Helium isotopes in peridotite mineral phases from Hyblean Plateau xenoliths (south-eastern Sicily, Italy)

Citation:
Sapienza, G, Hilton DR, Scribano V.  2005.  Helium isotopes in peridotite mineral phases from Hyblean Plateau xenoliths (south-eastern Sicily, Italy). Chemical Geology. 219:115-129.

Date Published:

Jun

Keywords:

african plate, basalts, CO2 fluid inclusions, constraints, fluid inclusion, fluids, geochemistry, geothermal, He isotopes, Hyblean Plateau, mantle, metasomatism, mount-etna, noble-gases, peridotite xenoliths, radiogenic he-4, upper-mantle peridotites

Abstract:

Fourteen He isotope and abundance analyses have been performed on olivine, orthopyroxene, and clinopyroxene grains from selected spinel-facies peridotite xenoliths collected from the Hyblean Plateau, southeastern Sicily. These peridotites consist of protogranular-textured, spinel-facies lherzolites and lherzolitic harzburgites. Microthermometric data on olivine and pyroxenes reveal that the fluid inclusions (FIs) are nearly pure CO2, with densities ranging from 0.76 to 1.15 g/cm(3). The densest inclusion trail occurs in an orthopyroxene grain and indicates a trapping-pressure of 0.98-1.14 GPa (estimated equilibration temperature of 1000 degrees C for pure CO2 system) corresponding to a depth of similar to 36-41 km (for an average estimated density of the lithospheric column of 2.85 g/cm(3)). These are minimum estimates. The near-unimodal distribution Of CO2 densities indicates that these xenoliths have been stored at the pressure interval of 0.75-0.95 GPa-corresponding to a depth of similar to 27-35 km (crust-mantle boundary or just below). En route to the surface, they have not had any significant interaction with shallower volatile reservoirs. He trapped in CO2-rich fluid inclusions ranges in abundance from 1.8 to 86.6 X 10(-9) cm(3) STP/g, with clinopyroxenes usually showing higher He contents than coexisting olivines and orthopyroxenes. The inter-mineral differences are due to physical properties of minerals. In contrast, He isotope ratios cover a narrow range (-7.3 +/- 0.3R(A), where R-A=air(3) He/He-4), indicating isotopic equilibration between the mineral phases. He composition indicates a MORB-type source for the metasomatic agent(s), influenced by relatively minor radiogenic production in the source. Therefore, in closed-system conditions, 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 Central Mediterranean area. The most pristine He signature for the mantle endmember is found on Pamelleria Island (Sicily Channel): this He ratio is similar to that found in Hyblean peridotite minerals, testifying to a common source history. We conclude, therefore, that Hyblean xenoliths can be used as a powerful tool in defining the geochemical features of this portion of the upper mantle. (c) 2005 Elsevier B.V. All rights reserved.

Notes:

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DOI:

10.1016/j.chemgeo.2005.02.012