Viscous Fingering During Replenishment of Felsic Magma Chambers by Continuous Inputs of Mafic Magmas: Field Evidence and Fluid-Mechanics Experiments

Vegetation Island outcrops (Terra Nova Intrusive Complex, Antarctica) offer a unique example of the replenishment of a felsic magma chamber fossilized at the initial stages of intrusion of a mafic magma. The morphology of interfaces between the mafic and the felsic magma ranges from rounded to finger-like, and their quantification by means of fractal dimension indicates a wide variability of morphological complexity. Fluid-mechanics experiments of viscous fingering have been performed by injecting water + glycerin solutions with different viscosity ratios into pure glycerin using the Hele-Shaw cell. The fact that interface morphologies between the injected and the host fluid are identical to those observed on outcrops indicates that the latter shows the development of viscous fingering processes during the initial stages of intrusion of the mafic magma into the felsic magma chamber. The fractal dimension of the simulated structures was measured, and a very good exponential empirical relationship between the logarithm of viscosity ratio and fractal dimension has been derived. The empirical relationship is used to estimate viscosity ratios of natural structures by using measured values of fractal dimension. Results indicate that in the same magmatic system, a wide range of viscosity ratio existed between the two magmas. These results are used to reconstruct the mechanism of replenishment of the felsic magma chamber as characterized by continuous heating of the resident felsic magma by continuous inputs of the mafic magma.