BIG CHANGES AT KĪLAUEA VOLCANO: THE IMPACT OF 2018 SUMMIT COLLAPSE ON MAGMATIC PROCESSES RECORDED BY THE 2020 ERUPTION (Invited Presentation)

The unprecedented 2018 summit collapse at Kīlauea and subsequent 2020 Halema‘uma‘u eruption provide an unparalleled opportunity to understand how collapse events impact the volcano’s shallow reservoir system and dominant magmatic processes. Glass and olivine were analyzed from tephra ejected by several explosions on 20-21 December within a few hours of the eruption onset. The olivine population is bimodal with zoned and non-zoned phenocrysts. Normally zoned olivine with dominantly Fo₈₈ cores (forsterite; [Mg/(Mg+Fe)x100]) have 30-50 μm wide Fo₈₂ overgrowth rims that have skeletal textures. Two skeletal xenocrysts (Fo₇₄ and Fo₈₁) are also reversely zoned up to Fo₈₂. The Fo₈₂ rims and a population of non-zoned Fo₈₂ crystals are in Fe-Mg equilibrium with the glass (average MgO of 6.9±0.4 wt% (1σ), Mg# [Mg/(Mg+Fe²⁺)] of 0.57). These textures and compositions reflect undercooling after intrusion of magma to shallow levels in the plumbing system. In the years prior to the 2018 collapse, non-zoned Fo₈₁ olivine and low MgO glasses (e.g., 6.8 wt%) reflected continuous mixing and compositional buffering of magma recharge into several km³ of shallowly stored magma. The 2020 olivine lack evidence of mixing with stored magma, indicating that the intrusion occurred in a disrupted system, and/or it bypassed any stored magma remaining from the Halema‘uma‘u reservoir, which facilitated summit collapse and was at least partially drained by the 2018 eruption. Non-zoned olivine for several years prior to 2018 precludes diffusion studies, but the normally zoned 2020 olivine constrain the timescales of magma movement through the current system. Diffusion model results cluster around 60 days before eruption, which aligns well with 22-24 October seismic swarms under the Ka‘ōiki fault zone W of Kīlauea’s summit. These timescales and the olivine data show that the summit collapse in 2018 has led to substantial changes in both Kīlauea’s shallow plumbing system and its magmatic processes.