Gond. Geol. Mag., V. 29(1 and 2), June and December, 2014. pp.19-28

Study of Fluid Inclusions: Methods, Techniques and Applications

K. R. Randive1*, K. R. Hari2, M. L. Dora3, D. B. Malpe1 and A. A. Bhondwe1

1Post Graduate Department of Geology, RTM Nagpur University, Nagpur, India 2Department of Geology, Pt. Ravishankar University, Raipur, India

3Geological Survey of India, NER, Shillong, India *Email: randive101@yahoo.co.in

ABSTRACTFluid inclusions are small volumes of paleofluids

trapped in minerals which provide indispensable information about geological processes, from high temperatures at depth towards low temperatures near the Earth’s surface. These inclusions are trapped gases, liquids or crystals, either trapped singularly (one-phase) or as a heterogeneous mixture of more than one phase (multi-phase) in a single cavity. Depending upon the timing of entrapment of liquid in the crystals, fluid inclusions are classified as primary, secondary or pseudosecondary. The inclusions occur either as isolated, clustered, or trail bound; those occurring in groups form the Group of Synchronous Inclusions (GSI) having similar composition and time of entrapment. The composition of trapped fluid varies greatly; commonly detected constituents include H2O, CO2, CH2, H2S, Cl, Br, F, I, N2, S, Na, K, Ca, Mg and Fe. There are several instruments used in the study of fluid inclusions, but the basic study is carried out using heating-freezing stages and Laser Raman Microprobe. The study of fluid inclusions reveal geologically important information such as temperature, pressure, salinity, density and depth of trapping; and thereby providing direct information about the conditions at which given minerals and rocks are formed.

INTRODUCTION

Fluid inclusions are inclusions in minerals that are filled with fluid (gas and liquid), and sometimes with one or more solid phases. They result from defects in crystals during their growth which lead to the entrapment of fluid in their surroundings. The trapped fluids may be liquid, vapor, or supercritical fluid, and the composition of the trapped fluid may include essentially pure water, brines of various salinity, gas or gas-bearing liquids, and silicate, sulfide or carbonate melts, among others (Roedder, 1984; Shepherd et al., 1985; Bodnar, 1994; Andersen et al., 2001; Samson et al., 2003). Compared to rock forming minerals, fluid inclusions are short-lived objects. Their formation is instantaneous, or at least very short at geological scale. Fluid inclusions are part of the rock, they occupy roughly the same volume as most accessory minerals; therefore, they are as deserving as that of the study any rock-forming mineral. The

fact that they are fluid is just a question of reference temperature, i.e. if the observation could be done close to absolute zero, everything would be solid (Tauret, 2001). Several terminologies are frequently used in the study of fluid inclusions, important ones are explained in Table 1.

Fluid inclusions occur in different patterns, e.g. they are found isolated in a host crystal trapped during initial crystallization or in trails along former micro-fractures or grain boundaries. The study of abundance, orientation and chemistry of fluid inclusions provides a history of formation and alteration over time, contributing to both fundamental processes in geology as well as exploration for mineral deposits. Although, in practice fluid inclusions are used mainly in studies related to the metallogeny of mineral deposits due to its applied character, this type of study is becoming increasingly frequent in different branches of geology and in some related sciences. Important areas of the study includes: (1) ore deposits, (2) hydrothermal environments, (3) sedimentary environment and diagenetic processes, (4) metamorphic terranes with different metamorphic grades, (5) volcanic areas, (6) mantle environments, (7) petroleum exploration, (8) tectonic history of deformed areas, (9) gemology, (10) paleoclimatology, (11) paleohydrogeology, (12) extraterrestrial environ-ments, (13) weathering of historical monuments, (14) analysis of air-inclusions in ice cores, (15) ambar, etc (Hollister and Crawford, 1981; Roedder, 1990; De Vivo and Frezzotti, 1994; Shmulovich et al., 1995; Bodnar, 2003).

CLASSIFICATION OF FLUID INCLUSIONS AND SILICATE-MELT INCLUSIONS

Classification of Fluid Inclusions

There are many ways to classify fluid inclusions (Roedder, 1984; Goldstein, 2003), but one of the most useful classification schemes relates the timing of formation of the inclusion relative to that of the host mineral (Bodnar, 2003). Based on their origin, fluid inclusions are of following types:

K. R. Randive and Others20

Table 1: Terminology commonly used in the study of fluid inclusions

Sr. No.

Terminology / phrase used Meaning and explanation of the term

1. Monophase Inclusions The inclusions having a single phase at the room temperatures, e.g. liquid, gas

2. Biphase Inclusions The inclusions having two phases in the same inclusion at room temperature, e.g. liquid + gas; liquid + crystals

3. Polyphase Inclusions The inclusions having more than two phases in the same inclusions at the room temperature; e.g. liquid + gas + crystals; liquid + vapor + solid(1)+solid(2)

4. Homogenous Inclusions The inclusions composed of a single phase

5. Heterogenous Inclusions The inclusions composed of multiple phases (more than one phase)

6. Common abbreviations Although there is no specific rule to abbreviate the commonly used terminologies; following are followed by notionV = vapor; L = liquid; S = solid; VL = vapor + liquid (liquid dominated); LV = liquid + vapor (gaseous bubble with trapped liquid); L2V = two immiscible liquids + vapor; S3LV = three different solids + liquid + vapor; use same logic with any combination of solid, liquid and gaseous phases.

7. Trapping Temperature (Tt) The temperature of trapping or formation of a fluid inclusion.

8. Homogenization Temperature (Th) The temperature at which a fluid inclusion gets transformed from a heterogenous (multi-phase) to a homogenous (one-phase) state.

