Petrological, Textural, and Volatile Constraints on the Architecture of Mauna Loa’s Magmatic System over 170 years (1852–2022)

Abstract

Mauna Loa’s reawakening in 2022 highlighted the limited availability of petrological constraints characterizing the geometry and dynamics of pre-eruptive storage at this volcano. In particular, there has been relatively little prior work constraining pre-eruptive volatile budgets, mineral zoning and textures. To address this, we examine olivine- and orthopyroxene-hosted melt inclusions, their mineral hosts, and matrix glasses from seven Mauna Loa eruptions spanning 170 years (1852–2022). We present the first extensive melt inclusion dataset from Mauna Loa with direct Raman measurements of vapor bubble CO2. Olivine- and orthopyroxene-hosted melt inclusions contain 0.3–0.5 wt% H2O and 100–800 μg/g CO2. Melt inclusion saturation pressures reveal that the dominant reservoir is located at ~ 2–5 km depth, consistent with fluid inclusion barometry and geophysical constraints. This depth has remained consistent over the past 170 years, although some eruptions appear to have tapped a narrower range of depths than others. Even the high Mg# orthopyroxene phenocryst-hosted melt inclusions record saturation pressures of < 150 MPa (< 6 km depth), which is inconsistent with a deep sub-Moho origin proposed by previous studies that rely on thermodynamic modeling. Our Raman measurements also provide an opportunity to assess the accuracy of different methods to reconstruct the amount of CO2 in vapor bubbles, with applications to melt inclusion datasets published without direct Raman measurements. In general, models that partition CO2 between the melt and bubble using the bubble volume prior to pre-eruptive quenching (e.g. MIMiC) systematically underestimate CO2 contents by a median of ~ 100–200 μg/g (depending on the model and KD choice) and thus underestimate storage depths by ~ 0.5–1.5 km. While these methods are useful for first-order estimates of bubble CO2 in published datasets, new studies should measure bubble CO2 by Raman spectroscopy or use melt inclusion rehomogenization. The zoning and textures of the crystals that host the melt inclusions provide additional constraints on the plumbing system to supplement melt inclusion barometry. Orthopyroxene crystals exhibit varied zoning patterns, including oscillatory zoning that records repeated fluctuations in magmatic conditions, and sector zoning indicating rapid, kinetically controlled crystal growth. While faster Fe-Mg diffusion in olivine results in a simpler zoning record, complexly zoned olivines with higher-Fo ‘shoulders’ from 1852, 1868 and 1950 also record the injection of more primitive melts at shallow levels. Electron backscatter diffraction (EBSD) reveals that touching crystals in orthopyroxene glomerocrysts exhibit only very weakly systematic crystallographic orientation relationships, perhaps due to rapid aggregation in a dynamic system. Overall, our data are consistent with a large, shallow (~2–5 km) pre-eruptive reservoir repeatedly recharged by primitive magma injections that homogenized prior to eruption due to their small volume relative to the overall size of this reservoir. This chemical complexity is effectively recorded in mineral and melt inclusion compositions (orthopyroxene zoning in particular) but is obscured in glass and whole-rock analyses.