A Simple Mechanical Recipe for Andesites

Mechanical mixing of mafic and silicic magmas is often cited as one way to make andesite, the predominant rock type of Earth's continental crust. Petrologic and geochemical observations require extensive mixing of basaltic and highly silicic magmas to produce andesites. However, such mixing is thought to be mechanically impossible due to the effective viscosity contrast between the two magmas. The extent to which these magmas have to be stirred must depend on the initial scale length at which mafic or silicic magmas are introduced into magma chambers. Here we show that understanding how to efficiently mix two compositionally and thermally distinct magmas may provide a method to make andesites and bridge the famous "Daly gap". We use analog experiments to examine the impact of the viscosity ratio between injected and host magma on the length scales of magma mixing. To capture the combined effects of cooling and solidification on the rheology of dense, mafic intrusions in a straightforward way that can be scaled to the natural magmatic case, we introduce buoyant, rheologically complex plumes with different physical properties into a lower viscosity fluid undergoing simple shear flow. By carefully varying the buoyancy, effective viscosity, and yield strength of the injected material, as well as the strength of the imposed shear in the ambient fluid layer we characterize the deformation of the injections over a wide range of conditions expected to occur in natural systems. We show that large differences in magma viscosity and/or rheology favor the breakup of mafic dikes entering a silicic chamber into blobs that are initially centimeters to tens of centimeters in size. Extensive fractal analysis of the size distributions of thousands of enclaves from Cascade lava flows supports this physical picture and demonstrates that these initial magmatic blobs undergo further fragmentation to an even smaller scale that decreases as the effective viscosity contrast between the injected and host magma increases. We argue that arc andesites are a natural consequence of the mechanical behavior of mafic or silicic dikes as they enter crustal magma chambers. More broadly, our work predicts conditions where thorough mechanical mixing of silicic and relatively mafic magmas is inevitable and where compositional stratification is favored. We will discuss implications for essential differences between the compositional structures of arc and continental magma chambers and compare our predictions with observations from Cascade volcanoes, the Bishop and Bandelier Tuffs and other examples.