Essentially pure CO2 fluid inclusions have been found in orthopyroxene, clinopyroxene and olivine of 11 spinel lherzolite nodules in nepheline basalts from Pali (Koolau Range) and Salt Lake Crater, Oahu, Hawaii. Most of the inclusions occur along healed fractures (are thus secondary) and are often associated with different proportions of silicate glass ; some contain a dark globule, probably a sulfide. They represent the coexistence of two or three immiscible fluid phases — silicate melt, sulfide melt, and dense supercritical CO2. Only a few ones, not associated with glass, are most likely primary ; they are generally bigger in size and show clear decrepitation phenomena. 361 secondary inclusions were studied ; homogenization temperatures (Th) of vapour (V) and liquid (L) CO2 range from — 54.2 to + 31.1 °C (95 % Th : L + V → L ; 5 % Th : L + V → V), yielding CO2 densities from 1.16 to 0.08 g/cm3 with preferential distribution near 1.11, 0.96, 0.89, 0.79 and 0.74 g/cm3. Assuming a trapping temperature of 1 000 °C, the corresponding trapping pressure for a pure CO2 system would lie in the range 0.2 to 11.5 kbar, i.e. a depth of 1 to 43 km. The CO2 fluid inclusions give evidence that the spinel lherzolite nodules originate in the upper mantle and represent accidental fragments randomly sampled by the host nepheline basalt magma. The distribution of CO2 densities reflects the succession of fracturing episodes during the ascent.