9. Melting Temperature (Tm) It refers to any temperature at which, when the the inclusions are heated (often measured after attaining minimum temperature in a system by freezing, by gradually raising temperature). Of the particular interest however, are following melting temperatures, viz., eutectic temperature, initial melting temperature and final melting temperature.

10. Eutectic Temperature (Te) The minimum temperature at which liquid is stable in a specified system. In an isobaric (constant pressure) system, the eutectic temperature is a unique value at which different phases coexist. Such combination of phases coexisting at a unique temperature are said to represent “eutectic composition”.

11. Initial Melting Temperature (Ti) The temperature at which a liquid is first observed due to progressive heating of solid-bearing fluid inclusion. In gas-bearing inclusion this temperature does not necessarily correspond to the eutectic or apparent eutectic temperature.

12. Incipient Melting Temperature (Ti) It is synonymous with initial melting temperature mentioned above.

13. Final Melting Temperature (Tm) The temperature at which a solid is completely melted (dissolved or dissociated) by progressive heating of a fluid inclusion.

14. Temperature of Fusion (Tf) Known as a temperature of fusion, it is synonymous with final melting temperature discussed above. Abbreviation ‘Tf’ is also used for denoting ‘temperature of formation’; therefore one needs to be cautious of the indicated term.

15. Nucleation Temperature (Tn) The temperature of first appearance of a particular phase during microthermometry is said to be its ‘nucleation temperature’. For e.g. if a homogenous liquid cools to form a first crystal, the temperature is denoted by Tn (L LS) or simply Tn (Solid). Similarly, nucleation can also take place during heating, e.g. Tn(SV SLV)

16. Decripitation Temperature (Td) The temperature at which a fluid inclusion irreversibly ruptures, bursts, or ‘decrepitates’. This can happen either due to heating, which causes extreme internal overpressure due to fluid expansion; or upon cooling due to extreme internal overpressure caused by crystallization of ice. The decrepitation causes loss of the contents of inclusion and reduction of inclusion volume.

17. Metastability In the thermodynamic sense, “metastability” refers to a system state characterized by a Gibbs free energy that is not the lowest attainable under the specific conditions (e.g. T-Vm-X conditions). For fluid inclusions this means that another phase assemblage should be present instead of the one observed. e.g. the persistence of ice above 0°C upon heating of fluid inclusions is a cse of metastability.

18. Salinity It is the summation of total number of solutes, including electrolytes (e.g. NaCl, CaCl2) and non-electrolytes (e.g. CO2, H2S), in an aqueous solution. In the multicomponent fluid inclusions compositions are not known, therefore an indirect method is used wherein phase transitions are measured. The phase transitions are sensitive to the overall salinity (e.g. Tm(ice), Tm(hydrohalite), Tm(halite), Tm(clathrate)). In such cases, salinity is conventionally reported as NaCl or CaCl2 equivalents.

19. Equivalent weight-fraction (e.g. NaCl Equivalent)

Salinity is conventionally expressed as equivalent weight fractions. e.g. weight-NaCl-equivalent of 25% means that the inclusion shows phase transitions that are consistent with a mass fraction of 25% NaCl in the aqueous phase.

20. Pressure Correction The difference between homogenization temperature and estimated temperature of entrapment of the fluid inclusion is called as Pressure Correction.

Study of Fluid Inclusions 21

(1) Primary: fluid inclusions which are formed 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. These are very good indicators of the condition of crystallization of host minerals.(2) Secondary: fluid inclusions which are trapped in the fractures which are 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.(3) Pseudosecondary: fluid inclusions which are trapped during formation of the host minerals are referred to as pseudosecondary. These inclusions occur along trails that end abruptly against grain boundaries or one of the growth zones.

Classisfication of Silicate Melt InclusionsLinqui and Clocchiatti (1985) proposed a scheme of classification for silicate-melt inclusion based on degree of evolution achieved by the melt, as the above mentioned scheme of classification based on origin of inclusion, it may not sufficiently reflect the physico-chemical condition of formation of silicate-melt inclusions of the type occurring in volcanic setting.

The silicate-melt inclusions are classified as (1) Non-evolved: These fluid inclusions have regular cavity shapes with or without a small shrinkage bubble. The tapped silicate melt remains almost in eqillibrium with the host minerals and daughter mineral seldom nucleate. These are formed when magma crystallizes near the surface, leading to rapid cooling.(2) Slightly Evolved: These fluid inclusions too have regular cavity shapes with or without a shrinkage bubble. The epitaxial growth of host minerals like mica generally occur within inclusions, mostly adjacent to the cavity walls. Such inclusions form towards the periphery of crystal.(3) Evolved: In these fluid inclusions, the silicate melt does not remain in equilibrium with the host crystal, leading to the growth of several daughter crystals. The silicate glass occurs as a residue in these inclusions. They are trapped at a great depth under pressure exceeding to 2 kbars.

ROEDDER’S RULES

For obtaining reliable information about the original trapping conditions of fluid inclusions, either primary or secondary, following assumptions were laind down (Roedder, 1981, 1984), which are referred to as “Roedder’s Rules”.(1) The inclusions traps a single, homogenous liquid,(2) Nothing is added to, or removed from, the inclusion following trapping.(3) The inclusion volume remains constant following trapping, i.e. represent an isochoric system.

If the effects of pressure are insignificant or known; the origin of fluid inclusion is known (by detailed fluid petrography); the determination of temperature of homogenization (Th) are both precise and accurate.

CRITERIA FOR RECOGNITION OF FLUID INCLUSIONS

The fluid or melt inclusions are commonly identified as primary or secondary on the basis of detailed petrological study. Primary inclusions are to be identified accurately, all other inclusions becomes secondary. Zoned crystals provide important guides for identifying primary fluid inclusions, important criteria are listed below (Van Den Kerkhof and Hein, 2001).

