THE 2021-22 HUNGA TONGA-HUNGA HA’APAI ERUPTION: INVOLVEMENT OF TWO HIGHLY DEPLETED MAGMAS IN A VEI 6 ERUPTION

The eruption of Hunga Tonga-Hunga Ha’apai on January 15^th, 2022 was the largest in the last 140 years. It emanated from an active caldera system that produces highly trace-element-depleted magmas, typical of the wider South Pacific Tonga-Kermadec island arc. Tephra deposits, sampled on Tongatapu and surrounding islands, are andesitic and comprised of light pumice, dark pumice, and volcanic glass. Plagioclase, clinopyroxene, orthopyroxene, and rare olivine phenocrysts also occur. The pumices and non-vesicular glass have similar highly depleted major and trace element compositions (e.g., K₂O=~0.5 wt%, Ba= ~140 ppm, Sr= ~160 ppm). They also share similar ⁸⁷Sr/⁸⁶Sr ratios (0.7037 to 0.7038) but the ¹⁴³Nd/¹⁴⁴Nd ratio of the dark pumice is significantly less radiogenic (0.51281 vs ~0.51300) and retains a different (²³⁰Th)/(²³²Th) ratio, indicating at least two magmas were involved in the eruption. All glass types are highly U-enriched (i.e., (²³⁸U)/(²³²Th) = ~1.85) typical of those resulting from arc-related, slab fluid additions. The two different andesitic magma compositions erupted have trace elements and radiogenic isotopes typical of Tonga-Kermadec island arc magmas. In contrast to pumice and glass (i.e., melts), elemental compositions and isotope ratios of accompanying plagioclase and clinopyroxene (cpx) crystals are variable. Most plagioclase crystals (~An_88-93), analyzed as single crystals and insitu, have ⁸⁷Sr/⁸⁶Sr ratios that are generally similar to the pumices and glass (~0.7037). However, glasses are heterogeneous but have a distinct mode at ~Mg#45, which would be in equilibrium with cpx of ~Mg#75. The mode of cpx compositions is however more mafic (~Mg#80-90). Thus, cpx crystals are likely to originate from more mafic magmas and could be relics from a deeper root zone of the magmatic system. Variable cpx Mg# are accompanied by a range of ⁸⁷Sr/⁸⁶Sr ratios (~0.7035 to 0.7039). Overall, the magmas (and their crystals) involved in the Hunga Ha’apai eruption reflect highly variable signatures that support the presence of at least two compositions consistent with tapping diverse parts of the magma system not observed in earlier eruptions.