- © The Mineralogical Society Of America
The physical properties of partially molten rocks are directly coupled to the grain-scale as well as the broader scale distribution of the melt phase. At the grain scale, if melt forms isolated pockets in a silicate matrix, its influence on plastic flow is generally relatively minor. In contrast, if the melt wets all of the grain-grain interfaces, it may dramatically lower the viscosity of the rock. At the broader scale, deformation will localize to form shear zones in melt-rich regions that commonly develop in partially molten rocks. In turn, the melt distribution evolves during plastic deformation of a partially molten rock. Melt located in triple junctions takes on a pronounced preferred orientation. In addition, melt often segregates during plastic deformation to form melt-enriched bands separated by melt-depleted regions.
This chapter develops two interrelated themes with emphasis on the relationship between melt distribution (structure) and the plastic flow (property) of partially molten rocks. The first section concentrates on thermodynamic constraints and experimental observations on the distribution of melt in a non-deforming rock, that is, in a partially molten rock exposed simply to a hydrostatic state of stress. The second section then builds on these boundary conditions to introduce theoretically predicted and experimentally determined flow laws describing plastic deformation of partially molten rocks. Finally, the third section examines the influence of deformation (i.e., a non-hydrostatic state of stress) on melt distribution and the associated implications for the rheological properties of partially molten crustal and mantle rocks.
MELT DISTRIBUTIONS IN NON-DEFORMING ROCKS
Under a hydrostatic state of stress, the melt distribution in a partially molten rock is governed by the relative values of the solid-melt and solid-solid interfacial energies, γsm and γss. (NB: Although the term melt is used throughout this paper, the designation fluid or liquid could equally well be substituted.) The melt distribution …