Terrestrial magmatism covers 8 orders of magnitude oxygen fugacity

from Carmichael (1991)

-30

-25

-20

-15

-10

-5

0

5

0.0005 0.0006 0.0007 0.0008 0.0009 0.001

1/T (K)

Pd-PdO

Ir-IrO2

Rh-Rh2O3

Os-OsO2

Ru-RuO2

HM

Cu-Cu2O

MnO-Mn3O4

Re-ReO2

Ni-NiO

FMQ

Co-CoO

W-WO2

Fe-FeO

FQI

Cr-Cr2O3

1400 130012001100 1000 900 800

Compiled from work of O’Neill, Pownceby, Holzheid and others

Chondritic metals become Co- and Ni-poor and Fe-richChondritic silicates become Fe-, Ni-, Co-poor

from Arculus et al. (1990)

Olivine composition changes with oxygen fugacity such that it has an enormous stability field that depends upon equilibria with Fe metal and Fe oxide

2Fe3O4 + 3SiO2 = 3Fe2SiO4 + O2

FeSiO3 + Fe + ½ O2 = Fe2SiO4

from Nitsan (1974)

from Buening and Buseck (1973) from Chakraborty et al. (2004)

Diffusion of major, minor and trace elements in crystalline solids is strongly dependent upon oxygen fugacity

from Righter (2003)

Metal-silicate partition coefficients are fO2 dependent

Five different approaches in high pressure research

A) Sliding sensors (Taylor et al. 1992; Rubie et al., 1993)

B) Sample composition helps to set fO2 (Rubie, 1999)

C) Fluid or buffer in capsule (King et al. 2000)

D) Capsule imposes fO2 (Frost et al. 2004)

E) Assembly imposed (COMPRES development?) (Dobson and Brodholt, 1999)

Respond to fO2 of environment and record in either metal or oxide solid solution

Problems:Cannot really control fO2 using this approach, but at least it can be known

from Taylor et al. (1992)

A) Sliding sensors

A) Sliding sensors

Fe2SiO4 = 2Fe + SiO2 + O2

Ni2SiO4 = 2Ni + SiO2 + O2

Mg2SiO4 + SiO2 = Mg2Si2O6

from Rubie et al. (1993)

fO2 calculated from td and a-x data.

B) Sample composition participates

from Rubie (1999)

Si:Fe ratio in metal of metal/silicate experiments was varied to vary the fO2 imposed upon sample

Problems: Si in metal causes non-ideal behavior and therefore potentially not natural

C) Fluid or buffer in capsule

from Holloway et al. (1992); Pawley et al. (1992); King et al. (2000)

CO2 or O2 sources have been used to fix fO2 in capsules

Problems: -C migration into Pt and reduction over time- fluids dissolve into other phases

C) Fluid or buffer in capsule

from Rubie (1999)

Ni-NiO mix has been used to fix fO2 in capsules

Problems: NiPt alloying

D) Capsule imposed

Re capsules have been used to carry out experiments at higher fO2, because Re-ReO2 buffer is much higher than IW or QFM

Problems: buffer never verified and buffer can react with sample

from Frost et al. (2004)

D) Capsule imposed

from Arculus et al. (1990)

Graphite capsules can be used to buffer oxygen fugacity

C-CO-CO2 is below IW at 1 bar, but is very pressure sensitive

At 10 kb 3 log units higher…..

D) Capsule imposed

from LaTourette and Holloway (1994)

……by 80 kb, oxygen fugacity is buffered about 6 log fO2 units above that of 1 bar

from Luth (1993)

Diamond can also participate in buffering equilibria, but its hardness becomes a problem for later sample preparation –

grinding and polishing

Best approach?

Buffer is in pressure medium

Advantages:- Long lasting- Doesn’t react with sample

from Dobson and Brodholt (1999)

E) Pressure medium imposed

Castable octahedragasket and pressure medium are the same

(Dobson and Brodholt, 1999)=> high failure rate

Ni-, Fe-, Re-doped pressure medium – cast or injection molded?

Pre-cast with spacersgasket and pressure medium are different

(maybe an area for COMPRES development??)=> lower failure rate ?

3 short 1 short, 2 long 3 long2 short, 1 long

4 cubes with teflon and pyrophyllite spacers

4 cubes with balsa wood spacers

strips ofteflon tape

Hybrid ?