oxygen fugacity oxidation state€¦ · fe3o4 – 6 log a fe2sio4 fe3+/sfe . d log f o 2 (fmq)...
TRANSCRIPT
? oxidation state oxygen fugacity
oxidation state
Fe2+, Fe3+, Fe0 3d6, 3d5, 3d6 4s2
Cr2+, Cr3+ 3d4, 3d3
V2+, V3+, V4+, V5+ 3d3, 3d2, 3d1, 3d0
oxygen fugacity
Castellan 1971
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
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?
olivine
spinel
opx
6 Fe2SiO4(olivine) + O2 = 3 Fe2Si2O6(opx) + 2 Fe3O4(spinel) ΔG = ΔG° + 3 RT ln aFe2Si2O6 + 2 RT ln aFe3O4 – 6 RT ln aFe2SiO4 – RT ln fO2 log fO2 = -log K + 3 log aFe2Si2O6 + 2 log aFe3O4 – 6 log aFe2SiO4
Fe3+/SFe
Dlo
g fO
2 (FM
Q) intuitive relation
between oxidation state and oxygen fugacity
J
? oxidation state oxygen fugacity
Rob Lavinsky, iRocks.com
olivine opx
spinel cpx
Rob Lavinsky, iRocks.com
olivine opx
spinel cpx
olivine opx
spinel cpx
0.00 0.10
0.11 0.09 whole rock Fe2O3 = 0.3 wt%
Fe3+/ΣFe
Δlog
fO2 (
FMQ
)
0 100
0
-5
+5
olivine
high oxygen fugacity low Fe3+
? oxidation state oxygen fugacity
Dan Frost
bridgmanite ferropericlase
bridgmanite ferropericlase
Dan Frost
bridgmanite ferropericlase
McCammon (1997) Lauterbach et al. (2001) Frost et al. (2004)
Velocity (mm/s) Velocity (mm/s)
McCammon et al (1997)
former bridgmanite ferropericlase
Velocity (mm/s) Velocity (mm/s)
McCammon et al (1997)
bridgmanite ferropericlase
3.9 0.02 whole rock Fe2O3 = 3.9 wt%
Fe3+/ΣFe
Δlog
fO2 (
FMQ
)
0 100
0
-5
+5 low oxygen fugacity high Fe3+
bridgmanite
? J
+ Fe0
oxidation state oxygen fugacity
why should you care?
oxidation state oxygen fugacity
physical properties elasticity, electrical conductivity, thermal diffusivity, rheology, viscosity, … → LLSVPs, plumes, MiMOSAs, lithosphere
chemical properties melting, element partitioning, …
volatile speciation carbon, hydrogen, nitrogen, … → slab processes
Earth differentiation including core formation
evolution of Earth’s atmosphere