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.