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Fluid inclusions occur in a composite xenolith from the Lunar Crater Volcanic Field, Nevada, U.S.A. The xenolith is an amphibole-bearing wehrlite that is cut by an andesine-amphibole vein. The compositions of individual fluid inclusions in both portions of the xenolith have been determined using microthermometry and micro Laser-Raman spectroscopy. Fluids in the host wehrlite are nearly pure CO2 (>99 mol%) whereas those in the vein contain from 8.5 to 12.0 mol % CO in CO2. Chemical modelling shows that the composition of the vein fluids at Troom is representative of the composition at the high P, T conditions of trapping. Graphite has not been observed by optical microscopy in any of the fluid inclusions. Graphite is probably absent (although stable at T<800° C) most probably because of the kinetically unfavorable CO decomposition reaction and rapid quenching. By combining the measured fluid compositions with fluid P-V-T data and the chemical equilibrium CO2?CO +1/2 O2, we have calculated the oxygen fugacity of the fluid inclusions at 1200° C: log $$f_{O_2 }$$ ?8.6 (vein) and -6 (host). If the $$f_{O_2 }$$ of the fluid in the vein represents that in equilibrium with the magma that crystallized to produce the vein, then the $$f_{O_2 }$$ of the basalt magma is near QFM at 1200° C and 10.3 kbar. This is similar to values reported for extrusive basaltic lavas. If the much lower intrinsic oxygen fugacity-values for divines and spinels from alkali basalt nodules are representative of upper mantle conditions, then oxidation of basaltic magmas must occur in the upper mantle prior to ascent to the surface. Implications for the origin of CO2-rich fluids and carbon isotope geochemistry are also discussed.