The effect of oxygen fugacity on the rheological evolution of crystallizing basaltic melts

Storage and transport of silicate melts in the Earth's crust and their emplacement on the planet's surface occur almost exclusively at sub-liquidus temperatures. At these conditions, the melts undergo crystallization under a wide range of cooling-rates, deformation-rates, and oxygen fugacities (fO₂). Oxygen fugacity is known to influence the thermodynamics and kinetics of crystallization in magmas and lavas. Yet, its influence on sub-liquidus rheology remains largely uncharted. We present the first rheological characterization of crystallizing lavas along natural cooling paths and deformation-rates and at varying fO₂. Specifically, we report on apparent viscosity measurements for two crystallizing magmatic suspensions 1) at log⁡fO₂ of −9.15 (quartz–fayalite–magnetite buffer, QFM, −2.1) and 2) in air. These fugacities span a range of reduced to oxidized conditions pertinent to magma migration and lava emplacement. We find that: 1) crystallization at constant cooling-rates results in a quasi-exponential increase in the apparent viscosity of the magmatic suspensions until they achieve their rheological cut off temperature (T_cutoff), where the melt effectively solidifies 2) the rheological departure and T_cutoff increase with increasing fO₂ and 3) increasing fO₂ results in decreased crystallization-rates. Based on the experimental results and by comparison with previous rheological isothermal studies we propose a generalisation of the effect of fO₂ on the dynamic rheological evolution of natural magmatic and volcanic suspensions. We further discuss the implications for magmatic transport in plumbing and storage systems (e.g. conduits, dikes and magma chambers) and during lava flow emplacement.