Carbonic inclusions
Introduction
Gas compositions of non-aqueous fluid inclusions in rocks can be generally represented by the system CO2–CH4–N2. Pure gas compositions (mostly CO2, sometimes CH4 or N2) are commonly found in fluid inclusions, but in many cases, gas mixtures occur. Besides these three gas compounds and water, no other fluid compounds in the system C–O–H–N are normally stable in rocks at room temperature, or they occur in very low concentrations. However, at high temperatures during metamorphism, compounds like CO may be important (e.g. Huizenga, 2001). CO2 is the most common carbonic component in high-grade metamorphic rocks (Touret, 1977); considerable amounts of CH4 may form in rocks which have been chemically re-equilibrated during retrograde conditions: the oxygen fugacity being buffered by the rock minerals is reduced towards lower temperatures and may cause a change of the composition of the fluid, which is in equilibrium with the rock. As a consequence of low oxygen fugacity, methane is a common gas compound in diagenetic and low-grade metamorphic rocks (Mullis, 1979). Nitrogen is an important gas component in many rock types, from early diagenetic towards high-grade metamorphic rocks (Dubessy and Ramboz, 1986) and assumed to form by the breakdown of NH4+-containing feldspar and mica during prograde metamorphism. N2-rich fluid inclusions have been frequently found in paragneissic eclogite Andersen et al., 1990, Klemd et al., 1992. Carbonic fluids in rocks may be of magmatic origin (Touret, 1992), formed by the decomposition of organic matter, by oxidation reactions involving graphite or as a reaction product of decarbonization in calc-silicate rocks. In granulite rocks from Bamble (Norway), isotopic evidence has been found for juvenile CO2 Hoefs and Touret, 1975, Van den Kerkhof et al., 1994. During the metamorphic evolution, fluids are normally repeatedly re-equilibrated and may be re-trapped at subsequent stages during uplift. Being controlled by the mineral buffer (in respect of fO2, fN2, etc.) fluid compositions normally result in (pseudo)binary gas mixtures of CO2–CH4 (reducing) or CO2–N2 (oxidizing) Bakker and Jansen, 1993, Huizenga, 1995, Huizenga, 2001; ternary mixtures of about equal amounts of CO2, CH4 and N2 are rare in natural fluid inclusions (Van den Kerkhof, 1988).
The first step and aim of microthermometry experiments (eventually combined with Raman analysis) is the determination of the fluid molar volume (or the density) and the composition, the key parameters to calculate the trapping pressure and temperature (isochore calculation) with the use of an appropriate equation of state (e.g. Holloway, 1977, Kerrick and Jacobs, 1981, Bottinga and Richet, 1981, Brown, 1989). Fig. 1 shows isochores for CO2, CH4 and N2, derived from the literature.
CO2 with admixtures of other components can be identified by melting temperatures (Tm) markedly lower than −56.6°C, the triple point of CO2. Methane and nitrogen in carbonic inclusions have considerable effect also on the homogenization temperature (Th). Burruss (1981) introduced the so-called VX diagram, showing the variation of the molar volume (V) with the composition (X) for a given temperature, to demonstrate these effects for the system CO2–CH4. With the help of these diagrams, the molar volume and composition of fluid inclusions can be derived directly from Tm and Th data. The accuracy of the diagrams (besides for CO2–CH4 also constructed for CO2–N2 and CO2–H2S, etc.) has been improved by new experimental data and the development of more accurate equations of state. Furthermore, the compositional and volumetric ranges of application could be extended and cover now the properties of most natural fluid inclusions. The phase transitions in carbonic inclusions with more than 20 mol% CH4 and/or N2 in the mixture deviate considerably from ‘pure’ fluids Guilhaumou et al., 1981, Van den Kerkhof, 1988. In this paper, an overview of most important phase behavior of gas mixtures in fluid inclusions is given. It is shown that the phase behavior during microthermometry runs is uniquely given for any possible combination of molar volume and composition.
