? 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