Abstract
On 15 Jan 2022, Hunga Volcano in the Tonga-Kermadec oceanic arc generated the most explosive eruption recorded in the last 140 years. The deposits are andesitic (whole rock SiO2 57 wt %) comprised of blocky, poorly vesicular glassy ash with subordinate vesicular pumice ash and fine lapilli. Phenocrysts (from 5-10% by volume) are plagioclase, orthopyroxene, clinopyroxene, and very rare olivine. They are commonly euhedral or fractured and are most abundant in the 500-1000 µm size fraction. Plagioclase phenocrysts have high-anorthite cores (~An90-93) surrounded by more sodic oscillatory-zoned overgrowths (~An80-85). Clinopyroxene phenocryst cores have average Mg# values of 79 (range = 68.0 to 87.5), and orthopyroxene cores have a mode at Mg# 75 (range = 63.4 to 82.7). Overgrowth mantles on pyroxene range from 100 to 300 microns and may be more or less magnesian than the cores, but rims are normally zoned, and the outermost edges are similar to groundmass pigeonitic pyroxenes. Preliminary thermobarometry estimates from equilibrium cpx-melt pairs indicate temperatures of ~1110-1130 °C and pressures of ~150-200 MPa for the modal cpx composition. Olivine crystals are euhedral with homogenous cores up to Fo93 and thin, normally zoned rims. Chondrite-normalized Rare Earth Element and primitive mantle-normalized trace elements in the glass are in similar to whole-rock values from the 2014/2015 Hunga eruption, with slightly lower incompatible element abundances. These data demonstrate assembly of heterogeneous magmas to drive this eruption and tap deeper parts of the magma system than ever seen before – likely due to extreme decompression. While there is no evidence for ‘mafic magma recharge’ immediately preceding eruption, phenocryst compositional variability, microlite texture contrasts, and ubiquitous mingling textures all indicate that different andesitic magmas mingled both before and during the eruption. This mingling may have driven development of extreme gas-pressure to trigger the very violent onset of this event.