The P- V̄-T-X-f O 2 evolution of H 2O-CO 2-CH 4-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 WO 4) vein within the low-grade metamorphic aureole of the Borne granite (French Massif Central) have been studied by microthermometry and Raman spectrometry. The bulk V̄-X properties of the aqueous-carbonaceous inclusions have been derived using the equation of state of HEYENet al. (1982) for the low-temperature CO 2-CH 4 system. A P-T path has been proposed for their trapping using the equations of state of JACOBS and KERRICK (1981a) for the H 2O-CO 2-CH 4 system. Two main episodes were reconstructed for the history of the aqueous-carbonaceous fluid. (1) Primary H 2O-CO 2-CH 4 vapourrich inclusions in quartz indicated the early circulation of a low-density fluid (65 mole% H 2O-34 mole% CO 2-1 mole% CH 4 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 H 2O with a concomitant increase in density. The fO2 of the H 2OCO 2-CH 4 fluid, estimated from the equilibrium CO 2 + 2H 2O CH 4 + 2O 2, 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 PCO 2, 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 H 2O and CH 4 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 H 2O, CO 2 and CH 4 in fluid inclusions, which lie in the gap of the V̄-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 PCO 2, 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.