Fluid inclusions

This chapter has been compiled from different sources, references at the end. The newest, comprehensive source is the book Geofluids by Hurai et al. (2015). Much information and many other references can be found in that book.

Definition. Fluid inclusions are defined as „hermetically closed objects in natural and synthetic

materials which enclosed, during and after their formation, the surrounding fluid” (Leeder et al. 1987). To clarify the term fluid, we must note that in geosciences, fluids are

- melts of any composition, including silicate, sulfide, oxide, carbonate, etc. melts - liquids, such as water - gases, such as CO2 - supercritical fluids (see below).

Methods for the study of fluid inclusions Normally, one would first define the objects of the study (i.e., the fluid inclusions) and then introduce the methods which can tell us about their properties. In this case, we will make an exception to this rule. We need to know about the methods, especially about microthermometry, when we will discuss the composition of the enclosed fluids and the phase transitions inside the fluid inclusions.

The methods for the study of fluid inclusions can be divided into two large groups: destructive and non-destructive.

Non-destructive methods include microthermometry, Raman spectroscopy, and X-ray fluorescence spectrometry. Microthermometry is a special technique developed specifically for the fluid inclusion studies. In a small heating-freezing table attached to a rotating stage of a microscope, the inclusions can be heated to high temperature (up to 600 ºC, in special devices even higher) or cooled to low temperatures (down to -170 ºC). During heating or cooling, the inclusion can be observed visually and temperature of any phase transition can be recorded by the scientist. We will discuss the possible phase transition below.

Raman spectroscopy belongs to the group of vibrational spectroscopic techniques, together with infrared spectroscopy. This method should be familiar to you from other classes and I will not describe the principles here. Some examples will be given below.

X-ray fluorescence (XRF) spectrometry is a well-known method for bulk analyses of rocks. Fluid inclusions can be also measured by XRF but because they are small, they have to be measured with a tiny beam of X-rays, that is, with a microbeam. Such measurements are possible at large research facilities called synchrotrons. Here, each inclusion can be measured separately. The detection limits are relatively low.

Destructive methods include crush-leach analysis, decrepitation analysis, or investigation of the fluid inclusions by laser-ablation inductively coupled-plasma mass spectrometry or scanning electron microscope. As the name suggests, the inclusions are destroyed in the process and cannot be re-used for further analyses.

The idea of crush-leach analysis is very simple. A sample (100-500 mg) is cleaned and broken to smaller pieces. Afterwards, the pieces are immersed in pure water and ground to a powder. During grinding or crushing, the content of the fluid inclusions is released into this water, so to say “leached”. The aqueous sample is then analyzed by suitable methods, for example inductively coupled-plasma mass spectrometry (ICP-MS) or ion chromatography. The major drawback of this method is that all inclusions in a sample are analyzed together. The analytical result may be difficult to interpret if there are several generations of fluid inclusions with different chemical compositions in the sample.

The decrepitation method is not being used very much today. In this method, it was assumed that the fluid inclusions explode when we heat them up to the formation temperature. This explosion is called decrepitation and can be detected, for example by acoustic devices. As the crush-leach analysis, this method analyzes all fluid inclusions together. In addition, fluid inclusions do not have to decrepitate near their formation temperature. The decrepitation temperature depends also on the size of the inclusions and the mechanical properties of the mineral host.

Laser-ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS) is used to measure the chemical composition of individual inclusions. Therein lies its great advantage when compare to the crush-leach analysis. The host mineral is gradually drilled by a laser beam until the fluid inclusion is open. Its content is then fed into the ICP-MS spectrometer and analyzed for a suite of elements. Thank to the great sensitivity of the modern instruments, even trace elements can be analyzed in the small volume of a fluid inclusion.

Scanning electron microscopy (SEM) is a well-established method in geosciences. This method can be used to determine the chemical composition of the daughter minerals in a fluid inclusion. The sample is broken, coated with carbon and inserted into an SEM. Somewhere along the broken surface, there will be also fluid inclusions exposed, with their daughter minerals still inside the little cavities. These minerals can be then analyzed by the standard energy-dispersive systems inside an SEM.

Fluids encountered in the crust

There are two common fluid types in the crust: silicate magmas and hydrothermal fluids, dominated by aqueous phase, sometimes also CO2. Note that there is no sharp line between these two fluid types. Pegmatites, for example, form from fluids which can be described as water-rich silicate magmas or high-temperature, concentrated hydrothermal fluid.

