Page 43 - Microtectonics

3.4 · Intracrystalline Deformation 31
Along grain contacts, pressure solution may occur in deformation, are hampered. However, the process may
a thin fluid film between grains (Rutter 1976), possibly also be important at higher metamorphic grade (Wintsch
enhanced by an etched network of microcracks in the and Yi 2002). The effect of pressure solution is particu-
contact surface (Gratz 1991; den Brok 1998), or it may larly clear in the development of differentiated crenula-
occur by dissolution undercutting of ‘island structures’ tion cleavage at low to medium metamorphic grade, as
that are surrounded by fluid-filled channels, forming a explained in Sect. 4.2.7.3 (see also Bell and Cuff 1989).
stress-supporting network between grains (Ray 1982; Pressure solution in quartz or calcite seems to be enhanced
Spiers et al. 1990; Lehner 1995). The dissolved material by the presence of mica or clay minerals at grain bounda-
can diffuse away from the sites of high solubility down a ries of these minerals (Houseknecht 1988; Hippertt 1994;
stress-induced chemical potential gradient to nearby sites Dewers and Ortoleva 1991; Hickman and Evans 1995).
of low solubility by stress-induced solution transfer, usu- Details of pressure solution are described in Durney (1972),
ally referred to as solution transfer. Redeposition of the Elliott (1973), Gray and Durney (1979a), Rutter (1983),
dissolved material may occur at free grain boundaries Groshong (1988), Knipe (1989), den Brok (1992, 1998),
that are in contact with the fluid. Newly precipitated Wheeler (1992), Shimizu (1995), and den Brok et al.
material may be of a different mineral composition or (1998a,b, 2002). Box 3.2 lists evidence for pressure solu-
phase as compared to the dissolved material; this is tion in thin section.
known as incongruent pressure solution (Beach 1979;
McCaig 1987). Alternatively, the fluid with dissolved 3.4 3.4
material can migrate over a larger distance and deposit Intracrystalline Deformation
material in sites such as veins or strain shadows (Chap. 6),
1
or even migrate out of the deforming rock volume . Pres- Crystals can deform internally without brittle fracturing
sure solution and solution transfer of material are domi- by movement of so-called lattice defects, a process known
nant at diagenetic to low-grade metamorphic conditions as intracrystalline deformation (Figs. 3.9, 3.10; Box 3.3).
where fluids are abundant and deformation mechanisms Lattice defects in crystals can be grouped into point de-
favoured at higher temperatures, such as intragranular fects and line defects or dislocations (Figs. 3.11, 3.13, 3.14).
Point defects are missing or extra lattice points (atoms or
Box 3.2 Evidence for pressure solution molecules) known respectively as vacancies and intersti-
tials (Fig. 3.11a). Line defects may be due to an ‘extra’ half
Evidence for the action of pressure solution is the presence of lattice plane in the crystal. The end of such a plane is
truncated objects such as fossils, detrital grains, pebbles and
idiomorphic phenocrysts (McClay 1977; Rutter 1983; House- known as an edge dislocation (Fig. 3.11b). Besides edge
knecht 1988; Figs. 3.7, 3.8, ×Photo 3.8), truncation of chemical dislocations, screw dislocations exist where part of a crys-
zoning in crystals such as garnet or hornblende (Berger and tal is displaced over one lattice distance and is therefore
Stünitz 1996) and the displacement of layering on certain planes twisted (Fig. 3.11c). Edge and screw dislocations can be
(Figs. 4.4, 4.21). In the latter case, however, the possibility of interconnected into dislocation loops (Figs. 3.11d, 3.14); they
slip along the contact should also be considered; if the contact
is indented, the displacement is most probably due to pressure are end members of a range of possible dislocation types.
solution (Fig. 4.21). Spherical grains may form indenting con- Dislocations can also split into partial dislocations, sepa-
tacts. Equally sized grains will be in contact along relatively rated by a strip of misfitted crystal lattice known as a stack-
flat surfaces, while small grains tend to indent into larger grains ing fault.
(Blenkinsop 2000). Planes on which pressure solution occurred Dislocations cannot be directly observed by optical
are commonly rich in opaque or micaceous material, which is
left behind or deposited during the solution process (Figs. 3.7, microscopy, only by TEM (McLaren 1991; Sect. 10.2.4;
4.20). A spectacular example are stylolites, highly indented sur- Figs. 3.13, 3.14, 10.11). However, they can be made visible
faces where material has been dissolved in an irregular way, indirectly by etching of pits where they transect a pol-
allowing the wall rocks to interpenetrate (Box 4.3; Fig. B.4.4). ished surface, or by decoration techniques; in olivine,
The opposite process, deposition of material from solution, decorated dislocations can be made visible by heating a
can be visible as new grains, fibrous vein fill or fibrous over- sample in an oxidizing environment (Kohlstedt et al. 1976;
growth of grains in strain shadows (Chap. 6). New grains grown
from solution may be recognised by lack of intracrystalline Karato 1987; Jung and Karato 2001).
deformation structures (Sect. 3.4), well defined crystallo- A dislocation is characterised by a Burgers vector
graphically determined crystal faces, and growth twins. New (Figs. 3.11d, 10.11), which indicates the direction and
grown rims of material in optic continuity with older parts of minimum amount of lattice displacement caused by the
a grain are also common but may be difficult to distinguish, dislocation. The Burgers vector can be imagined by draw-
except by cathodoluminescence (Sect. 10.2.1). Fluid inclusion
trails (Sect. 10.5) can also reveal the presence of overgrowths. ing a square circuit around the dislocation from atom to
atom, with an equal number of atoms on each side of the
square; in an intact crystal this circuit would be closed,
but around a dislocation the loop is not closed – the miss-
1 Something to remember when drinking mineral water. ing part is the Burgers vector (Fig. 3.11d).