The geochemistry of carbon in mantle peridotites

Carbon abundances have been determined in mantle xenoliths from alkalic basalts and kimberlites and interpreted in terms of the nature and distribution of the C-rich phases. Anhydrous Cr-diopside Group I spinel lherzolites from basalts typically contain 15–50 ppm C, and amphibole-bearing ones have only marginally higher concentrations (40–100 ppm). Carbon abundances in Al-augite Group II pyroxenites are not significantly different from those of the Group I rocks. Although most LREE-depleted lherzolite xenoliths contain less C than enriched samples, there is no clear relationship between abundances of C and the incompatible trace elements.In the suite of deformed cumulate peridotite and dunite xenoliths of the 1801 Kaupulehu flow of the Hualalai volcano, Hawaii, C abundances are clearly related to texture, modal composition, and style of deformation. The most C-rich rocks are wehrlites in which the clinopyroxenes deformed more brittly and thus possess higher fluid inclusion and crack densities than the surrounding olivines.Regardless of their lithology, all xenoliths from kimberlites (including both peridotites and eclogites) are C-rich compared to those from basalts. Most of the C in these xenoliths exists as calcite or carbonaceous matter associated with serpentine veins and was thus probably contributed by the kimberlite host. Primary carbonates are extremely rare in all xenoliths, although occasionally they have been observed as daughter products in fluid inclusions.Although most C exists as inclusions of CO₂-rich vapor, condensed carbonaceous matter also appears to occur in all rocks as discrete platy grains and as a film on natural surfaces such as grain boundaries and cracks.