Ž . Lithos 55 2001 101–114 www.elsevier.nlrlocaterlithos Thermodynamic modelling of C–O–H fluids Jan-Marten Huizenga ) Department of Geology, Rand Afrikaans UniÕersity, PO Box 524, Auckland Park 2006, Johannesburg, South Africa Received 7 March 2000; accepted 25 April 2000 Abstract H O, CO , CH , CO, H and O are the most important species in crustal fluids. The composition of these C–O–H 2 2 4 2 2 fluids can be calculated if the pressure, temperature, carbon activity, and either the oxygen fugacity or the atomic HrO ratio of the fluid is known. The calculation methods are discussed and calculation results are illustrated with isobaric T– X , P– T , i Ž . and isobaric–isothermal ternary C–O–H diagrams. Fluid inclusion compositions, in particular, the X r X qX CO CO CH 2 2 4 ratio, can be used for C–O–H model calculations. However, care should be taken about possible post-entrapment changes, which may have modified the chemical composition of the fluid inclusion. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Fluid speciation; Thermodynamic modelling; C–O–H system; Fluid inclusions 1. Introduction Many metamorphic and igneous fluids can be described by three components: carbon, oxygen and Ž . hydrogen the C–O–H system . The principal fluid species of geological interest in this system are: Ž H O, CO , CH , CO, H and O French, 1966; 2 2 4 2 2 . Ohmoto and Kerrick, 1977; Holloway, 1987 . French Ž . Ž . 1966 and Ohmoto and Kerrick 1977 have demon- strated that this system can be used to calculate the composition of crustal fluids as a function of P , fluid Ž fluid . T , and either the oxygen fugacity f or the HrO O 2 atomic ratio of the fluid. C–O–H model calculations have been used by many authors for a variety of geological subjects, such as fluid equilibria at low- Ž . grade metamorphism e.g., Holloway, 1984 , gran- ) Fax: q 27-11-489-2309. Ž . E-mail address: jmh@na.rau.ac.za J.-M. Huizenga . Ž ulite genesis e.g., Glassley, 1982; Lamb and Valley, . 1985; Skippen and Marshall, 1991 and devolatilisa- Ž tion reactions in graphitic rocks Connolly and Ce- . sare, 1993; Connolly, 1995 . This paper reviews the method for C–O–H model calculations. The results are shown in different dia- grams and illustrate the influence of different vari- Ž fluid . ables i.e. P , T , f , atomic HrO ratio on the fluid O 2 fluid speciation. The applications and limitations of C–O–H model calculations to fluid inclusion studies will also be discussed. 2. Calculation method 2.1. Basic principles The calculation method used follows the proce- Ž . dure given by French 1966 and is based on the 0024-4937r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0024-4937 00 00040-2