ABSTRACT
The oxidation state of eleven mantle-derived ilmenite-bearing xenoliths from four different localities was studied using O2-specific solid electrolytes. Repetitive calibrations of the experimental apparatus with the iron-wustite (IW) buffer indicate that the precision of the measured fO2's at 1312 ± 1 K is 2.37% (±0.01 log units). Results obtained with the IW and Ni-NiO buffers agree with values reported by other investigators (e.g., Darken and Burry, 1945; Kemori et al., 1979).
Experiments on blank systems (no sample) demonstrated that the oxygen sensors that include Al203 elements (SIRO2) are reactive to free oxygen. They also indicate that the fO2 of equilibration of an Ar-filled SIRO2 sensor depends, among other factors, on the fO2 of the previous run.
Results from some natural samples indicate that: (a) the fO2 developed by the blank system can modify the composition of the sample during the experiments; and (b) the criteria of stability of the Emf with time, and thermal reproducibility of the measured fO2's are not sufficient to demonstrate equilibrium. To circumvent these problems, an experimental procedure was developed to reverse and bracket the equilibrium fO2's.
Good agreement was obtained between electrochemically derived fO2's on Mg-ilmenite-titanomagnetite intergrowths and values calculated using the iron-titanium oxide geothermometer-oxybarometer. However, the electrochemical determinations are systematically about 0.6 log fO2-units more oxidized. This bias may be ascribed to the effects of the geikielite component of the iimenitess.
Auto-oxidation was observed in clinopyroxene-ilmenite intergrowths. Whether this behavior is observed, however, depends on the thermal treatment given to the sample during the experiment.
All of the results are characterized by redox conditions at or above the FMQ (or EMOG) buffer at one bar and are in agreement with those of Haggerty and Tompkins (1983) and Arculus et al. (1984) for similar samples. The measured intrinsic fO2's of a homogeneous Mg-ilmenite from Monastery Mine, which showed good reversibility and a continuous log fO2 - 1/T pattern from 800°C to 1150°C during slow heating, agree very well with data obtained independently by McMahon (1984) using gas equilibration techniques at 1 atm. The agreement indicates that electrochemically derived fO2's are not high pressure relicts but rather are equilibrium values at 1 atm.

Loureiro, D., 1986. Electrochemical determination of the oxygen fugacity of mantle-derived ilmenite megacrysts. Unpublished PhD dissertation, State University of New York at Albany. 375pp., +xvi
University at Albany Science Library call number:  SCIENCE MIC Film QE  40 Z899 1986 L68
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