6.5 Future challenges
Using numerical models to invert for the parameters relevant to the
expression of the effective matrix viscosity is a powerful approach to
test the mechanisms that are at play during compaction in mushes and
quantify extraction rates. However, more experiments, at both low and
high T + P conditions, must be performed to investigate how the rheology
of compacting systems vary systematically as a function of particle
shape, size, and distribution. Further experimental work is also
required to investigate the partitioning of sliding between viscous and
friction endmembers and their respective grain size and temperature
dependence.
Melt loss signatures in plutons have been investigated at the grain and
field scale using a suite of investigative techniques. Signatures of
melt loss solely by compaction include (1) large differences in bulk
SiO2 in samples from the base to the top of the
intrusion, while the matrix forming phenocrysts population remains
relatively constant, (2) gradients in melt fraction sensitive trace
elements, and (3) variations in abundance of late-stage crystallizing
phases associated with the development of crystal or shape preferred
orientation in matrix forming phenocrysts (Bachl et al. , 2001,
Claiborne et al. , 2006, Cornet et al. , 2022, Fiedrichet al. , 2017, Garibaldi et al. , 2018, Hartung et
al. , 2017, Tegner et al. , 2009, Walker Jr et al. , 2007).
Furthermore, physical textures suggestive of compaction include shape
preferred orientation in matrix forming crystals if compaction was
accommodated by repacking (Fiedrich et al. , 2017, Garibaldiet al. , 2018), or zoning truncations and overgrowths encroaching
into locally melt-rich areas when accommodated by GBD (Holness et
al. , 2017). A combination of field, experimental, and modeling analyses
is key to provide an accurate physical lens to interpret chemical trends
in magmatic systems. If plutonic systems fail to record chemical and
physical signatures consistent with compaction by repacking or GBD,
other mechanisms may be required to explain efficient fractionation and
melt segregation in magma reservoirs (i.e., melt-rock reactive transport
(Pec et al. , 2020) or crystal hindered settling).