Given that most ore deposits are formed by the action of hydrothermal fluids, we will further consider the chemical composition of these. Obviously, the most common and abundant component of these fluids is H2O. Intentionally, I did not write water. H2O can be present as liquid (water), gas (steam), or supercritical fluid. Another two abundant components are CO2 and NaCl. Let’s explore phase diagrams of systems which include these components to understand how can we measure something that we cannot reach – the fluids enclosed in their inclusions.

Phase relationships in simple systems: One component systems H2O and CO2

Figure 1 shows the phase diagrams for H2O and CO2. Every substance, if it does not decompose upon heating, will eventually melt and boil or sublime (melting denotes the transformation from solid to a liquid, sublimation the transformation from solid to a gas). The three forms (solid, liquid, gas) co-exist in

equilibrium only in a single point called the triple point. Figure 1 shows the temperature and pressure of the triple point for H2O and CO2.

Another important point on the phase diagram is the critical point (see Figure 2a). Above this point, the substance exists only as a supercritical fluid, a kind of peculiar state between liquid and gas. Supercritical fluid can have liquid- or gas-like density. There is no phase transformation, visible or not, when the density of the supercritical fluid changes from liquid-like to gas-like, or vice versa. We know from everyday experience that there is a visible phase transformation when liquid changes to a gas, for example when water boils. One could imagine the supercritical fluid this way.

The curve which connects the triple and critical point is called the condensation curve. Liquid and gas co-exist along this curve.

The phase transitions observable in these simple systems are temperature of melting and temperature of homogenization. After freezing, the fluid inclusion will contain a solid (for example, ice) and gas (water vapor). During heating, at a certain point, solid and gas will co-exist with liquid (for example, water). This transition will be seen as melting in a microscope. In closed, constant-volume systems such as fluid inclusions, the transition occurs in the triple point. Hence, the measurement of melting can tell us something about the system. Of course, fluid inclusions are very rarely made of one component and if they are, we probably know already what is it. Temperatures of triple points for selected systems are summarized in Table 1. Table 1. Temperatures of triple and critical points, pressures of critical points (after Hein 1990).

triple point critical temperature critical pressure (ºC) (ºC) (bar)

H2O 0.015 374.15 220.4 CO2 -56.6 31.1 74.0 SO2 -72.7 157.8 78.8 H2S -83.6 100.4 90.2 COS -138.2 104.8 65.7 C2H6 -172 32.1 49.0 CH4 -182.6 -82.5 46.3 CO -205 -140.2 35.0 N2 -210 -147.1 13.0

Temperature of homogenization (Thom) is much more interesting phase transition from the point of

fluid inclusion studies. As the name suggests, this is the temperature at which the fluid inclusion homogenizes. At room temperature, many one-component fluid inclusions would contain liquid (for example, water) and a gas bubble. Upon heating, these two phases would merge at some temperature and become one phase. Let us examine a few possibilities shown graphically in Figure 2.

Figure 2b shows a fluid inclusion filled with pure water with density () of 0.9 g·cm–3. Because a fluid inclusion is a constant-volume system, its volume and its density cannot change. If we vary temperature (for example, in a microthermometry stage), pressure cannot vary independently, but along lines which plot on the phase diagram, for example in Figure 2b. These lines are called isochores. As long there are two phases (liquid+gas) in the fluid inclusion, its P-T path must coincide with the condensation curve. The gas bubble will be smaller and smaller, until it disappears at some temperature and the fluid

inclusion will be filled by liquid. Upon further heating, the inclusion must follow the isochore for = 0.9 g·cm–3, also plotted in Figure 2b. If we keep heating, no further phase transitions will be visible.

Figure 2c shows another example. In this case, we will observe the behavior of a pure-water fluid inclusion with density of 0.08 g·cm–3. At room temperature, this inclusion will be mostly filled with water

vapor, with a smaller amount of water. The walls of the inclusion appear black because the refraction indices of the host mineral (mostly quartz) and water vapor are very different. Upon heating, the gas phase will expand until the liquid completely disappears. This is the Thom. Upon further heating, nothing

will be observed, and the constant-volume system inside the inclusion must follow the isochore for = 0.08 g·cm–3.

Figure 2d shows the final, rare possibility. In this case, the fluid inclusion has density of 0.322 g·cm–3. This is the critical density of water, meaning that the system must pass upon heating through the critical point. We see (in Figure 2d) that neither liquid nor gas are expanding. Only the interface between those two will become fainter as the temperature increases; this is because their properties, including refraction indices, are approaching each other. In a critical point, the inclusion homogenizes. Upon

further heating, the inclusion will follow the isochore for = 0.322 g·cm–3. Temperatures and pressures of critical points in selected systems are summarized in Table 1.