Single Crystal

(1) The inclusions or group of inclusions in three dimensional space are oriented parallel to the crystal margins (Fig. 1a)(2) Isolated inclusions usually occurring away from the cluster of other inclusions. Distance is usually > 5X diameter (Fig. 1b).(3) The large inclusions in relation to host crystals(4) Those inclusions that contains a solid phase, which also occurs as solid inclusion in the host crystal.

Zoned Crystals

(1) Inclusions which occur in the core of prismatic or columnar crystals

Fig.1. a) Crystal showing fluid inclusions developed during formation of crystal: (I) Isolated, (II) inclusions parallel to growth zone, (III) Inclusions inside the growth zone, and (IV) Inclusions crossing across the growth zone; b) Isolated two-phase inclusions; c) Inclusions forming cluster within the crystal; d) Inclusions forming trails

(2) The inclusions which occur along the inter-section of growth planes.(3) The inclusions which are related to skeletal or spiral growth.(4) Those inclusions that occur in the growth zones.(5) The fluid inclusions, which are caused by solid or melt inclusions or intergrowth.(6) Those inclusions that occur along the healed crack of an older growth zone.(7) Those inclusions that occur after solid or melt inclusion or intergrowth.

FLUID INCLUSION ASSEMBLAGE (FIA) / GROUP OF SYNCHRONOUS INCLUSIONS (GSI)

Goldstein and Reynolds (1994) introduced the concept of Fluid Inclusion Assemblage (FIA) to describe a group of fluid inclusions that were all trapped at the same time. An FIA thus defines most finely discriminated fluid inclusion trapping event that can be identified based on petrography (Goldstein, 2003).This implies that the inclusions in the FIA were all trapped at approximately the same temperature and pressure, and all trapped a fluid of approximately the same composition. Therefore, the FIA represents a ‘fluid event’ in the history of the system, and the fluid in the inclusions making up the FIA represents the fluid that was present during that event (Bodnar, 2003). Similar to this, Fonarev et al. (1998) proposed that, a Group of Synchronous Inclusions (GSI) corresponds to a limited number of (typically between 10 and 20) of inclusions formed at the same time, which will serve as the test population to evaluate the homogeneity of microthermometric data (notably Th).

FLUID TYPESA fluid type is characteristic of a group of

inclusions, which either occur isolated, form clusters or trails. They are composed of similar type of fluids and trapped at the same time. Touret (2001) defined the fluid type as follows: “a set of inclusions having (roughly) the same chemical composition (e.g. low salinity, CO2-rich, etc.), eventually variable density (in nature, a sufficiently great number of inclusions will never have the same density), approximately trapped at the same time (the term ‘approximate’ indicates that the precise timing of the inclusion formation is rarely known).”

ISOLATED, CLUSTERED AND TRAIL-BOUND INCLUSIONS

The isolated inclusions are, in principle, primary. However, additional criteria related to crystal growth are often necessary to confirm their primary nature (Fig. 1b). The clustered inclusions are typically comprised of a group of 10-20 neighboring inclusions (Fig. 1c). Such clusters may have quite different origins such as neighboring isolated cavities, or by transportation of a

former larger cavity. These clusters may be formed by decrepitaion of inclusions by explosion or implosion. The Trail-bound inclusions, occurring in the surface of a former micro-crack, are certainly secondary (Fig. 1d). These inclusions are far more abundant than early, isolated or clustered cavities.

Trail Terminology

The trail-bound inclusions either remain confined to a single mineral or cut across different grains or phases. Depending upon the observed trails in minerals different terminologies used are given below (Fig. 2).(a) Transgranular: a trail of fluid inclusions cutting across different mineral grains throughout the rock.(b) Intergranular: a trail of fluid inclusions crossing the grain boundary and continues into another mineral grain.(c) Intragranular: A trail of fluid inclusions confined to a particular mineral grain. In such cases the trail of fluid inclusions either, (i) remain confined to the crystal interior, (ii) remain within grain-boundary and crystal interior, or (iii) continues from grain-boundary to grain-boundary.(d) Interphase: a trail of fluid inclusions starts from one phase and continuous into another phase.(e) Transphase: a trail of fluid inclusions cutting across different phases.

Inter-granular Decoration of Fluid Inclusions

The fluid inclusions often remain confined to a certain textural feature, and are said to ‘decorate’ different inter-grain textures. Following are commonly noticed (Fig. 3).(a) Cleavage Plains: fluid inclusions occurring along the cleavage plains.(b) Deformation Lamellae: the fluid inclusions that are confined to and occur along deformation lamellae.(c) Deformation Bands: the fluid inclusions occurring along the deformation bands.

Fig.2. Terminologies used for fluid inclusions (I) Transgranular, (II) Intragranular (grain boundary to grain boundary), (III) Intragranular (grain boundary to crystal interior, (IV) Intragranular (crystal interior), (V) Interphase, (VI) Transphase, and (VII) Intergranular

K. R. Randive and Others22

(d) Sub-grain Boundaries: the fluid inclusions that are confined to sub-grain boundaries.(e) Twin Lamellae: the fluid inclusions which occur along the twin lamellae.

MODIFICATION OF FLUID INCLUSIONS

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 are discussed below.(1) Recrystallization: crystallization of fluids and re-crystallization of solids trapped within fluid inclusions can lead to substantial change in the fluid inclusion morphology and lead to the development of negative crystal shapes.(2) Stretching and Necking-down: necking-down is a typical dissolution-precipitation process, which finally leads to negative crystal shapes. The 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.(3) Explosion/Implosion Decrepitation: 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’. Similarly, when the pressure is considerably reduced (underpressure), the inclusion gets collapsed. Such decrepitation is known as ‘implosion decrepitation’.(4) Leakage: sometimes, originally trapped fluid in the inclusion gets leaked-out due to external processes such as deformation.

