Fluid inclusion constraints on the geometry of the magmatic plumbing system beneath Mauna Loa – Part I: Lavas and tephras

There are few petrological constraints on magma storage depths at Mauna Loa, Hawai‘i. Yet understanding the geometry of the magmatic plumbing system is critical for interpreting geophysical signals of unrest at this very high-threat volcano. We address this gap by examining CO₂-rich fluid inclusions (FI) in lava and tephra from seven eruptions (8870 ± 56 14C yr BP, 1852, 1855, 1868, 1949, 1950, and 1984), supplemented with published data from 2022. Raman spectroscopy was used to determine FI densities, from which entrapment pressures were calculated using a CO₂-H₂O equation of state. Most FI record pressures of ~ 0.25–1.25 kbar (~ 2–5 km depth below the summit), consistent with geophysical estimates from the past 40 years. In summit eruptions, FI hosted in more evolved olivine and orthopyroxene clots (Fo and Mg# < 84) record slightly shallower pressures than those in more primitive olivines (Fo > 84) from rift zone eruptions, suggesting a crystal-poor evolved cap near the top of the reservoir (~ 2 km). The similarity in storage depths across all eight eruptions indicates that Mauna Loa’s magmas have tapped a quasi-stable reservoir over the past two centuries, and possibly over 10 kyr. Electron backscatter diffraction reveals deformations to the crystal lattice in Fo82-83 olivines, likely due to deformation during storage in mush piles. The intensity of deformation is comparable to that seen at Kīlauea, implying that mush pile stress may be decoupled from edifice size or longevity. Finally, SO₂ contents in FI increase from ~ 2 mol% at 2 kbar to ~ 15 mol% at 0.5 kbar, suggesting sulphur degassing begins far deeper than the 0.2–0.3 kbar commonly assumed for Hawaiian systems. This validates the newest generation of S degassing models (e.g., Sulfur_X), and explains precursory SO2 emissions in the ~3 hours prior to the onset of the 2022 eruption (Esse et al. 2025).