What did we learn from this description? The homogenization temperature in a constant-volume system is a function of the density of that system. This holds whether we are dealing with one- or more-component systems. Density of the fluids is an important parameter for geological interpretations.

Phase relationships in simple systems: Two component system H2O-NaCl Figure 3a shows the phase diagram for the system H2O-NaCl. NaCl is the most common electrolyte in the aqueous fluids in the Earth and therefore is this phase diagram of great importance for fluid inclusion studies. Very often, we want to know the concentration of NaCl in the aqueous phase, that is, its salinity. Salinity can be determined by microthermometric measurements.

Let us examine the phase diagram and think about the phase transition which will be visible in a fluid inclusion filled with H2O and NaCl. In Figure 3b, a portion of the phase diagram is shown. We will concentrate now on the fluid inclusions marked with the Roman numerals I-III. These are the usual crustal fluids with H2O and some, but not too much NaCl. We such inclusion is exposed to low temperatures, its content will freeze, as shown in Figure 3b, inclusion I. Upon heating, we will see the so-called “first melting”, or the eutectic transition. Since you are very familiar with eutectic phase diagrams, I will not explain this phenomenon more closely. It is only important to note that the eutectic temperature differs for different systems. The following table summarizes some of the systems and their eutectic temperatures. System Eutectic temperature, Te (in ºC) H2O-Na2SO4 -1.2 H2O-KCl -10.6 H2O-NaCl -21.2 H2O-MgCl2 -35 H2O-CaCl2 -55 H2O-LiCl -74

A quick glance at this table tells us that the type of the electrolyte can be determined from the eutectic temperature. Unfortunately, nature is not always so simple and we have often mixed systems. The measurement of the eutectic temperature, however, is helpful, and can be taken at least as a guiding information about the electrolytes present.

After the eutectic melting and with the increase of the temperature, more and more ice will melt. At some point, only a few ice crystals will remain, as shown in Figure 3b, inclusion II. The last ice crystal will melt at a specific temperature, denoted as Tm,ice. This temperature is directly proportional to the

concentration of NaCl in a H2O-NaCl fluid inclusion, that is, to its salinity. The variations of the ice melting temperature, Tm,ice, are caused by the well-known freezing point depression.

After melting of the last ice crystal, the inclusion will contain liquid (aqueous brine) and a gas bubble, as shown in Figure 3b, inclusion III.

Another example in Figure 3b shows the behavior of a high-salinity fluid (inclusions IV-VII). After freezing, the inclusion will consist of solid mixture of ice and hydrohalite (NaCl·2H2O) and a gas bubble. First melting will be observed at the eutectic temperature, Te = –21.2 ºC. The remaining solid, however, will not be ice but hydrohalite. Hydrohalite will transform to halite at the peritectic temperature of T = 0.1 ºC. Upon further heating, the halite crystal will disappear at a certain temperature, denoted, not quite correctly, as the temperature of halite melting, Tm,halite. In this case, this temperature can be used to determine the salinity of the fluid inside this fluid inclusion.

There is an intermediate case, not shown in Figure 3b. In this case, hydrohalite disappears before transforming to halite at the peritectic temperature. You can figure out this case on your own.

What did we learn from this description? The temperature of eutectic melting can give us a hint about the electrolytes present in an aqueous solution. The final melting temperature gives us the concentrations of these electrolytes – the salinity of the fluid.

Fluids trapped in the fluid inclusions Fluid inclusions enclose the fluid at some point in the geological history. In an ideal case, the composition of a fluid inclusion does not change over time, upon cooling and release of pressure. It should be clear from the previous discussion of the phase changes that new phases may appear as the temperature of the fluid inclusions (that is, their host) changes. Fluid inclusions are therefore commonly described by the number of phases which are observable at room temperature.

Some fluid inclusions may be filled by a single phase. This phase could be an aqueous brine (H2O + electrolyte) or gas (water vapor, methane). The two cases can be recognized relatively easily just by observation in a microscope. Because of the great difference in the refraction indices between minerals and gases, the gaseous inclusions appear to be dark.

More common and often more interesting are two-phase fluid inclusions. Most frequently, they consist of a brine and gas, usually water vapor. They are designated as L+V (liquid+vapor) inclusions. Inclusions with a similar appearance may be found in volcanic rocks; the gas bubble is present but the rest of the inclusions is made of solidified glass. These are melt inclusions which preserve the magma in its original chemical composition. Studying fluid inclusions, you can actually see magma!

