oxygen fugacity
?
oxidation state
oxidation state
Fe2+,
Fe3+,
2+
Cr ,
3+
Cr
2+
V ,
3+
V ,
Fe0
4+
V ,
3d6, 3d5, 3d6 4s2
3d4, 3d3
5+
V
3d3, 3d2, 3d1, 3d0
oxygen fugacity
Equilibrium configuration defined where G reaches a minimum, or dG = 0
For a gas at P,T:
dG = -S dT + V dP
PV = n RT
at constant T:
dG = V dP = RT/P dP = RT d lnP
∫ dG = ∫ RT/P dP
ΔG = ΔG° + RT ln P
for an ideal gas, f = P
and for gases at low pressure, f ≈ P
Fugacity is measured in units of pressure
Castellan 1971
what is the oxygen fugacity of air?
At 1 bar assume ideal behaviour, so f ≈ P and fi ≈ Pi = xi Ptotal
Hence fO2 = xO2 × Ptotal = 0.209476 × 1 bar
so log fO2 = -0.68
xO2 of air is essentially constant with temperature, hence log fO2 (air) is also insensitive to
temperature
what is the oxygen fugacity of spinel peridotite?
6 Fe2SiO4(olivine) + O2 = 3 Fe2Si2O6(opx) + 2 Fe3O4(spinel)
olivine
ΔG = ΔG° + 3 RT ln aFe2Si2O6 + 2 RT ln aFe3O4
– 6 RT ln aFe2SiO4 – RT ln fO2
opx
spinel
Dlog fO2 (FMQ)
log fO2 = -log K + 3 log aFe2Si2O6 + 2 log aFe3O4 – 6 log aFe2SiO4
J
Fe3+/SFe
intuitive relation
between oxidation
state and oxygen
fugacity
oxygen fugacity
?
oxidation state
olivine
opx
cpx
spinel
Rob Lavinsky, iRocks.com
olivine
opx
cpx
spinel
Rob Lavinsky, iRocks.com
olivine
opx
0.00
cpx
0.10
spinel
0.11
0.09
whole rock Fe2O3 = 0.3 wt%
high oxygen fugacity
low Fe3+
Δlog fO2 (FMQ)
+5
0
olivine
-5
0
Fe3+/ΣFe
100
oxygen fugacity
?
oxidation state
bridgmanite
ferropericlase
Dan Frost
bridgmanite
ferropericlase
Dan Frost
bridgmanite
Velocity (mm/s)
ferropericlase
Velocity (mm/s)
McCammon (1997)
Lauterbach et al. (2001)
Frost et al. (2004)
McCammon et al (1997)
former bridgmanite
Velocity (mm/s)
ferropericlase
Velocity (mm/s)
McCammon et al (1997)
bridgmanite
3.9
ferropericlase
0.02
whole rock Fe2O3 = 3.9 wt%
low oxygen fugacity
high Fe3+
Δlog fO2 (FMQ)
+5
0
bridgmanite
-5
0
Fe3+/ΣFe
100
+ Fe0
oxygen fugacity
?
J
oxidation state
why should you care?
oxidation state
oxygen fugacity
physical properties
volatile speciation
elasticity, electrical conductivity,
thermal diffusivity, rheology,
viscosity, …
→ LLSVPs, plumes, MiMOSAs,
lithosphere
chemical properties
melting, element partitioning, …
carbon, hydrogen, nitrogen, …
→ slab processes
Earth differentiation
including core formation
evolution of Earth’s
atmosphere