On the mechanics and melt segregation of crystal-rich systems

Abstract:

There is controversy about the duration, dynamics and physical conditions of magmatic systems, and the means by which silicic melts are extracted from host mushes. Conflicting geological, geochemical and geophysical evidence suggest that magmas and migmatites are either short-lived or can persist in a crystal-rich state for up to a million years. In addition the mechanisms of melt migration from magma mushes is poorly understood, as the processes of porosity reduction are not uniformly consistent with the commonly invoked crystal-plastic or viscous compaction. Porosity reduction (and hence melt expulsion) requires a series of “micro-discrete” processes controlled by crystal shape, melt buoyancy and the nature of imposed shear on the mush. We will first review the expressions of crystal-rich processes as preserved in an intact deep arc crustal section from the Famatinian orogeny in Argentina, and describe the progressive scales of melt extraction and migration that culminate in the formation of canonical, compositionally stratified arc crust. This will motivate us to introduce and exemplify measures of microstructural contact, force and mineral shape fabric and their anisotropy using the respective tensors. We will introduce two dimensionless groups, the viscous and Sommerfeld numbers, which can be used to describe the state of the mush and the controls on mixing and modes of mechanical lock-up. We will close with a proposal of an integrated research program at the crystal-to-outcrop scale combing discrete numerical and Paterson rig experiments, with EBSD, color CL and other measure of microstructures, to aid progress in understanding the distinct character of magmatic processes and products.