Fourteen He isotope and abundance analyses have been performed on olivine, orthopyroxene, and clinopyroxene grainsfrom selected spinel-facies peridotite xenoliths collected from the Hyblean Plateau, southeastern Sicily. These peridotitesconsist of protogranular-textured, spinel-facies lherzolites and lherzolitic harzburgites. Microthermometric data on olivine andpyroxenes reveal that the fluid inclusions (FIs) are nearly pure CO2, with densities ranging from 0.76 to 1.15 g/cm3. The densestinclusion trail occurs in an orthopyroxene grain and indicates a trapping-pressure of 0.98–1.14 GPa (estimated equilibrationtemperature of 1000 8C for pure CO2 system) corresponding to a depth of ~36–41 km (for an average estimated density of thelithospheric column of 2.85 g/cm3). These are minimum estimates. The near-unimodal distribution of CO2 densities indicatesthat these xenoliths have been stored at the pressure interval of 0.75–0.95 GPa—corresponding to a depth of ~27–35 km (crust–mantle boundary or just below). En route to the surface, they have not had any significant interaction with shallower volatilereservoirs.He trapped in CO2-rich fluid inclusions ranges in abundance from 1.8 to 86.6 10 9 cm3 STP/g, with clinopyroxenesusually showing higher He contents than coexisting olivines and orthopyroxenes. The inter-mineral differences are due tophysical properties of minerals. In contrast, He isotope ratios cover a narrow range (~7.3F0.3RA, where RA=air 3He/4He),indicating isotopic equilibration between the mineral phases. He composition indicates a MORB-type source for themetasomatic agent(s), influenced by relatively minor radiogenic production in the source. Therefore, in closed-systemconditions, the calculated helium residence time is 790 Myr.Geophysical and geochemical studies suggest the existence of a common fluid reservoir in the mantle beneath the CentralMediterranean area. The most pristine He signature for the mantle endmember is found on Pantelleria Island (Sicily Channel): this He ratio is similar to that found in Hyblean peridotite minerals, testifying to a common source history. Weconclude, therefore, that Hyblean xenoliths can be used as a powerful tool in defining the geochemical features of thisportion of the upper mantle.
|Titolo:||Helium isotopes in peridotite mineral phases from Hyblean Plateau xenoliths (south-eastern Sicily, Italy)|
|Data di pubblicazione:||2005|
|Appare nelle tipologie:||1.1 Articolo in rivista|