METHODS OF STUDY OF FLUID INCLUSIONSSample Preparation

A Fluid Inclusion Section (FIS) is a doubly polished loose plate, of the size of normal thin section (approximately 3 x 5 cm), but significantly thicker than those used for petrological studies. The quality of polishing, the ideal thickness (typically between 90 and 120 µm), depending on transparency of the host crystal and the inclusion size and abundance, are very important factors (Touret, 2001).

Fluid Inclusion Petrography

The textural relationship between fluid inclusions and the host rock is studied in detail in fluid petrography. It begins with careful study of the different populations of fluid inclusions in such way that the fluid inclusions analyzed is a true representative of the characteristic fluid of the process. The fluid inclusions that are observed in a crystal can be the result of different fluids with which the crystal has interacted throughout its history. Therefore, in order to ensure the representativeness of the inclusion, it is necessary to conduct a proper petrographic study that selects the representative inclusions and avoids the temptation of focusing solely on the more aesthetically beautiful, best-formed or larger fluid inclusions. A petrographic study must provide information about populations of fluid inclusions, such as number of inclusions, their type, and chronology with respect to geological events, etc. A thorough and systematic petrographic study is a key for proper decision-making about the type of fluid inclusions to be examined further (Roedder, 1981; 1984; Tauret, 2001).

Microthermometry

The microthermometric study of fluid inclusions is done using a heating and cooling stage, a device that allows increasing or decreasing temperature over a wide range, between –200°C and +1500°C approximately. This stage is placed on a microscope, so that the phase changes occurring as well as the temperature at which these changes taking place can be observed. 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, therefore, helps to associate the composition of the fluid inclusion with a chemical system (Macdonald and Spooner, 1981; Shepherd, 1981; Rosso and Bodnar, 1994; Bakker, 2001; Fall et al., 2011).

Raman Microprobe

Raman Spectroscopy is routinely associated with the study of fluid inclusions to determine the composition of fluids. A confocal Raman microscope, uses an optical

Fig.3. Fluid inclusions within different textural locations also known as decorations, (I) along cleavage planes, (II) along twinning lamelle, (III) along subgrain boundaries, (IV) along deformation lamelle, and (V) along deformation bands

Study of Fluid Inclusions 23

arrangement that inserts a limiting aperture at an image plane. This approach serves to limit the Raman signal entering the spectrograph to a very specific, sharply in focus, volume in the sample. The resulting Raman spectrum is characteristic of that isolated region alone, eliminating or strongly reducing Raman signals from out-of-focus regions in the field of view. By definition a fluid inclusion is surrounded by a host mineral, yet a Raman spectrum of the contents of the inclusion alone can be obtained by focusing at a plane inside the specimen and bringing the inclusion into focus. The analysis is completely non-destructive, and spectra of volumes as small as a few microns in size can be obtained (Dubessy et al., 1989; Roedder, 1990; Van Den Kerkhof and Olsen, 1990; Tauret, 2001).

Apart from the above routine techniques, several special techniques are also used for the study of fluid inclusions. Table 2 lists all, but their detailed description is beyond the scope of present paper.

INFORMATION PROVIDED BY FLUID INCLUSIONSFluid inclusions provide a wealth of information

on the geofluids that influenced the petrogenesis of rocks. Such inclusions provide important information of temperature, pressure, density, salinity and composition of

fluids (Roedder, 1979; Kreulen, 1987; Touret, 1987; 1992; Santosh et al., 1991; Zhang and Frantz, 1987; Newton, 1989; Diamond, 1994; Vityk and Bodnar, 1995; Mukherjee and Sachan, 2001; Bakker and Diamond, 2006).

Temperature of Entrapment of Fluid

One of the most important aspects of fluid inclusions study is its ability to measure the temperature of entrapment of fluids in the system by direct measurement using microthermometry. In this technique two-phase inclusions are heated till they homogenize. The temperature at which homogenization takes place, is the minimum temperature of entrapment of that fluid. If the pressure of the system is known (by separate barometric estimates) then the temperature of entrapment can be estimated from intersection of isochore passing through the temperature of homogenization.

Pressure of the Fluid SystemIt is very difficult to measure the pressure of fluid system directly; however, it can be estimated using separate barometric techniques such as phase relations or geobarometry.

Density of FluidsThe density of the fluid can be calculated by knowing the

Table 2: Summary of methods employed in the study of fluid inclusions

Sr. No.

Name of the Technique Purpose Technique Type

1. Petrological Microscope Primary study of inclusions, their abundance and chronology

Non-Destructive Optical

2. Cathodoluminiscence Mincroscopy / SEM Cathodoluminiscence (CL-SEM)

Textural relations with host mineral, secondary quartz

Non-Destructive Optical

3. Ultraviolate Microscopy (UV) Detection and study of hydrocarbons Non-Destructive Optical4. Infrared Microscopy Visualization of fluid inclusions in semi-opaque

and opaque minerals (e.g. cassiterite, chromite, sphalerite, pyrite)

Non-Destructive Optical

5. Scanning Electron Microscopy (SEM) Morphology of inclusions Non-Destructive Optical6. Transmission Electron Microscopy (TEM) Microfractures around fluid inclusions /dislocations Non-Destructive Optical7. Atomic Force Microscopy (AFM) Topology and irregularities at the atomic scale Non-Destructive Optical8. Microthermometry Composition and molar volume Non-Destructive thermometric9. Laser-excited micro-Raman

spectrometryComposition of non-aqueous fluids, identification ofdaughter crystals

