Natural versus experimental control of oxidation state: Effects on the composition and speciation of C-O-H fluids

Journal Article · · American Mineralogist; (USA)

OSTI ID:7189185

Luth, R W ^[1]

---

1. Carnegie Institution of Washington, DC (USA)

In high-pressure, solid-media apparatus, the oxidation state of a sample is usually controlled by imposing a known hydrogen fugacity (f{sub H2}) on the system. The oxidation state of natural systems, in contrast, is often defined by an internally or externally fixed oxygen fugacity (f{sub O2}). This contrast has fundamental implication for the application of experimental results to natural systems containing C-O-H fluids. The composition and speciation of a C-O-H fluid were calculated for a given pressure, temperature, and either an imposed f{sub H2} or an imposed f{sub O2}. The requisite fugacity coefficients were calculated with the modified Redlich-Kwong equation of state (MRK EOS) of Holloway. Comparison of the composition and speciation of the fluid calculated with the two sets of constraints illustrates the differences between the imposed-f{sub O2} and the imposed-f{sub H2} cases. To achieve fluid compositions with values of f{sub O2} near those of the quartz-fayalite-magnetite (QFM) buffer, as proposed for many regions of the lower crust and upper mantle, the oxidation state of an experiment that contains a mixed-volatile fluid must be controlled by a buffer that has a higher value of f{sub O2} than does QFM, because of the imposition of f{sub H2} rather than f{sub O2} on the sample. Furthermore, the f{sub O2} of the buffer required to maintain the f{sub O2} of the sample at QFM increases with decreasing {chi}(H{sub 2}O) in the vapor; such continuous variation in the f{sub O2} of the buffer required to maintain a constant f{sub O2} of the sample is not attainable with present experimental applications of the f{sub O2} buffers.

• OSTI ID: 7189185
• Journal Information: American Mineralogist; (USA), Journal Name: American Mineralogist; (USA) Vol. 74:1-2; ISSN AMMIA; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALI METAL COMPOUNDS
ALKANES
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
CALCULATION METHODS
CARBON
CARBON COMPOUNDS
CARBON DIOXIDE
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
EARTH MANTLE
ELEMENTS
EQUATIONS
EQUATIONS OF STATE
FLUIDS
GEOTHERMAL SYSTEMS
HYDROCARBONS
HYDROGEN
HYDROGEN COMPOUNDS
IRON COMPOUNDS
IRON ORES
IRON OXIDES
MAGMA SYSTEMS
MAGNETITE
METALS
METHANE
MINERALS
NICKEL
NICKEL COMPOUNDS
NICKEL OXIDES
NONMETALS
ORES
ORGANIC COMPOUNDS
OXIDATION
OXIDE MINERALS
OXIDES
OXYGEN
OXYGEN COMPOUNDS
QUARTZ
RESERVOIR FLUIDS
RESERVOIR PRESSURE
RESERVOIR TEMPERATURE
SILICON COMPOUNDS
SILICON OXIDES
SODIUM COMPOUNDS
SODIUM OXIDES
TRANSITION ELEMENT COMPOUNDS
TRANSITION ELEMENTS
WATER