settings. Current tectonic models have shown that the TVZ is situated within a rifting arc and hosts magmatic systems dominated by distinct rhyolite types, that are likely to have evolved under different conditions than the subordinate andesites. Therefore, a new appraisal of the existing models is required to further understand the origin of the spatial compositional diversity observed in the geothermal fluids and its relationship to the structural setting. Here, we use volatile concentrations (i.e. H 2 O, Cl, B) from rhyolitic and andesitic mineral-hosted melt inclusions to evaluate the magmatic contribution to the TVZ geothermal systems. The andesite and two different types of rhyolites (R1 and R2) are each distinct in Cl/H 2 O and B/Cl, which will affect volatile solubility and phase separation (vapor vs. hydrosaline liquid) of the exsolved volatile phase. Ultimately, these key differences in the magmatic volatile constituents will play a significant role in governing the concentration of Cl discharged into geothermal systems. We estimate bulk fluid compositions (B and Cl) in equilibrium with the different melt types to show the potential contribution of ‘parent’ fluids to the geothermal systems throughout the TVZ. The results of this analysis show that the variability in fluid compositions partly reflects degassing from previously unaccounted for distinct magma source compositions. We suggest the geothermal systems that appear to have an ‘arc-type’ andesitic fluid contribution are actually derived from a rhyolite melt in equilibrium with a highly crystalline andesite magma. This model is in better agreement with the current understanding of magma petrogenesis in the central TVZ and its atypical rifted-arc tectonic setting, and show that the central TVZ records an arc, not back-arc, fluid signature. [ABSTRACT FROM AUTHOR]