3.2.4.6 Coarsening in almost completely solid materials: metamorphic
and sub-liquidus plutonic rocks
Coarsening occurs in metamorphic rocks where it is commonly referred to as
‘grain-boundary area reduction’ (GBAR) or Ostwald ripening. In deformed
metamorphic rocks the major difference from igneous rocks is that excess energy
is derived from unsatisfied bonds both at the surface of the crystals and around
defects within the crystals. The balance of these factors is controlled by the grain
size distribution, grain shape and defect concentration. All will be reduced as
the texture equilibrates (see Sections 3.2.5 and 4.2.2). Coarsening can occur
during deformation, but its effect is counteracted by grain size reduction
processes. GBAR is most evident in rocks that have undergone ‘static recrystal-
lisation’, generally after deformation has ceased. If the process is to produce
measurable effects then a fluid is necessary: one dominated by water at lower
temperatures and by silicate at higher temperatures.
Coarsening of crystals suspended in a liquid, or free to move, is clearly very
different from coarsening in a material that is almost completely solid: the
presence of an independent second phase (or phases) may ‘pin’ the position of
the grain boundary and limit coarsening in that direction (Berger, 2004,Berger&
Herwegh, 2004). Such a situation occurs in all polymineralic rocks and may be
important even in almost monomineralic rocks that only contain a small
amount of a second phase. This effect is sometimes referred to as Zener-
influenced coarsening. Modelling of the influence of second phases suggest
that such CSDs have a lognormal distribution, rather than the Gaussian
distribution produced by the LSW models.
The physical processes governing GBAR are identical to those of coarsening
described previously. However, the saturation temperature of a phase in a
metamorphic rock does not correspond to its liquidus temperature in magma,
but to its upper stability limit. Hence, coarsening should be most important
at higher temperatures, close to the stability limit of the phase. In metamorphic
rocks kinetic effects must always be considered, particularly where fluid is not
abundant. Hence, there has been much discussion concerning the applicability
of coarsening and GBAR (Miyazaki, 1996, Carlson, 1999, Miyazaki, 2000).
3.2.5 Crystal deformation and fragmentation
3.2.5.1 Crystal deformation
Solid-state mechanical deformation of crystals and subsequent recovery of
strain energy can result in grain size reduction (e.g. Karato & Wenk, 2002).
This process differs from cataclasis in that at no point are the crystals
68 Grain and crystal sizes