LOW VISCOSITY OF OF CRYSTAL- AND BUBBLE-BEARING LAVA MEASURED AT KILAUEA 2018 ERUPTION CONDITIONS

Lavas are three-phase materials consisting of bubbles, crystals, and melt. Melt composition, temperature, and crystallinity have typically been viewed as the three most important characteristics determining lava rheology, which in turn affects emplacement styles and rates. Traditional methods for measuring the high-temperature viscosity of crystallizing basalts result in samples which can have different mineral assemblages and textures to the natural samples, and are typically bubble-free.

We present a new technique for the measurement of in situ, high-temperature three-phase isothermal (HTTPI) lava viscosity, and apply it to samples from the 2018 eruption of Kilauea. This new experimental technique begins at sub-liquidus temperatures, preserving some original phenocrysts, primarily olivine and plagioclase. A short experimental duration allows for the retention of most of the original bubble population (19-31% compared to 36% in the starting material), documented in quenched post-experiment samples.

The observed rheological behavior in these experiments, conducted at syn-eruptive temperatures and strain rates (0.4-18 s^-1), should be representative of the lava flows. We measure viscosities of 106 Pas at 1150˚C, 178 Pas at 1115˚C, and 1050 Pas at 1105˚C. These are much lower than traditional bubble-free experiments held for hours until they reach a viscosity of ~14,000 Pas at 1115˚C, even though our HTTPI experiments retain a higher crystal fraction of mostly pyroxenes and oxides. This demonstrates that the effect of bubbles, which exceeded 80 volume % near the fissure 8 vent, is very important in crystal-bearing systems, and this third phase must be included in realistic lava flow rheology models. Future experiments should focus on measuring samples that preserve original textures.