Gravity instabilities in magma chambers: rheological modelling

Abstract

The physical study of gravity instabilities in magma chambers provides a method to estimate the rheological properties of magmas. Many layered intrusions exhibit gravity instability structures, such as magmatic load casts. We present here a study based on structures occuring in veined diorites in the layered plutonic complex of Guernsey. Wavelengths of gravitational perturbations are determined by spectral analysis. We relate the wavelengths to the rheological properties of the magmas by a perturbation enhancement model. The time scale for the gravity instabilities to develop is calculated to be about 10 2 s, which is about the characteristic time scale for solidification in these rocks; this is why these structures are preserved in the rock. The calculated length and time scales are compatible with the geometry of the observed structures and the crystallization kinetics of magma chambers, showing that the structures are appropriately modeled as gravity instabilities. This mechanical approach, together with petrological study and direct computation of magma viscosity, is likely to provide several constraints on petrogenetic modelling. The fact that the observed structures can be interpreted as gravity phenomena is clear evidence that the observed layering is a primary igneous feature rather than the result of any metasomatic process.