There are different kinds of three-phase inclusions. One possibility is L1-L2-V inclusions, made of aqueous liquid, CO2 liquid, and gas. They are typical for environments where elevated pressure created conditions for the formation of liquid CO2, such as metamorphic environments or ore deposits related to metamorphic processes. Another possibility are aqueous brine-oil-gas inclusions, found in sedimentary rocks. They enclosed oil which migrated from the source rocks to the reservoirs. Finally, we can encounter L+V+S inclusions which consist of liquid, gas (usually water vapor) and a solid. Because they can be quite variable, we will discuss them separately in the next paragraph.

The L+V+S inclusions may represent three- or multiple phase systems. Beside the liquid and gas phase, they may have one or many solid phases. Most common solid in such inclusions is halite, essentially always in the form of euhedral, cube-shaped crystals. Many other minerals have been found enclosed in fluid inclusions. Some pegmatites, for example, are famous for fluid inclusions full of various exotic minerals.

There are two kinds of solids enclosed in the fluid inclusions. One kind are the solids which crystallized from the fluid after the fluid inclusion was sealed, during cooling. An example could be the halite crystals

which we already mentioned. Such crystals are called the daughter crystals. Another possibility is that the fluid, already at the time of formation of fluid inclusions, contained crystals of solids. Such crystals may have been randomly trapped by some inclusions whereas other ones have no such crystals. These are trapped crystals.

Petrography of fluid inclusions

According to the relationship of the enclosed fluid to the fluid from which the observed mineral formed, we recognize three types of inclusions:

1. Primary inclusions. They have trapped or enclosed the original fluid from which the mineral

formed. 2. Secondary inclusions. They have been trapped by the mineral later and likely contain a fluid that

had nothing to do with the formation of the mineral. They tend to occur in trails which cross the grain boundaries of the minerals.

3. Pseudosecondary inclusions. In their appearance, they are similar to the secondary inclusions. They also occur in trails but, at least in theory, these trails should not intersect the grain boundaries. Pseudosecondary inclusions are thought to contain the primary fluid. Their formation will be discussed later.

Pitfalls in the study of fluid inclusions in ore deposits ● Relationship between fluid inclusions and ore minerals: Fluid inclusions are most often studied in quartz, less commonly in other minerals, such as calcite or fluorite. Yet, we want to learn something about the formation conditions of ore minerals, not necessarily about quartz. Therein lies a problem that the fluids which precipitates quartz may not be the same as the fluids which precipitates the ore minerals. An ideal solution to this problem is to study the fluid inclusions directly in the ore minerals. Given that most ore minerals are opaque, this could be difficult. Sphalerite is transparent and the inclusions in this mineral have been studied with some success. Other minerals, such as cinnabar, are also transparent, but they decompose upon heating, in this case releasing mercury vapor, creating a serious health risk. Another possibility is to observe and measure the properties of fluid inclusions in infrared light; many opaque ore minerals are transparent to infrared radiation. These include sphalerite, pyrargyrite, wolframite, cinnabar, stibnite, chalcocite, enargite, molybdenite, tetrahedrite–tennantite, hematite, and even non-arsenian pyrite. ● Post-entrapment modifications – necking down: Fluid inclusions strive to achieve an ideal form, meaning a shape with the lowest energy. This shape is the negative crystal shape, a shape of a crystal typical for the host mineral. Especially long fluid inclusions have a tendency to change their shape over long geological time and separate into two or several shorter fragments. This process is called necking down. The inclusions which underwent this process are useless in terms of providing information about the fluids and formation conditions of the host or any other mineral. ● Post-entrapment modifications – diffusion: Small atomic or molecular species, such as H2 or He, may be able to diffuse in or out of the fluid inclusions. It was observed that fluid inclusions in some porphyry-copper deposits have chalcopyrite as a daughter mineral. Obviously, the chemical constituents of this mineral must have been dissolved in the fluid at the time of the formation of these fluid inclusions. Yet, chalcopyrite did not dissolve when the inclusions were heated in a microthermometry stage.

Mavrogenes and Bodnar (1994) found out that when such samples were exposed to high-pressure H2 atmosphere for prolonged time, chalcopyrite was dissolving upon heating. They concluded that during the geological time, H2 diffused out of the inclusions, changing therefore the redox state of the included system. Diffusion could possibly affect also larger species, especially if the host mineral contains many grain boundaries or defects. Diffusion is much faster and much more effective along such defects.