Section snippets
Phase transitions in pure gas inclusions
Contrary to water, carbonic inclusions show phase transitions always at lower temperatures than the geological conditions at which they are formed. Above 31.1°C (the critical temperature of CO2), gas inclusions are always in a supercritical state and essentially monophase. This means that phase separation at geological conditions is unimportant as far as water is not considered. Aqueous inclusions are normally heated in order to study the phase behavior of fluid inclusions at the conditions of
The binary systems CO2–CH4 and CO2–N2
Carbonic fluid inclusions, which are not pure in composition in most cases, can be considered as binary or pseudobinary mixtures of CO2–CH4 or CO2–N2. Fig. 4 shows the main elements of a PT phase diagram for the system CO2–CH4 including phase transitions of a fluid inclusion with about 30 mol% CH4 and molar volume of about 50 cm3/mole. Compared to the pure systems (cf. Fig. 2), additional forms appear in these phase diagrams: the ternary solid–liquid–gas (SLG) phase equilibria and the critical
VTX-phase diagrams
Current diagrams mostly show projections of pressure (P), temperature (T), and composition (X), i.e. isopleths (constant X) in a PT diagram. However, fluid inclusions have constant volume (V) and constant bulk composition (X) whereas the temperature and pressure are variable; the pressure cannot be directly determined in fluid inclusions. Therefore, an alternative type of phase diagrams has been proposed which basically represents projections of the molar volume (V), the temperature (T) and the
Isochoric sections of the binary TX diagram
For CO2-rich inclusions with less than 20 mol% of other gas compounds, it is mostly impossible to distinguish between CO2–CH4 or CO2–N2-dominated mixtures without additional Raman analysis. However, for fluid inclusions with >80 mol% of N2 or CH4, the phase behavior is directly indicative for one of the two systems. These differences are illustrated by the phase diagrams shown in Fig. 6.
The principle forms in the diagrams are (1) the melting and sublimation curves, (2) the homogenization point
The interpretation of microthermometry data: the application of VX diagrams
VX diagrams are highly powerful for the interpretation of microthermometry data as the bulk VX properties (as well as the VX properties of the individual phases) can be directly deduced from phase transition temperatures. Additional Raman analysis (qualitative or quantitative) is not always essential as far as the system is known. The VX diagram was popularized by Burruss (1981) for the application to fluid inclusion studies and several diagrams have been published since then by compilation of
Tracing VX properties of the individual phases during microthermometry runs: bubble nucleation in CO2–CH4 fluid inclusions as an example
The VX diagram can be used also to determine the VX properties and relative volume proportions of the individual phases during cooling and warming experiments. Even highly complicated sequences of phase transitions can be easily reconstructed by graphical methods for any chosen V and X. As an example, we describe below a CO2–CH4 inclusion with XCH4=0.60 and V=49 cm3/mole (Fig. 11). After freezing (at about −85°C), bubble nucleation (SL→SLG) can be observed during subsequent warming. The
A classification of phase transition sequences: H- and S-type inclusions
The type of phase transitions directly reflects the composition and molar volume (density) of fluid inclusions. The description of complex phase behavior in fluid inclusions can be easily simplified. Van den Kerkhof, 1988, Van den Kerkhof, 1990 proposed a division in H-type fluid inclusions with Th>Tm (showing homogenization as the final phase transition) and S-type fluid inclusions with Th<Tm (showing solid disappearance, i.e. melting or sublimation as the final phase transition on warming),
Conclusion
Phase diagrams, which graphically present the composition, molar volume and the temperature of a system without direct indication of the pressure (and therefore exotics in physics and chemistry) are highly useful and uniquely developed for the study of fluid inclusions. During the last decades, the parallel development of technical equipment for the micro-analysis of fluid inclusions, i.e. improved heating/freezing stages and spectrometers, together with the progress made in the theoretical
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