Mantle-derived xenoliths from Hyblean area, Sicily, southern Italy.

Mantle-derived xenoliths occur in some Miocenic basaltic diatreme pipes and Quaternary lava flows from the Hyblean Plateau, southeastern Sicily, Italy. About 70 % of Hyblean mantle xenoliths consist of spinel harzburgites and harzburgitic lherzolites commonly with protogranular texture. Glassy patches are quite common in the Hyblean ultramafic xenoliths providing useful information on natural partial melting and metasomatic processes of mantle rocks. They have a moderate depleted character (e.g., CaO<3.5wt%, Al2O3<2wt%, Al2O3/TiO2=20-57). Some LREE (LaN/YbN=9.2-18.8) and other incompatible elements enrichment and the very rare occurrence of modal phlogopite, are likely due to metasomatic event(s). Fluid inflow into peridotite system is also recorded by numerous secondary fluid inclusions of pure CO2 with densities from 0.756 to 1.145 g/cm3, indicating an entrapment pressure of 0.9-1.1 GPa. Two-pyroxene geothermometry yields equilibration temperature from 950 to 1050°C. Pyroxenite xenoliths, belonging to both Cr-diopside and Al-augite series, make up about 30 % of the Hyblean ultramafic xenoliths. They consist of: a) spinel-bearing Cr-diopside websterites, b) spinel-garnet Al-diopside websterites, c) Opx-bearing spinel clinopyroxenites, d) Opx-bearing garnet clinopyroxenites. Group (a) websterites often occur as veins in the peridotites (composite xenoliths). The garnet (Py54.5Alm42Gr3.5) from group (b) websterites forms reaction coronas between spinel and pyroxene grains: garnet-pyroxene geothermobarometry indicates P=1.0 GPa (ca), T= 750 °C (ca), whereas garnet and pyroxene pairs from group (d) clinopyroxenites indicate P,T ranges from P=1.4 GPa - T=1050 °C to P=1.9 GPa -T=1150 °C. A number of xenoliths with relatively large size allowed laboratory measurements of Vp and Vs and seismic anisotropy at a confining pressure up to 600 Mpa and temperature up to 600 °C: Vp average is 7.15 km/s for peridotites and 7.46 km/s for pyroxenites; Vs average is 4.02 Km/s for peridotites and 4. 22 km/s for pyroxenites.The Hyblean upper mantle suffered melting episodes as shown by its sligtly depleted character and underwent both cryptic (LREE, Sr enrichment) and modal (phlogopite) metasomatic events. It was intruded by early extracted melts that give pyroxenites at different depth. Pyroxenite xenoliths, belonging to both Cr-diopside and Al-augite series, make up about 30 % of the Hyblean ultramafic xenoliths. They consist of: a) spinel-bearing Cr-diopside websterites, b) spinel-garnet Al-diopside websterites, c) opx-bearing spinel clinopyroxenites, d) opx-bearing garnet clinopyroxenites. Group (a) websterites often occur as veins in the peridotites (composite xenoliths). The garnet (Py54.5Alm42Gr3.5) from group (b) websterites forms reaction coronas between spinel and pyroxene grains: garnet-pyroxene geothermobarometry indicates P=1.0 GPa (ca), T= 750 °C (ca), whereas garnet and pyroxene pairs from group (d) clinopyroxenites indicate P,T ranges from P=1.4 GPa - T=1050 °C to P=1.9 GPa -T=1150 °C. A number of xenoliths with relatively large size allowed laboratory measurements of Vp and Vs and seismic anisotropy at a confining pressure up to 600 Mpa and temperature up to 600 °C: Vp average is 7.15 km/s for peridotites and 7.46 km/s for pyroxenites; Vs average is 4.02 Km/s for peridotites and 4. 22 km/s for pyroxenites.The Hyblean upper mantle suffered melting episodes as shown by its sligtly depleted character and underwent both cryptic (LREE, Sr enrichment) and modal (phlogopite) metasomatic events. It was intruded by early extracted melts that give pyroxenites at different depth.