Non-Destructive Vibrational Spectroscopic

10. Fourier Transform Infrared Spectroscopy (FTIR)

Detection of H2O, hydroxyl, CO2, etc Non-Destructive Vibrational Spectroscopic

11. Fluorescence Spectroscopy Detection of hydrocarbons Non-Destructive Vibrational Spectroscopic

12. Mechanical crushing of samples Non-aqueous fluids, qualitative Destructive Mechanical13. Acoustic emission (AE) Decrepitometer Finger print of fluid inclusion content Destructive thermionic14. Gas chromatography Bulk composition of fluid Destructive 15. Mass spectrometry (MS) Bulk fluid inclusion and isotope composition Destructive 16. Crush and leach combined

with micro.chemical analysis, AAS, etc.Composition of aqueous fluids, element ratios, dissolved daughter crystals

Destructive

17. EPMA or SIMS (openedinclusions)

Identification of daughter minerals, composition of fluid inclusions (freezing method)

Destructive Single inclusion

18 LA-ICPMS Composition including trace elements Destructive Single inclusion

K. R. Randive and Others24

temperature of final melting of (Tmf) and homogenization temperature (Th) of the fluid. Placing these values into equations of state for fluids of known composition (i.e. knowing which system to deal with) can help to determine the density of the fluid and to calculate isochores.

Salinity of FluidsSalinity can be measured by observing the depression of the freezing point of the aqueous fluids in the inclusion. The salinity is expressed as NaCl equivalent since, the presence of other ions such as Ca+2, cannot be determined and will greatly influence the salinity estimates. For determination of salinity, the stage is cooled with the help of liquid N2 during cooling; the phase changes in the inclusion are carefully monitored. After the liquid in the inclusion completely solidifies, the stage is heated slowly while the inclusion is observed. The temperature at which the last piece of solid melts (Tmf) is then recorded. This will correspond to the freezing temperature of the inclusion. Knowing the freezing points of pure H2O and CO2, the recorded freezing point (Tmf) is inserted into an equation of state (one of the form: PV= nRT). This freezing point depression for the system under observation is directly related to the amount of impurities present in this system, which provides information on the concentration of salts in this fluid (i.e. salinity of the fluid).

Composition of Fluids

Information about the composition of the fluids can be obtained indirectly by measuring the first melting temperature (also known as the eutectic melting temperature Te or Tme). After freezing the inclusion, it is heated slowly while being carefully observed under the microscope. The temperature of first melting of solid (ice) is recorded. Comparing this temperature to eutectic melting points on published phase diagrams for binary and ternary systems are used to estimate the composition of the fluid. Fluid composition can also be determined by Laser Raman Spectroscopy.

P-T History of the Sample

Careful study of the textural relations of fluid inclusion assemblage may provide important clues to the P-T history of the sample. For example, significant isothermal decompression would cause some of the larger inclusions to “explode” resulting in a decrepitated inclusion surrounded by a number of satellite inclusions around it. Isobaric cooling can also produce distinct textures.

Gas Composition of the Inclusions

Gases are common and are very important constituents of fluid inclusions. CO2, CH4, N2, H2S, and inert gases are the most commonly present gases in the inclusions. Microthermometry is useful in determining the presence and quantity of CO2 using freezing measurement temperatures on the CO2-Clathrate. Methane can be determined by its low liquefaction temperature below

-56°C. But most measurements of the gases species require Laser Raman microprobe.

APPLICATION IN MINERAL EXPLORATION

The use of fluid inclusions in mineral exploration has received varying degrees of attention over the years. The more direct uses of fluid inclusions in exploration mainly rely on defining an empirical relationship between some inclusion characteristic and mineralization. Methods for using fluid inclusions to assist target selection on a regional scale or for more localised definition of likely zones of focussed fluid flow or ore shoots can be subdivided into three categories (Wilkinson, 2001):1. The occurrence or relative abundance of a specific inclusion type2. Systematic variations in microthermometric properties3. Systematic variations in other properties (e.g. descrepitation behavior, inclusion chemistry).

Occurrence or Relative Abundance of a Specific Inclusion Type

The occurrence of CO-bearing inclusions has been suggested as a favourable characteristic for exploration of vein–gold deposits (e.g. Ho, 1987). Rankin and Alderton (1983) used this type of approach to correlate the relative abundance of specific inclusion types in granite samples with the distribution of Sn–W–Cu mineralization in the Cornish ore field. It was found that there was an empirical relationship between inclusion abundance in granite samples and mineralized areas.

A similar approach has been used in porphyry–copper systems, where the occurrence of hypersaline fluid inclusions containing several daughter minerals and/or vapour-rich inclusions has been linked to ore formation (Roedder, 1971; Nash, 1976; Bodnar, 1981). Another example is in epithermal precious metal systems, where the occurrence of cogenetic vapour and liquid rich inclusions may be used to identify zones of boiling and ore mineral precipitation (Kamilli and Ohmoto, 1977).

Systematic Variations in Microthermometric Properties

One of the most important use of inclusion data is the use of inclusion homogenization measurements to map out thermal zonations, an approach of direct relevance to exploration in epithermal and intrusion-centred hydrothermal mineralization. Utilising coupled salinity and homogenization measurements can also provide important information concerning the anatomy of a hydrothermal system, with exploration implications (Fig. 4). Everett et al. (1999) showed how inclusion homogenization temperatures declined and fluid salinity decreased within the Silvermines fault zone with increasing distance from the Zn–Pb–Ba deposits in Ireland. Interestingly, homogenization temperatures

Study of Fluid Inclusions 25

also decreased but fluid salinities generally increased on moving into the footwall of the same fault zone.

