Insights from Disequilibrium and Equilibrium Crystallization Rheological Experiments on the 2018 Kilauea LERZ Eruptive Sequence

Magma rheology influences the dynamic transport of stored magma from subvolcanic depths to the surface and its further flow expression until final cooling and emplacement. Behind all of this is a temperature- chemistry- and crystallinity-dependent rheology which can be measured and/or estimated in the field and determined under controlled conditions in the laboratory. Here, we have determined the rheology of both molten and partially crystallized samples of the eruption products of both Phase I and Phase II of the 2018 LERZ eruptive vents sequence. Experiments were conducted on samples of 8 vents using concentric-cylinder rheometry in air in the 1500 - 1200°C temperature range. The liquid viscosities range from 2 Pa s to 60 Pa s. The Phase II samples are systematically ca. 0.5 log₁₀ units less viscous than the Phase I samples. For Phase I we see minor variations in the viscosity of six remelted samples (< 9 Pa s at any given temperature), whereas for Phase II there is no detectable variation between the two samples measured. To quantify the influence of crystallisation on the viscosity these samples, experiments conducted in air are of limited value. For this reason, we have initiated a series of viscosity determinations under reducing conditions (fO₂ = -7) in order to stabilise and reproduce the actual crystallisation sequence observed in nature. Initial crystallisation experiments under equilibrium conditions (isothermal) provide a basis for the interpretation of rheological data as a function of crystallinity, as obtained from SEM images of quenched products. Finally, in order to approach most closely the actual highly dynamic natural conditions of the eruption, we are also conducting experiments under disequilibrium crystallisation conditions (constant cooling rate). These data represent the most extensive rheological characterisation of a basaltic eruption sequence to date.