Bulk composition, molar volume and isochore calculations

Apart from a subordinate number of aqueous fluid inclusions, the majority of inclusions shows melting of a solid phase within a temperature range of –59.2 to –56.6

4. Analytical methods

of the triple point of pure CO2 (-56.6 °C) by up to 2.6 °C is symptomatic of the presence of small quantities of additional gases such as CH4 or N2. However, nitrogen was the only additional gaseous species detected in CO2-dominated inclusions by Raman analysis. The presence of N2 could not always be confirmed by Raman spectrometry. In other cases the amount of N2 detected by Raman spectrometry was lower than expected from graphic estimations where homogenisation and melting temperatures were transferred into volume-composition (VX) properties using the diagrams provided by Thiery et al. (1994). Vice versa, Raman spectrometric measurements sometimes proved the presence of accessory gases, where microthermometry had indicated a pure carbonic fluid phase.

Possible reasons for these discrepancies can be summarised as following:

• The quality of Raman measurements is affected by many parameters e.g. the quality of wafer, inclusion size, shape and its position within the sample or its density (cf. Burke, 2001). No standards exist for the calibration of gas mixtures or the internal standard deviations of the Raman equipment. Although analytical conditions and methodology were tried to be optimised, it can not be completely excluded that one or another factor had a negative effect on the quality of the measurements, leading to less accurate results.

• Due to the mineral colour (e.g. in garnet), the presence of many small solid or fluid inclusions that becloud single crystals (often the case in feldspar), or the darkish appearance of many CO2-dominated inclusions, the quality of the observation of a phase transition may be limited. Consequently, the melting temperature measured by microthermometry does not correspond exactly to the actual content of the fluid inclusion although, the accuracy of the stages was determined to ± 0.2 °C for low temperature measurements and special techniques (e.g. cycling) had been applied.

• The use of VX diagrams to graphically determine fluid compositions or molar volumes is afflicted with a relatively large error. The VX values incorporated in the published diagram for CO2-N2 fluid inclusions are based on an experimental reproduction of TPX-data. Deviations caused by inaccuracies may be sizeable especially in the critical region. For some inclusions, no data on composition or molar volume can be derived as no point of intersection exists for the curves of the melting- and homogenisation temperatures measured.

The deviation between compositional values derived from Raman analysis and those determined graphically may be as high as 7 mole % (e.g. incl. no. 2176-2-24).

Albeit, the fact that the results attained by applying several methods do not always agree with each other, the error concerning the exact fluid composition lies within acceptable analytical limits. Particularly, if one considers the errors that occur during

molar volume/density and isochore calculations that result from the limited applicability of available equations of state.

Densities were calculated for all fluid inclusions whose composition could have been derived from either method, and thus guide values were obtained for inclusion assemblages. Nevertheless, to avoid possible miscalculations or misinterpretations, isochores were calculated only for representative fluid inclusions whose composition known from Raman microanalysis is in close accordance with results from graphic estimates.

The bulk compositions, molar volumes/densities and isochores of individual fluid inclusions were calculated from microthermometry data, Raman analysis and volume fraction estimates using the software packages Fluids (Bakker, in press) and Clathrates (Bakker, 1997). The programs provide several equations of state (EOS) for the calculations of PVTX properties of varying fluid systems. In consideration of the particular fluid system and the accuracy and limitations of the corresponding equations of state, the suitable equations were chosen for further calculations (see below). During all isochore computations the program took into account the compressibility and expansion of the hostmineral with the volumetric data for quartz taken from Hosieni et al. (1985) and from Berman (1988) for all other minerals. If the amount of N2 detected by Raman spectrometry or graphic estimations did not exceed 2 mol%, fluid properties were calculated as being equivalent to pure CO2. The error in molar volume calculations resulting from this assumption is by far smaller than liquid-vapour equilibrium calculations with published equations of state.

Pure CO₂

Molar volumes of pure CO2 fluid inclusions were obtained from the homogenisation temperatures using the equation of Duschek et al. (1990), and isochore calculations are based on the equation of state of Span & Wagner (1996).

CO2-N2

VX properties of mixed CO2-N2 fluids were determined from homogenisation temperatures and Raman data according to the EOS of Thiery et al. (1994), based on the modelling of PTX-conditions by the Soave-Redlich-Kwong EOS (Soave, 1972) and molar volumes by the Lee-Kesler correlation (Lee & Kesler, 1975). Isochores were calculated applying the EOS of Duan et al. (1992, 1996).

H2O-salt (NaCleq)

Fluid salinities (in wt% NaCleq) were determined from final ice melting temperatures applying the data of Bodnar (1993). The EOS for bulk fluid density and isochore calculations given by Zhang & Frantz (1987) was used for further computations.

4. Analytical methods

H₂O-CO₂±N₂±salt

Complex fluid systems containing mixtures of several gases (e.g. CO2-CH4-N2), H2O and salt are not yet accurately investigated by experimental studies. Consequently, data on solvus PVTX properties are not available and homogenisation temperatures can not be directly transformed into bulk molar volumes/densities but depend on estimation of volume fractions of the fluid phases present.

If clathrate melting in presence of a heterogeneous carbonic phase was observed in complex fluid systems, the program Q2 from the software package Clathrates (Bakker, 1997) was used for the calculation of bulk fluid properties applying the EOS of Duan et al. (1992, 1996) and Thiery et al. (1994) (H2O-CO2-N2-salt) or Duschek et al. (1990) (H2 O-CO2-NaCl) in combination with volume fraction estimates. Isochores were calculated according to Bowers & Helgeson (1983) modified by Bakker (1999).

In salt free complex systems (H2O-CO2-N2), volume fraction estimates were combined with volumetric properties calculated after Thiery et al. (1994) whereas isochores were calculated with the equation of state of Holloway (1977).