Reconstructing Magma Storage Depths for the 2018 Kı̄lauean Eruption From Melt Inclusion CO₂ Contents: The Importance of Vapor Bubbles

Abstract

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multiparameter data set provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed high‐ and low‐Fo). Melt inclusion entrapment pressures reveal that low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir (∼2‐km depth), while many high‐Fo olivines (>Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir (∼3–5‐km depth). Melt inclusions in high‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favored the growth of a CO2‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median = 93%). If this CO2‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by secondary‐ion mass spectrometry and Raman spectroscopy, combined with a suitable H2O‐CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.

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

“…The initial FeO t content was estimated by determining a relationship between olivine Fo content and melt FeO t contents by calculating the equilibrium olivine Fo content for co-erupted matrix glasses (see Text S2 and Figure S3 in Supporting Information S1). While olivine-hosted melt inclusions erupted in the later stages of F8 (28 May-1 August) have experienced up to 35% PEC (Wieser et al, 2021), the maximum amount of PEC experienced by olivines erupted at the earlier fissures examined in this study is 10% (median = 4%).…”

Volatile metal emissions from volcanic degassing and lava–seawater interactions at Kīlauea Volcano, Hawai’i

“…However, this is often not possible as it requires sampling in hazardous or inaccessible environments (e.g., active lava flows or recent tephra falls). Here, where data availability and quality allowed for the elements of interest, we use an average composition of basaltic matrix glass from two 2018 samples (one overflow sample, one lava channel sample) collected close to the time and location at which the magmatic plume was sampled 105 . The elements in these matrix glass samples were measured by electron microprobe and LA-ICP-MS (laser ablation-ICP-MS) 105 .…”

“…Here, where data availability and quality allowed for the elements of interest, we use an average composition of basaltic matrix glass from two 2018 samples (one overflow sample, one lava channel sample) collected close to the time and location at which the magmatic plume was sampled 105 . The elements in these matrix glass samples were measured by electron microprobe and LA-ICP-MS (laser ablation-ICP-MS) 105 . Where elements were not measured in these 2018 samples, either a 1971 summit composition 53 or preferred BHVO values were used (Supplementary Data 1 Table S14).…”

“…Model inputs are major and trace species gas concentrations (kmol), pressure, temperature and the phases expected in the plume. Using the method from Helz and Thornber 107 , the MgO content (6.50 ± 0.13 wt% 105 ) of Fissure 8 lavas give an equilibrium temperature of the magmatic gases emitted at the time of plume sampling of~1145 ± 1:6 3:6°C . Further cooling of gases may occur during adiabatic expansion of bubbles; the extent of this cooling depends on the size of the bubbles, with bubbles of radius 2 m producing a cooling of~100°C 108 .…”

Explosive activity on Kīlauea’s Lower East Rift Zone fuelled by a volatile-rich, dacitic melt

“…By the 28 th of May (Phase 3), activity had focused at a single fissure (F8, recently renamed Ahuʻailāʻau) which developed a fast-flowing lava channel, and erupted material with even higher bulk MgO contents resulting from the incorporation of antecrystic olivine crystals(Fig. 2a; Gansecki et al, 2019;Wieser et al, 2021b). For brevity, to distinguish between this main phase of activity at Ahuʻailāʻau (F8), and the earlier, more MgO-poor material, we refer to activity in early Phase 1, Late Phase 1 and Phase 2 as "early fissures", and activity at Ahuʻailāʻau after May 28 th as Phase 3.…”