OXYGEN FUGACITY OF THE DIAMOND PLUS C-O FLUID ASSEMBLAGE AND CO2 FUGACITY AT 8-GPA

We have bracketed the oxygen fugacity (fO(2)) of the diamond + C-O flu id buffer (CCO) relative to the wustite-magnetite (WM) and nickel-nick el oxide (NNO) buffers at 8 GPa and 950-1550 degrees C using a Walker- style multi-anvil press. The intersection of CCO with WM is between 10 50 degrees and 1150 degrees C and thus the log fO(2) of CCO at 1100 de grees C is constrained to be -5.10 +/- 0.59. From 1100 degrees to 1550 degrees C CCO is between NNO and WM, and below 1100 degrees C CCO is more oxidized than both NNO and WM. Although the intersection of CCO a nd NNO was not located, previous studies indicate that CCO has shallow er slope than NNO and with this constraint the log fO(2) of CCO at 155 0 degrees C is -1.76 +/- 0.95. The fO(2) of CCO at 8 GPa and 950-1550 degrees C can be expressed as log fO(2) - 8.4 +/- 0.8 - 18570 +/- 7000 /T(degrees K) and is consistent with other recent experiments. The res ults of this study define the fO(2) of experiments conducted in graphi te 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 similar to 8 GP a probably requires an olivine-free local environment such as eclogite . CCO lies in the reduced half of the range of estimated mantle fO(2) values and thus diamond will be stable only in the more reduced region s. CO2 fugacities estimated from these results are at the low end of t he range predicted by equations of state at low temperature but show a greater thermal expansion for CO2 and, hence, a greater increase in f CO(2) with temperature than the equations of state. This results in lo wer predicted decarbonation reaction temperatures near 8 GPa compared to existing equations of state.