Ž . Lithos 55 2001 49–68 www.elsevier.nlrlocaterlithos Carbonic inclusions Alfons Van den Kerkhof a, ) , Regis Thiery b ´ ´ a IGDL, UniÕersity of Gottingen, Goldschmidtstrasse 3, 37077 Gottingen, Germany ¨ ¨ b UMR 6524 ‘Magmas et Volcans’, OPGC, Blaise Pascal UniÕersity 5, rue Kessler, 63038 Clermont-Ferrand, France Received 15 September 1999; accepted 4 April 2000 Abstract The paper gives an overview of the phase relations in carbonic fluid inclusions with pure, binary and ternary mixtures of the system CO –CH –N , compositions, which are frequently found in geological materials. Phase transitions involving 2 4 2 liquid, gas and solid phases in the temperature range between y1928C and 318C are discussed and presented in phase Ž . diagrams PT , TX and VX projections . These diagrams can be applied for the interpretation of microthermometry data in Ž . order to determine fluid composition and molar volume or density . q 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluid inclusions; Carbon dioxide; Methane; Nitrogen; Microthermometry 1. Introduction Gas compositions of non-aqueous fluid inclusions in rocks can be generally represented by the system Ž CO –CH –N . Pure gas compositions mostly CO , 2 4 2 2 . sometimes CH or N are commonly found in fluid 4 2 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 Ž . CO may be important e.g. Huizenga, 2001 . CO is 2 the most common carbonic component in high-grade ) Corresponding author. Ž . E-mail address: akerkho@gwdg.de A. Van den Kerkhof . Ž . metamorphic rocks Touret, 1977 ; considerable amounts of CH may form in rocks which have been 4 chemically re-equilibrated during retrograde condi- tions: 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 conse- quence of low oxygen fugacity, methane is a com- mon gas compound in diagenetic and low-grade Ž . metamorphic rocks Mullis, 1979 . Nitrogen is an important gas component in many rock types, from early diagenetic towards high-grade metamorphic Ž . rocks Dubessy and Ramboz, 1986 and assumed to form by the breakdown of NH q -containing feldspar 4 and mica during prograde metamorphism. N -rich 2 fluid inclusions have been frequently found in parag- Ž neissic eclogite Andersen et al., 1990; Klemd et al., . 1992 . Carbonic fluids in rocks may be of magmatic Ž . origin Touret, 1992 , formed by the decomposition 0024-4937r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0024-4937 00 00038-4