Oxygen fugacity of the diamond +C-O fluid assemblage and CO2 fugacity at 8 GPa

Details Oxygen fugacity of the diamond +C-O fluid assemblage and CO2 fugacity at 8 GPa Oxygen fugacity of the diamond +C-O fluid assemblage and CO2 fugacity at 8 GPa

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.128..439l&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 128, no. 3-4, p. 439-451

Other

6

Diamonds, Earth Atmosphere, Earth Mantle, Oxygen, Peridotite, Buffers (Chemistry), Eclogite, Surface Temperature, Thermal Expansion

Scientific paper

We have bracketed the oxygen fugacity (fO2) of the diamond +C-O fluid buffer (CCO) relative to the wuestite-magnetite (WM) and nickel-nickel oxide (NNO) buffers at 8 GPa and 950-1550 C using a Walker-style multi-anvil press. The intersection of CCO with WM is between 1050 and 1150 C and thus the log fO2 of CCO at 1100 C is constrained to be -5.10 +/- 0.59. From 1100 to 1550 C CCO is between NNO and WM, and below 1100 C CCO is more oxidized than both NNO and WM. Although the intersection of CCO and NNO was not located, previous studies indicate that CCO has shallower slope than NNO and with this constraint the log fO2 of CCO at 1550 C is -1.76 +/- 0.95. The fO2 of CCO at 8 GPa and 950-1550 C can be expressed as log fO2 = 8.4 +/- 0.8 - 18570 +/- 7000/T (K) and is consistent with other recent experiments. The results of this study define the fO2 of experiments conducted in graphite capsules and regions of the mantle saturated with diamond and a C-O fluid. CCO is more oxidizing than diamond-carbonate buffers and thus the existence of CO2-rich fluids in natural samples at approx. 8 GPa probably requires an olivine-free local environment such as eclogite. CCO lies in the reduced half of the range of estimated mantle fO2 values and thus diamond will be stable only in the more reduced regions. CO2 fugacities estimated from these results are at the low end of the range predicted by equations of state at low temperature but show a greater thermal expansion for CO2 and, hence, a greater increase in fCO2 with temperature than the equations of states. This results in lower predicted decarbonation reaction temperatures near 8 GPa compared to existing equations of state.

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