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 δ13CCO2 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 δ13C and CO2/3He variations, ranging from -4‰ to -2‰ 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.