The petrologic and degassing behavior of sulfur and other magmatic volatiles from the 2018 eruption of Kīlauea, Hawaiʻi: melt concentrations, magma storage depths, and magma recycling

Abstract

Kīlauea Volcano's 2018 lower East Rift Zone (LERZ) eruption produced exceptionally high lava effusion rates and record-setting SO 2 emissions. The eruption involved a diverse range of magmas, including primitive basalts sourced from Kīlauea's summit reservoirs. We analyzed LERZ matrix glasses, melt inclusions, and host minerals to identify melt volatile contents and magma storage depths. The LERZ glasses and melt inclusions span nearly the entire compositional range previously recognized at Kīlauea. Melt inclusions in Fo 86-89 olivine from the main eruptive vent (fissure 8) underwent 70-170 °C cooling during transport in LERZ carrier melts, causing extensive post-entrapment crystallization and sulfide precipitation. Many of these melt inclusions have low sulfur (400-900 ppm) even after correction for sulfide formation. CO 2 and H 2 O vapor saturation pressures indicate shallow melt inclusion trapping depths (1-5 km), consistent with formation within Kīlauea's Halemaʻumaʻu and South Caldera reservoirs. Many of these inclusions also have degassed δ 34 S values (− 1.5 to − 0.5‰). Collectively, these results indicate that some primitive melts experienced near-surface degassing before being trapped into melt inclusions. We propose that decades-to-centuries of repeated lava lake activity and lava drain-back during eruptions (e.g., 1959 Kīlauea Iki) recycled substantial volumes of degassed magma into Kīlauea's shallow reservoir system. Degassing and magma recycling from the 2008-2018 Halemaʻumaʻu lava lake likely reduced the volatile contents of LERZ fissure 8 magmas, resulting in lower fountain heights compared to many prior Kīlauea eruptions. The eruption's extreme SO 2 emissions were due to high lava effusion rates rather than particularly volatile-rich melts.

Evolving magma temperature and volatile contents over the 2008–2018 summit eruption of Kīlauea Volcano

“…3). That this temporal variation is similar to the inferred primary magma's total volatile mass fraction of 1 to 2 wt % (36,42) suggests strong variations in the outgassing regime (14). The only volatile species with continuous emission measurements that can be compared with X and X avg is SO 2 .…”

“…Continuous gravity data are only available over limited time segments but constrain the density of magma in the lava lake and suggest temporal variation of up to 1500 kg/m 3 ( 12 ). Analysis of erupted products provides limited temporal and spatial resolution but suggests that Halema‘uma‘u magma consists of near-liquidus (1150° to 1300°C) crystal-poor basalt outgassed in CO 2 with respect to the primary mantle magma ( 13 , 14 ). Subsurface magma volatile contents are also indirectly informed by continuous gas emissions ( 13 , 15 , 16 ).…”

The sulfur solubility minimum and maximum in silicate melt

“…Our model calculations place the SS min for this Hawaiian basalt composition at 1200 °C at SSmin fO2  FMQ+1.2. For comparison, this is slightly more oxidised than Hawaiian basalts (FMQ-0.5 to +1.0; Moussallam et al 2016;Brounce et al 2017;Lerner et al 2021) and more oxidised than mid-ocean ridge basalts (MORB), but within the range measured in arc and ocean island basalts (OIB) (e.g., Cottrell et al, 2021). The position of SSmin fO2 will depend on T, composition of the silicate component, and additional volatile components (e.g., H2O, CO2).…”

Explosive Activity on Kīlauea's Lower East Rift Zone Fueled by a Volatile‐Rich, Dacitic Melt

“…The fractionation trajectory for Zr is calculated assuming incompatible behavior. It has been suggested, based on the presence of high forsterite olivines incorporated within Phase 3 lavas, and the bimodal distribution of melt inclusion saturation pressures at 1-2 and 3-5 km, that this magma originated from the two magma reservoirs identified by geophysical imaging beneath Kīlauea's summit (Lerner et al, 2021;Wieser et al, 2021), with a hydraulic connection linking these reservoirs to Ahuʻailāʻau (Gansecki et al, 2019;Neal et al, 2018;Patrick et al, 2019). Conversely, Pietruszka et al (2021) suggest that major and trace element differences between Phase 3 lavas and those erupted at Kīlauea's summit and Puʻuʻōʻō prior to the onset of the 2018 eruption rules out a summit origin, and instead suggest that this MgO-rich magma had accumulated in the Middle East Rift Zone (MERZ) downrift of Puʻuʻōʻō over ∼10 years prior to 2018.…”

“…The starting composition used for the modeling lies within the range of whole-rock compositions from various early fissures from Gansecki et al (2019), justifying this choice. The initial Fe 3+ /Fe T ratio was set at 0.15 (Lerner et al, 2021;Moussallam et al, 2016), and models were run without a buffer, allowing for changes in oxidation state following Fe-Ti oxide saturation. The pressure was set at 650 bars, based on the range of saturation pressures measured in Early Phase 1 melt inclusions (see Section 4.5).…”