Interpretation of fluid inclusion data in hydrothermal systems ● Fluid salinity: Salinity of the fluid and the nature of the electrolytes is one of the fundamental variables in the studies of ore deposits (Fig. 4). In many previous descriptions of individual types of ore deposits, we have noted that the salinity of their fluids varies from almost pure water to high-temperature solutions with more salt than water. Hence, salinity can tell us a lot about the fluid origin and evolution in the depth.

Even of greater interest may be trends on a homogenization temperature – salinity plots (Fig. 5). From these plots, we can possibly explain the observations made by other methods (petrology, isotope geochemistry, etc.) and learn much about the fluids. We can distinguish processes such as boiling, effervescence, mixing of fluids, or simple cooling (Fig. 5).

Trace elements do not contribute much to the bulk salinity of the fluids but they are of interest on their own. Using advanced techniques of the analysis of fluid inclusions (e.g., LA-ICP-MS), individual inclusions may be analyzed for their metal (e.g., Cu, Zn, Pb) content. Doing so, we gain data about the concentration of metals in ore-forming fluids which circulated in the crust many millions year ago, in a depth of several kilometers and under high pressure. ● Volatile content: Volatiles such CO2, CH4, N2, Ar, and He are useful tracers for magmatic, metamorphic, and sedimentary environments. They can be analyzed by bulk methods, for example crushing the samples and analysis of the released gases by gas chromatography. Much more powerful are the analyses of the individual inclusions, especially with the non-destructive Raman spectroscopy. ● Density: Density of the fluids can be calculated from the known composition and homogenization temperature. Density is an important variable which can predict if the fluids are buoyant or if they should sink deeper in the crust. Hence, density and density contrast to the host rocks, pressure gradients, hydrostatic and lithostatic pressure can also enter our models of ore deposit formation. ● Trapping of fluid inclusions from a homogeneous fluid: Obviously, inclusions which trapped the same homogeneous fluid should have a “homogeneous look”. This means that the ratio of the phases present in such a population of fluid inclusions should be very similar (see Fig. 6). These inclusions should have similar properties, for example similar temperatures of phase transitions. Upon heating, the inclusions will homogenize to a single phase, for example liquid. We can return to Figure 2b but must point out that the following discussion applies to fluid inclusions which homogenize to the liquid phase in general, not only to pure water fluid inclusions. Once the fluid inclusion homogenizes to liquid, it will depart the

condensation curve and move in the P-T space along an isochore, in case of Figure 2b along the = 0.9 g·cm–3 isochore. Note that the temperature of homogenization, marked by the point of departure from the condensation curve, is different from the real formation, or trapping temperature, marked by a small cross in Figure 2b. The difference between the two temperatures is caused by the pressure which existed during the entrapment of the fluid. For this reason, the homogenization temperature in a population of fluid inclusions trapped from a homogeneous fluid in the minimum temperature of formation. The difference between the two temperatures is often called the pressure correction.

● Trapping of fluid inclusions from a heterogeneous fluid: In case of a heterogeneous fluid, other rules apply. Most common reasons for a transition of a homogeneous fluid to a heterogeneous fluid are boiling and effervescence. Everyone is surely familiar with boiling. Effervescence is the separation of a gas phase from a liquid upon pressure release. Effervescence can be easily observed if you shake a bottle with mineral water or champagne and then open it. An example of fluid inclusions trapped from a heterogeneous boiling fluid in shown in Figure 7.

Without going into details, the formation temperature can be determined by homogenization of the end-member phases in a heterogeneous fluid. In the above examples, these would be liquid and vapor. Inclusions which trapped random portions of the phases (for example, liquid and vapor), will yield temperatures higher than the formation temperature. Cited and recommended literature Andersen, T., Frezzotti, M.L., Burke, E.A.J. eds. 2001: Fluid inclusions: phase relationships - methods-

applications (special issue). Lithos 55, 320 pp. De Vivo, B., Frezzotti, M.L., 1994: Fluid inclusions in minerals: methods and applications. Short course of

the working group (IMA) "Inclusions in Minerals" (Siena) Fluids Research Laboratory, Department of Geological Sciences, YPI, Blacksburg.

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

Hollister, L.S., Crawford, M.L., eds., 1981: Short course in fluid inclusions: application to petrology. Mineralogical Association of Canada, 304 pp.

Hurai, V., Huraiova, M., Slobodnik, M., Thomas, W., 2015: Geofluids. Developments in Microthermometry, Spectroscopy, Thermodynamics, and Stable Isotopes. Elsevier.

Leeder, O., Thomas, R., Klemm, W., 1987: Einschlüsse in Mineralien. VEB Deutscher Verlag für Grundstoffenindustrie, Leipzig, 180 pp.

