Computerised Clathrate Stability Model: Specification of Metamorphic Fluids in Inclusions
VX properties of a fluid that has been trapped in crystals during metamorphism may give substantial information on geological processes that occurred within these rocks. Non-destructive analytical methods of individual fluid inclusions, such as Raman Spectroscopy and microthermometry may give sufficient information for a complete description of these bulk VX properties. Clathrate melting temperature obtained from microthermometry is an important phase change that occur at relative low temperatures (below room T) in inclusions containing H2O, gases such as CO2, CH4, and N2, and salts. Clathrate melting conditions define both the density of the gas-rich phase and the salinity of the aqueous solution, which can be related to bulk properties by its relative volume fraction estimates. Clathrate stability conditions are modelled according to a combination of ideal localised adsorption and statistical thermodynamics (Waals and Platteeuw, 1959), and are calculated within the programs Q2, DENSITY, and ICE (in turbo C++). The program Q2 calculates fluid properties at clathrate melting in the presence of two gas-rich phase, i.e. liquid and vapour (Th>Tclath). The program ICE uses the final ice melting temperature and clathrate melting temperature in the presence of one low density gas-rich vapour. The program DENSITY uses the metastable absence of a clathrate phase to estimate the density of the gas-rich phase (Th<Tclath). Prerequisites for each program are the Raman analysis of a homogeneous gas-rich phase. The volume fraction estimations are optional, but are required for bulk fluid properties estimates. ICE and DENSITY are improved versions of the algorithms originally developed by Dubessy et al. (1992) and Diamond (1992), respectively.
Selected fluid inclusions from Galicia (G) and the Spanish Central System (S) which display a large range of composition and density are used to illustrate the application of these computer codes.
In Galicia, metamorphic fluids have been circulating in granitic rocks hosting gold mineralisation (Boiron et al., 1996). The bulk composition of the fluid evolved from early CO2-rich fluids equilibrated with host rock mineral assemblages to late H2O-CO2-CH4-rich fluids of low gas density. This fluid evolution illustrated changes in the P-T conditions from early sulphide deposition in quartz veins (450°C, 150-300 MPa) towards epithermal conditions (250-300°C, <75 MPa). In the Spanish Central System, fluids are associated with wolframite and sulphides mineralisations in granites (Vindel et al., 1995) and are characterised by H2O-CO2-CH4-NaCl. This fluid is progressively enriched in CH4 and diluted by an aqueous fluid. The fluid is assumed to have originated in the surrounding metamorphic rock in equilibrium with graphite. Chlorite geothermometry indicates maximum trapping conditions of 330 ± 30°C. Isochore calculations from VX properties of the fluid inclusion obtained by the programs indicate a trapping pressure of 100 ± 20 MPa.
Computer codes allow accurate calculations of the V-X properties of fluids present in inclusions in the absence of global homogenisation temperatures. Salinity is now possible to calculate precisely using melting temperature of clathrate and / or ice. This new advancements in computer modelling are very useful for a better understanding of the P-T conditions of fluid migration through rocks during retrograde metamorphism.
This study has been supported by the program Human and Capital Mobility (contract CT930198) Hydrothermal/ Metamorphic water-rock interaction in crystalline rocks. E. Vindel and J. Lopez are thanked for the use of their samples from the Spanish Central System.
Boiron, M.C., Cathelineau, M., Banks, D., Yardley, B., Noronha, F. & Miller, M., Geochim. Cosmochim. Acta. (1996)
Diamond, L.W., Geochim. Cosmochim. Acta 56, 19-41 (1992).
Dubessy, J., Thiery, R., Canals, M., Eur. J. Mineral. 4, 873-884 (1992).
Vindel, E., Lopez, J.A., Boiron, M.C., Cathelineau, M. & Prieto, C., Eur. J. Mineral. 7, 675-688 (1995).
Waals, J.H., Platteeuw, J.C., Adv. Chem. Phys. 2, 1-57 (1959).
Table 1: Temperatures in °C, Vm in cc/mole, composition in mole %.