The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon

Abstract

New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (−5.1%., s = ± l.4%. vs PDB) is indistinguishable from that of 13 kimberlite pipes (−4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (−5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O 18 by several permil with respect to those of carbonates from the subvolcanic type of carbonatite. The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC 13 values from −5.8 to −7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (−5.8 to −7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C 13 content (−2.4 to −4.4%.). If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as −5.2%. and the range is −2 to −8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.