The carbon‐isotope signature of ultramafic xenoliths from the H yblean P lateau (southeast S icily, I taly): Evidence of mantle heterogeneity

We investigated the carbon isotope composition of mantle source beneath the Hyblean Plateau (southeast Sicily, Italy) by studying CO 2 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 δ 13 C CO2 results with information about noble gases (He and Ar) obtained in a previous investigation of the same products. Although Ar/CO 2 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 δ 13 C and CO 2 / 3 He variations, ranging from −4‰ to −2‰ and from 10 9 to 10 10 , 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.