MICROSTRUCTURAL AND BAROMETRIC CONSTRAINTS ON THE GEOMETRY AND DYNAMICS OF THE MAGMATIC PLUMBING SYSTEM BENEATH MAUNA LOA, HI, THE WORLD’S LARGEST VOLCANO

The 2022 eruption of Mauna Loa on Hawaiʻi Island ended 38 years of quiescence, and reinforced to the volcanological community how little is known about the geometry and dynamics of the plumbing system of Earth’s largest active volcano. The relative paucity of petrological and geophysical work is particularly striking relative to its smaller neighbor, Kīlauea. Here, we provide petrological constraints of the geometry of the magmatic plumbing system obtained by examining crystal cargoes from basalts erupted in 1852, 1855, 1868, 1949, 1950, 1984 and 2022 from the summit, SWRZ and NERZ. Samples range from picritic to aphanitic. We use confocal Raman Spectroscopy to analyze small pockets of CO₂-rich fluid trapped within olivine crystals termed fluid inclusions, coupled with Energy Dispersive Spectroscopy to determine the chemistry of the host crystal. Fluid inclusion barometry provides a precise determination of the pressures at which fluids were trapped within crystals, and thus the depths at which crystals grew or stalled for prolonged periods. Analyses of ~200 fluid inclusions from these seven eruptions show that magma is predominantly stored at 1-5.5 km depth below the summit, which aligns well with existing geophysical constraints. Interestingly, for the two aphanatic (1949, 1984), our albeit sparse dataset yields shallower storage pressures of ~1-2 km vs. the strongly picritic eruptions (1852, 1868) dominated by 2-5.5 km. Previous work based on the presence of high Mg# orthopyroxene has suggested that there may regions of deeper magma storage at Mauna Loa (e.g., Gaffney et al. 2002). To further investigate this hypothesis, we will present new fluid inclusion data from these samples to compare to olivine-hosted fluid inclusions.

Textural and chemical work at neighboring Kīlauea has led to suggestions that many crystals erupted at Hawaiian volcanoes are antecrysts, scavenged from seismically-imaged olivine-rich mush zones. To assess the likely provenance of the grains hosting fluid inclusions from Mauna Loa, we perform electron-backscatter diffraction. Most grains yielding fluid inclusion depths >3.5 km show clear deformation structures, indicating derivation from a mush pile. Interestingly, Mauna Loa fluid inclusion pressures and deformation intensities are very similar to Kīlauea, despite vast differences in edifice size.