Systematic Variations in Other Properties

The chemistry of fluids related to mineralization show a distinct chemical signature, with systematic variations in relative abundances of Na, Ca, S, Ba, Sr and Rb being correlatable with mineralized and unmineralized areas. Another empirical relationship between inclusion chemistry and mineralization was demonstrated by Haynes and Kesler (1987) in a study of MVT mineralization in east Tennessee. They showed that the Ca/Na ratios of inclusion decrepitates as measured by SEM-EDS analysis were higher in ore-related dolomites than in barren dolomites.

FLUID INCLUSIONS STUDY OF PALAEOGENE – NEOGENE – HOLOCENE ROCKS

Fluid inclusions have been effectively applied in the study of geothermal fields, where hydrothermally precipitated minerals were found to be suitable phases for such study. The entrapped fluids provide reasonable estimates of the Pressure-Temperature-Salinity conditions of the geothermal water and be reasonably good geothermometers (Browne et al., 1976; Sasada et al., 1986; Magro et al., 1998; Lutz et al., 2002; Ruggieri et al., 2004). Fluid inclusions are among the potential tools for the exploration of geothermal fields (Leach, 1981) and also for the study of genesis of minerals such as anhydrite (Maramatsu et al., 2000).

Other areas where fluid inclusions find exclusive importance are hydrocarbons in the petroliferous basins and the areas of recent and sub-recent active volcanoes (Frezzhotti, 2001). The fluid inclusions in petroliferous

rocks have been used to determine the physical limit of petroleum migration and to reconstruct the geothermal history of the sedimentary basin (Burruss, 1981; Pagel et al., 1986; McLimans, 1987); such study has recently been undertaken in sandstones of Jaisalmer basin, Rajasthan (Verma et al., 2012).

The studies of fluid and melt inclusions had been effectively used to explain important petrogenetic processes; such as, liquid immiscibility (e.g. La Gomera, Canary Islands; Frezzhotti et al., 2002), crustal anatexis and crust-magma interaction (e.g. Aeolian Arc, Southern Italy; Frezzhotti et al., 2004). The technique of fluid inclusions study is very much there and finds applications across the geological sciences. It is however, difficult to specify their application in geologically recent rocks. Nevertheless, the examples cited above provide vital clues for using fluid inclusions in the study of Palaogene – Neogene – Holocene rocks. The recently active volcanoes and geothermal fields are particularly very promising.

SUMMARYThe study of fluid inclusions has come a long way

since their initial description by Sorby (1857). New instrumentation has facilitated deeper focus in the study by way of understanding their composition, morphology, phase relations and thermodynamic properties. Moreover, laboratory simulations of the synthetic fluid inclusions allowed predicting exact conditions of entrapment. Nevertheless, it is of paramount importance to identify and classify the fluid inclusions by careful petrographic study. Microthermometry aided by Raman Spectroscopic study helps to estimate composition and T-P-X composition of the fluid system, which is used further for construction of isochors and evolution of the fluid system.

Fig.4. Homogenization temperature–salinity diagram illustrating typical ranges for inclusions from different deposit types after Willkinson (2001)

K. R. Randive and Others26

Andersen, T., Frezzotti, M.L. and Burke, E.A.J. (2001). Fluid inclusions: Phase relationships - methods- applications (special issue). Lithos, v.55(1-4), pp.320.

Bakker, R.J. (2001). Fluids: new software package to handle microthermometric data to calculate isochores (available from the author)

Bakker, R.J. and Diamond, L.W. (2006). Estimation of volume fractions of liquid and vapor phases in fluid inclusions, and definition of inclusion shapes. Amer. Mineral., v.91, pp.635-657.

Bakker, R.J. and Jansen, J.B. (1990). Preferential water leakage from fluid inclusions by means of mobile dislocations. Nature, v.345(6270), pp.58-60.

Bodnar, R.J. (1981).Use of fluid inclusions in mineral exploration: comparison of observed features with theoretical and experimental, Colorado. Econ. Geol., v.66, pp.98–120.

Bodnar, R.J. (1994). Philosophy of fluid inclusions analysis. In: De Vivo and M.L. Frezzotti (Eds.), Fluid Inclusions in Minerals and Applications. Virginia Tech, Blacksburg, VA, pp.1 – 6.

Bodnar, R.J. (2003). Introduction to fluid inclusions. In: I. Samson, A. Anderson and D. Marshall (Eds.), Fluid Inclusions: Analysis and Interpretation. Mineral. Assoc. Canada, Short Course 32, pp.1-8.

Browne, P.R.L., Roedder, E. and Wodzicki, A. (1976). Comparison of past and present geothermal waters from a study of fluid inclusions, Broadlands Field, N.Z., Proc. Int. Sym. on water-rock. Interaction, pp.140-149.

Burruss, R.C. (1981). Hydrocarbon fluid inclusions in studies of sedimentary diagenesis. In: L.S. Hollister and M.L. Crawford (Eds.), Short course in fluid inclusion: Application to petrology. Mineral. Assoc. Canada, v.6, pp.138-156.

Cordier, P., Doukhan, J.C. and Ramboz, C. (1994). Influence of dislocations on water leakage from fluid inclusions in quartz: a quantitative reappraisal. Eur. Jour. Mineral., v.6, pp.746- 752.

De Vivo, B. and Frezzotti, M.L. (1994). Fluid inclusions in minerals: methods and applications. Short course of the working group (IMA) “Inclusions in Minerals” Pontignano/Siena 1-4 Sept. 1994, Fluids Research Laboratory, Department of Geological Sciences, YPI, Blacksburg, VA, USA,

Diamond, L.W. (1994). Salinity of multivolatile fluid inclusions determined from clathrate hydrate stability. Geochim. Cosmochim. Acta, v.58, pp.19-41.

