MAGMA STORAGE DEPTHS DURING THE GROWTH AND EVOLUTION OF THE YOUNGEST HAWAIIAN VOLCANO: KAMAʻEHUAKANALOA

Kamaʻehuakanaloa (Kamaʻehu), typifies the pre-shield stage of Hawaiian volcanism. Over the past ~400 kyrs (Garcia et al., 2006), it has drifted closer to the plume’s focal center, accompanied by a progressive transition from alkalic to tholeiitic magmas from 40-2 ka (Pietruszka et al., 2021). Kamaʻehu’s plumbing system is poorly understood - the absence of permanent OBS, and seismic stations only onland to the north, means that seismic constraints are challenging. Previous work suggests a magma storage region for the 1996 lavas at ~8-9 km based on MELTS modelling and the clustering of earthquakes (EQ) near ~5-7 km (Garcia et al, 1998; Caplan-Auerbach & Duennebier, 2001), while scarce tholeiite melt inclusion data (CO₂ from glass phase only) record depths of ~2-4 km bsl (Schipper et al., 2011). Interestingly, CO₂ fluid inclusions (FI) in dunite ultramafic xenoliths erupted in alkalic lavas record pressures corresponding to ~8-17 km bsl (Roedder, 1983).

Kamaʻehu's deeply dissected east-flank section spans ~100-150 ka, corresponding to ~40% of its extrusive history (Guillou et al., 1997; Garcia et al., 2006), making it a unique opportunity to assess variations in magma storage depths and the geometry of the plumbing system during the volcano’s evolution. Here, we analyzed ~400 FI – small pockets of exsolved CO₂ fluid trapped in olivine crystals – from 5 alkaline to tholeiitic submarine lavas from the east flank stratigraphic section (2000, 1410, 1270, 1215 and 1080 m bsl). We measured the CO₂ density and composition of the fluids using Raman spectroscopy and microthermometry, and calculated pressures using an equation of state at temperatures inferred from SEM-EDS measurements of host chemistry. Our data indicate magmas were stored at a wide range of depths; tholeiitic samples at the top of the section show a relatively uniform depth distribution between ~2.5 to 13 km bsl, while the oldest alkalic sample has more of a bimodal distribution, at ~5-9 and ~10-17 km bsl. Our results support that magma storage may shallow slightly as magma flux increases during migration towards the plume’s center, though it is interesting that erupted crystal cargoes show similar depth distributions despite large changes in erupted melt chemistry (alkalic to tholeiitic). This may imply that magma storage was relatively stable with changing flux, and/or that crystal cargoes are largely antecrystic.