Parnell, J., ed., 1994: Geofluids: Origin, Migration and Evolution of Fluids in Sedimentary Basins. Geological Society Special Publication 78, 372 pp.

Roedder, E., 1984: Fluid inclusions. Reviews in Mineralogy 12, 644 pp. Samson, I., Anderson, A., Marshall, D., 2003: Fluid inclusions - Analysis and Interpretation. Short Course

Series 32, Mineralogical Association of Canada, 374 pp. Shepherd, T.J., Rankin, A.H., Alderton, D.H.M., 1985: A practical guide to fluid inclusion studies. Blackie-

Glasgow, 239 pp. Shmulovich, K.I., Yardley, B., Gonchar, G.G., 1995: Fluids in the crust. Equilibrium and transport

properties. Chapman & Hall, 323 pp. Walther, J.V., Wood, B.J., eds. 1986: Fluid-rock interactions during metamorphism. Advances in Physical

Chemistry 5, Springer-New York Wilkinson, J.J., 2001: Fluid inclusions in hydrothermal ore deposits. Lithos 55, 229-272.

ice

water

steam

su

pe

rcritica

lflu

id

triple point(0.01 ºC, 0.0006 MPa)

critical point(374 ºC, 22.06 MPa)

-200 600-100 0 100 200 300 400 500

50

0

10

20

30

40

temperature (ºC)

pre

ssure

(M

Pa)

s.f.

temperature (ºC)-100 -80 -60 -40 -20 0 20 40 60

CO (s)2

CO (l)2

CO (g)2triple point

(-56.6 ºC, 0.511 MPa)

critical point(31.1 ºC, 7.3 MPa)

log p

ressure

Fig. 1. Phase diagram for H O (upper panel) and CO (lower panel).2 2

50 450100 150 200 250 300 350 4000

1000

100

200

300

400

500

600

700

800

900

c.p.

1.0

0.9

5

0.9

0.8

5

0.8

0.7

5

0.7

0.6

5

0.6

0.5

50.5

0.4

50.

40.

3

0.2

0.15

0.1

0.08

0.060.05

0.030.04

0.01

temperature (ºC)

pre

ssure

(bar)

c.p.

0.08

0.9

c.p.

0.32

2

a b

c d

Fig. 2. a) Part of the phase diagram of H O with the condensation curve (thick line), critical point2

(c.p.) and isochores for H O fluids with different density. The density (in g·cm ) is indicated on2-3

each isochore. b,c,d) Examples of behavior of H O fluid inclusions with different density upon2

heating. The trapping conditions are indicated by a little cross. The ratios of the liquid and gasphases shown are approximate. The variables on the axes and the scale is the same as in a).Details in text.

0 10010 20 30 40 50 60 70 80 90-40

40

-30

-20

-10

10

20

30

0

L

ice + L hydrohalite+ L

ice + hydrohalite

hydrohalite + halite

halite + L

weight % NaCl

tem

pera

ture

(ºC

)

0 10 20 30 40-40

40

-30

-20

-10

10

20

30

0

weight % NaCl

tem

pera

ture

(ºC

)

I

II

III

Tm,ice

TeIV

V

VI

VII

Fig. 3. a) Phase diagram for the system H O-NaCl. b) A portion of the2

system H O-NaCl with two compositions and the appearance of the fluid2

inclusions at different temperatures. Details in text.

a

b

Fig. 4. Summary homogenization temperature–salinity diagram illustratingtypical ranges for inclusions from different deposit types. From Wilkinson (2001).

Fig. 5. Schematic diagram showing typical trends in Th –salinityspace due to various fluid evolution processes. From Wilkinson (2001).

Fig. 6. A population of fluid inclusions which trapped a homogeneousfluid. Note that the volume ratios of liquid/gas are approximately thesame in all inclusions. The image fromhttp://www.geochem.geos.vt.edu/fluids/. Scale unknown.

Fig. 7. Two fluid inclusions trapped from a heterogeneous (boiling)fluid. The larger inclusion contains only water vapor, the smaller oneliquid and gas. Epithermal system near , Slovakia. Photo J.Dolná VesMajzlan.

100 m�

Fluid Inclusions – Flüssigkeitseinschlüsse An Introduction – Eine Einführung

Rohstoffgeologie MGEO 2.2 SoSe 2017 Flüssigkeitseinschlüsse und Mikrothermometrie

Was sind Flüssigkeitseinschlüsse?

