The carbon-isotope signature of ultramafic xenoliths from the Hyblean Plateau (southeast Sicily, Italy): Evidence of mantle heterogeneity

We investigated the carbon isotope composition of mantle source beneath the Hyblean Plateau
(southeast Sicily, Italy) by studying CO2 in fluid inclusions from ultramafic xenoliths recovered in some
Miocene diatremes. In order to constrain the processes influencing the isotopic marker of carbon we combined
d13CCO2 results with information about noble gases (He and Ar) obtained in a previous investigation
of the same products. Although Ar/CO2 and He/Ar ratios provide evidence of Rayleigh-type fractional
degassing, the isotopic geochemistry of carbon is poorly influenced by this process. Mixing related to metasomatic
processes where MORB-type pyroxenitic melts permeate a peridotite mantle probably contaminated
by crustal fluids inherited from a fossil subduction can explain the measured d13C and CO2/3He
variations, ranging from 24&to 22& and from 109 to 1010, respectively. Simple mass-balance calculations
highlighted that the Hyblean peridotite source was mainly contaminated by the carbonate source, being
carbonate and organic matter present at a ratio that varied within the range from 7:1 to 4:1.

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