Fluid inclusion planes

Fluid inclusions consist of nm to mm sized cavities filled by fluids, which may also contain solid material. They occur as isolated inclusions, in clusters, and in planes. They are common in thin section, where they may appear as dark in-clusions which reveal their fluid contents on examination at higher magnifications. The most common fluids are aqueous, saline or with possible admixtures of

sulphur compounds, or more complex hydrocarbons.

An important distinction is made between primary fluid in-clusions that form during growth of the original minerals of a rock, and secondary inclusions that form subsequently (e.g.

Roedder 1984). Primary inclusions can be recognized be-cause they are disposed on euhedral crystal forms, whereas secondary inclusions cut across crystal growth features.

Fluid inclusion planes (FIPs) are planes of fluid inclusions that often have a strong preferred orientation on a microscopic scale, and may also have regionally consistent orientations (Fig. 3.13, Fig. 2.5).

The fluids in any set of FIPs are generally composition-ally homogeneous, and may be distinct from fluids in other sets of FIPs that occur in the same rock. FIPs form by trap-ping of a fluid during precipitation of microcrack fillings.

The microcracks are usually extension microcracks formed by the mechanisms described in Section 2.3. Healing of mi-crocracks in quartz can occur as rapidly as micrometres/day (Smith and Evans 1984). The healing rate depends on tem-perature, concentration of the fluid, and microcrack di-mensions (e.g. Brantley 1992). Healing leaves a plane of cylindrical or spherical fluid inclusions along the former mi-crocrack, firstly by forming cylindrical tubes of fluid paral-lel to the microcrack tip (necking down), and pinching off or ovulation to form approximately spherical or negative crys-tal shapes (Fig. 3.14). Isolation of spheres occurs because grain boundary migration rates depend on the thickness of the fluid phase (the microcrack can be considered as a type of grain boundary): slower migration rates occur in thicker fluid films. A local thickening of the microcrack will slow

34 CHAPTER 3. DIFFUSIVE MASS TRANSFER BY SOLUTION

the grain boundary migration rate at that point, isolating a fluid inclusion behind the rest of the more rapidly migrating boundary (e.g. Urai 1983). FIPs are primary evidence for the importance of fluids and DMT during deformation. They form perpendicular to and can be very useful in kinematic analysis, because they can constrain both the orientation of and the fluid pressure (e.g. Lespinasse and Cathelineau 1995). They can also give evidence for fluctuating fluid pres-sures associated with stress cycling and therefore probably with earthquake faulting (e.g. Robert et al. 1995).

3.12 Microveins

Microveins are microscopic tabular zones of secondary min-eral growth. The rich variety of microvein textures observable under the microscope can allow very detailed interpretations of their formation. CL is valuable for analysis of microveins, because variations in fluid chemistry and temperature during microvein filling can be reflected in the luminescence, delin-eating delicate growth features (e.g. Dietrich and Grant 1986, Urai et al. 1991).

One of the most important features of a microvein is the orientation of the opening vector, the vector which connects points that were originally joined before microvein opening.

The opening vector may be perpendicular to the wall (an extension microvein) or have components of displacement both perpendicular and parallel to the walls (a shear micro-vein). The opening vector can be determined from displaced markers, or from some features of microvein fillings, as de-scribed below. The opening vector can be considered for the net opening history of the microvein (the cumulative open-ing vector) or for individual increments of openopen-ing (the in-cremental opening vector).

Common microvein fillings are carbonates, quartz, chlor-ite and epidote. The texture of the filling may be massive, equant (blocky; Plate 20), fibrous, laminated, euhedral (idio-morphic), or botryoidal. Filling textures are a function of nuc-leation and growth kinetics, the rate of microvein opening, the geometry of the opening vector and the geometry of the walls.

Euhedral crystal terminations and botryoidal textures are dia-gnostic of growth into open space, and are useful in ore par-agenetic studies for discriminating epithermal environments.

Euhedral crystal faces can be recognized by growth zoning in the crystal, which may be defined by fluid or solid inclusions.

Wilson (1984) suggests that considerable variation in crystal

orientation and grain size is characteristic of such “free-face”

growth.

In addition to the major filling phases, many microveins also contain inclusions of the same mineralogy as the wall rock. Lines of inclusions parallel to the microvein margins are known as inclusion bands, while those at higher angles to the margins are inclusion trails (Figs. 3.15, 3.16).

Inclusion bands and trails may form by two mechanisms:

overgrowth of wallrock fragments, and fracturing of the wall rock followed by incorporation of fragments into the filling.

The latter mechanism may occur where irregularities on mi-crovein walls obstruct opening, and must be broken off for opening to occur (e.g. Urai et al. 1991). The presence of numerous inclusion bands is evidence for a cyclic process of microcrack opening followed by filling, a process known as crack-seal (Ramsay 1980). Each inclusion band represents one cycle. The width between inclusion bands is

for many rock types, and there may be up to thousands of bands in a microvein (Ramsay and Huber 1987). Inclusion trails are markers of the position of particular points on the microvein margin at successive opening positions: they are therefore parallel to the opening vector (Fig. 3.15b).

