The P-V?-T-X-f O2 evolution of H2O-CO2-CH4-bearing fluid in a wolframite vein: Reconstruction from fluid inclusion studies

Aqueous-carbonaceous and later pure aqueous fluid inclusions in quartz from a ferberite (Fe_.95Mn_.05 WO₄) vein within the low-grade metamorphic aureole of the Borne granite (French Massif Central) have been studied by microthermometry and Raman spectrometry. The bulk V?-X properties of the aqueous-carbonaceous inclusions have been derived using the equation of state of Heyenet al. (1982) for the low-temperature CO₂-CH₄ system. A P-T path has been proposed for their trapping using the equations of state of Jacobs and Kerrick (1981a) for the H₂O-CO₂-CH₄ system. Two main episodes were reconstructed for the history of the aqueous-carbonaceous fluid. (1) Primary H₂O-CO₂-CH₄ vapourrich inclusions in quartz indicated the early circulation of a low-density fluid (65 mole% H₂O-34 mole% CO₂-1 mole% CH₄ and traces of N₂: d = 0.35 gcm^?3) at around 550° ± 50°C and 700 ± 100 bar. Fluid cooled approximately isobarically to 450°-400°C and was progressively diluted by H₂O with a concomitant increase in density. The fO₂ of the H₂OCO₂-CH₄ fluid, estimated from the equilibrium CO₂ + 2H₂O CH₄ + 2O₂, first ranged from 10^?22 to 10^?27 bar, close to the Q-F-M buffer. Within analytical errors, these values were consistent with the presence of graphite in equilibrium with the fluid. (2) A drop in PCO₂, and therefore a drop in fO₂, was recorded by the secondary liquid-rich inclusions in quartz. The inclusions, formed at and below 400°C, were composed of H₂O and CH₄ only, and fO₂ at that stage was below that fixed by the graphite-fluid equilibrium. This second episode in the fluid-rock system could be explained by the drop of temperature below the blocking temperature of the graphite-fluid equilibrium. According to this interpretation, the blocking of the graphite-fluid equilibrium occurred at T ≥ 370°C and probably at 400°C on account of the pressure correction. Mass spectrometric data show that ferberite contains H₂O, CO₂ and CH₄ in fluid inclusions, which lie in the gap of the V?-X properties of the aqueouscarbonaceous fluid in quartz. Deposition of ferberite probably occurred at around 400°C, the previously inferred blocking temperature, resulting from either the drop in PCO₂, the drop fO₂ and/or the related pH-increase.It is concluded that the existence of a blocking-temperature for the graphite-fluid chemical equilibrium may be a critical factor for maintaining a stable fluid pressure gradient in geothermal systems occurring under greenschist facies conditions in graphite-bearing rocks.