Other
Scientific paper
Jan 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49..205r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 1, pp.205-219
Other
18
Scientific paper
Aqueous-carbonaceous and later pure aqueous fluid inclusions in quartz from a ferberite (Fe .95 Mn .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 -X properties of the aqueous-carbonaceous inclusions have been derived using the equation of state of et 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 and (1981a) for the H 2 O-CO 2 -CH 4 system. Two main episodes were reconstructed for the history of the aqueous-carbonaceous fluid. (1) Primary H 2 O-CO 2 -CH 4 vapourrich inclusions in quartz indicated the early circulation of a low-density fluid (65 mole% H 2 O-34 mole% CO 2 -1 mole% CH 4 and traces of N 2 : 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 2 O with a concomitant increase in density. The f O 2 of the H 2 OCO 2 -CH 4 fluid, estimated from the equilibrium CO 2 + 2H 2 O 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 f O 2 , was recorded by the secondary liquid-rich inclusions in quartz. The inclusions, formed at and below 400°C, were composed of H 2 O and CH 4 only, and f O 2 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 2 O, CO 2 and CH 4 in fluid inclusions, which lie in the gap of the -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 f O 2 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.
Dubessy Jean
Ramboz Claire
Schnapper D.
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