Page 56 - Microtectonics

44 3 · Deformation Mechanisms

Box 3.7 Evidence for solid-state diffusion creep and grain boundary sliding
A process like solid-state diffusion creep is expected to leave few Linking up of grain boundaries along several grain widths
traces. Therefore, few microstructures have been proposed as evi- (White 1977; Stünitz and Fitz Gerald 1993; Zelin et al. 1994).
dence for diffusion creep. The process may give rise to strongly Diamond-shaped or rectangular grains formed by straight and
curved and lobate grain boundaries between two different min- parallel grain boundary segments, often in two directions
erals at high-grade metamorphic conditions (Fig. 3.33; Gower and throughout a sample (Lister and Dornsiepen 1982; Drury and
Simpson 1992). Another possible effect is the erasure or modifi- Humphreys 1988; Fliervoet and White 1995; Hanmer 2000). This
cation of chemical zoning or fluid inclusion density and content is also known as a reticular grain aggregate. Such boundaries
in grains. Ozawa (1989) suggested that sector Al-Cr zoning ob- are especially conspicuous in monomineralic aggregates of
served in spinel grains in peridotite is formed by unequal diffu- minerals such as quartz or calcite, for which this structure is
sivity of these ions when spinel is deformed by solid-state diffu- unusual.
sion creep in the Earth’s mantle. The presence of diffuse contacts between strongly flattened
Grain boundary sliding has also been suggested as a defor- fine-grained monomineralic aggregates of two different min-
mation mechanism in rocks but is equally elusive. Solid-state erals. This may be a mixing effect of grain-boundary sliding
diffusion creep combined with grain boundary sliding may pre- (Tullis et al. 1990; Fliervoet et al. 1997; Hanmer 2000; Brodie
vent development or cause destruction of a lattice-preferred ori- 1998b).
entation. If a fine-grained mineral aggregate has undergone high The presence of anticlustered distribution of mineral phases
strain but consist of equant grains and lacks a clear lattice-pre- in a deformed fine-grained aggregate (Boullier and Gueguen
ferred orientation, or has a lattice-preferred orientation that 1975, 1998a; Rubie 1983, 1990; Behrmann and Mainprice 1987;
cannot be explained by dislocation activity, this may be taken as Stünitz and Fitz Gerald 1993; Newman et al. 1999; Brodie
indirect evidence for dominant grain boundary sliding as a de- 1998b, p 403). This may be due to selective nucleation of one
formation mechanism (White 1977, 1979; Boullier and Gueguen phase in triple junction voids between grains of another phase,
1975; Allison et al. 1979; Padmanabhan and Davies 1980; Behr- formed during grain boundary sliding (Kruse and Stünitz
mann 1983, 1985; Behrmann and Mainprice 1987; Stünitz and 1999).
Fitz Gerald 1993; Rutter et al. 1994; Fliervoet and White 1995; In the TEM, possible indications for grain-boundary sliding
Fliervoet et al. 1997; Bestmann and Prior 2003). On the other are a low dislocation density in grains; a lath shape of grains,
hand, the presence of a preferred orientation cannot be used as and the presence of voids along grain boundaries (Fig. 3.36;
proof against the action of grain boundary sliding (Rutter et al. Gifkins 1976; White and White 1981; Behrmann 1985; Behr-
1994; Berger and Stünitz 1996). mann and Mainprice 1987; Tullis et al. 1990; Fliervoet and White
Other, less reliable evidence for grain boundary sliding is: 1995).

3.9 3.9 Although the three mechanisms of dynamic recrystalli-
Competing Processes During Deformation sation are described separately, there are transitions and
they can operate simultaneously under certain conditions,
At low temperature, minerals deform by brittle deforma- (Sect. 9.9; Fig. 9.10; Lloyd and Freeman 1994). In solid solu-
tion but there are many indications that pressure solu- tion minerals such as feldspars, amphiboles and pyroxenes,
tion and brittle processes occur together in low-grade however, BLG and GBM recrystallisation is not only driven
deformation. Pressure solution is slow and may not be by internal strain energy (Sect. 3.12), but also by chemical
able to accommodate faster bulk strain rates, especially if driving potentials associated with differences in composi-
diffusion paths increase in length when solution surfaces tion of old and new grains (Hay and Evans 1987; Berger
become more irregular with time, as in stylolites (Gratier and Stünitz 1996; Stünitz 1998). This is specifically impor-
et al. 1999). Fracturing can temporarily relieve stresses and tant where recrystallisation takes place at other (lower)
increase possibilities for pressure solution. Such combined metamorphic conditions as during the formation of the
slow and fast processes may also cooperate in other com- older, recrystallising minerals. Any link between tempera-
binations. Kinking and twinning commonly are associ- ture and changes in recrystallisation mechanism as outlined
ated with brittle fracturing as well. above depends on these differences in composition.
At more elevated temperature, crystalplastic deforma- There are two main types of deformation based on dis-
tion is initiated but the conditions at which this happens location creep, depending on the accommodating process
not only depend on temperature, but also on strain rate and (Sellars 1978; Zeuch 1982; Tullis and Yund 1985); climb-ac-
fluid pressure in the rock. During deformation of a crys- commodated dislocation creep (Yund and Tullis 1991) as-
talline material, continuous competition exists between sociated with SGR recrystallisation (Guillopé and Poirier
processes that cause distortion of the crystal lattice and 1979), and recrystallisation-accommodated dislocation
processes such as recovery and recrystallisation that re- creep where grain boundary migration is the accommo-
duce the dislocation density. Recrystallisation during ac- dating mechanism (Tullis and Yund 1985; Tullis et al. 1990).
tive deformation such as the BLG, SGR and GBM recrys- There are indications that, with increasing temperature, the
tallisation discussed above are known as dynamic recrys- accommodating mechanism in quartz is first BLG recrys-
tallisation (Figs. 3.27–3.33; ×Video 11.6a,b,e). Box 3.8 lists tallisation when dislocation climb and recovery is difficult,
evidence for dynamic recrystallisation in thin section. which then switches to SGR recrystallisation at the onset