Hunga-Tonga-Hunga-Ha’apai Jan 15, 2022 eruption: Assembly of heterogeneous magma sources recorded in melt inclusions from plagioclase, clinopyroxene and orthopyroxene.

The 15 Jan 2022 eruption of Hunga-Tonga-Hunga-Ha’apai was the largest explosive volcanic event in the last 30 years. These islands represent the subaerially exposed summit of the Hunga Volcano, merged into a single land mass during the most recent eruption in 2014-2015. The 2022 eruption likely represents a 1-in-1000-year event for the Hunga Volcano, with the previous large-magnitude eruption occurring in ~1100 CE during a series of caldera-forming events. The 2022 erupted magma is plagioclase-, orthopyroxene- and clinopyroxene-bearing basaltic andesite to andesite dominated by blocky, poorly vesicular glassy ash with lesser amounts of vesicular pumiceous ash and fine lapilli. Melt Inclusions (MIs) hosted in plagioclase, clinopyroxene and orthopyroxene are abundant and glassy, some displaying shrinkage bubbles, with no evidence of secondary crystallization along the walls or within the MI glass. The groundmass glass and MI in the three main phenocryst phases were analysed for major, trace and volatile element concentrations to enable identification of magmatic sources and to better constrain processes happening at depth. Preliminary data indicate that plagioclase phenocrysts range from An₉₃ to An₇₈, and MI range from 54.1 to 58.7 wt % SiO₂, with MgO from 2.5 to 5.3 wt %. Clinopyroxene phenocrysts range from En₄₂ to En₅₀, and MI range from 51.6 to 65.1 wt % SiO₂, with MgO from 1.1 to 5.7 wt %. Orthopyroxene phenocrysts range from En₆₈ to En₇₇, and MI range from 55.7 to 59.6 wt % SiO₂, with MgO from 2.5 to 5.3 wt %. Clinopyroxene MI span the full range of SiO₂ compositions observed from the Hunga Volcano, from the host 2022 event (SiO₂: ~57.5 wt %), the 1100 CE event (SiO₂: ~60 wt %), the 2014-2015 event (SiO₂: ~60.5 wt %), and the most evolved 2009 event (SiO₂: ~63 wt %) and extend an additional ~4 wt % SiO₂ to more mafic compositions. Orthopyroxene MI most closely resemble the 1100 CE event and the average groundmass glass compositions of the 2022 event. Plagioclase MI overlap the least silicic compositions observed in the 2022 groundmass glass (58.6 wt% SiO₂) and extend down to 54 wt % SiO₂, overlapping the main field of clinopyroxene MI. Both plagioclase and clinopyroxene MI tend to show higher MgO as compared to the 2022 groundmass glass at the same SiO₂ concentration, whereas orthopyroxene shows lower MgO than the groundmass glass. SO₃ in MI ranges up to 1600 ppm, significantly higher than the 2022 groundmass glass which averages 200 ppm, with both plagioclase and clinopyroxene MI preserving the highest observed concentrations. In contrast, Cl concentrations in MI extend to 2000 ppm, with the highest values in orthopyroxene and clinopyroxene, and plagioclase MI are lower and generally overlie the main groundmass glass concentrations (~1300 ppm). F was below detection limits. We postulate that clinopyroxene crystals reflect a more primitive basaltic andesite magma, whereas orthopyroxene crystals were likely derived from the magmatic remnants of the 2009 and 2014/2015 events in the upper magma system, and plagioclase crystals were sourced from the full range of magma sources.