Coupled crystallisation and chaotic mixing of basaltic and dacitic magmas under sub-liquidus conditions

Constraining the magmatic processes that control how magmas differentiate is essential for understanding reservoir dynamics before and during eruptions. Crystallisation and mixing are the two primary processes governing the evolution of magma reservoirs. However, the influence of crystallisation on the physical and chemical mixing of magmas remains poorly constrained, limiting our understanding of textural and chemical evolution of eruptible magma prior to eruptions. Here, we present an experimental study investigating the simultaneous occurrence of crystallisation and dynamic magma mixing using basaltic and dacitic end members at sub-liquidus conditions. Our experiment directly captures the interaction of crystallisation and magma mixing under dynamic conditions, revealing how these processes produce enclave disaggregation, filaments, and compositional gradients in the melt. Our experiment reproduces the interaction of mafic and felsic magmas, and the derived processes of mixing while the crystallisation proceeds. The results indicate that basaltic magmas crystallise rapidly, forming crystal-rich mafic enclaves within a felsic host and producing basaltic andesitic to andesitic melts. Advection promotes stretching and folding, which enhance both chemical exchanges and physical magma mixing effects, leading to enclave disaggregation and the formation of crystal clusters in disequilibrium with the surrounding melt, within a few hours in the investigated experimental setup. We used the parameter (normalized variance) to quantify the mixing efficiency and differential elemental mobilities. This indicates that crystallisation of the mafic magma can promote the evolution of melt compositions that enhances magma mixing efficiency with the more evolved end-member.