Formal: Ein Defekt in einem Kristall in Form eines Einschlusses Entstehung: a) bei der Neubildung eines Minerals in Anwesenheit eines „Fluids“

b) bei der „Heilung von Brüchen/Rissen“ in bereits existierenden Mineralen

Henry Clifton Sorby (1826 – 1908) Fig.: Sorby (1858)

Flüssigkeitseinschlüsse sind chemisch/physikalische Speicher - Mikrothermometrie - Salinität - Haupt- und Spurenelementzusammensetzung - Isotopen

Mikrothermometrie: Untersuchung der Flüssigkeitseinschlüsse um Informationen über die geologischen Ereignisse zu erhalten, welche zur Bildung der Wirtsminerale/Gesteine geführt haben.

Mikrothermometrie: Physikalisch-chemische Daten der Flüssigkeitseinschlüsse (P-T-X)

Charakteristika Flüssigkeitseinschlüsse sind in der Regel (sehr) klein • Exemplare >mm sind Museumsstücke • 3 – 20 µm ist die typische Größe die zur Mikrothermometrie

• Die Fluidphase kann eine Flüssigkeit oder Dampf sein, kann eine wässrige Lösung sein, kann Volatile wie CO2, CH4, N2, etc., flüssige Kohlenwasserstoffe, oder Schmelze enthalten

Hunterian Collections Universität Glasgow

Audétat et al., 2008 DOI: 10.2113/gsecongeo.103.5.877

Klassifizierung der Flüssigkeitseinschlüsse

2 phasig (Flüssigkeit + Gas) - Flüssigkeitsdominierend - Gasdominierend

Fig.: Goldstein (2003)

Klassifizierung der Flüssigkeitseinschlüsse

3 phasig (Flüssigkeit + Gas + Festphase)

Wilkinson 2001

Klassifizierung der Flüssigkeitseinschlüsse

CO2-führende Einschlüsse

Klassifizierung der Flüssigkeitseinschlüsse

Erdöl-führende Einschlüsse

Dutkiewicz et al., 1998 doi:10.1038/27644

Klassifizierung der Flüssigkeitseinschlüsse

Schmelzeinschlüsse

Anderson 2003

Bildung von primären und sekundären Flüssigkeitseinschlüssen

Primär

Einschlüsse die sich während des Kristallwachstums bilden. Meist entlang

von Wachstumszonen

Sekundär

Einschlüsse die sich nach dem Kristallwachstum bilden. Meist in

remineralisierten „geheilten“ Mikrorissen im Mineral

2 m

m

Audétat et al., 2008 DOI: 10.2113/gsecongeo.103.5.877

Fig.: Goldstein (2003)

Mikrothermometrie – Was? Wie? Warum?

Schmelztemperatur Stoffe die den Schmelzpunkt erniedrigen? z.B. CO2 -> CH4, H2S, N2 Die Temperatur der ersten Schmelze eines Stoffgemischs z.B. H2O + KCl + NaCl ist gleich der Eutektischen Temperatur des Kompenentensystems und somit ein Hinweis auf dessen Zusammensetzung

Homogenisierungstemperatur = die Temperatur zu welchem der Einschlussinhalt von der Mehrphasigkeit in die Einphasigkeit übergeht! Gehen wir davon aus, dass das Fluid bei zur Zeit der Einschlussbildung Einphasig war, muss die Homogenisierungstemperatur gleich der Mindestbildungstemperatur sein!

Um aussagekräftige Informationen über die Bedingungen während des Fluideinschlusses zu Erlangen, müssen die Einschlüsse 3 Annahmen erfüllen, welche als „Roedders Regeln“ bezeichnet werden.

1. Ein Einschluss ist wirklich ein EINschluss = eine einzelne homogene Phase zum Zeitpunkt des Einfangens

2. Nichts wurde nach dem „Einfangen“ des Einschlusses entfernt oder hinzugefügt.

3. Das Volumen des Einschlusses blieb nach dem „Einfangen“ konstant = Isochores System

Befolgt dieser Einschluss Roedder‘s Regeln?

Vorführender

Präsentationsnotizen

Sind die Phasen Homogen oder finden sich Ansammlungen von Einschlüssen? Ist die Anzahl und der Anteil der Phasen gleich? Verhalten sich die Einschlüsse während der Mikrothermometrischen Untersuchung gleich?

