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Terrestrial magmatism covers 8 orders of magnitude oxygen fugacity

Terrestrial magmatism covers 8 orders of magnitude oxygen fugacity. from Carmichael (1991). Compiled from work of O’Neill, Pownceby, Holzheid and others. Chondritic metals become Co- and Ni-poor and Fe-rich Chondritic silicates become Fe-, Ni-, Co-poor. from Arculus et al. (1990).

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Terrestrial magmatism covers 8 orders of magnitude oxygen fugacity

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  1. Terrestrial magmatism covers 8 orders of magnitude oxygen fugacity from Carmichael (1991)

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

  3. Chondritic metals become Co- and Ni-poor and Fe-rich Chondritic silicates become Fe-, Ni-, Co-poor from Arculus et al. (1990)

  4. 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)

  5. Diffusion of major, minor and trace elements in crystalline solids is strongly dependent upon oxygen fugacity from Buening and Buseck (1973) from Chakraborty et al. (2004)

  6. Metal-silicate partition coefficients are fO2 dependent from Righter (2003)

  7. Five different approaches in high pressure research • 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)

  8. A) Sliding sensors 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)

  9. A) Sliding sensors from Rubie et al. (1993) Fe2SiO4 = 2Fe + SiO2 + O2 Ni2SiO4 = 2Ni + SiO2 + O2 Mg2SiO4 + SiO2 = Mg2Si2O6 fO2 calculated from td and a-x data.

  10. B) Sample composition participates 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 from Rubie (1999)

  11. C) Fluid or buffer in capsule • 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 from Holloway et al. (1992); Pawley et al. (1992); King et al. (2000)

  12. C) Fluid or buffer in capsule Ni-NiO mix has been used to fix fO2 in capsules Problems: NiPt alloying from Rubie (1999)

  13. 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)

  14. D) Capsule imposed 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….. from Arculus et al. (1990)

  15. D) Capsule imposed ……by 80 kb, oxygen fugacity is buffered about 6 log fO2 units above that of 1 bar from LaTourette and Holloway (1994)

  16. Diamond can also participate in buffering equilibria, but its hardness becomes a problem for later sample preparation – grinding and polishing from Luth (1993)

  17. E) Pressure medium imposed Best approach? Buffer is in pressure medium Advantages: - Long lasting - Doesn’t react with sample from Dobson and Brodholt (1999)

  18. Castable octahedra gasket and pressure medium are the same (Dobson and Brodholt, 1999) => high failure rate Pre-cast with spacers gasket and pressure medium are different (maybe an area for COMPRES development??) => lower failure rate ? Hybrid ? Ni-, Fe-, Re-doped pressure medium – cast or injection molded?

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