Paradoxically, microstylolites sub-parallel to microvein margins have also been described, particularly along inclu-sion bands (e.g. Cox 1987). The shortening demonstrated by such microstylolites could be part of the crack-seal cycle if fluid pressures decreased to less than lithostatic in part of the cycle, causing the microvein to close and experience com-pressional stress. These microstylolites may also be due to a later deformation, unrelated to the microvein formation.

Laminated microveins have planar bands, often composed of phyllosilicates, sub- parallel to the margins. The bands

36 CHAPTER 3. DIFFUSIVE MASS TRANSFER BY SOLUTION

may form as inclusion trails where the opening vector has a large component parallel to the margin (Cox 1987). In this situation there is no distinction between inclusion trails and bands.

Fibrous microveins are particularly rewarding for kin-ematic interpretation. Fibrous fillings are common and form by progressive microvein opening at a rate that can be matched by crystallization of the filling. Fibres can grow by at least five different filling sequences: these need to be carefully established before kinematic interpretations can be made. Syntaxial growth means growth from the wall rock towards the microvein centre on both sides of the fracture (Fig. 3.17a, Plate 21). The diagnostic features of syntaxial growth are two separate bands of fibres on either side of a central suture. The fibres are not continuous across the suture.

The microvein fill is similar to the wall rock and may be in crystallographic continuity with it. Fibre widths generally in-crease in the direction of growth due to competition between fibres, which ensures that the faster growing and therefore larger fibres overgrow and eventually isolate the slower and smaller fibres from the precipitating solution (e.g. Smith 1964). Therefore in syntaxial growth, fibre widths may in-crease towards the suture. By contrast, antitaxial growth oc-curs from the microvein towards the wallrock, and may occur symmetrically on both sides of the microvein (Fig. 3.17b) or asymmetrically on only one side (Fig. 3.17c, 3.18). The dia-gnostic feature of antitaxial growth is fibre continuity across the microvein. Antitaxial growth is characterized by fillings of different material from the wall rock, and no crystallo-graphic relationship between the filling and wall rock.

Sym-metric antitaxial growth may result in a median line of inclu-sions along the centre of the microvein, unlike asymmetric growth. Fibre widths can also be used to distinguish sym-metric growth (fibre widths increase symsym-metrically in two opposite directions towards the wall rock) from asymmetric growth, in which the widths increase unidirectionally across the microvein. Composite growth means both syntaxial and antitaxial protions in the same microvein (Fig. 3.17d). Non-systematic growth (“ataxial” - Passchier and Trouw 1996) histories may involve fracture at any point in the crystal fibres, which are termed stretched crystal fibres by Durney and Ramsay (1973). They exhibit none of the systematic fea-tures described for the other four categories of growth above (Fig. 3.17e), and the sides of the fibres have distinctive in-terlocking teeth, dividing then fibres into tablets (Plate 22).

The lack of directional growth indicators (e.g. fibre widening direction) in these microveins is diagnostic.

In many fibrous microveins, the fibres grow parallel to the incremental opening vector: Such tracking or displacement-controlled fibres can be used to deduce incremental strain histories with great effect. They can be recognized because fibres connect markers across the microvein, because they are parallel to inclusion trails (Urai et al. 1991), or because they have a constant orientation between microvein walls of variable shape (Plate 23). The strain history deduced from the fibres can be plotted on a diagram showing rotation of the incremental strain on the horizontal axis against strain on the vertical axis (a cumulative incremental strain history or cish diagram, e.g. Fisher and Anastasio 1994, Hedlund et al.

1994).

38 CHAPTER 3. DIFFUSIVE MASS TRANSFER BY SOLUTION

Non-tracking fibres do not track the incremental opening vector (e.g. Durney and Ramsay 1973). Non-tracking fibres can be recognized because they do not connect markers across the microvein (Fig. 3.19). However, markers may not be connected even by tracking fibres if a micro-vein has an early history of shear displacement before filling

occurred. Inclusion trails may be used to distinguish track-ing and non-tracktrack-ing fibres in this case. Non-tracktrack-ing growth occurs because fibre growth directions are determined by the orientation of the growth surface, like face-controlled pres-sure fringes. Fibre boundaries are either perpendicular to the growth surface (Fig. 3.19), or along the bisector of two ad-jacent growth surfaces. The fibre boundaries have no fixed relationship to the opening vector. The tracking efficiency, or degree of match between the opening vector and the fibre, is determined by the angle between the opening vector and the growth surface and the shape of the growth surface (Fig. 3.19, Urai et al. 1991). These fibres can only be par-allel to the opening vector when when the tracking efficiency is 1.

Fibres in microveins with any of the above growth histor-ies are often curved. The curvature can be primary (formed during crystal growth), or secondary (due to subsequent de-formation). Primary curvatures can form in both tracking and non-tracking fibres due to rotation of the incremental open-ing direction with respect to the previously formed part of the fibre, and can be recognized because the curved fibres show no evidence of strain or recovery.

All low-temperature microvein fillings form by precipita-tion from soluprecipita-tion, and are therefore excellent evidence for DMT via solution. Microveins are a conspicuous feature of greenschist-facies and lower grade deformation because both cataclasis (fracture) and DMT via solution are required for their formation. At higher grades, other deformation mech-anisms operate, and microvein textures have low preservation potential because of recrystallization.