CO2 Fluid Inclusions in Ultramafic Xenoliths from the Iblean Plateau, Sicily, Italy

B. De Vivo, A. Lima and V. Scribano
Dipartimento di Geofisica e Vulcanologia, Largo S. Marcellino 10, 80138 Napoli, Italy
Istituto di Scienze della Terra, Corso Italia 55, 95129 Catania, Italy.

Abstract: 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.

Keywords: CO2 • fluid inclusions • ultramafic xenoliths • Iblean Plateau • Sicily

Mineralogical Magazine; June 1990 v. 54; no. 375; p. 183-194; DOI: 10.1180/minmag.1990.054.375.05
© 1990, The Mineralogical Society
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