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

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. Plain Language Summary: Pockets of frozen magma trapped within olivine crystals, termed "melt inclusions," can provide information about the depths at which magma is stored beneath the surface prior to a volcanic eruption. This is because the amount of CO2 and H2O that can be dissolved in a melt is dependent on the pressure, and therefore the depth. We examine melt inclusions from lava flows produced during the 2018 eruption of Kı̄lauea Volcano. Previous work, based on geophysics, has shown that magma is stored in two main reservoirs at Kı̄lauea, located at ∼1–2‐ and ∼3–5‐km depth. However, because many melt inclusions host almost all of their CO2 within a vapor bubble, which is rarely measured, previous petrological estimates of magma storage depths at Kı̄lauea do not align with the depths of the two reservoirs identified by geophysics. In this study, we measure the amount of CO2 in the glass and the bubble using secondary‐ion mass spectrometry and Raman spectroscopy, respectively. By adding these two measurements together, we can reconstruct the amount of CO2 that was present when melt inclusions were trapped. Calculated depths align remarkably well with geophysical estimates, and demonstrate that the 2018 eruption was supplied by both magma storage reservoirs. Key Points: Petrological, gaseous and geophysical observations can be reconciled by a model where Fissure 8 was supplied from two summit storage reservoirs (∼1–2‐ and 3–5‐km depth)Extensive post‐entrapment crystallization of melt inclusions within high‐Fo olivines (Fo > 81.5) caused ∼90% of the CO2 to enter the vapor bubbleRaman analyses of vapor bubbles combined with choice of a suitable H2O‐CO2 solubility model is required to accurately determine magma storage depths [ABSTRACT FROM AUTHOR]