Strain partitioning during partial melting and crystallizing felsic magmas

Abstract

The rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization. Solid to liquid and liquid to solid transitions are not rheologically identical, contradicting the concept of a unique rheological critical melt percentage. We use four thresholds, separating the successive stages according to the melt fraction. For the melting transition: (1) the liquid percolation threshold connects the melt pockets at about 8% of melt, thus allowing local magma displacement at the metric scale; and (2) for greater melt percentages (20–25%), a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase, leading ultimately to granitic bodies. For the solidification transition: (3) the first formation of a rigid framework of crystals which can accumulate stress, although the melt can still flow, corresponds to the rigid percolation threshold (55% solids); and (4) the system becomes totally locked at about 72–75% solid, which corresponds to the particle locking threshold. The present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds. On a variety of scales, non-coaxial deformation is preferentially accommodated by the lower-viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern. Strain partitioning also increases the scale length of the weakest phase during melting and magma ascent, whereas strain partitioning decreases the scale length during crystallizing. Strain partitioning also affects the migration of melt, by raising the particle locking threshold and lowering the melt escape threshold. For three-dimensional deformations, such as transpression, the direction of melt segregation (e.g. horizontal) may differ from the direction of melt migration (e.g. vertical).