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

Abstract:

The 2018 lower East Rift Zone (LERZ) eruption of Kīlauea Volcano, and the accompanying collapse of the summit caldera, marked the most destructive episode of activity in the last 200 years. The eruption was exceptionally well-monitored, with numerous types of data collected, including continuous geodetic data capturing the caldera collapse, frequent measurements of SO₂ emission rate, and an extensive lava sampling campaign for the duration of the eruption. The multi-parameter dataset provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kīlauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium between the phenocrysts and co-erupted carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8. Melt inclusion entrapment pressures reveal that more evolved olivines (Fo<81.5) crystallized at ~2 km depth within the shallower Halema’uma’u reservoir, while more primitive olivines (Fo>81.5) crystallized within the deeper South Caldera (SC) reservoir at ~3–5 km depth. Crucially, melt inclusions in primitive olivines experienced extensive post-entrapment crystallization, driving the growth of a vapor bubbles in the trapped melt. Raman spectroscopy reveals that vapor bubbles contain up to 99% of the total melt inclusion CO₂ budget (median=93%). Measurements of CO₂ in only the glass phase of the inclusion would have underestimated entrapment depths by up to 60 (median=11) and failed to recognize the contribution of magma supplied from the SC reservoir. We also show that reconstruction of bubble CO₂ contents using the Equation of State method applied to measured bubble volumes overestimates entrapment pressures for inclusions where a substantial proportion of the bubble grew during syn-eruptive quenching. Overall, we demonstrate that Raman measurements of bubbles, along with careful choice of a suitably calibrated H₂O-CO₂ solubility model, is vital to place accurate constraints on the depths of magma storage regions supplying volcanic eruptions.