Experimental and thermodynamic constraints on the magmatic variables governing pre-eruptive conditions at Hunga volcano: Development of a new equilibrium orthopyroxene-clinopyroxene thermometer

The cataclysmic eruption of Hunga volcano (Tonga-Kermadec arc system) on 15 January 2022, the most powerful explosive volcanic event of the 21st century, underscores the critical need to constrain the pre-eruptive magmatic conditions governing arc volcanoes with shallow marine calderas. In this study, we present results from isobaric-isothermal crystallization experiments conducted on a basaltic andesite representative of primitive magmas at Hunga volcano. Experimental runs were performed at pressures of 200 and 300 MPa, over a temperature range of 1000–1130 ◦C, melt water contents of 0.6–6.4 wt%, and oxygen fugacities between +0.7 and + 3.4 log units relative to the nickel‑nickel oxide buffer. Temperature and melt-water content are the primary controls on modal phase assemblages and compositional trends in the experimental charges, exerting a dominant influence on the cotectic crystallization of clinopyroxene and plagioclase, as well as on the stability of orthopyroxene and abundance of magnetite. By integrating experimental data with thermodynamic modeling, orthopyroxene-clinopyroxene thermometry, and plagioclase-based hygrometry, we document that both pre-2022 and 2022 eruptive products at Hunga volcano reflect the differentiation of basaltic andesitic to andesitic magmas at pre-eruptive temperatures of ~1050–1130 ◦C and melt-water contents up to ~3 wt%. Under the investigated experimental conditions, however, extensive crystallization of plagioclase and magnetite at ~1000 ◦C drives the host basaltic andesitic melt toward more evolved dacitic compositions. Dacites have not been reported at Hunga volcano, but they occur in low-temperature, magnetite-bearing phenocryst assemblages on other Tongan islands, particularly in the northern part of the arc. Collectively, our experimental-thermodynamic approach provides compelling evidence for polythermal and polybaric plumbing systems at intra-oceanic arc volcanoes, highlighting the pivotal role of phase stability and mineral chemistry in controlling the differentiation pathways of magmas and the transition from tholeiitic to calc-alkaline affinity.