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).