CONSTRAINING VOLATILE SYSTEMATICS AND MAGMA STORAGE DEPTHS FROM MINERAL-HOSTED MELT INCLUSIONS AT MAUNA LOA (HI)

Mauna Loa is the largest active volcano on Earth, erupting on average every five years since the beginning of written records in 1843, and is a significant hazard to lives and infrastructure in Hawaii. Yet, despite the risk, Mauna Loa remains understudied relative to nearby Kīlauea. The geometry and dynamics of the magmatic plumbing system supplying melt to the surface, along with volatile systemics (e.g., S, Fl, Cl), remain poorly constrained. Although geophysical datasets indicate a magma storage zone at ~3 – 4 km, petrological constraints on magma storage depths remain sparse.

Melt inclusions (MIs) are droplets of magmatic liquid that become trapped inside growing crystals and can provide insight into volatile budgets and degassing amounts, as well as the depths of magma storage prior to an eruption. This is because the amount of CO₂ and H₂O that can be dissolved in a melt is dependent on pressure and therefore depth. In this study, we present preliminary results from Raman spectroscopy, SIMS, and EPMA analyses of olivine- and pyroxene-hosted MIs, sulfides, and matrix glasses from the 1852, 1855, 1868, 1949, 1950, 1984, and 2022 Mauna Loa eruptions. Raman analyses of melt inclusion-hosted vapor bubbles reveal low CO₂ concentrations (< 650 ppm; 302 ppm average) compared to the average vapor bubbles from the 2018 Kīlauea eruption (~590 ppm)— although a few crystals from the 1950 eruption have up to 2150 ppm, and CO₂ densities of ~0.02-0.15 g/cm³. SIMS MI glass analyses show 412-7 ppm CO₂ and 4800-2600 ppm H₂O. For MIs with vapor bubbles, up to 85% of the CO₂ budget is contained within the vapor bubble. Combined CO₂ concentrations of MI glass from SIMS analysis and ramaned vapor bubbles (where present) are less than 550 ppm. Mineral and MI chemistry, along with existing literature data, are used to reconstruct a liquid line of descent for Mauna Loa magmas and correct for post-entrapment crystallization (PEC) of olivine, including a rigorous assessment of uncertainty arising from choice of PEC model. We also examine sulfide occurrence and chemistry to provide context to melt inclusion sulfur contents and inform sulfide saturation models, allowing us to gain insights into the atmospheric sulfur burden of Mauna Loa eruptions. These data provide vital new constraints on volatile degassing and magma storage depths, with implications for the interpretation of volcanic hazards at Mauna Loa.