Textural variations in Fluid Inclusions

Fluid inclusions are a common feature of minerals. When a mineral grows in the presence of a fluid phase, some of the fluid may be trapped as imperfections in the growing crystal to form fluid inclusions. Alternatively, tiny blebs of fluid can become trapped in healed fractures within a crystal. These inclusions normally range in size from <5 m to >100 m and are usually only visible in detail by microscopic study and in transparent minerals like quartz the inclusions visible in naked eye, but they are rare. Roedder (1984) and some other workers use the term fluid inclusion to describe only those inclusions, that have trapped a fluid and have remained in the fluid state during cooling to ambient temperatures. The broader and more modern approach that will be followed here is to use the term 'fluid inclusion' to refer to any inclusion that trapped a phase that was a fluid at the temperature and pressure of formation, regardless of the phase state of the inclusion as observed at laboratory conditions.

The minerals in which fluid inclusions can be observed are theoretically, all minerals deposited from the fluids. But there are certain minerals which will be opaque to the visible range of the light. Thus only minerals that are transparent to the visible range of light can be used to see the fluid inclusions. Mostly inclusions are studied in quartz, fluorite, halite, calcite, apatite, dolomite, sphalerite, barite, topaz and cassiterite and even in some are metamorphic minerals like garnet and cordierite.

Most ore minerals like sphalerite, cassiterite, wolframite and sulphide minerals like pyrite, molybdenite, stibnite are opaque under visible light, but actually they are transparent under near-infrared light. The use of infrared microscopy for microthermometric analyses of fluid inclusions in ore minerals was introduced by Campbell et al. (1984) and later subsequent studies were carried out by Campbell and Robinson-Cook (1987). Such studies of fluid inclusions in opaque ore minerals were used to obtain reliable information about depositional conditions of ore bearing fluids (Rosière and Rios, 2004; Ni, P. et al., 2008; 2015).

The relationship between fluid inclusions and the host rock is the subject of “fluid inclusion petrography”. Inclusion assemblages/types may be used to infer the timing of formation of an inclusion/ a group of inclusions relative to the timing of formation of other groups of inclusions and in some cases relative to the growth of the crystal (Roedder, 1984).

The Classification schemes relate the timing of formation of the fluid inclusion (FI) relative to that of the host mineral and later stress conditions in the system. (Bodnar, 2003). The major classification of the fluid inclusions is based on (1) when they were trapped relative to mineral growth, and (2) room temperature phase properties.

Based on their origin, FI are of following types:

Primary: fluid inclusions which were trapped during the formation of the enclosing crystal are primary in origin. They are generally trapped along the growth zones and crystal faces, or tends to occur solitary or isolated.

Secondary: fluid inclusions which are trapped in fractures which developed after the formation of host mineral and caught due of healing of fractures. These inclusions occur as trails or clusters, which often cut across the grain boundaries.

Pseudosecondary: Pseudosecondary inclusions are formed by the healing of fractures in the minerals during the growth of the mineral. These inclusions occur along trails that end within the grain interior or one of the growth zones.

Microthermometry involves freezing and heating (liquid N2 temperature to 600ºC) of FI (simple or sequential runs) while observing phase changes through a binocular microscope or on a computer screen, thereby determining the fluid composition, salinity and density. It is simple, cheap but tedious job that enables determination of gross chemistry and physical properties of the trapped phases. In Geological Survey of India, Heating cum Freezing Stage are used in many applications to obtain Micro Thermometric data from FI where controlled heating/freezing rates and reproducibility are needed. As a normal working protocol, the fluid inclusions are cooled to the lowest temperature that the stage can achieve and subsequently, the phase changes taking place are observed while the temperature rises towards room temperature again. The first change of phase that should be observed in the liquid phase is the appearance of the first liquid in the cavity (eutectic temperature). This temperature is characteristic of each system and it will help us to find the composition of the fluid inclusions. Further heating of the fluid inclusions, where all the ice will melt and become liquid and this temperature provides the salinity of the fluid inclusions. Again the inclusions will be further heated up the all phase present in the inclusion will become homogenized and also in some cases solid dissolution and this temperature provides the homogenisation temperature and pressure.

The data obtained can be used with the help of the experimental data in pertinent systems to constrain the chemical (salinity, gross chemistry) and physical (density, temperature and pressure) parameter of the fluids. The constraints are in general, semi quantitative for comparing complex multi-component natural fluids to simplified experimental systems.

A Raman Spectroscopy is a high-sensitivity rapid, non-destructive vibrational spectroscopic technique based on inelastic scattering or Raman scattering of monochromatic light, usually from a laser source and is utilized for fingerprinting chemical species. Raman spectroscopy can be very effective in fluid inclusion identification.

Quartz is one of the most common minerals in the Earth’s crust and occurs in a wide variety of geological environments including many igneous and metamorphic rocks. It is generally colourless and transparent or translucent and belongs to the trigonal crystal system.

The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted or more commonly intergrown with adjacent crystals of quartz and other minerals as to only show part of this shape or even to lack obvious crystal faces. Quartz has the lowest potential for weathering in the Goldich dissolution series and consequently it is very common as residual mineral in stream sediments and residual soils. Generally a high presence of quartz in sedimentary rock suggests a “mature” rock. Quartz is also very common in hydrothermal systems and veins. Quartz often contains well preserved fluid inclusions.

Primary assemblages indicate that the fluid inclusions were formed during the growth or recrystallization of the host mineral. They are generally trapped along the growth zones and crystal faces or tends to solitary or isolated. Primary fluid inclusions are formed at the time of mineral growth. These inclusions show assemblages such as isolated fluid inclusion and clustered fluid inclusions.

