Re-equilibration of CO2fluid inclusions at controlled hydrogen fugacities

摘要: Natural, pure CO2 inclusions in quartz and olivine (c. Fo90) were exposed to controlled fH2 conditions at T= 718–728°C Ptotal= 2 kbar; their compositions monitored (before after exposures) by microsampling Raman spectroscopy (MRS) microthermometry. In both minerals the graphite–methane buffer (fH2= 73 bar), fluid speciations record diffusion of hydrogen into inclusions. quartz, room-temperature products euhedral isolated (EI type) are carbonic phases with molar c. CO2(60) + CH4(40) plus graphite (Gr) H2O, whereas anhedral along secondary fractures (AS Gr-free contain H2O range CO2(10) CH4(90). EI type evolved CO2(90–95) CH4(5–10) without Gr, AS have a from CO2(90) CH4(10) ± Gr CH4(50) H2(50) Gr; neither nor any hydrous species was detected optical microscopy or MRS olivine-hosted products. Differences composition between among texturally distinct populations probably arise variations initial densities, as all apparently equilibrated ambient fH2. These relations suggest that compositional variability given natural sample does not require entrapment multiple generations fluids. addition, absence would extraction oxygen fluids, which case re-equilibration mechanisms may be dependent on structure host mineral. Many same samples re-exposed identical P–T using Ar pressure medium, yielding fH2= 0.06 bar. most inclusions, fluids returned persisted Reversal prior exposure bar did occur but types olivine, minor produced expense CH4 and/or graphite. The observed non-reversibility previous attributed to: (1) slower fluid–solid reactions compared homogeneous phase; (2) depressed activities due poor ordering; (3) low fO2 second run.