The Iblean Plateau (Southeastern Sicily, Italy) consists of a thick Meso-Cenozoic carbonate sequence with interbedded volcanic horizons (alkaline and tholeiitic basalts). The alkaline basalts contain ultramafic (peridotites and pyroxenites) and mafic xenoliths. The peridotites are spinel-bearing lherzolites and lherzolitic harzburgites, with porphyroblastic to protogranular texture. Pyroxenites consist of Cr-diopside-bearing and Al-augite-bearing websterites. The mineral chemistry of the nodules indicates temperatures between 700 and 1050°C.

Fluid inclusions containing CO2 and (sometimes) various proportions of silicate glass have been studied in olivine, orthopyroxene and clinopyroxene. The secondary inclusions occur as trails of CO2-rich inclusions, often cross-cutting deformation lamellae. The few primary inclusions, generally empty, show clear evidence of decrepitation. Of the 390 inclusions examined, 97% homogenized to the liquid phase (Th → L = −43.9 to +30.9°C); 3% homogenized to the vapour phase (Th → V = + 20.5 to +30.3°C, yelding CO2 densities in the range 0.20–1.13 g/cm3. Assuming a trapping temperature of 1100°C, the corresponding trapping pressure for a pure CO2 system lies in the range 0.6–11.0 kbar, i.e. a depth of ∼2.2 to 42 km.

The majority of CO2 trapping events in the xenoliths occurred from 2.2 to 11.0 kbar, with no major trapping events at pressures less than 2.3 kbar, indicating the absence of a shallow magma reservoir below the Iblean Plateau.

Ultramafic xenoliths enclosed in Plio-Quaternary alkali basalts from Tallante near Cartagne (southern Spain) are composed mainly of spinel lherzolites which are probably upper mantle residues. In many xenoliths, the spinel lherzolite is cut by pyroxenite or gabbroic anorthosite veinlets generally 0.2–3 cm thick. The clinopyroxenite veinlets were formed by high-pressure crystal-liquid segregation from alkali basalt magmas formed earlier than the host basalts, whereas mantle metasomatism played a role in the genesis of gabbroic anorthosites. Close to the contact with the veinlets, the spinel lherzolites are enriched in Ca, Fe, and some incompatible elements including light REE due to the migration of a fluid from the veinlets into the surrounding lherzolites. The host alkali basalts were derived from a heterogeneous, incompatible element-enriched upper-mantle source probably similar in composition and nature to the composite xenoliths, but were formed in a garnet stability field.