Bull. Minéral.
(1988), 111, 257-266
Phase transitions and molar volumes of C02-CH4-N2 inclusions
by Alphonse M. van den KERKHOF
Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085,
1081 HV Amsterdam, The Netherlands.
Transitions de phase et volumes molaires des inclusions de composition CO2-CH4-N2.
Introduction
The application of analytical techniques to the study of fluid inclusions gives not only more insight into fluid compositions, but also opens the possibility for more reliable determinations of the molar volume. By means of Raman mi¬ crospectroscopy (“Raman analysis’’), it is pos¬ sible to measure the composition of the non-aqueous part of single inclusions containing gases in the C-O-H-N-S system. The use of a multichannel detector enables systematic and ac¬ curate measurements within a reasonable time (Burke and Lustenhouwer, 1987). The accuracy of the calculated compositions is generally better than 5 mole-percent. The combination of Raman analysis and microscope studies of phase beha¬ viour in fluid inclusions at low temperature (“microthermometry”) is most powerful.
Recent studies indicate that non-aqueous me-tamorphic and hydrothermal fluids are not exclusively composed of C02 : a wider compo¬ sitional range has been found for many fluids (Dhamelincourt et al., 1979 ; Guilhaumou et al., 198.1 ; Dubessy, 1985 ; Pasteris et al.,
1986 ; Darimont, 1986 ; van den Kerkhof, 1987). In particular, N2 and CH4 have been de¬ tected in variable amounts. By studying diffe¬ rent metamorphic rock types, it has been found that many inclusions can be approached as bi¬ nary systems. Mixtures between C02-N2 and CO2-CH4 appear to be representative of many natural fluids (Figure 1).
Microthermometry
Phase behaviour in the low temperature region expresses both the composition and the molar volume of fluid inclusions. Detailed microther¬ mometric observations are therefore necessary for reliable interpretations : not only homogeni¬ zation temperatures (Th) must be measured, but also a careful study of melting behaviour should be made. It is generally necessary to measure melting temperatures (Tm) with an accuracy of better than 0.5 °C in order to check the purity of C02. Furthermore, cooling to lower temperatu¬ res (to — 160 °C) is recommended to study phase behaviour in the low temperature region.
