We explore an alternative interpretation of the centrifuge data in which
the low melt fraction (< 0.3) centrifuge results are assumed
anomalous compared to deformation experiments on partially molten rocks.
Under this assumption, we suggest that the compaction in the centrifuge
experiments at intermediate melt fractions (> 0.3) is
accommodated primarily by particle rearrangements (repacking) with minor
GBD contribution. This approach allows us to test whether the
discrepancy between the centrifuge data at intermediate melt fractions
and earlier rock mechanics experiments at lower melt fraction can be
caused by a transition from repacking to GBD as the rate-limiting
compaction mechanism with decreasing melt fraction. It is important to
note that while this interpretation would provide a framework to
characterize compaction over a wide range of melt fraction, it does not
reconcile the discrepancy between the low melt fraction centrifuge data
and previous rock mechanics experiments.
The distinction between GBD and repacking is important as upscaling the
rheology to natural conditions in silicic, crustal magma systems
involves different grain size dependence and therefore will yield
significantly different melt extraction timescales. At melt fractions
below 0.3, effective compaction rates (or compaction viscosities)
inferred from the centrifuge experiments (Bagdassarov et al. ,
2009, Connolly et al. , 2009) and measured by deformation
experiments (Allan et al. , 2013, Hirth & Kohlstedt, 1995, Meiet al. , 2002, Renner et al. , 2003, Scott & Kohlstedt,
2006, Zimmerman & Kohlstedt, 2004) display several orders of magnitude
of difference. While we do not directly address compaction at melt
fractions below 0.3, given the agreement between the experiments
designed to directly measure compaction rate (Allan et al. , 2013,
Hirth & Kohlstedt, 1995, Mei et al. , 2002, Renner et al. ,
2003, Scott & Kohlstedt, 2006, Zimmerman & Kohlstedt, 2004), we make
the assumption here that they provide reliable estimates of compaction
rates at low melt fraction (Fig. 1 ). Because of the limited
amount of experimental data published for compaction rates at melt
fraction > 0.3, we use both the olivine centrifuge
experiments at intermediate melt fractions and the repacking experiments
of Hoyos et al. (2022) to assess the role of repacking on compaction in
this regime. This suite of experiments consists of olivine-MORB melt
aggregates that are compacted in a centrifuge apparatus. Crystal-melt
separation initially occurs by sedimentation followed by crystal matrix
compaction. Interestingly, the melt fractions in this suite of
experiments remains above the minimum melt fraction of ca. 0.3,
consistent with the minimum melt fraction obtained in natural silicic
systems (Lee & Morton, 2015). These observations suggest that the
physics governing the minimum amount of trapped melt in the centrifuge
experiments and in silicic, crustal magma systems reflect the same
processes.
The hypothesis that compaction in the centrifuge experiments may be
accommodated by repacking is inspired by a suite of analog experiments
on suspensions performed at or near ambient pressures and temperatures
(Boyer et al. , 2011, Cassar et al. , 2005, Costa et
al. , 2009, Faroughi & Huber, 2015, Hoyos et al. , 2022, Jopet al. , 2006). For instance, the experiments of Boyer et
al. (2011) included applied shear stresses to suspensions housed in a
shear cell and revealed that at intermediate fractions (ca. 0.4 – 0.6),
phase separation is dominated by a combination of hydrodynamic and
particle-particle interactions (repacking).
The purpose of this paper is first to establish whether continuum models
of compaction can be used to model repacking phenomena (section 2.2).
The second objective is to constrain the rheology associated with
repacking by developing a compaction model and applying it to the high T
+ P centrifuge experiments and to the low T + P experiments of Hoyoset al. (2022) (section 3). The experiments of Hoyos et al.(2022), unlike those of Boyer et al. (2011), were performed under
conditions of uniaxial compression with no macroscopic shear component.
We demonstrate that a transition between compaction by repacking to
grain boundary-controlled diffusion near the maximum packing of the mush
can explain the sharp increase in effective resistance to compaction
between the centrifuge and partially molten rock experiments and is
consistent with petrologic observations in natural magma chamber
systems. We then simulate a composite grain boundary-controlled
diffusion and repacking composite rheology and apply the composite
rheology to high strain rate transients measured in Renner et al.(2003). Finally, we extrapolate our results to conditions relevant to
silicic, crustal magma systems in the continental crust.