Magmatic degassing dynamics at Halema'uma'u Crater, Kīlauea, Hawaii

• Illustrate spattering and lava lake surface degassing. • Characterize the degassing process inside the lava lake activity. • Partial degassing of magma within the lava lake. • Production over time of a large volume of partially degassed magma. • Equilibrium temperature and redox state are consistent with gas-rock buffering Best regards. Lava lake activity within Halema‘uma‘u crater on Kīlauea volcano, Hawaii, between 2010 and 2018 provided a remarkable opportunity to observe the dynamics of magmatic degassing occurring in both quiescent and lava-spattering degassing regimes. We collected open-path FTIR absorption spectra of magmatic gas in December 2015, when distributed lake surface degassing and spattering activity occurred on the SE margin of the lava lake. We quantified seven volcanic gas species, H 2 O, CO 2 , SO 2 , HCl, HF, CO and OCS, distinguishing between spattering and lake surface degassing. Passive and solar traverse measurements allowed quantification of compositions and relative SO 2 emission rates of 60 % from spattering and 40 % from lake surface degassing. Spattering gas has a CO 2 /SO 2 molar ratio of 0.88 compared with 0.56 for lake surface degassing, consistent with a partial sulphur loss from magma during spattering. We propose that spattering is the result of continuous formation of coalescing gas bubbles driven by downwelling lava lake crust, which promotes gas loss from 10s-100 s of metres within the lake. Spattering degassing provide a mechanism for partial degassing of magma within the Halema‘uma ‘u lava lake, and production over time of a large volume of partially degassed magma. This is in agreement with petrological models indicating that summit-derived partially degassed magma accumulated in the MERZ in the decade prior to the 2018 LERZ eruption. Calculations of equilibrium temperature and redox state are consistent with gas-rock buffering.