Abstract
The eruption of Hunga Tonga on January 15th, 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. Pumices and non-vesicular glass have similar highly depleted major and trace element compositions (e.g., K (sub 2) O= approximately 0.5 wt%, Ba= approximately 140 ppm, Sr= approximately 160 ppm). They also share similar (super 87) Sr/ (super 86) Sr ratios (0.7037 to 0.7038) but the (super 143) Nd/ (super 144) Nd ratio of the dark pumice is significantly less radiogenic (0.51281 vs approximately 0.51300) and retains a different ( (super 230) Th)/( (super 232) Th) ratio, indicating at least two magmas were involved in the eruption. In addition, the more radiogenic Nd magmas retain very different Pb isotope ratios, confirming the presence of at least three end-magma member magmas. Pb isotopes also suggest the interplay of both Lau Basin and Pacific mantle sources. All glass types are highly U-enriched (i.e., ( (super 238) U)/( (super 232) Th)= approximately 1.85) typical of arc-related, slab fluid additions and radium isotope disequilibria suggest magma generation and transit to the surface in less than 3200 years. In contrast to pumice and glass (i.e., melts), isotope ratios of accompanying plagioclase and clinopyroxene (cpx) crystals are variable. Most plagioclase crystals, analyzed as single crystals and in situ, have (super 87) Sr/ (super 86) Sr ratios that are similar to the pumices and glass ( approximately 0.7037). However, glasses are heterogeneous but have a distinct mode at approximately Mg, which would be in equilibrium with cpx of approximately Mg, but the mode of cpx compositions is more mafic ( approximately Mg-90). Along with the common calcic plagioclase ( approximately An (sub 88-93) ), cpx crystals are likely to originate from more mafic magmatism and could be relics from a deeper root zone of the system. Variable cpx Mgre accompanied by a range of (super 87) Sr/ (super 86) Sr ratios ( approximately 0.7035 to 0.7039). Overall magmas, and their accompanying crystals, involved in the Hunga eruption reflect highly variable signatures that support the presence of at least two compositions with complex petrogenetic characteristics.
Oral presentation.