7. Conclusions
Our numerical analysis of laboratory experiments suggest that in
magmatic systems at intermediate melt fractions (ca. 0.3 – 0.6), melt
loss accommodated by repacking is likely a more efficient process than
grain-boundary diffusion-controlled creep until the melt fraction
diminishes towards the maximum packing fraction. We find that
particle-particle friction exerts a dominant control over the
dissipation of energy associated with compaction in this regime. At melt
fractions below the maximum packing fraction, individual grains
comprising the matrix are no longer able to rotate or translate freely,
and further melt redistribution should be accommodated by grain-boundary
diffusion-controlled creep or possibly other intracrystalline creep
processes. However, the low T + P experiments of Hoyos et al.(2022) demonstrate that force chains can in some cases reduce the
ability of granular media to repack and introduce ephemeral,
intermediate jamming states. A consequence of jamming under high enough
T + P conditions could be to augment momentarily the melt fraction at
which the rheology of compacting columns transition from repacking to
grain-boundary diffusion-controlled creep. Constraining the rheology of
granular media when deformation is accommodated by repacking and when it
is accommodated by grain-boundary diffusion-controlled creep is crucial
to understand how melt is extracted in magmatic systems. Furthermore,
the control exerted by particle shape and size distributions on jamming
may have important ramifications for how melt is accommodated by
repacking versus grain-boundary diffusion-controlled creep.
Geologically, in regions of partially molten rock where melt is largely
redistributed by grain-boundary diffusion-controlled creep, its
fingerprint is recorded by virtue of crystal overgrowths, zoning
truncations, etc. (Holness, 2018). In regions where, instead, melt is
redistributed largely by repacking, such signatures may be absent
(Holness, 2018), weak, or in isolated regions where ephemeral jamming
states persisted due to the buildup of force chains. Several plutons in
the geologic rock record have been investigated and show evidence of
significant phase separation (Cornet et al. , 2022, Fiedrichet al. , 2017, Hartung et al. , 2017, Lee & Morton, 2015,
Tavazzani et al. , 2020). Such evidence includes a plethora of
chemical compositions along unmixing lines between chemical endmembers
(cumulate and extract) (Cornet et al. , 2022, Hartung et
al. , 2017, Schaen et al. , 2018, Tavazzani et al. , 2020),
shape-preferred orientation in matrix forming crystals that increases in
intensity towards the base of the pluton (Fiedrich et al. , 2017,
Garibaldi et al. , 2018), and gradients in melt fraction obtained
by trace elements or modal mineral abundance (Fiedrich et al. ,
2017, Gelman et al. , 2014, Hartung et al. , 2017, McKenzie,
2011, Tegner et al. , 2009). Future work integrating numerical
models, textural and chemical sample analysis, and experiments will be
crucial to understand how melt is segregated across a wide spectrum of
melt fraction, the timescales required to construct large eruptible
magma-rich horizons, and to interpret chemical signatures of igneous
bodies.