Understanding the dynamics of magmatic gas behavior in lava fountains during the Kīlauea 2018 rift eruption via FTIR, MultiGAS, and petrology

Abstract:

Volcanic gases have been measured in Hawaii for over a century and the modern framework for understanding volatile behavior at Kīlauea Volcano was established nearly four decades ago. More recently, technological advances have enabled long-duration and high-temporal resolution gas composition and emission rate measurements. These gave insight into both short-term variability in gas emissions and secular changes in gas chemistry. However, the last major lava fountaining episodes at Kīlauea were in 1986, precluding study of such eruptions with the new technologies until 2018.

During the 2018 eruption of Kīlauea, lava fountains at Fissure 8 provided the unprecedented opportunity for a multi-instrument gas measurement campaign to investigate dynamic gas-related processes associated with fountaining. We deployed UV spectrometers to determine SO₂ emission rate as well as a Fourier Transform Infrared spectrometer (FTIR) and MultiGAS instruments to determine gas composition. Petrologic measurements of melt inclusions, lavas, and tephra supplemented the gas measurements.

Kīlauea magmas release CO₂-rich gas at the summit; during rift eruptions, magma migrates downrift, where SO₂-rich gas is released upon eruption. FTIR, MultiGAS, and petrologic measurements all indicate that this two-stage model held true during the 2018 eruption. Our data show that lava fountains were driven by shallow, H₂O- and SO₂-rich, CO₂-poor gas rather than by CO₂-rich gas slugs of deeper origin as seen at some other volcanoes.

CO₂/SO₂ ratios (~0.6-1) estimated by petrologic models and determined by FTIR measurements of the fountain gas differ slightly from those measured ~200-800 m downwind by a MultiGAS (~0.3) mounted on an unoccupied aircraft system (UAS) and from gas measurements made during rift eruptions in the 1970s and 1980s. This may be evidence of syn-eruptive, non-equilibrium degassing of slow-diffusing S within the fountain lava. FTIR measurements of gas emitted by the proximal lava channel more closely match MultiGAS ratios, possibly indicating that sulfur degassed to equilibrium by that point.

Each measurement technique independently indicates that the 2018 lava fountains were driven by shallow degassing and outgassing. Taken together, the data offer insight into finer-scale, diffusion-limited degassing processes.