Carbon isotope anomaly in marbles associated with eclogites from the Dabie Mountains in China

High delta(13)C values, up to +5.7 parts per thousand (PDB), were measured in marbles associated with ultrahigh-pressure eclogites in the Dabie Mountains, East China. Such a carbon isotope anomaly is interpreted to represent the carbon isotope composition of premetamorphic limestone (i.e., the protolith of the marbles), because there is no known process to enrich carbonate in C-13 during regional metamorphism. The high delta(13)C also corresponds to high delta(18)O of up to +22.9 parts per thousand (SMOW), suggesting that premetamorphic delta(13)C values would be about +5 to +6 parts per thousand for their protolith (limestones). The carbon isotope anomaly in the protolith of the marbles implies a local change in the redox state of water during deposition of the limestones. Preservation of the unusually high positive delta(13)C values in the marbles supports the previous conclusion that there was no significant isotopic exchange with the mantle during subduction of supracrustal rocks to mantle depths of greater than 100 km.