Dubessy, J., Poty, B. and Ramboz, C. (1989). Advances in C-O-H-N-S fluid geochemistry based on micro-Raman spectrometric analysis of fluid inclusions. Eur. Jour. Mineral., v.1, pp.517- 534.

Everett, C.E., Wilkinson, J.J., Boyce, A.J., Ellam, R.M., Gleeson, S.A., Fallick, A.E. (1999). The genesis of Irish-type Zn–Pb deposits: Characterisation and origin of the principal ore fluid. In: C.J. Stanley (Ed.),

Mineral Deposits: Processes to Processing. Proc. 5th Biennial SGA Meeting and 10th Quadrennial IAGOD Symposium, London A.A. Balkema, Rotterdam, pp.845–848.

Fall, A., Tattitch, B. and Bodnar, R. (2011). Combined microthermometric and Raman spectroscopic technique to determine the salinity of H2O-CO2-NaCl fluid inclusions based on clathrate melting. Geochim. Cosmochim. Acta., v.75, pp.951-964.

Fonarev, V.I., Touret, J.L.R. and Kotelnikova, Z.A. (1998). Fluid inclusions in rocks from the Central Kola granulite area (Baltic Shield). Eur. Jour. Mineral., v.10, pp.1181 – 1200.

Frezzotti, M.-L. (2001). Silicate melt inclusions: applications to petrology. In: T. Andersen, M.-L. Frezzotti and E. Burke (Eds.), “Fluid inclusions: Phase relationships – methods – applications”, Lithos, v.55, pp.289-300.

Frezzotti, M.L., Tauret, J.L. and Neumann, E.R. (2002). Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands. Eur. Jour. Mineral., v.14, pp.891-904.

Frezzotti, M.L., Peccerillo, A., Zanon, V. and Nikogosian, I. (2004). Silica-rich melts in quartz xenoliths from Vulcano Island and their bearing on processes of crustal anatexis and crust-magma interaction beneath the Aeolian Arc, Southern Italy. Jour. Petrol., v.45, pp.3–26.

Goldstein, R.H. (2003). Petrographic analysis of fluid inclusions. In: I. Samson, A. Anderson and D. Marshall (Eds.), Fluid Inclusions: Analysis and Interpretation. Mineral. Assoc. Canada, Short Course Ser.32, pp.9-53.

Goldstein, R.H. and Reynolds, T.J. (1994). Systematics of fluid inclusions in diagenetic minerals. Society for Sedimentary Geology. SEPM, Tulsa, Oklahoma. Short Course 31, 199p.

Haynes, F.M. and Kesler, S.E. (1987). Fluid inclusion chemistry in the exploration for Mississippi Valley-type deposits: an example from east Tennessee. App. Geochem., v.2., pp.321–327.

Ho, S.E. (1987). Fluid inclusions: their potential as an exploration tool for Archean gold deposits. Geol. Dept. Univ. Ext., Univ. Western Australia Publ. 11, pp.239–263.

Hollister, L.S. and Crawford, M.L. (Eds.) (1981). Short course in fluid inclusions: application to petrology. Mineralogical Association of Canada, Calgary, 304p.

Leach, T.M. (1981). An evaluation of fluid inclusions as a geothermal exploration tool. New Zealand Geothermal Workshop, 1981, pp.475-478.

Kamilli, R.J. and Ohmoto, H. (1977). Paragenesis, zoning, fluid inclusion and isotopic studies of the Finlandia Vein, Colqui district, Central Peru. Econ. Geol., v.72, pp.950–982.

Lutz, S.J., Moore, J.N., Blamey, N.J.F. and Norman, D.I. (2002).

REFERENCES

Study of Fluid Inclusions

ACKNOWLEDGEMENTS

We thank Prof. S. J. Sangode for inviting us to write a paper in the special issue on “Advances in Geoscientific

Methods, Techniques and Applications”. KRR and DBM acknowledges the partial financial support through UGC-SAP DRS-I grants received by the Department of Geology, RTM Nagpur University.

27

Fluid-inclusion gas chemistry of the Dixie Valley (NV) geothermal system. Proc. Twenty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 28-30, 2002, SGP-TR-171.

Macdonald, A.J. and Spooner, E.T.C. (1981). Calibrations of a LINKAM TH600 programmable heating-cooling stage for microthermometric examination of fluid inclusions. Econ. Geol., v.76, pp.1248-1258.

Magro, G., Ruggieri, G., Panichi, C., Boiron, M.C. and Cathelineau, M. (1998). Noble gases and carbon isotopic composition of fluid inclusions from the Lorderello geothermal field (Italy), preliminary data. Goldschimdt Conference Volume, Tuolouse 1998, pp.939 – 940.

Mclimans, R.K. (1987). The application of fluid inclusions to migration of oil and diagenesis in petroleum reservoir. App. Geochem., v.2, pp.585-603.

Mukherjee, B.K. and Sachan, H.K. (2001). Discovery of coesite from Indian Himalaya: A record of ultra-high pressure metamorphism in Indian continental crust. Curr. Sci., v.81(10), pp.1358 – 1361.

Nash, J.T. (1976). Fluid inclusion petrology-data from porphyry copper deposits and applications to exploration. US Geol. Surv. Prof. Paper 907D, 16p.

Pagel, M., Walgenwitz, F. and Dubessy, J. (1986). Fluid inclusion in oil and gas bearing sedimentary formation. In: J. Bunus (Eds.), Thermal modelling in sedimentary basin, Techniq, Paris, pp.565-583.

