Abstract
The eruption of Hunga volcano on January 15th, 2022 emanated from an active caldera system that produces highly trace-element-depleted magmas, typical of the wider Tonga-Kermadec island arc. Tephra deposits, sampled on Tongatapu and surrounding islands, are andesitic and composed of light pumice, dark pumice, and dark vesicle-poor 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= approximately 0.5 wt%, Ba= approximately 140 ppm, Sr= approximately 160 ppm). They also share similar 87Sr/86Sr ratios ( approximately 0.7038) but the 143Nd/144Nd ratio of the dark pumice is significantly less radiogenic (0.5128 vs approximately 0.5130). This pumice also retains different (230Th)/(232Th), confirming involvement of two magmas. All glass types are highly U-enriched (i.e., (238U)/(232Th)= approximately 1.85) typical of those resulting from subduction-related, slab fluid additions. The two magma types also retain high (226Ra)/(230Th) ratios (> or =3.0) confirming rapid extraction, differentiation, and transit to the surface. In contrast to pumice and glass (i.e., melts), elemental compositions and isotope ratios of accompanying plagioclase and clinopyroxene crystals are variable. Most plagioclase crystals ( approximately An88-93), analyzed as single crystals, have 87Sr/86Sr ratios that are generally similar to pumices and glass ( approximately 0.7037) despite variable An contents. Glasses however, are heterogeneous but have a distinct mode at Mgapproximately 45, which would be in equilibrium with cpx of Mgapproximately 75. The mode of cpx compositions is however more mafic (Mgapproximately 80-90). Thus, cpx crystals are likely to originate from more mafic magma and could be relics from a deeper root zone of the magmatic system. Variable cpx Mgare accompanied by a range of 87Sr/86Sr ratios ( approximately 0.7035 to 0.7039). Overall, magmas and crystals reflect highly variable signatures that support the presence of at least two andesitic magmas consistent with tapping diverse parts of the system not observed in earlier Hunga eruptions.