Analogue experiments to investigate magma mixing within dykes.

Multiple magmas residing in plumbing systems that feed fissure eruptions can physically and chemically interact and mix during storage, transport, and eruption. The extent and success of such mixing ultimately control the physical properties (e.g. density and viscosity) of the magma, the eruptive conditions, and thus the associated hazards. Analogue experimental studies have previously investigated magma interactions in plumbing systems typically with pipe-like or chamber-like geometries (i.e. cylindrical or cuboidal respectively) and immiscible fluids that represent magma mingling. However, these findings are difficult to extrapolate to high aspect ratio geometries typical of dykes that characterise fissure systems. Here, we present results from a high aspect ratio experimental setup to explore magma mixing within dykes. Using an array of miscible fluid pairs, representing magmas of differing composition, we found that flow is initially localised towards the centre of the system and mixing occurs at the interface between the two fluids, spreading laterally out over time. The mixing interface is generally greater, and mixing is more rapid when the starting physical properties of the two fluids are more similar. Furthermore, a dyke-like geometry facilitates mixing to a greater degree relative to a chamber-like system. We explore the implications of the mixing dynamics on diffusive and crystal exchange between magmas, the transport of magmas through the crust, and the evolution of physical and chemical properties of interacting magmas. The mixing ratio trends of our experimental data are similar to near-real time geochemical mixing data from the Kīlauea 2018 eruption, suggesting a future avenue for understanding the complexities of mixing during magma ascent. Supplementary Information: The online version contains supplementary material available at 10.1007/s00445-025-01809-0.