Parnell, J. (Eds.) (1994). Geofluids: Origin, migration and evolution of fluids in sedimentary basins. Geol. Soc. Spl. Pub., v.78, pp.372.

Rankin, A.H. and Alderton, D.H.M. (1983). Fluid inclusion petrography of S.W. England granites and its possibilities in mineral exploration. Mineral. Deposita, v.18, pp.335–347.

Roedder, E. (1979). Fluid inclusions as samples of ore fluids. In: H.L. Barnes (Ed.), Geochemistry of hydrothermal ore deposits. 2nd ed. Wiley- New York, pp.684-737.

Roedder, E. (1981). Problems in the use of fluid inclusions to investigate fluid-rock interaction in igneous and metamorphic processes. Fortschr. Mineral., v.59, pp.267 -302.

Roedder, E. (1984). Fluid inclusions. Reviews in Mineralogy. Mineral. Soc. America, Washington., v.12, 644p.

Roedder, E. (1990). Fluid inclusion analysis – Prologue and epilogue. Geochim. Cosmochim. Acta, v.54, pp.495-507.

Roedder, E. (1971). Fluid inclusion studies on the porphyry-type ore deposits at Bingham, Utah, Butte, Montana, and Climax, Colorado. Econ. Geol., v.66, pp.98–120.

Rosso, K.M. and Bodnar, R.J. (1995). Microthermometric and Raman spectroscopic detection limits of CO2 in fluid inclusions and the Raman spectroscopic characterization of CO2. Geochim. Cosmochim. Acta, v.59, pp.3961-3975.

Ruggieri, G., Giolito, C., Manzella, A. and Boiron, M.C. (1998). Application of fluid inclusion to the study of Bag Nore Geothermal field (Tuscany, Italy). Geothermics, v.33(5), pp.675-692.

Samson, I., Anderson, A. and Marshall, D. (Eds.) (2003). Fluid inclusions - Analysis and Interpretation. Mineralogical

Association of Canada. Short Course Series, v.32, 374p.Santosh, M., Jackson, D.H., Harris, N.B.W. and Mattey,

D.P. (1991). Carbonic fluid inclusions in South Indian granulites: evidence for entrapment during charnockite formation. Contrib. Mineral. Petrol., v.108, pp.318–330

Sasada, M., Roeder, E. and Belkin, H.E. (1986). Fluid inclusions from the drill hole DW-5, Hohi Geothermal Area, Japan: evidence of boiling and procedure for estimating CO2 content. Jour. Volcan. Geotherm. Res., v.30, pp.231-251.

Shepherd, T.J. (1981). Temperature-programmable heating-freezing stage for microthermometric analysis of fluid inclusions. Econ. Geol., v.76, pp.1244-1247.

Shepherd, T.J., Rankin, A.H. and Alderton, D.H.M. (1985). A practical guide to fluid inclusion studies. Blackie-Glasgow., 239p.

Shmulovich, Ki., Yardley, B. and Gonchar, G.G. (1995). Fluids in the crust. Equilibrium and transport properties. Chapman & Hall, 323p.

Sorby, H.C. (1858). On the microscopic structure of crystals, indicating the origin of minerals and rocks. Geol. Soc. London Quart. Jour., v.14, pt.I, pp.453-500.

Sterner, S.M. and Bodnar, R.J. (1989). Synthetic fluid inclusions - Vll. Re-equilibration of fluid inclusions in quartz during laboratory-simulated metamorphic burial and uplift. Jour. etam. Geol., v.7(2), pp.243-260.

Touret, J.L.R. (1987). Fluid inclusions and pressure-temperature estimates in deepseated rocks. In: Helgeson (Ed.), Chemical transport in metasomatic processes. NATO ASI Series C: Mathematical and Physical Sciences, v.218, pp.91-121.

Touret, J.L.R. (1992). CO2 transfer between the upper mantle and the atmosphere: temporary storage in the lower continental crust. Terra Nova, v.4, pp.87-98.

Touret, J.L.R. (2001). Fluid inclusions in metamorphic rocks. Lithos, v.55, pp.1-25.

Van Den Kerkhof Am and Hein, U.F. (2001). Fluid inclusion petrography. In: T. Andersen, M.L. Frezzotti and E.A.J. Burke (Eds.), Fluid inclusions: phase relationships-methods applications (special issue). Lithos., v.55(1-4), pp.320.

Van Den Kerkhof Am and Olsen, S.N. (1990). A natural example of superdense CO2 inclusions: Microthermometry and Raman analysis. Geochim. Cosmochim. Acta, v.54, pp.885-901.

Varma, Dhananjai, Jadhav, G.N., Biswal, T.K., Jena, S.K. and Sharma, N. (2012). Characterization of Hydrocarbon-bearing Fluid Inclusion in Sandstones of Jaisalmer Basin, Rajasthan: A Preliminary Approach. Jour. Geol. Soc. India, v.80, pp.505-514

Vityk, M.O. and Bodnar, R.J. (1995). Do fluid inclusions in high grade metamorphic terranes preserve peak metamorphic density during retrograde decompression? Amer. Mineral., v.80, pp.641-644

Wilkinson, J.J. (2001). Fluid inclusions in hydrothermal ore deposits. Lithos, v.55, pp.229–272

Zhang, Y.G. and Frantz, J.D. (1987). Determination of homogenization temperatures and densities of supercritical fluids in the system NaCl-KCl-CaCl2-H2O using synthetic fluid inclusions. Chem. Geol., v.64, pp.335-350.

K. R. Randive and Others

(Received : 25 March 2015; Revised form accepted : 8 May 2014)

28