Vorführender

Präsentationsnotizen

Erfüllen die FI alle drei Annahmen kann ein FI auf Homogenisierungstemperatur und Salinität hin untersucht werden! Temperatur: Wir haben hier einen Auschnitt für das PT Diagram für reines Wasser; Es handelt sich um ein Isochores System = das Volumen; die Dichte bleibt imemr gleich und Zustandsänderungen können nur bei gleichbleibeder Dichte erfolgen Dichten in kg/m3; kritischer Punkt kein Unterschied zwischen den Dichten der Gas und der Flüssigphase – an diesem Punkt gibt es quasi keine unterschiedlichen Phasen!

Vorführender

Präsentationsnotizen

Dichten in g/cm3

Vorführender

Präsentationsnotizen

Ab der Homogeniesierungstemperatur beginnt die Isochore auf der Siedekurve ihre Fortsetzung in das homogene Zustandsfeld!

Roedder and Bodnar (1980)

Vorführender

Präsentationsnotizen

Je mehr Phasen sie in eine System intregrieren um so komplizierter wird das System! Das ist ein verzerrtes PTX Diagramm des Systems H2O NaCl; Der Eutektische Punkt (= Punkt der Phasengleichheit) für dieses System liegt bei -21.2°C und 23.2 wt.% NaCl ; Wir schauen uns jetzt einen kleinen Auschnitt

Vorführender

Präsentationsnotizen

Auschnitt aus dem dem PTX Diagram für das System NaCl-H2O; Sie haben verschiedene Stabilitätsfelder in Abhängigkeit der Salinität und der Temperatur; Hydrohalit usw: Jetzt blenden wir aber bitte aus, dass wir schon wissen, dass wir in diesem System bei dieser Salainität sind und betrachten den Flüssigkeitseinschluss! Wir wollen bei diesem Einschluss wissen: A ist er überhaupt Salin? Welches Salz ist darin gelöst und zu welcher Konzentration? In salinen System sind dabei die wichtigsten Informationen – bei welcher Temperatur habe ich die erste Eisschmelze und zu welcher Temperatur die letzte?

Die Temperatur der ersten Schmelze = die eutektische Temperatur

NaCl -21.2°C KCl -10.7°C CaCl2 -49.8°C MgCl2 -33.6°C NaCl-KCl -22.9°C NaCl-CaCl2 -52.0°C NaCl-MgCl2 -35.0°C

Die Temperatur der letzten Schmelze = Hinweis auf die Salinität

Welche Untersuchungsmethoden gibt es noch?

Laser-Raman-Spektroskopie

Zerstörungsfreie Technik Untersuchung der Vibration der Molekularbindungen = Informationsgewinn über die Art von Volatilen und Festphasen

Welche Untersuchungsmethoden gibt es noch?

LA-ICPMS

Albrecht et al., 2014 DOI: 10.1039/C4JA00015C

Guo and Audetat 2016 DOI: 10.1016/j.gca.2016.11.029

LA-ICPMS Fluid Inclusion Data Skarns Zn 5000 – 10,000 ppm Pb 500 – 5,000 ppm Ag 5 – 50 ppm Porphyries Cu 2000 – 10,000 ppm Mo 500 – 1,500 ppm Au 80 – 800 ppb Ulrich et al. 1999 Williams-Jones and Heinrich 2005 Klemm et al. 2008 Samson et al., 2008

Warum untersuchen wir Flüssigkeitseinschlüsse? Flüssigkeitseinschlüsse liefern Informationen über: - Temperatur bei der Erzentstehung - Druck bei der Erzentstehung - Fluidentwicklung (bei der Entstehung von Vererzung, aber nicht nur) - Lokalisierung von Fluidbewegungen (meist fossil) - Identifizierung von Zonen hochgradiger Vererzung - Abgrenzung von Mineralisationsevents und Lagerstättentypen

Aus: Wilkinson 2001

Aus: Wilkinson 2001

Aus: Hedenquist et al., 1998

Einige weitere Punkte... - Können Flüssigkeitseinschlüsse für die Exploration von Lagerstätten wichtig sein?

• z.B. CO2 – haltige FI typisch für Qz-Au-Ganglangerstätten (Ho, 1987) • Co-genetisches Auftreten von V + L –reichen FI Anzeiger für „boiling-zones in Epithermalen Lagerstätten (Kamilli and Ohmoto, 1977) • Bestimmte Variationen in der Th wurde als typisch für orogene Au-Ag Lagerstätten beschrieben (Spooner, 1981; Vikre, 1989) • Tochterminerale können Anzeiger für Erzreiche Zonen sein

Zusammenfassend: FI und deren Analyse können eine kleine aber wichtige Rolle in der Exploration spielen! Und die Zukunft?

MORGEN

Eine kurze Übung bei Zeiss