Enhancement of Magma Mixing Efficiency by Chaotic Dynamics: an Experimental Study

Magma mixing is common in the Earth. Understanding the dynamics of the mixing process is necessary for dealing with the likely consequences of mixing events in the petrogenesis of igneous rocks and the physics of volcanic eruptive triggers. We present a new apparatus to perform chaotic mixing experiments in systems of melts with high viscosity contrast. The apparatus consists of an outer and an inner cylinder, which can be independently rotated at finite strains to generate chaotic streamlines. The two cylinder axes are offset. Two end-member silicate melt compositions were synthesized from oxide and carbonate components and used in the experiments: (1) a peralkaline haplogranite and (2) a haplobasalt. The viscosity ratio between these two melts was of the order of 103. Experiments have been performed for ca. 2 h, at 1,400°C under laminar fluid dynamic conditions [Re ~ 10^(-7)]. Optical analysis of post-experimental samples revealed a complex pattern of mingled filaments forming a scale-invariant (i.e. fractal) distribution down to the μm-scale, as commonly observed in natural samples. This is due to the development of stretching and folding of the two melts in space and time. Chemical analysis showed that the original end-member compositions had nearly entirely disappeared from the filaments generated by the chaotic flow field. In addition, strong non-linear correlations in inter-elemental plots were observed. The generation of thin layers of compositionally widely contrasting interfaces strongly enhanced chemical diffusion producing a remarkable modulation of compositional fields over a short-length scale. Notably, diffusive fractionation generated highly heterogeneous pockets of melt, in which depletion or enrichment of chemical elements occurred, depending on their potential to spread within the magma mixing system. Results presented in this work offer new insights into the complexity of processes expected to be operating during magma mixing and may have important petrological implications. In particular: (1) it is shown that, in contrast with current thinking, magmas with highly contrasting rheology can mix, thus extending significantly the spectrum of geological conditions under which magma mixing processes can occur efficiently; (2) the geochemical modelling of mixing process cannot be performed using the classical linear two-end-member mixing model; and (3) the chemical compositions on short-length scales represent snapshots within the process of chaotic mixing and, therefore, must be considered as transient compositions. Their connection to the composition of original end-members is, thus, not trivial. This implies that microanalysis on short-length scales may provide misleading information on the parental composition of magmas.