Minimum trapped melt fraction and maximum packing fraction
An important result presented here is that the minimum trapped melt is ca. 30%. Lee and Morton (2015) analyzed plutons in the Peninsular Ranges Batholith and similarly relied on trace element compositions to estimate trapped melt fraction. They found similar results with 20-30% of residual melt trapped in cumulate at the root of high silica granites, arguing on that basis that hindered settling was likely responsible for melt-crystal separation. We propose that the lower limit of ca. 30% trapped melt is due to the transition in phase separation mechanisms from a hydrodynamic (melt-crystals interactions) and friction (crystal-crystal interactions) repacking dominated compaction regime to a compaction regime dominated by viscous creep such as grain-boundary diffusion-controlled creep. Repacking is efficient at intermediate melt fractions, as individual grains do not need to be deformed to accommodate pore closure (Boyer et al. , 2011). However, as the maximum packing fraction is approached, this is no longer the case and the resistance to pore space closure associated with repacking diverges (Boyer et al. , 2011). At melt fractions lower than the maximum packing fraction, pore space can only be closed by deformation of grains. Pore closure at melt fractions lower than the maximum packing fraction is much less efficient than pore space closure by repacking at intermediate melt fractions. This common minimum trapped melt fraction inferred for SMB and the Peninsular Ranges Batholith suggests that once compaction by repacking reaches the maximum packing fraction (around 0.3 trapped melt), further compaction involves processes that are too slow to be active before the thermal death of these plutons.