285-4 The Rheological Visco-Elasto-Plastic Regime of Magma Mingling

Abstract:

Magma mingling, broadly part of magma-mixing processes, occurs when the physical interactions between mafic and felsic melts do not proceed to total mixtures. Such incomplete mixtures leave well-described heterogeneous structures in outcrop including enclaves, interfingering of different magmas, magma-fille fractures, and dikes; many of these features are thought to drive explosive eruptions. Magma-mingling studies often use physical (a.k.a. analog) modeling as a visualization and parameter-scaling tool. Such experiments typically mix fluids of contrasting viscosity to produce lobate plumes and viscous fingers. Yet, it is well known that otherwise viscous flows also undergo brittle deformation. Approaches to modeling ‘brittle-ductile’ transitions include a high-enough Deborah number  – the ratio of relaxation to flow – that the magma will behave as a true brittle solid, and/or spring-and-dashpot models that haved both elastic and viscous components. Here, we present the results of experiments wherein we inject low-viscosity fluid into a visco-elastic-plastic carbomer (trademarked Carbopol) gel in a quasi-2D Hele-Shaw cell, essentially two plexiglass plates. The injection experiments produce a wide range of mesoscopic structures, ranging from pooling lenses, to branching patterns with tipped-out fluid-filled fractures, to interfingering of the fluid and gels. These experiments bracket the “finger-fracture transition”, and also illustrate widely varying ranges of recovery/elasticity/memory. The well-characterized controls on the experimental system’s rheology allow us to define a broader rheological phase space that encompasses a range of elastic, plastic, and viscous forces. Applying this approach to the magma mingling problem shows that the commonly observed outcrop structures all formed as the ratio of these forces approached unity. Therefore, the conspicuous mesoscale magma-mingling structures should be viewed as having formed in a rather specific rheological regime for an instability, rather than in widely disparate regimes of different instabilities.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.