Isolated primary fluid inclusions are thought to represent the earliest generation of inclusions and the term “primary” is often used to denote an isolated fluid inclusion or occur randomly in three dimensional networks.

The clustered inclusions are typically comprised of a group of 10-20 neighbouring inclusions and such clusters may have quite different origins such as neighbouring isolated cavities, or by transformation of a former larger cavity.

Secondary fluid inclusions are trapped in the fractures which developed after the formation of host mineral and trapped due of healing of such fractures. These trail-bound/secondary inclusions either remain confirmed to a single mineral or cut across different grains or phases. These inclusions are far more abundant than early, isolated or clustered ones. Depending upon the observed trails in minerals, different terminologies are used.

Trans granular: A trail of fluid inclusions cutting across different mineral grains in a field view.

Intergranular: A trail of fluid inclusions crossing the grain boundary and continuing into another mineral grain. The fluid inclusions often remain confined to a certain textural feature, and are said to ‘decorate’ different inter-grain textures.

Cleavage Plains : Fluid inclusions occurring along the cleavage plains.

Deformation Lamellae : the fluid inclusions that are confined to and occur along deformation lamellae.

Deformation Bands: the fluid inclusions occurring along the deformation bands.

Deformation of grain Boundaries: deformation of grain boundaries through grain boundary migration may also lead to inclusion of fluids that were present along grain boundaries as fluid inclusion arrays and also the fluid inclusions that are confined to grain and sub-grain boundaries.

Twin lamellae : the fluid inclusions which occur along the twin lamellae.

Pseudosecondary inclusions are formed by the healing of fractures in the minerals during the growth of the mineral. These inclusions occur along trails that end abruptly against grain boundaries or one of the growth zones. These trail-bound inclusions either remain confined to a single mineral. An Intragranular fluid inclusions trail either remains (i) confined to a crystal interior or (ii) remain within grain-boundary and crystal interior or (iii) continues from grain-boundary to grain boundary.

The intersection features in trails of homogeneous secondary inclusions are observed and these intersections of trails are correlated with the chronological implications.

Originally trapped fluid inclusions are sometimes modified because of several processes subsequent to their trapping. These modifications cause reduction in their volume, change in morphology and often loss of originally trapped fluid. It is very important to identify such modifications and study them (Sterner and Bodnar,1987; Bekker and Jensen, 1990; Cordier et al., 1994; Parnell, 1994). Commonly known mechanisms for modifications of fluid inclusions are discussed below:

Crystallization of fluids and recrystallization of solids trapped within fluid inclusions can lead to substantial change in the fluid inclusion morphology and while process of healing cracks or healed fractures, it is common to see that some are have negative crystal shape and some do not. So negative-crystal-shaped inclusions have been interpreted based on their location and/or distribution within the crystal. Even mainly on the results of crystal growth and some synthetic fluid inclusion studies, it is well known that secondary inclusions with a negative-crystal-shape are also common (cf. Sterner & Bodnar, 1984).

Necking-down is a typical dissolution-precipitation process. This phenomenon of necking-down corresponds to the evolution of decreasing temperature, of a large tubular inclusion into a series of small inclusions, which are initially connected by capillaries.

Due to changes in pressure-temperature conditions, large fluid inclusions get busted or ‘decrepitated’ into smaller inclusions. When pressure increases (overpressure), the decrepitation is said to be ‘explosion decrepitation’.

Some fluid inclusions shows numerous microtubes or micro fractures, which radiate from the walls to give a hairy appearance and these features called as “annular” which is also a explosion texture (Vityk and Bodnar, 1995 ).

Similarly, when the pressure is considerably reduced (under pressure), the inclusion gets collapsed. Such decrepitation is known as ‘implosion decrepitation’.

Calcite is the most stable polymorph of calcium carbonate (CaCO3) and very common non-silicate rock-forming mineral which frequently contains well preserved fluid inclusions. Primary, secondary and pseudo secondary fluid inclusions can easily be identified in calcite. Also originally trapped fluid inclusions are sometimes modified because of physical processes subsequent to their trapping. These modifications cause reduction in their volume, change in morphology and often loss of originally trapped fluid.

The following are some representative photographs of fluid inclusions in calcite.

Different types of fluid inclusions have been studied in orthoclase feldspar (KAlSi3O8). This section presents a series of microphotographs of textural variants of fluid inclusions in orthoclase.

Barite occurs in many depositional environments, and can be genetically biogenic, hydrothermal or evaporation related. Barite has been reported from lead-zinc deposits, from hot spring deposits and in association with hematite ore. It is often associated with the minerals anglesite and celestine. It has also been identified in meteorites. This section presents a series of microphotos of textural variants of fluid inclusions in barite.

Beryl is a hexagonal beryllium aluminium cyclosilicate. Pure beryl is colourless, but being frequently tinted with minor impurities, beryl can exhibit colour variations from green, blue, yellow to red (the rarest). There are also several important gemstone varieties of beryl that include emerald and aquamarine. This section presents a series of microphotos of textural variants of fluid inclusions in beryl.

Light transmissivity of garnet can range from the gemstone-quality transparency to the opaque varieties used for industrial purposes as abrasives. This section presents a series of microphotos of representing textural variants of fluid inclusions in garnet.

Sphalerite is an important ore mineral for zinc. This section presents a series of micro photos on textural variants of fluid inclusions in sphalerite.