Experimental constraints on the deformation and breakup of injected magma

Abstract

The injection, breakup and stirring of dikes entering convecting silicic magma chambers can govern how they grow and differentiate, as well as influence their potential for eruption at the surface. Enclaves observed in plutons may preserve a record of this process and, thus, identifying and understanding the physical processes underlying their formation is a crucial issue in volcanology. We use laboratory experiments and scaling theory to investigate the mechanical and rheological conditions leading to the deformation and breakup of analog crystal-rich dikes injected as discrete plumes that descend into an underlying imposed shear flow. To scale the experiments and map the results across a wide range of natural conditions we define the ratio S of the timescale for the growth of a gravitational Rayleigh–Taylor (R–T) instability of the sheared, injected material to the timescale for settling through the fluid layer and the ratio Y of the timescales for shearing and lateral disaggregation of the particle–fluid mixture (yielding). At low S (<3) and high Y (>40), descending plumes are stretched and tilted before undergoing R–T instability, forming drips with a wavelength that is comparable to the initial diameter of the injection. At low Y (<40) and S values that increase from ∼3 as Y →0, an injection yields in tension before a R–T instability can grow, forming discrete particle-fluid blobs that are much smaller than the initial injection diameter and separated by thin filaments of the original mixture. At high S (>3) and high Y (>40), injections remain intact as they settle through the layer and pond at the floor. Applied to magma chambers, our results do not support the production of a continuum of enclave sizes. Indeed, from scaling analyses we expect the two breakup regimes to form distinct size populations: Whereas enclaves formed in the R–T regime will be comparable to the injection size, those formed in the tension regime will be much smaller. We show that enclave size distributions observed in the field can potentially be used to infer the Y−S conditions for the magma chamber at the time of injection. In addition, these observations can constrain aspects of the styles of flow, stirring, and mixing within the magma chamber, as well as the rheological contrast between the injected and host magma at the time of enclave formation. Our work shows that the contrast in composition between the injected and host magmas will have a strong effect on the mingling structures that are likely to be generated.